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Review article

Selection for mastitis resistance in dairy cattle: a review with

focus on the situation in the Nordic countries

*

Bjørg Heringstad , Gunnar Klemetsdal, John Ruane

Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 As, Norway Received 22 April 1998; received in revised form 2 March 1999; accepted 15 June 1999

Abstract

The literature concerning selection for mastitis resistance in dairy cattle is reviewed and the reasons for including mastitis resistance in dairy cattle breeding programs are described. The current situation in Denmark, Finland, Norway and Sweden is described with emphasis on the data recording schemes and the choice of models used for breeding value estimation. The use of clinical mastitis data and somatic cell counts in selection for mastitis resistance as well as implications and prospects for the future are discussed.  2000 Elsevier Science B.V. All rights reserved.

Keywords: Dairy cattle; Selection; Mastitis

1. Introduction death, leading to large economic losses. The minor

pathogens are coagulase-negative staphylococci and Mastitis is a complex disease which can be simply Corynebacterium bovis. Infections with such patho-defined as an inflammation of the mammary gland gens can lead to moderate inflammation of the resulting from the introduction and multiplication of mammary gland and a slightly increased SCC and pathogenic micro-organisms in the mammary gland. they only rarely lead to changes in milk composition, The causative bacteria can be classed as major or greatly reduced milk yield or clinical mastitis (Har-minor pathogens (Harmon, 1994). The major patho- mon, 1994).

gens include Staphylococcus aureus and Streptococ- There is evidence that the spectrum and

pro-cus agalactiae (both of which are contagious) and portions of mastitis causing bacteria are changing

coliforms, streptococci and enterococci (all of which over time (e.g. Myllys et al., 1998) and also that come from the cow’s environment, i.e. bedding, geographic differences exist. For example, pre-manure and soil). The major pathogens can cause dominating types of S. aureus seem to be specific clinical disease, with changes in milk composition, to each of the Nordic countries (Aarestrup et al., an increase in somatic cell counts (SCC) and even 1997).

According to Harmon (1994) clinical mastitis is characterised by swelling or pain in the udder, milk *Corresponding author. Tel.:147-6494-8000; fax: 1

47-6494-with an abnormal appearance and, in some cases, 7960.

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even death. In addition, bacteria are present in the 2. Reasons to improve mastitis through milk, the milk yield is much reduced, and the milk breeding

content is altered considerably. Subclinical mastitis

does not lead to visible changes in the milk or udder. 2.1. High frequency It is characterised by reduced milk yield, altered milk

composition and the presence of inflammatory com- Mastitis is one of the most frequent diseases ponents and bacteria in milk. affecting dairy cattle. Generally, the incidence of Schukken et al. (1997) describe infection patterns clinical mastitis per cow-year varies between 20 and of the major mastitis-causing pathogens by using an 40%. In 1993, the number of cases of clinical example with E. coli, S. aureus and non-agalactiae mastitis (defined as treatment by a veterinarian of streptococci. They claim that E. coli infections one cow with clinical mastitis) per 100 cow years mainly cause clinical mastitis, that S. aureus in- was 56, 32, 30 and 21 in Denmark, Finland, Norway fections mainly cause sub-clinical mastitis and that and Sweden respectively (Forshell et al., 1995). In non-agalactiae streptococci have an infection pattern the Nordic countries, antibiotics are administered with both subclinical and clinical appearances. Ac- only by veterinarians; therefore, disease recording is cordingly, most E. coli infections should be captured quite reliable. It should be noted, however, that for by clinical mastitis records, while S. aureus in- many reasons (e.g. the trait can be defined in fections should also be reflected in changes of SCC, different ways, and mastitis frequency can be in-which increases as a result of infections with major fluenced by the type of feeding and management pathogens (e.g. Reneau, 1986). systems used) it is difficult to compare mastitis This shows that mastitis is caused by many incidences across countries or even across farms different micro organisms changing over time and within country.

location. Mastitis is a disease which shows differ- Comparable results in the literature from other ent infection patterns; from subclinical mastitis countries are not easily available because they do not with no clinical signs to peracute clinical mastitis have a system for registering health data. In surveys that may cause death of the animal. The duration from California, Michigan and Ohio incidences of of a mastitis case varies from a few days to long mastitis were found to be 30, 33 and 37 cases per duration chronic or subclinical infections which can 100 cows per year respectively (Gardner et al., 1990; last for weeks or months. Thus, udder health prob- Kaneene and Hurd, 1990a; Miller and Dorn, 1990). lems are not easily expressed as one single trait, These estimates include both veterinary-treated and but observationally clinical mastitis has the advan- owner-reported mastitis.

tage of being possible to record in national health

recording systems, and by utilising records of treat- 2.2. High costs ment of clinical mastitis in genetic evaluation, one

is selecting for all traits involved in the cows Mastitis is the most costly dairy cattle disease immunological performance. (Waage, 1989; Kaneene and Hurd, 1990b; Miller and Denmark, Finland, Norway and Sweden are the Dorn, 1990). Costs due to clinical mastitis include only countries with well-established, national re- veterinary and treatment costs, reduced milk pro-cording systems for health data in dairy cattle and duction during the remaining part of the lactation, the only countries which include clinical mastitis the loss of milk that has to be discarded due to directly in their breeding programs. Thus, the main contamination with antibiotics, early culling, extra goal was to review developments and status of labour, decreased milk quality and increased disease breeding for increased mastitis resistance in the risk in the future.

