Diagnostic and public health investigation of Mycobacterium tuberculosis infection in a dog in Ontario, Canada
By Luke Haydock
Over the past couple of millennia, there are perhaps few other diseases that rival tuberculosis for the scope and impact of its effects on the health of the global population. From Anne and Emily Brontë, to George Orwell, to Chopin, and Charles IX of France, the victims of the tuberculosis are littered throughout the annals of human history. Even today, in a world where many consider tuberculosis a disease of the past, Mycobacterium tuberculosis (the causative species of most human tuberculous mycobacterial infections), remains highly relevant. According to the World Health Organization, approximately 1.6 million deaths per year are attributed to tuberculosis globally and, prior to the onset of the COVID-19 pandemic, tuberculosis was regarded as the leading infectious killer in the world. With such a significant impact on global human health, one may ponder whether we should have similar concerns for our nearest and dearest companion species. Man’s best friend. Our loving and loyal dogs.
In a recent article published in the Journal of Veterinary Diagnostic Investigation (Haydock et al., 2022), a group of veterinary practitioners and researchers at the Ontario Veterinary College, along with their collaborators at the Central Toronto Veterinary Referral Clinic, Toronto Public Health, Public Health Ontario, and the Canadian Food Inspection Agency detail the diagnostic investigation and subsequent public health fallout of a case of Mycobacterium tuberculosis infection in a domestic dog.
Tuberculosis has long been recognized in dogs and other canid species. However, the vast majority of cases have historically been associated with infection by Mycobacterium bovis, a closely related but distinct pathogen from Mycobacterium tuberculosis. In general, Mycobacterium bovis is host-adapted to cattle, being the causative agent of bovine tuberculosis, but is known to jump the species barrier to other animals and, rarely, to humans (in which cases it is referred to as “zoonotic tuberculosis”). Exposure to infected cattle, often via the ingestion of raw (unpasteurized) milk, or other susceptible species (e.g. Badgers in the United Kingdom, Brushtail possums in New Zealand) is the main mode of transmission for this form of tuberculosis. Mycobacterium tuberculosis, on the other hand, is adapted to humans and infections in dogs are comparatively rare and sporadic. Among the handful of published case reports on canine Mycobacterium tuberculosis infections, the majority are associated with a history of close contact with an infected human. The subject of the Haydock et al., 2022 article was a 4-year-old, female, mixed-breed dog who lived in central Toronto and, interestingly, had no known history of exposure to a tuberculosis-affected human.
This case highlights to veterinary practitioners the importance of collecting a complete and thorough history, especially when dealing with animals with abnormal presentations of rare and potentially infectious diseases. At initial presentation, little was known of this dog’s past. It was known that she had been adopted from a rescue agency approximately 18 months prior to the onset of illness. But it was not until after a protracted course of illness and this animal’s eventual euthanasia, that Mycobacterium tuberculosis infection was confirmed and a further investigation into this animal’s past prior to her arrival at the rescue agency was initiated. Subsequent genomic profiling and public health follow-up allowed this Mycobacterium tuberculosis strain to be linked back to an almost identical human infection based out of Quebec. Incidentally, it was at this point that some important details of this animal’s past came to light. Prior to her rescue, this dog had been living amongst a remote community in northern Quebec with a historically high rates of active tuberculosis infections. This information could have been pivotal in the early identification of Mycobacterium tuberculosis infection in this dog, and the subsequent public health risk posed by this animal could have been greatly lessened. Fortunately, in this case, from a public health perspective, it appears that dogs do not serve as efficient vectors for dog-to-human transmission of Mycobacterium tuberculosis. Despite close contact with her owners for almost 2 years and exposure to a horde of veterinary personnel (over 50 people), a lengthy public health investigation revealed no evidence of human transmission of Mycobacterium tuberculosis that could be linked back to this dog’s illness (see details in Table 1).
This case served as a reminder that an animal’s origin (or sites or recent travel) needs to be strongly considered when dealing with the investigation of potentially infectious diseases, especially when the pathogen in question carries a zoonotic risk. Additionally, this case highlighted the relative need for more readily available and sensitive means of detecting Mycobacterium tuberculosis infections in dogs, as veterinary practitioners in most parts of the world are left with minimal options when searching for quick and easy way to screen dogs who have been exposed to people with tuberculosis. Especially given that traditional means of screening for human tuberculosis (e.g. tuberculin skin test) are of limited reliability in dogs.
Article Details
Diagnostic and public health investigation of Mycobacterium tuberculosis infection in a dog in Ontario, Canada
Luke A. J. Haydock, Anthony C. G. Abrams-Ogg, J. Scott Weese, Michael R. Goldstein, Amy B. Clifford, Adrian Sebastian, Elizabeth H. Rea, Frances B. Jamieson, Carla Duncan, Olga Andrievskaia, Mirjana Savic, Durda Slavic, Robert A. Foster, Christopher J. Greenwood, Tamara L. MacDonald, Jacqueline E. Scott, and Andrea Sanchez
First published online January 25, 2022
DOI: .10.1177/10406387221074706
Journal of Veterinary Diagnostic Investigation
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