New Zealand Free Of Brucella Canis, MAF Investigation Finds
A MAF investigation has concluded that New Zealand is free of Brucella canis, the cause of canine brucellosis.
Since August 2000 MAF had been investigating suspicious clinical signs and serology results in dogs to determine whether
the bacterial pathogen Brucella canis was introduced into New Zealand.
Final test results show that none of the dogs tested in the investigation were infected with the disease, even though
initial serology tests produced positive results. Such false positives can occur with serology tests, but the level
experienced during the investigation was unprecedented and remains unexplained.
Brucella canis primarily affects dogs, but it can also infect humans. Human cases are sporadic and the clinical signs
(headaches, chills, nausea and weight loss) are mild. There has never been a reported case of the disease in New
Zealand, although it is common in the United States.
The investigation began when a St Bernard dog in Wellington with an inflamed testicle was blood tested and found to be
seriologically positive for Brucella canis (i.e. the blood sample showed evidence of Brucella canis antibodies). The
test used in New Zealand is called the card agglutination test, and is marketed as a commercial test kit for detecting
antibodies to Brucella canis.
Tracing and testing of dogs that were in contact with the suspect dog raised suspicions of false positive results due to
the high number of positive serology results. By the end of the investigation, one hundred and thirteen dogs were tested
at MAF's National Centre for Disease Investigation (NCDI) using the card agglutination test, and 54 had given positive
results.
The health status of these dogs was eventually clarified through use of a range of other tests. These included two
different serology tests performed at Cornell University in the USA; culture of the blood of test-positive dogs in an
attempt to isolate the organism; and the development of a gene probe to look for bacterial DNA in samples. No evidence
of true infection was found through this work.
When the final results of all the tests were compared, the interpretation that best fits the evidence is that:
· none of the dogs involved in this investigation were truly infected with Brucella canis, providing further evidence
that this disease does not occur in New Zealand; however
· there is a source of cross-reactivity in New Zealand dogs that manifests during particular serological test techniques
for Brucella canis.
· the source of the cross-reactivity for Brucella canis in New Zealand dogs is unknown. It could result from stimulation
of the canine immune system from a variety of potential sources: the environment, food, an infectious agent, or
vaccines, for example. MAF assumes that the cross-reactivity is inconsequential aside from its impact on interpreting
test results. Therefore, MAF will focus further research efforts on developing screening test capability to overcome
this interaction.
Serology is commonly used as an initial screening test for many infectious diseases. Serological tests detect antibodies
rather than the target organism itself, in this case the bacteria Brucella canis. The presence of antibodies is taken to
indicate a previous exposure to the target organism, and is the result of the host's immune response to infection.
However, there is always the potential that antibodies stimulated in the animal due to another reason will cross-react
to a serological test, leading to a false positive result. The potential for serologically-based techniques to detect
cross-reactivity varies between test techniques and tests for different organisms. This can create difficulties during
disease investigations and can require confirmatory testing of screening test-positive animals. During the Brucella
canis investigation, MAF emphasised the presumptive nature of the test results and the need for confirmation.
The Ministry of Agriculture and Forestry would like to thank the St Bernard owning and breeding community and the New
Zealand Kennel Club for their assistance and co-operation, which has been vital to the investigation.
For further information contact: Matthew Stone, Programme Co-ordinator Exotic Disease Response, Animal Biosecurity Ph
04-498-9884, 025-332-509.