Nordic countries with main emphasis on traits, data Estimates of mastitis costs vary depending on recording and genetic evaluation. Another objective assumptions and country. According to Steine was to identify shortcomings as a basis for future (1996a) the estimated costs per case of clinical

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on all the costs mentioned above. Sender et al. is expected to negatively affect mastitis resistance. (1992) estimated the economic losses in Finland due Thus, Strandberg and Shook (1989) showed that to one case of clinical mastitis to be 1000 FIM (215 breeding for increased production under a traditional US$), based on the value of discarded milk, vet- progeny testing programme, without selection for erinary costs, medicine costs and extra labour costs. mastitis, results in a genetic increase of 0.02 cases of Costs of clinical mastitis reported by US farmers mastitis per cow per year, assuming a genetic vary from 108 to 122 US$ per case, based on drugs correlation between mastitis and milk yield of 0.30. and veterinarian costs, preventive costs, costs of The rate of change in mastitis may seem low, but the extra labour, culling and milk loss (Kaneene and increase is alarming from a long-term perspective. Hurd, 1990b; Miller and Dorn, 1990). Including disease resistance in breeding programmes It is expensive to replace a diseased animal. is therefore needed to counteract the undesirable Mastitis increases culling rates and replacement correlated responses resulting from selection for milk costs. Udder health problems are a major reason for production alone.

culling dairy cattle. For example, in Finland, Norway

and Sweden, udder health problems were the reason 2.4. Total economic merit for culling in 35, 19 and 22%, respectively, of cases

in milk-recorded herds. It was the main reason in Selection for increased mastitis resistance contri-Finland and the second most important reason for butes to reduced production costs and is consistent culling in Norway and Sweden (Maaseutukeskusten with the goal of maximising genetic improvement for Liitto, 1997; SHS, 1995; NML, 1997). In the US, total economic merit.

mastitis is the third most important reason for In Norway it is more profitable to use broad premature culling of dairy cattle (Shook, 1989). breeding goals than breeding goals including pro-duction traits only (Steine, T., unpublished results). 2.3. Genetic correlation to milk production Simulation studies have shown similar results, with higher genetic gain for the overall economic value It is generally accepted that a positive genetic when selection for mastitis resistance is included in correlation exists between mastitis susceptibility and the breeding scheme than when selecting for milk milk yield. Estimates of the genetic correlation based yield only (Strandberg and Shook, 1989; Sender et on Nordic data range from 0.24 to 0.55 (Simianer et al., 1992; Colleau and le Bihan-Duval, 1995). al., 1991; Lund and Jensen, 1996; Sander Nielsen et

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al., 1996; Poso and Mantysaari, 1996; Luttinen and 2.5. Antibiotics and vaccination Juga, 1997), with an average of 0.43. All these

studies are based on relative large field data sets. In a Mastitis is the most frequent reason for antibiotic review by Emanuelsson (1988), the genetic correla- use in lactating dairy cattle (e.g. Guterbock et al., tion between milk production and clinical mastitis 1993). Genetic improvement of mastitis resistance was shown to vary from slightly negative to 0.66, may reduce the need for treatment and, consequently, with a mean of 0.30 from seven studies. The highest the use of antibiotics and also reduce risk of residues value was obtained from a relatively large field data in dairy products. Antibiotic resistant bacteria are a set (Madsen et al., 1987) and also the average of growing problem. In a study of bovine mastitis in Nordic estimates of 0.43 was higher than the mean Finland, Myllys et al. (1998) reported that the of the studies reviewed by Emanuelsson (1988). This proportion of S. aureus strains resistant to at least suggests that estimates based on large field data sets one antibacterial drug increased from 36.9% in 1988 tend to show higher genetic correlations between to 63.6% in 1995 for and from 26.6% to 49.7% for milk production and mastitis compared with those coagulase-negative staphylococci. Multiresistance based on smaller data sets. was also increased. The problem of antibiotic resist-If mastitis is ignored in a breeding programme, the ance makes any possible permanent improvement in large weight traditionally placed on milk production a cow’s mastitis resistance through breeding even

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more important. In addition, few antibiotics are (Elleby and Veirup, 1977). The current health-re-effective against Gram-negative bacteria. For exam- cording system in Denmark started in 1986 and was ple, coliform mastitis appears to be unaffected by introduced nation-wide in 1990. Recordings are

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antibiotic treatment (Pyorala et al., 1994). Mastitis primarily made by veterinarians.

