Tuberculosis (TB) is a very important public health issue.  Caused by the bacterium Mycobacterium tuberculosis, TB is likely the most widespread human infectious disease. Although it primarily affects people, it can rarely affect various animal species as well, including pets. The relevance of dogs to human TB is unclear: we know dogs can (rarely) be infected by M. tuberculosis, but it remains uncertain whether infected dogs pose a risk to their human contacts (presumably dogs become infected in the first place from one of their human contacts, who is therefore also a source of exposure to other people).  There is little information available about this risk.

People working in the veterinary field are at increased risk of exposure to many infectious agents. For this reason, there are standard practices that are (or at least should be) used to reduce unexpected exposures. A good example of such an unexpected exposure and the consequences thereof is presented in a case report from Switzerland recently published in Veterinary Microbiology (Posthaus et al 2010). 

The report describes a seven-year-old Ibizan hound that was referred to the University of Bern’s small animal clinic because of lethargy and vague neurological abnormalities. The dog was originally imported from an unknown country in southern Europe and was adopted from a shelter at two years of age. The animal was ultimately euthanized after it became very ill, with severe neurological disease and respiratory arrest. On necropsy, masses were identified in the brain and liver, and infection with Mycobacterium avium complex was suspected. However, subsequent testing identified M. tuberculosis, triggering an investigation of all human and animal contacts.

Testing included a blood test called the IGRA, which detects people who are either infected or exposed. The test can’t differentiate between a recent infection and a past infection/exposure, so a positive result needs to be interpreted in conjunction with an evaluation of the person’s history, to determine if previous exposure is likely.

The Ibizan’s two owners tested negative. None of the veterinary clinicians that worked with the dog before its death were positive (or more specifically, none were positive and the result thought to be related to exposure to the dog), including the three people who were involving in intubating the animal (placing a breathing tube – a potentially high-risk procedure in an animal with TB). In contrast, six pathologists who were involved with the necropsy were positive, including the three who were most closely involved. Two of these individuals had no apparent risk of previous exposure, so infection from the dog was suspected

Additionally, a cat that lived in the house tested positive.  Because of the potential that the cat was infected and concerns about transmission to people, it was euthanized.

This case had some rather interesting findings:

• TB infection in a dog. Clinical disease caused by TB in dogs is rare, and signs can be quite vague, as in this animal. This was not a case of classical respiratory tract TB.
• Infection occurred in the dog well after its presumed time of exposure. The source of infection was unknown, but given the family’s history, it is reasonable to suspect that the dog arrived in the country already infected. This shows how diseases like TB with potentially long incubation periods can catch you by surprise. The dog was treated with high doses of steroids initially, possibly triggering re-activation of dormant TB because of immunosuppression.
• Infection of people with TB while conducting a necropsy, demonstrating the occupational risks that can be involved in such procedures. The use of power tools to open the skull is commonplace in veterinary pathology, and may have played a role here by aerosolizing the bacterium. The institution has now changed standard necropsy practices to reduce the risk of subsequent exposures.
• There was no apparent infection of any people when the dog was alive. Presumably, it was not shedding much, or any, of the TB bacterium in its respiratory tract.
• Possible transmission of TB from the dog to a cat. However, it can’t be stated with certainty that the dog and cat were not infected by the same unknown source after the dog was adopted. Testing was not performed on the cat after it was euthanized, so it is possible that it was not infected with TB, but with another related Mycobacterium.

This case doesn’t indicate that we are dealing with a new issue. Rather, it is possible that there are many more dogs that are apparently healthy but infected with TB. These animals may pose little risk to people, especially if they don’t have respiratory disease, but it’s an area where information is limited.  More attention needs to be paid to the risk of interspecies transmission of TB, how to identify it, how to reduce the risk, and how to manage infected animals. There’s debate about whether exposed but healthy animals like the cat in this case should be euthanized because of the potential (but completely unknown) public health risk. Without good research data, we can’t provide reliable answers to questions about the risks.

Image: Ibizan Hound (click for source)