13 July 2018
This is part of a series of posts from our own Shane Hanlon’s disease ecology class that he’s currently teaching at the University of Pittsburgh Pymatuning Laboratory of Ecology. Students were asked to write popular science posts about (mostly) wildlife diseases. Check out all the posts here.
By Josh Donovan
Throughout human history, dogs have played many important economic and social roles. From hunting and trapping, to herding, to simple companionship, wherever humans have traveled throughout history they have brought and bread dogs. Over the course of history humans have created hundreds of specialized breeds, each bread for a specific purpose, and originating from all corners of the globe. All corners of the globe except for the Americas however, with a recent study showing that modern breeds in the Americas only can trace 2-4% of their ancestry to the original dogs of the Americas, even though archaeological evidence shows that domesticated dogs were in the Americas as early as 10,000 years ago.
To answer the question of what happened to these forgotten breeds, some researchers point to selective breeding after European arrival being responsible, with Europeans preferring the dogs they had brought with them as opposed to indigenous dogs. However, due to having such low ancestry in common, more recently scientists have begun to point to Canine Transmissible Venereal Tumor (CTVT) disease as the most likely cause of the almost extinction of American dog breeds either before or during European arrival in the 15th century.
CTVT is one of the oldest identified cancers, with genetic testing revealing it to be largely unchanged for almost 10,000 years, though confined to small populations of dogs and only within the last 500 years was it found to have spread across the globe. The disease was first described in the late 1870’s when it was found to be one of the few transmissible cancers, with Devil facial tumor disease being one of the only forms. CTVT causes large, rose colored tumors of the internal and external genitalia of both male and female dogs and is normally spread via intercourse, though it can be spread through other forms of contact.
These tumors can affect the ability of affected dogs to mate and eventually can cause mortality, if left untreated, though most dogs are not very sensitive to the disease. While in the modern day this disease is more a concern for dogs kept as pets, in the past, with the absence of treatment, this transmissible cancer wreaked havoc on both wild and domestic dogs.
Recently, genetic testing has revealed that CTVT originated in dogs more closely related to those indigenous to the Americas rather than Eurasian dogs, prompting two theories on how this disease resulted in the disappearance of American dogs. The first is that the disease came with the original dogs to reach the Americas from Asia millennia ago and the second is that the disease spread across Europe and Asia before reaching the Americas, like so many other diseases, with European arrival. In either case, researchers believe that the disease’s similarity in genetic makeup to American dogs allowed the cancer to more easily ‘slip by’ the dogs immune systems, becoming very virulent and causing high mortality. The immune systems of Eurasian dogs, however, could more easily recognize the much more foreign cancer and could mount a stronger defense, limiting both infection rates and mortality.
The increasing use of genetic analysis to uncover information on the lineage of modern and historic disease provides not only great insight into how diseases have co-evolved with their hosts over time, but provides us with models to be able to predict how diseases can evolve into the future, both in terms of their effect on animals, but more importantly how they affect humans. In the modern age of a very interconnected world, with diseases able to spread very quickly, being able to understand and predict how diseases can rapidly evolve and change is necessary to prevent an occurrence such as this historic elimination of indigenous american dogs from occurring again.