vaccination can be viewed as an alternative strategy In Finland, a health-recording system was started ¨ to genetic improvement (Rinehart et al., 1996). for all dairy farms in the spring of 1982 (Grohn et However, problems remain in verifying its short- and al., 1986). The veterinarian records the date, diag-long-term effects. In view of the adaptive nature of nosis, treatment and medicine used on the health bacteria, it is questionable whether a single vaccina- card. Cases handled over the telephone may also be

tion would offer prolonged protection. Even if recorded by the dairy farmer (Grohn et al., 1986). effective vaccination is available at low costs which In Norway, a disease recording system was tested may be more cost effective in the short run, genetic in one part of the country starting in 1970. The improvement has more advantage in the long run. «health card system» was introduced on a national basis in 1975. Since then, diseases have been re-corded in most milk-rere-corded herds in Norway 2.6. Ethics and animal welfare

(Solbu, 1983). In 1996, 98% of cows in the milk-recording system were included in the disease-re-The ethical aspects of disease are related to animal

cording system (NML, 1997). Each cow has her own welfare considerations and consumer interests.

Con-health card, and only veterinarians record data on sumer interest in production methods and concern

this card. about animal welfare are growing. In general,

con-In Sweden, the recording of disease treatments by sumers want products produced by healthy animals

veterinarians started in a single province in 1971 with as little use of antibiotics or other drugs as

(Lindhe et al., 1978), and the system was introduced possible. Even if economic losses due to mastitis

on a nation-wide scale in 1984 (Eriksson and Wre-could be offset by additional production, ethical

tler, 1987). The veterinarian records the disease considerations might not allow us to ignore the

diagnosis and the ID number of treated animals on a impact of selection for increased production on the

special form and then sends the data to the central health status and general welfare of cows (Shook,

data base. 1989; Solbu and Lie, 1990).

3.2. Data and models used for breeding value estimation of mastitis resistance

3. Situation in the Nordic countries

Mastitis resistance is taken into account in Nordic

3.1. Data recording breeding programmes by including a breeding value

for mastitis in the total merit bull index. In Norway, The Nordic health-recording systems, which have estimated breeding values for mastitis were first been established over the last 20 years, combine data calculated in 1978, whereas in Sweden, Finland and from three different sources; veterinary, milk record- Denmark breeding values were first published in ing and AI records. Each case of a disease treated by 1984, 1986 and 1992 respectively. In the evaluation, a veterinarian is recorded on a health card (Denmark, Denmark uses data from 1990 onwards; Finland and Finland and Norway) or on a special form (Sweden). Sweden use data from 1983 onwards, and Norway The information is then matched against other data in currently uses data only from the most recent batch the national milk recording system and stored on an of progeny tested bulls.

individual animal basis in a central data bank. The There are some differences between the Nordic same general methods are used for disease recording countries in the way mastitis resistance is defined, in in each of the four countries. the statistical models used and in the data used in the The first health-recording project in the Nordic evaluation (Ruane and Klemetsdal, 1996). However, countries began as a pilot study in Denmark in 1966 in all four countries, breeding value estimation is

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based on a sire model using field records of vet- data, but do no create systematic differences between erinary-treated cases of clinical mastitis, which is progeny groups.

considered as an all-or-none trait. Cows reported to To summarise, the models and data used for have had mastitis within a defined period of the breeding value estimation vary slightly between the lactation, from a few days before calving to about Nordic countries, but they nevertheless have far the middle of the lactation, are treated as diseased, more in common than they have differences. and the remainder are considered to be healthy. The

period used varies between countries, stretching from 3.3. Nordic breeding programmes 10 days before to 180 days after calving in Denmark,

7 days before to 150 days after calving in Finland, Dairy cattle breeding in the Nordic countries 15 days before to 120 days after calving in Norway, includes a breeding goal with functional traits of low and 10 days before to 150 days after calving in heritability, such as health and fertility, and is based Sweden. The main reason for using only a short on progeny testing of large daughter groups. For period of the lactation is to avoid bias due to culling example, the average size of daughter groups for AI of cows. In the first part of the lactation the culling bulls in 1992 was 90, 220, 250 and 140 in Denmark,

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rate is low and, according to Syvajarvi et al. (1986), Finland, Norway and Sweden respectively (Lindhe, two thirds of all mastitis treatments occur within two 1995). Participation in the milk and health-recording

months after calving. system is high. For example, in 1996, 90% of the

In Finland and Sweden, the reason for culling dairy cows in Norway were included in the milk-reported by the farmers is used as an additional recording system (NML, 1997). For the Nordic source of information about mastitis. Cows culled countries, an average of 45% of the milk-recorded

due to udder health problems before 150 days after cows are bred using young bulls (Lindhe, 1995). calving are treated as diseased, even if they were not Hence, most of the dairy cattle population is active recorded as having mastitis. Koenen et al. (1994) in the breeding programme. As a consequence, every found that heritability estimates for mastitis in year a relatively large number (450) of young bulls Swedish data were significantly higher when in- of red breeds can be progeny tested with large ´ formation on culling was included. In Denmark and daughter groups in the Nordic countries (Lindhe, Norway, the reason for culling is not used in the 1995). Another characteristic of the Nordic breeding genetic evaluation. systems is the co-operative cattle breeding organisa-In Denmark and Sweden, records are used from tions, which are owned by the dairy farmers, and first-lactation cows, whereas in Finland records are which thus take a longer-term view of cattle breeding used from the first three lactations. In Norway the and so include non-production traits in the breeding first lactation is analysed separately, although «re- goal.

