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If the same syndrome continues to be associated with deaths over subsequent days, then additional gross necropsies can be conducted on fewer representative animals one
or two animals per day in order to confirm the gross cause of death. If animals die with different signs or if initial gross necropsy does not identify a cause of
death, then a more comprehensive necropsy may be conducted in an attempt to identify the cause.
Digital images of standard views at necropsy and of any lesions provide an option that may allow peer review at a later date and discussion of possible explanations and
classification into causes. If there are concerns that deaths may be occurring with signalment, signs or gross
necropsy changes that are inconsistent with the recognised major causes of death, then consideration should be given to further investigation including preparation on the same
or subsequent voyages for collection of biological samples that can be imported back into Australia for examination by veterinary pathologists.
It is suggested that consideration be given to incorporating the use of a targeted or simplified gross necropsy protocol into future revisions of the Veterinary handbook for the live export
industry.
12.3 Pathogens associated with respiratory disease
This project developed and applied quantitative PCR qPCR methods, in conjunction with histology and serology, on selected samples to detect pathogens associated with BRD and
to improve our understanding of the epidemiology of BRD in export cattle. This is the first time that these methods have been applied in large scale testing on samples from
commercial export voyages.
The qPCR techniques developed during this study provided sensitive methods diagnostic sensitivity of 10 gene copies for most organisms for the detection of nucleic acids from
viruses and bacteria of interest in swab and tissue samples:
Viruses of interest: o
Bovine coronavirus BCoV, Betacoronavirus 1; o
Bovine herpesvirus 1 BoHV-1; o
Bovine viral diarrhoea virus BVDV; o
Bovine respiratory syncytial virus BRSV; o
Bovine parainfluenza virus 3 BPIV-3; and, Bacteria of interest:
o Histophilus somni;
o Mycoplasma bovis;
o Mannheimia haemolytica; and,
o Pasteurella multocida.
One or more pathogens were detected in two-thirds 130195 of animals from which lung samples were collected.
There was a significant correlation between detection of each of the four bacteria in lung samples and the presence of histologic evidence of pneumonia, confirming that the four
bacterial pathogens were all involved at some level in cases of respiratory disease.
All of the viruses were detected in samples, though at lower rates than for bacteria, and there was no statistical association between detection of any of the viruses and presence of
histologic evidence of pneumonia.
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These findings support our understanding of the different roles that viruses and bacteria play in the pathogenesis of BRD. Viral infection often starts the process and viruses are primary
pathogens that damage the respiratory tract and may also inhibit the immune system, thus facilitating secondary invasion by bacteria that may go on to cause a fatal bacterial
pneumonia, by which time the viruses may no longer be detectable.
86
Our detection of BCoV in BRD cases is the first time that BCoV has been demonstrated in live export cattle and only the second time that BCoV has been reported in Australian cattle
following a recent report of detection associated with an outbreak of respiratory disease in beef feedlot cattle in eastern Australia.
87
We detected BCoV in lung samples and nasal swabs from cattle that died and in 40 of nasal swabs from cattle in the assembly depot. An association between presence of BCoV in
nasal swabs and elevated risk of BRD and reduced performance has been reported in some studies of feedlot cattle
88
, while others have found no such association.
89
It is our view that BCoV is likely to be important in the pathogenesis of BRD in Australian live export cattle.
BVDV was detected in lung samples and nasal swabs collected from animals that died during voyages. In addition BVDV was detected in 3 of nasal swabs in pre-export
assembly depots and the 60 of animals tested in assembly depots had circulating antibodies against BVDV. This meant that 40 of animals were considered to be susceptible
to BVDV infection.
To the authors’ knowledge, this study is the first time that BVDV has been detected in individual animal nasal swabs from naturally exposed cattle with or without clinical signs of
BRD.
Disease testing requirements for export cattle are based on bilateral agreements between the importing country and Australia. Our findings suggest that current protocols for BVDV
may not prevent BVDV circulation amongst animals in the assembly depot and on board ship.
The presence of susceptible animals, circulating virus and the known association between BVDV infection and subsequent development of BRD suggests that BVDV is continuing to
pose a risk for BRD morbidity and mortality during export voyages.
The significance of BoHV-1 and BRSV infections is difficult to determine. These viruses were isolated infrequently from animals that died during voyages and neither virus was
significantly associated with deaths due to respiratory disease. In the pre-export assembly depots the nasal prevalence of both viruses was very low ~1. To the authors
’ knowledge, this is the first time that BRSV has been detected in nasal swabs from beef cattle over the
age of 4 months. Paired samples nine days apart in the assembly depot suggested that there was circulation of BRSV amongst animals in the assembly period. BRSV may be
contributing to BRD morbidity during export voyages.
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Panciera and Confer 2010; Taylor et al. 2010
87
Hick et al. 2012
88
Lathrop et al. 2000; Storz et al. 2000; Thomas et al. 2006; Fulton et al. 2011
89
Cho et al. 2001; Hasoksuz et al. 2005
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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.
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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
