When I’m giving talks about zoonotic diseaes to people in the human healthcare field, I sometimes mention tuberculosis (TB) as an example of a serious human disease with poorly defined (but theoretically important) risks of transmission between people and pets. TB is a very important disease of increasing of concern because of its resurgence in many areas and the spread of drug-resistant strains.

We don’t know much about TB and pets. There are some older studies that provide conflicting information, suggesting that Mycobacterium tuberculosis, the bacterium that causes TB, can be commonly or rarely isolated from dogs owned by TB patients.

A recent study from South Africa (Parsons et al. Research in Veterinary Science 2012) provides more information. The study involved two main components:

• For the first component, they examined 100 stray dogs in Cape Town, South Africa, for evidence of TB. The dogs were being euthanized for population control purposes so the researchers were able to do necropsies (post-mortem exams) to look for the bacterium and signs of disease that may not have been outwardly apparent. They isolated the bacterium from 4% of the dogs, with only one of those having any signs of disease. That shows that TB is present in dogs in the area, albeit at a low rate. The fact that 3 of 4 TB-positive dogs had no evidence of disease is both good and bad. It’s good for the dog’s health that illness doesn’t always occur (just like in people), but it also means that apparently healthy dogs can be carrying this concerning bacterium. The risk of transmission from healthy carriers isn’t known. It’s probably rather low since close and prolonged contact are required to transmit TB between people, and healthy carrier dogs are probably not shedding many TB bacteria through their respiratory tract. Greater concerns are probably present in dogs with TB infection of the lungs who are coughing and spewing TB bacteria into the air.
• The second component of the study involved testing of 24 dogs living with people with TB. They used two different tests: the TB skin test (a test that’s commonly used in people but one that’s been typically considered useless in dogs) and an interferon gamma release assay (IGRA)(a test more commonly used now in humans). They concluded (not surprisingly) that the skin test was pretty useless, but their data suggest the IGRA may be a good test for dogs. 50% of dogs in those households had evidence of TB exposure through IGRA, consistent with one older study that indicated transmission of TB from people to pets may be common.

What are the implications of all this?

For the average person and pet, not much. TB transmission requires close and prolonged contact with an infected individual. You don’t get it walking down the street behind someone with TB. 

The concern is in situations when people with TB may have contact with pets – the same concern as in situations when people with TB may have contact with other people. The potential for transmission is something to pay attention to in households where there is an infected person, or in populations where there may be high TB rates and common pet contact (e.g. some homeless populations).

Results of this study should be a reminder that when considering who’s potentially been exposed to a person with TB and making plans to reduce the risk of transmission, you need to consider all individuals – human and animal – with which the person has contact. Therefore, measures taken by people to avoid transmission of TB to other people should be equally applied to reducing transmission to pets. Pet exposure should always be considered, particularly when dealing with multidrug-resistant (MDR) or extensively drug-resistant (XDR) TB, since such strains are huge concerns in people and we don’t want to create canine or feline vectors of these strains. The study results also indicate that pets owned by TB patients may be at increased risk of disease from TB, not just at risk of being infected and harbouring the bug. Therefore, knowing that a pet has had close and prolonged contact with a person with TB is relevant to veterinarians when evaluating sick animals.