peated evaluations» are carried out based on the second- and third-lactation records of daughters of

potential bull sires. Other minor differences with 4. Selection for mastitis resistance respect to the models used for genetic evaluation also

exist and are discussed by Ruane and Klemetsdal Breeding for increased resistance to mastitis can

(1996). be performed by direct selection using clinical

Data quality is viewed high since antibiotics only mastitis records, by indirect selection using traits can be prescribed by veterinarians. The probability genetically correlated to mastitis or by a combination of false positives, i.e. mastitis records on healthy of both. The most commonly used indirect measures cows, are minimised. However farmers differ in their have so far been SCC and type traits as well as ability to observe mastitis and in their criteria for leakage and milking speed. Other indicators, such as calling a veterinarian, and this influences the prob- milk antitrypsin (Harmon, 1994) and electrical con-ability of false negatives, i.e. cows with mastitis that ductivity (Schukken et al., 1997) have also been are not treated. This reduces the mastitis frequency proposed. Only SCC will be dealt with here, as it is below the true mean, with effect on information in widely considered to be the most useful indirect

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measure of mastitis resistance (De Jong and Lan- estimates of clinical mastitis in agreement with the sbergen, 1996). Detection of putative Quantitative Nordic studies.

Trait Locus (QTL’s) for mastitis resistance may in It should be noted that heritability estimates of the future make marker assisted selection an alter- all-or-none traits are functions of incidence and native or supplementary selection strategy for im- differences in estimates between different studies proving mastitis resistance in dairy cattle. QTL’s for may be caused by real differences between popula-SCC have been reported (e.g. Reinsch et al., 1998) tions and countries, but also be due to somewhat and mapping of QTL’s for clinical mastitis has different definitions of mastitis traits. Therefore started in Norway. Among others, BoLA alleles may parameters should be estimated in the population and be potential candidate genes (e.g. Sharif et al., 1998). country where they are going to be used, and possibly multiple trait evaluation e.g. for clinical 4.1. Direct selection for clinical mastitis mastitis and culling should be done rather than lumping all data together in one mastitis variable. The most common approach when utilising mas- Mastitis defined as veterinary treatments of clini-titis data in genetic evaluation has been to consider cal cases is a trait that may change over time. mastitis as an all-or-none trait and apply linear Introduction of somatic cell count as a quality models, which assume normal distribution of the criteria for milk price, and the gradually strengthened data. An alternative is the threshold model which quality over time have increased the attention paid to takes into account the binary nature of the data mastitis by the farmers, and their criteria for calling a which can be advantageous for variance component veterinarian may have been changed.

and breeding value estimation (Gianola and Foulley, The definition of mastitis as an all-or-none trait

1983). does not fully utilise all information in the data,

The heritability of clinical mastitis has been since some cows have more than one case of mastitis estimated from several studies based on data from and also the date of treatment is known. Therefore the Nordic health-recording systems. Estimates from alternative modelling of mastitis data is an important analyses with traditional linear methods on the area of research, and development of test-day models observable scale range from 0.001 to 0.06, with most for longitudinal binary response (Rekaya et al.,

values in the interval 0.02–0.03 (Lindstrom and 1998) is one alternative that may be used to improve ¨ ¨

Syvajarvi, 1978; Philipsson et al., 1980; Solbu, 1984; modelling of field records of clinical mastitis. ¨ ¨

Jensen et al., 1985; Syvajarvi et al., 1986; Madsen et Another development is the joint analysis of data al., 1987; Emanuelsson et al., 1988; Koenen et al., across Nordic countries which should reveal the

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1994; Lund et al., 1994; Poso and Mantysaari, 1996; genetic correlation between countries and thus to Sander Nielsen et al., 1996; Heringstad et al., 1997; what degree clinical mastitis is the same trait in Luttinen and Juga, 1997). Heritability estimates of different Nordic countries (Ruane and Klemetsdal, clinical mastitis from analyses with threshold models 1996). A smaller correlation than for milk yield on the underlying scale are higher, ranging from 0.06 would be a first indication that such traits, genetic-to 0.12 (Simianer et al., 1991; Lund and Jensen, ally are more different between countries than milk 1996; Heringstad et al., 1997). traits are.

