Dogs & Leprosy

by Rebecca Bragg
How dogs contract leprosy is still unknown, but insect bites are suspected.

How dogs contract leprosy is still unknown, but insect bites are suspected.

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Before 1973, when a Rhodesian vet stumbled upon a disease he initially thought was tuberculosis, nobody suspected that dogs might be able to contract one of the most dreaded human diseases in history -- leprosy. Serious clinical investigation of canine leprosy didn't get underway until the end of the 20th century, so it's still in its infancy. But even though scientists haven't yet pieced together the complete puzzle, they know that the disease isn't nearly as debilitating in dogs as it used to be in humans, and that it can be cured with the same antimicrobial drugs.

Similarities and Differences in Humans and Animals

The ancient scourge of leprosy, or Hansen's disease, is now easily cured. But even today in countries where people can't afford the necessary drugs, victims of Mycobacterium leprae suffer devastating disfigurements from skin and nerve damage caused by the bacillus. Much is still unknown about the different strains of the microbe that infect companion animals -- M. lepraemurium in cats and another, related to M. simiae, in dogs -- but barring rare cases, the disease doesn't wreak the same havoc on their bodies. Typically, it presents as one or more hard but painless nodules, or granulomas, just under the skin. The only animal known both to harbor and suffer from the same strain of leprosy that infects humans is the armadillo. A study published in April 2011 in "The New England Journal of Medicine" found compelling evidence that infected armadillos can and have transmitted leprosy to humans in the southern United States.

Canine Leprosy Background

Dr. Richard Malik of the Faculty of Veterinary Science at the University of Sydney, a pioneer in the study of canine leprosy, believes that many cases "with inconspicuous or few lesions" may exist but fly under the radar of veterinary detection. It was identified in 1973 after Rhodesian veterinarian Richard Smith discovered a mycobacterial skin infection in two short-haired dogs. Fearing the highly transmissible mycobacterial disease tuberculosis, the vet euthanized and necropsied the dogs but found no evidence of TB. After news of this mysterious new condition spread among other vets, similar cases came to light. The clinical name canine leproid granuloma syndrome replaced the colloquial name canine leprosy. CLGS, now the most common canine mycobacterial disease in Australia, has been identified in New Zealand, Brazil and Europe. Since it's not a notifiable disease in the United States, no centralized records indicating prevalence are kept, but according to the 2014 "Textbook of Veterinary Internal Medicine," cases have been recorded in California, Florida, New York and Georgia.

Symptoms, Diagnosis and Treatment

If you feel one or more hard lumps on your dog's body, especially on his head or ear flaps, definitely let the vet investigate, although chances that they'll turn out to be leproid granulomas are remote. Still, subcutaneous lumps may be symptoms of many other nasty conditions, including abscesses, cysts, malignant and benign tumors and insect bites. If your vet has concerns about CLGS, a tissue sample can be sent to a lab for microscopic examination that will either confirm or rule out that diagnosis. Most leproid granulomas disappear within one to three months without treatment, presumably dispelled by the dog's own immune system, Malik says. Occasionally the lesions persist, ulcerate and become a chronic problem with potential to cause disfigurement, especially if secondary infections develop. In such intractable cases, treatment usually involves a combination of antimicrobial drugs similar to those used to cure human leprosy. Sometimes surgery is performed.

Mysteries and Challenges

Until answers are found to some key questions about CLGS, the condition will remain baffling, Malik says. Among the short-coated breeds most often affected, boxers and boxer mixes account for almost half of reported cases -- but nobody knows what makes them so oddly susceptible. The organism's natural environmental niche is also a mystery. One theory is that biting insects transmit the infection to dogs, but if that's so, where the bugs pick it up is an unanswered question. Another possibility is that the organism lives in soil and gets into the bodies of animals through small breaks in the skin. It's impossible to culture the infectious agents to conduct experiments on animal-to-animal transmission; but, armadillos and humans aside, these strains of mycobacteria appear to be species-specific.

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