Page 174 of 201
The seroprevalence for BPIV-3 87 in pre-export testing was the highest out of the four viruses tested. This, combined with a low nasal prevalence and lack of evidence for an
association between BPIV-3 and respiratory disease in Australian live export cattle suggests that BPIV-3 may be a common infection in young Australian cattle, but that it is not likely to
be an important cause of BRD in Australian live export cattle. This is because cattle may be likely to have been exposed as younger animals and to be recovered and immune by the
time they are exported.
The four bacteria of interest were detected in nasal and lung swabs from animals that died during voyages and in nasal swabs collected from cattle in the pre-export assembly depots.
Caution is required in interpreting nasal swab detection of bacteria, since all four bacteria can be found as commensal organisms in apparently healthy animals, as well as causing
respiratory disease as primary or, more commonly, secondary disease agents.
All bacteria were significantly associated with histological pneumonia in animals that died of BRD. In addition, detection of three of the four M. bovis, M. haemolytica and P. multocida
in nasal swab samples was significantly associated with pneumonia and with respiratory disease as the primary cause of death.
Our finding that there was a relatively higher prevalence of P. multocida compared to M. haemolytica in both voyage and pre-export samples supports recent reports that P.
multocida may be rising in importance as a cause of BRD.
90
In the pre-export assembly depot, at least 1 of the bacteria of interest was detected in up to 42 of animals, and 1 or more viruses and concurrent bacteria were detected in 38 of
cattle. The prevalence of M. bovis and M. haemolytica increased significantly between entry to the depot and retesting approximately 1 week later. This increase is likely to be due to a
combination of stress-induced proliferation of commensal bacteria and transmission of bacteria between animals.
The methods we developed for collecting, storing and analysing biological samples to detect pathogens were innovative and effective and have increased our understanding of the
epidemiology of BRD in export cattle and the pathogens most likely to be involved in severe disease and death. These methods have the potential to be useful in further studies of
infectious diseases in export animals both here and overseas.
12.4 Describing patterns of mortality in export cattle
A range of analyses were conducted on industry data derived from project activities and from other sources, including in particular the Shipboard Mortality Database SMDB and the
NLIS database for Western Australia.
Advanced statistical modelling was applied to aggregated datasets to describe long term patterns in mortalities in export cattle. Where possible analyses were conducted using
mortality rate as an outcome deaths per 1,000 cattle-days, which allowed direct comparisons to be made between voyages of different durations.
90
Rice et al. 2007; Welsh et al. 2004
Page 175 of 201
Voyage mortality percent has been the major method for reporting mortality in export livestock. It is simple and easy to calculate and does provide an overall measure of mortality
as a percentage of cattle loaded. A major problem with voyage mortality percent is that it does not account for variation in voyage duration.
There are distinctions in purpose for different measures of mortality that need to be understood. For regulatory purposes it may be appropriate to maintain reliance on a simple,
easily understood summary measure like the voyage mortality percentage. The measure is in fact described in regulatory requirements and does provide a voyage measure of
performance.
The problem with voyage mortality percentage is that it does not incorporate any adjustment for voyage length. There may be benefit in reporting both types of measures since this will
allow identification and distinction of short voyages that have a relatively high daily risk of mortality, but a low overall mortality percentage, because the daily mortality risk is operating
over fewer voyage days, and longer voyages that have a relatively low daily mortality risk, but still have a relatively high voyage mortality percentage solely because of the length of
the voyage.
For detailed scientific analyses aiming at combining data from many different voyages to compare mortality risk and identify those drivers that may be influencing mortality risk,
voyage mortality percentage is not appropriate and it is necessary to use a true incidence rate measure of mortality, such as voyage mortality rate or daily mortality rate. These
measures allow direct statistical comparisons of different voyages and of drivers that may be operating across voyages to modify mortality risk season, month, breed, age, weight, sex,
pre-export vaccination or treatment, ship factors, ocean conditions and other factors.
There were associations between mortality rate and various explanatory variables including year, port of loading, destination region and month of year.
Our findings indicated that mortality rate has progressively dropped over time and that mortality rates in the most recent period 2010-2012 were generally lower than in previous
time periods.
Mortality rate was higher for voyages to the Middle East and North Africa than for voyages to SE Asia, NE Asia and SE Europe. Voyages to NE Asia and SE Europe may have had more
breeder animals and these voyages may involve differences in animal selection and management that contribute to lower mortality risk.
Mortality rate was higher in Summer months, with this pattern most evident in cattle loaded in southern ports and destined for SE Asia. In contrast, there was little variation in mortality
rate over the course of the year for cattle loaded in northern ports and destined for SE Asia.
The same pattern was seen in southern loaded cattle exported to the Middle East and North Africa.
We have also described for the first time, the pattern of daily mortality rate over time during export voyages. Daily mortality rate is not constant during the voyage and rises
progressively from the start of the voyage to a peak at around week four of the voyage and then declines. This pattern is remarkably similar to plots of mortality rate over time derived
Page 176 of 201
from land-based cattle feedlots in Australia, which also show a progressive rise over time to a peak at around week four post induction of cattle into the feedlot.
91
The land based feedlot mortality pattern can be attributed to the influence of BRD primarily since BRD is the biggest
cause of death in feedlot cattle. The current project did not collect detailed data on BRD risk factors in export cattle. This was
largely due to difficulties in collecting such data and related information, given the design constraints of the project and our reliance on industry workers for much of the data
collection. Nonetheless, our findings indicate that the epidemiology of BRD in export cattle may be very similar to that described for land-based feedlot cattle and may be driven by
pathogen exposure associated with co-mingling around transport to the assembly depot and during assembly depot aggregation of animals prior to export. A range of other causal
factors associated with animals, pathogens, management and the local environment are then likely to influence exposure of animals to pathogens, development of infection and
disease and risk of mortality.
There are likely to be particular causal factors that operate on export vessels and that are not present in land-based feedlots. These include factors such as ventilation and air quality
in closed decks, accumulation of waste products faeces, ammonia, etc in pens during the voyage and effects associated with sea conditions and climate temperature, humidity
during the voyage. These factors may act as modifiers in a causal web where initial exposure and infection risk associated with co-mingling in the assembly period and early
part of the voyage is likely to be essentially the same as the causal web operating for BRD in land-based feedlots.
There are two important conclusions to be made concerning BRD epidemiology in export cattle that arise from the findings of the current project.
The first is that some caution is required to avoid over-interpreting the findings from this project because of limitations in design and difficulties in identifying specific drivers of BRD
occurrence and severity under export conditions. A logical consequence of this is to make the recommendation that further research is required to investigate key drivers of mortality
risk with a particular focus on BRD in export cattle. However, it is also important to consider this recommendation in light of return on investment and to design approaches that build on
those used in this project where research activities are embedded into routine business operations. This is discussed further in later sections of the discussion and in the
recommendations.
The second conclusion is that the findings of the current project provide convincing evidence that BRD in export cattle is very much the same disease caused by the same pathogens as
BRD in land-based feedlot cattle. While we are lacking detailed data to assess causal factors, we can and should embrace research findings and practical experience about BRD
derived from Australian cattle feedlots to identify and implement risk management strategies to reduce BRD occurrence and severity in export cattle.
91
Perkins 2013
Page 177 of 201
12.5 Mitigating BRD risk in export livestock
