Baylisascariasis Nematodes
Baylisascariasis Nematodes
Importance
Baylisascaris procyonis, an intestinal nematode of raccoons, can cause severe neurological and ocular signs when its larvae migrate in humans, other mammals and birds. Although clinical cases are rare in people, they are serious and very difficult to treat.
Severe disease has also been reported in other mammals and birds in zoos, as well as in poultry, pet birds, dogs and wild animals. In most cases, humans or animals with central nervous system (CNS) disease are left with irreversible neurological signs if they survive.
Other species of Baylisascaris, particularly B. melis of European badgers and B. columnaris of skunks, can also cause neural and ocular larva migrans in animals, and are potential human pathogens.
Transmission and Life Cycle
Raccoons are normally the definitive hosts for B. procyonis. Mature worms are found in the intestines, and release unembryonated eggs into the feces. This organism produces very large numbers of eggs; each worm is estimated to lay up to 179,000 eggs per day, and raccoons carry an average of 43-52 worms.
Higher worm burdens are usually found in juvenile raccoons than adults. The development of B. procyonis eggs to the infective stage, containing second stage larvae, occurs in the environment. Under optimal conditions, this process can take as little as 11 to 14 days; however, under natural conditions, it is estimated to take 2 to 4 weeks or longer.
B. procyonis eggs are very resistant to environmental conditions, especially in moist soil. Although they can be killed eventually by extreme heat and dryness, the eggs survive harsh winters, and under some conditions, they can remain viable for years.
Infections in Animals Species Affected
Raccoons are usually the definitive hosts for B. procyonis. Dogs can be definitive as well as intermediate hosts. Adult B. procyonis have also been found in two kinkajous. Coatimundis and ringtails, which are closely related to kinkajous, might be susceptible.
More than 90 species of mammals, particularly rodents (mice, guinea pigs, prairie dogs and other species), lagomorphs and primates, can act as intermediate, paratenic or dead-end hosts. Symptomatic cases have also been seen in lemurs, Australian marsupials, opossums, porcupines, woodchucks and fruit bats, as well as in carnivores such as dogs, foxes, sea otters and American badgers (Taxidea taxus).
Baylisascaris larva migrans has not been reported in livestock, and experimental infections with B. procyonis in sheep, pigs or goats resulted in little migration of the larvae. However, one case of larva migrans occurred in a newborn lamb that had been infected before birth.
Birds are highly susceptible; B. procyonis can affect at least 40 avian species. Clinical cases have been reported in chickens, bobwhite quail, commercial pheasants, cockatiels, cockatoos, macaws, parrots and other species. B. procyonis infections have not been reported in poikilotherms.
Other species of Baylisascaris can also be involved in neural larva migrans of animals. Skunks, which carry B. columnaris, were associated with one outbreak at a zoo. B melis, a parasite of badgers, can cause CNS signs in mice, ground squirrels and rabbits, as well as ocular larva migrans in mice. B. transfuga, which occurs in bears, is also reported to cause ocular and neural larva migrans in experimentally infected animals.