All the above studies were based on relatively The response to selection is proportional to the large sets of data on clinically observed mastitis additive genetic standard deviation and the accuracy under field conditions. Designed field studies (e.g. and intensity of selection (e.g. Cunningham, 1969). Lin et al., 1989; Lyons et al., 1991; Uribe et al., Despite the low heritability, the accuracy of selec-1995) have given somewhat higher estimates of tion, and hence the potential genetic gain, can be heritability for clinical mastitis. This may be due to quite high, especially when the progeny group size is more accurate recording of data. Other studies from large. Assuming a simple selection index (e.g. Cun-non-Nordic countries (Weller et al., 1992; Pryce et ningham, 1969) for a trait with heritability 0.03, al., 1997b) using field data have found heritability accuracies of selection are predicted to be 0.66, 0.78

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and 0.83 with progeny groups of 100, 200 and 300 Swanson (1996) found a weighted average heritabili-respectively. In addition, the genetic standard devia- ty for first lactation SCC of 0.11 (1/2 0.04). tion of mastitis resistance is reasonably large. For Recent heritability estimates of SCC range from 0.08 example, in Norway, daughters of the three bulls to 0.19 (Lund and Jensen, 1996; Sander Nielsen et

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with the worst index values for mastitis resistance al., 1996; Poso and Mantysaari, 1996; Boettcher et after progeny testing in 1995 had twice the mastitis al., 1997; Boichard and Rupp, 1997; Luttinen and

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frequency (35%) of daughters of the three bulls with Juga, 1997; Pryce et al., 1997a; Poso et al., 1997). the best index values (18%) (Steine, 1996b). The efficiency of SCC as a selection criterion to

Thus, effective direct selection for mastitis resist- reduce the frequency of clinical mastitis depends on ance can be expected as long as proper recording and the genetic correlation between the two traits, and a sufficiently large daughter groups are used for wide range of values have been cited in the litera-progeny testing. This has recently been demonstrated ture. Estimates vary from close to zero (Coffey et al., by Steine (1998), who observed a 5% reduction in 1986) to close to unity (Lund et al., 1994). Other mastitis frequency among daughters of bulls with the estimates based on Nordic field data vary between best estimated breeding values for mastitis compared 0.3 and 0.8, with an average of 0.60 (Madsen et al., to daughters of bulls with the best estimated breeding 1987; Emanuelsson et al., 1988; Philipsson et al.,

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values for milk yield. 1995; Lund and Jensen, 1996; Poso and Mantysaari, 1996; Sander Nielsen et al., 1996; Luttinen and Juga, 4.2. Indirect selection using somatic cell counts 1997). Mrode and Swanson (1996) concluded that the average genetic correlation between clinical Somatic cells consist of many types of cells, mastitis and SCC, based on values from the litera-including neutrophile leukocytes, macrophages, lym- ture, was roughly 0.7.

phocytes, eosinophils, and various epithelial cell The correlation between SCC and clinical mastitis types of the mammary gland (Kehrli and Shuster, indicates that although both are expressions of udder 1994). A microbial infection causes a rapid increase health, they are not the same trait. High SCC implies in the number of cells and a change in the relative increased cell count for a longer period, when proportions of cell types. Thus macrophages and infrequently recorded, and thus reflects more long-lymphocytes are the dominant types in a healthy duration or subclinical cases of mastitis, whereas use udder, whereas in a diseased udder more than 95% of clinical mastitis ignores subclinical cases. From of the somatic cells are neutrophile leukocytes, biology, SCC and clinical mastitis are also consid-transferred from the blood. These changes take place ered to be different traits, as S. aureus, the frequent in only a few hours and are part of the normal cause of long term subclinical mastitis, activates the host-defence mechanism (Kehrli and Shuster, 1994). specific immune system, while clinical mastitis being At the site of infection, neutrophiles phagocytose more frequently due to E. coli is of short term and kill pathogens and therefore make up one duration and therefore the innate immune system has important part of the innate, non-specific immune to play a key role (Schukken et al., 1997). Smith et system (e.g. Sordillo et al., 1997). Macrophages al. (1985) reported that the mean duration of en-constitute another important part of this system, vironmental mastitis infections is between 9 and 17 which interact in a complex manner with the specific days. If milk samples for SCC analysis are collected immune system, involving lymphocytes. monthly or every second month, clinical mastitis Several authors have reported estimates of genetic cases with rapid recovery may not necessarily be parameters for the bacterial indicator variable SCC, detected by SCC. According to Shook and Schutz using various definitions of the trait, different models (1994) the monthly sampling scheme for SCC will and different methods to combine the test-day re- detect only about 10–20% of these infections. cords. Colleau and le Bihan-Duval (1995) found a The efficiency of SCC also depends on progeny heritability estimate for log SCC of 0.09, pooled group size. Thus, given a heritability of 0.03 for from 39 literature values. In a review, Mrode and clinical mastitis and a heritability of 0.10 for SCC,

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use of simple selection index theory shows that the as a selection criterion based on the role certain genetic correlation between the two traits has to be somatic cells have in defence against udder patho-greater than 0.70, 0.77, 0.82, 0.85 and 0.89 with a gens. Coffey et al. (1986) refer to results suggesting progeny-group size of 50, 100, 150, 200 and 300 that selection for decreased SCC may reduce the respectively for indirect selection on SCC to be more cows’ ability to respond to infection. For cows with effective than direct selection on clinical mastitis. a low SCC it is not clear that reducing SCC further Thus, selection on SCC alone appears to be less will reduce mastitis (Kehrli and Shuster, 1994). effective than selection directly on clinical mastitis. Some observational results (Erskine et al., 1988) A more thorough analysis would require stochastic show that herds with SCC ,150 000 had more simulation as there are some problems with applying clinical mastitis than herds with SCC.700 000. simplistic formulae for predicting genetic response Miltenburg et al. (1996) also found a significant with binary traits (Foulley, 1992). higher incidence of clinical mastitis in herds with Currently, Denmark uses SCC as an additional low SCC (,150 000) than in herds with SCC. source of information in a multi-trait model to 250 000.

