Baylisascaris – the raccoon roundworm
I recently did a post-mortem on a young raccoon that was among a small group found dead in a wealthy neighbourhood in Toronto. The raccoon had died of parvovirus enteritis, which was interesting in itself. Infection with parvoviruses are more commonly seen in raccoons in rehabilitation centres; it occurs in the wild, but is not seen that often, perhaps because infected raccoons become profoundly ill and typically die quite quickly. In contrast, raccoons infected with Canine Distemper Virus, the most common infectious disease of raccoons in this part of the world, will be ill over a period of time, and will often be found out wandering around in daylight, often showing signs of neurological disease.
A post-mortem finding in this raccoon that was perhaps even more remarkable than parvovirus was the presence of large numbers of roundworms, individually, in clumps and in tangled masses, filling the damaged intestine. In all, I counted 315 of these 3-6 cm long worms, the most I have ever seen in a raccoon. The entire picture of the raccoon, the virus and the worms caused me to reflect upon the complexity of life in the urban environment. Here, among healthy and wealthy people and their well cared for pets, are large numbers of wild carnivores (common estimates of urban raccoon densities range from 25-50/km2), with some of them dying of a virus that they share with those pets, and carrying a parasite that is capable of causing devastating disease in both people and other wildlife species that inhabit the area.
Baylisascaris procyonis, the raccoon roundworm, inhabits the small intestine of raccoon, often in large numbers as in this case. The female adult worms shed eggs into the environment, and after a period of time (~2 weeks) larvae develop in the egg and they become infective. If the eggs are eaten at this time by a young raccoon, the larvae go through a brief migration in the intestinal wall and nearby tissues of the raccoon and return to the intestine to mature as adults, completing the cycle. If, however, they are eaten by another animal (including people) they go on a different migration, which takes them to the lungs and into the circulation, from where they are spread around the body of their new host. Here, in tissue, they set off an inflammatory response, which eventually walls them off. There, they wait, and if their host is eventually eaten by a raccoon, they will hatch out in the intestines of the raccoon and complete their life cycle.
The eggs of the raccoon may be eaten by a wide range of species, including birds, small mammals and people. One of the key features of the behaviour of the parasite in these paratenic hosts is their predilection for reaching the brain. In the brain, as they move, they cause significant damage and inflammation, resulting in changes of behaviour of the host. In the case of birds and small rodents, this may make them more susceptible to predation by raccoons, facilitating the completion of their life cycle. In the case of people, it causes a rare but devastating neurological disease.
It is a diabolically clever system that has evolved in order to maximize the chances for the survival and reproduction of the parasite. You might wonder what the odds are of a rodent finding a parasite egg in the environment. The odds are long, but some aspects of the behaviour of both raccoon and parasite work to shorten them. To begin with, the eggs are not randomly distributed in the environment, but tend to occur where raccoons have defecated. Raccoons tend to defecate repeatedly and communally in a limited number of locations, referred to as raccoon latrines. These are often found along the tops of woodpiles, fences, or in the crotches of trees. Raccoon feces may contain items – seeds, berries, corn kernels, etc. – which will draw the attention of foraging rodents and birds. While hunting for food, the rodent or bird may also inadvertently contact parasite eggs. If the raccoon who deposited the feces was infected with B. procyonis, the eggs will be there in vast numbers: it has been estimated that, on average, an infected raccoon will produce 20-25,000 eggs per gram of feces.
From the parasite’s point of view, the raccoon, the environment and the paratenic hosts are just parts of a grand scheme orchestrated to produce more parasites. There are many better known stories of life history strategies that have evolved under the constraints of natural selection, and we tend to marvel at them – for example, the migrations of the monarch butterfly or the Atlantic salmon – but we recoil in horror at the same phenomenon in a parasite. Is that wrong? Our emotional responses to the messier aspects of nature are complex. When we watch a nature documentary showing the leopard seals waiting off the ice shelf for the hungry penguins who want to return to the water, we feel for the penguins but probably also hold some admiration for the predatory seals. How should we feel about parasites? Should we be able to divorce our sense of aesthetics and self-interest from an appreciation of the complexity and completeness of the parasite’s strategy? At the very least, we should be interested in understanding it as completely as possible – to know the enemy – in order to better protect ourselves.
Article written by Dr. Doug Campbell