Introduction death, leading to large economic losses. The minor

Livestock Production Science 64 2000 95–106 www.elsevier.com locate livprodsci 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.: 1 47-6494-8000; fax: 1 47-6494- with an abnormal appearance and, in some cases, 7960. E-mail address : bjorg.heringstadihf.nlh.no B. Heringstad increased rectal temperature, lethargy, anorexia and 0301-6226 00 – see front matter  2000 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 9 9 0 0 1 2 8 - 1 96 B . Heringstad et al. Livestock Production Science 64 2000 95 –106 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 developments. mastitis in Norway are 3000 NOK 460 US, based B . Heringstad et al. Livestock Production Science 64 2000 95 –106 97 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 ¨ ¨ ¨ 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 98 B . Heringstad et al. Livestock Production Science 64 2000 95 –106 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 ¨ ¨ ¨ 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-recorded 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