increase the accuracy of breeding value estimation Even though linear relationships have been found for mastitis (Interbull, 1996). In Finland and between sire evaluations for SCC and clinical mas-Sweden, single trait evaluation is carried out for both titis (Philipsson et al., 1995, Rogers et al., 1998), by SCC and clinical mastitis and both traits are then a method with low power of detecting non-linear weighted in the total merit bull index (Eriksson, relationships, there is still uncertainty about the

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1991; Interbull, 1996; Poso and Mantysaari, 1996). effect of further reductions in SCC already at low In Norway, calculations using selection index theory levels. The relationship between sire evaluations for have shown that including SCC will not improve the the two traits, presented by Rogers et al. (1998), accuracy of breeding value estimation for mastitis covers only the range in which most of the sires resistance because daughter-group size are larger in occur, and the authors point out that extrapolation Norway than in the other Nordic countries (Steine, beyond this range should be done cautiously. A T., unpublished results), and SCC has thus not been further reduction of a low SCC by genetic selection used for genetic evaluation, although it is recorded. may impair the cows’ innate immune system, as In some non-Nordic countries where direct selection Schukken et al. (1994) found that cows that resisted on mastitis resistance is not an option, SCC is infection had a higher SCC prior to S. aureus included in the sire evaluation procedures as an challenge than cows that became infected. This may indirect measure of mastitis resistance (Interbull, indicate that a very low SCC is not optimal and that

1996). optimal udder health will not necessarily occur at the

Although SCC is an objective measure which can lowest possible level of SCC. It is therefore doubtful be standardised across regions and countries and is whether long-term selection should be for the lowest easy to record on a continuous scale, there are some possible concentrations of SCC in milk. A quantita-disadvantages associated with its use. Many studies tive or functional optimum SCC value, such that the have shown that the use of SCC alone to classify number of macrophages and the effectiveness of the udder quarters as infectious or non-infectious can be innate immune system is not reduced, may be better. unreliable (e.g. Harmon, 1994), and since phenotypic Although indirect selection on SCC involves some information is used for genetic evaluation, it is risks and shortcomings, in countries where direct important also from a genetic standpoint. In addition, selection for clinical mastitis is not an option it is the ability to use SCC as a predictor of mastitis better to use SCC than ignoring mastitis in the seems to be dependent on the level of mastitis in the breeding program. However research should con-herd. If the prevalence of mammary infections is tinue searching for alternatives. One alternative high, then SCC can be a good predictor, whereas in could be a simple health recording system, where herds with a low prevalence of mammary disease it mastitis is recorded for only a short period of first is less useful (Kehrli and Shuster, 1994). lactation. Mastitis in the period from 15 days before Reservations also exist regarding the use of SCC calving to 30 days after calving shows a heritability

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of the same size as mastitis from longer sampling dairy cattle population (Ruane and Klemetsdal, periods (Heringstad et al., 1999). 1996).

The national health recording systems in the Nordic countries are valuable data banks that can be 5. Conclusions and prospects for the future used for further investigating the genetic background of mastitis, and are of importance to both the Nordic Although the four Nordic countries (Denmark, and international communities alike (Ruane et al., Finland, Norway and Sweden) record information on 1997). Continued research is necessary to make both clinical mastitis and SCC, different approaches selection for mastitis resistance more effective. are used in breeding for increased mastitis resistance. Documentation of genetic trend for clinical mas-Norway, with large daughter groups, only includes titis in populations under long-term selection and the information on clinical mastitis to improve mastitis relationship between clinical mastitis and other im-resistance. Denmark utilises SCC as an additional portant traits are needed. Improved modelling of source of information for estimating breeding values clinical mastitis data, to make better use of in-for mastitis, while Finland and Sweden consider both formation available in the data, can contribute to traits in the breeding goal. more effective selection for mastitis resistance.

When only utilising information on clinical mas- Detection of possible QTLs for mastitis resistance titis in breeding, one is selecting for the resultant of may make marker assisted selection a complemen-all biological processes that improve mastitis resist- tary or alternative selection strategy.

ance. With SCC, the situation is different as a high value is indicative of a diseased udder while a low value is not necessarily an indicator of a healthy

References udder. This is because a steady reduction of SCC by

breeding may impair the innate immune system.

Aarestrup, F.M., Wegener, H.C., Jensen, N.E., Jonsson, O., Before relying too much on SCC in breeding

pro-Myllys, V., Thorberg, B.M., Waage, S., Thamdrup Rosdahl, V., grammes, a thorough examination of the linearity of _{1997. A study of phage- and ribotype patterns of} Staphylococ-the relationship between clinical mastitis and low cus aureus isolated from bovine mastitis in the Nordic coun-levels of SCC should be carried out. tries. Acta Vet. Scand. 38, 243–252.

Boettcher, P.J., Dekkers, J.C.M., Kolstad, B.W., 1997. An udder To ensure a continued high level of participation

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and 0.83 with progeny groups of 100, 200 and 300

Swanson (1996) found a weighted average

heritabili-respectively. In addition, the genetic standard devia-

ty for first lactation SCC of 0.11 (

1 /

2 0.04).

tion of mastitis resistance is reasonably large. For

Recent heritability estimates of SCC range from 0.08

example, in Norway, daughters of the three bulls

to 0.19 (Lund and Jensen, 1996; Sander Nielsen et

¨ ¨

¨

with the worst index values for mastitis resistance

al., 1996; Poso and Mantysaari, 1996; Boettcher et

after progeny testing in 1995 had twice the mastitis

al., 1997; Boichard and Rupp, 1997; Luttinen and

¨ ¨

frequency (35%) of daughters of the three bulls with

Juga, 1997; Pryce et al., 1997a; Poso et al., 1997).

the best index values (18%) (Steine, 1996b).

The efficiency of SCC as a selection criterion to

Thus, effective direct selection for mastitis resist-

reduce the frequency of clinical mastitis depends on

ance can be expected as long as proper recording and

the genetic correlation between the two traits, and a

sufficiently large daughter groups are used for

wide range of values have been cited in the

litera-progeny testing. This has recently been demonstrated

ture. Estimates vary from close to zero (Coffey et al.,

by Steine (1998), who observed a 5% reduction in

1986) to close to unity (Lund et al., 1994). Other

mastitis frequency among daughters of bulls with the

estimates based on Nordic field data vary between

best estimated breeding values for mastitis compared

0.3 and 0.8, with an average of 0.60 (Madsen et al.,

to daughters of bulls with the best estimated breeding

1987; Emanuelsson et al., 1988; Philipsson et al.,

¨ ¨

¨

values for milk yield.

1995; Lund and Jensen, 1996; Poso and Mantysaari,

1996; Sander Nielsen et al., 1996; Luttinen and Juga,

4.2. Indirect selection using somatic cell counts

1997). Mrode and Swanson (1996) concluded that

the average genetic correlation between clinical

Somatic cells consist of many types of cells,

mastitis and SCC, based on values from the

litera-including neutrophile leukocytes, macrophages, lym-

ture, was roughly 0.7.

phocytes, eosinophils, and various epithelial cell

The correlation between SCC and clinical mastitis

types of the mammary gland (Kehrli and Shuster,

indicates that although both are expressions of udder

1994). A microbial infection causes a rapid increase

health, they are not the same trait. High SCC implies

in the number of cells and a change in the relative

increased cell count for a longer period, when

proportions of cell types. Thus macrophages and

infrequently recorded, and thus reflects more

long-lymphocytes are the dominant types in a healthy

duration or subclinical cases of mastitis, whereas use

udder, whereas in a diseased udder more than 95%

of clinical mastitis ignores subclinical cases. From

of the somatic cells are neutrophile leukocytes,

biology, SCC and clinical mastitis are also

consid-transferred from the blood. These changes take place

ered to be different traits, as S. aureus, the frequent

in only a few hours and are part of the normal

cause of long term subclinical mastitis, activates the

host-defence mechanism (Kehrli and Shuster, 1994).

specific immune system, while clinical mastitis being

At the site of infection, neutrophiles phagocytose

more frequently due to E. coli is of short term

and kill pathogens and therefore make up one

duration and therefore the innate immune system has

important part of the innate, non-specific immune

to play a key role (Schukken et al., 1997). Smith et

system (e.g. Sordillo et al., 1997). Macrophages

al. (1985) reported that the mean duration of

en-constitute another important part of this system,

vironmental mastitis infections is between 9 and 17

which interact in a complex manner with the specific

days. If milk samples for SCC analysis are collected

immune system, involving lymphocytes.

monthly or every second month, clinical mastitis

Several authors have reported estimates of genetic

cases with rapid recovery may not necessarily be

parameters for the bacterial indicator variable SCC,

detected by SCC. According to Shook and Schutz

using various definitions of the trait, different models

(1994) the monthly sampling scheme for SCC will

and different methods to combine the test-day re-

detect only about 10–20% of these infections.

cords. Colleau and le Bihan-Duval (1995) found a

The efficiency of SCC also depends on progeny

heritability estimate for log SCC of 0.09, pooled

group size. Thus, given a heritability of 0.03 for

from 39 literature values. In a review, Mrode and

clinical mastitis and a heritability of 0.10 for SCC,

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use of simple selection index theory shows that the

as a selection criterion based on the role certain

genetic correlation between the two traits has to be

somatic cells have in defence against udder

patho-greater than 0.70, 0.77, 0.82, 0.85 and 0.89 with a

gens. Coffey et al. (1986) refer to results suggesting

progeny-group size of 50, 100, 150, 200 and 300

that selection for decreased SCC may reduce the

respectively for indirect selection on SCC to be more

cows’ ability to respond to infection. For cows with

effective than direct selection on clinical mastitis.

a low SCC it is not clear that reducing SCC further

Thus, selection on SCC alone appears to be less

will reduce mastitis (Kehrli and Shuster, 1994).

effective than selection directly on clinical mastitis.

Some observational results (Erskine et al., 1988)

A more thorough analysis would require stochastic

show that herds with SCC

,

150 000 had more

simulation as there are some problems with applying

clinical mastitis than herds with SCC

.

700 000.

simplistic formulae for predicting genetic response

Miltenburg et al. (1996) also found a significant

with binary traits (Foulley, 1992).

higher incidence of clinical mastitis in herds with

Currently, Denmark uses SCC as an additional

low SCC (

,

150 000) than in herds with SCC

.

source of information in a multi-trait model to

250 000.

increase the accuracy of breeding value estimation

Even though linear relationships have been found

for mastitis (Interbull, 1996). In Finland and

between sire evaluations for SCC and clinical

mas-Sweden, single trait evaluation is carried out for both

titis (Philipsson et al., 1995, Rogers et al., 1998), by

SCC and clinical mastitis and both traits are then

a method with low power of detecting non-linear

weighted in the total merit bull index (Eriksson,

relationships, there is still uncertainty about the

¨ ¨

¨

1991; Interbull, 1996; Poso and Mantysaari, 1996).

effect of further reductions in SCC already at low

In Norway, calculations using selection index theory

levels. The relationship between sire evaluations for

have shown that including SCC will not improve the

the two traits, presented by Rogers et al. (1998),

accuracy of breeding value estimation for mastitis

covers only the range in which most of the sires

resistance because daughter-group size are larger in

occur, and the authors point out that extrapolation

Norway than in the other Nordic countries (Steine,

beyond this range should be done cautiously. A

T., unpublished results), and SCC has thus not been

further reduction of a low SCC by genetic selection

used for genetic evaluation, although it is recorded.

may impair the cows’ innate immune system, as

In some non-Nordic countries where direct selection

Schukken et al. (1994) found that cows that resisted

on mastitis resistance is not an option, SCC is

infection had a higher SCC prior to S. aureus

included in the sire evaluation procedures as an

challenge than cows that became infected. This may

indirect measure of mastitis resistance (Interbull,

indicate that a very low SCC is not optimal and that

1996).

optimal udder health will not necessarily occur at the

Although SCC is an objective measure which can

lowest possible level of SCC. It is therefore doubtful

be standardised across regions and countries and is

whether long-term selection should be for the lowest

easy to record on a continuous scale, there are some

possible concentrations of SCC in milk. A

quantita-disadvantages associated with its use. Many studies

tive or functional optimum SCC value, such that the

have shown that the use of SCC alone to classify

number of macrophages and the effectiveness of the

udder quarters as infectious or non-infectious can be

innate immune system is not reduced, may be better.

unreliable (e.g. Harmon, 1994), and since phenotypic

Although indirect selection on SCC involves some

information is used for genetic evaluation, it is

risks and shortcomings, in countries where direct

important also from a genetic standpoint. In addition,

selection for clinical mastitis is not an option it is

the ability to use SCC as a predictor of mastitis

better to use SCC than ignoring mastitis in the

seems to be dependent on the level of mastitis in the

breeding program. However research should

con-herd. If the prevalence of mammary infections is

tinue searching for alternatives. One alternative

high, then SCC can be a good predictor, whereas in

could be a simple health recording system, where

herds with a low prevalence of mammary disease it

mastitis is recorded for only a short period of first

is less useful (Kehrli and Shuster, 1994).

lactation. Mastitis in the period from 15 days before

Reservations also exist regarding the use of SCC

calving to 30 days after calving shows a heritability

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of the same size as mastitis from longer sampling

dairy cattle population (Ruane and Klemetsdal,

periods (Heringstad et al., 1999).

1996).

The national health recording systems in the

Nordic countries are valuable data banks that can be

5. Conclusions and prospects for the future

used for further investigating the genetic background

of mastitis, and are of importance to both the Nordic

Although the four Nordic countries (Denmark,

and international communities alike (Ruane et al.,

Finland, Norway and Sweden) record information on

1997). Continued research is necessary to make

both clinical mastitis and SCC, different approaches

selection for mastitis resistance more effective.

are used in breeding for increased mastitis resistance.

Documentation of genetic trend for clinical

mas-Norway, with large daughter groups, only includes

titis in populations under long-term selection and the

information on clinical mastitis to improve mastitis

relationship between clinical mastitis and other

im-resistance. Denmark utilises SCC as an additional

portant traits are needed. Improved modelling of

source of information for estimating breeding values

clinical mastitis data, to make better use of

in-for mastitis, while Finland and Sweden consider both

formation available in the data, can contribute to

traits in the breeding goal.

more effective selection for mastitis resistance.

When only utilising information on clinical mas-

Detection of possible QTLs for mastitis resistance

titis in breeding, one is selecting for the resultant of

may make marker assisted selection a

complemen-all biological processes that improve mastitis resist-

tary or alternative selection strategy.

ance. With SCC, the situation is different as a high

value is indicative of a diseased udder while a low

value is not necessarily an indicator of a healthy

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