The raccoon worm Baylisascaris procyonis is a roundworm that occurs in raccoons and raccoon-like animals (procyonidae).
The life cycle of this raccoon giant roundworm is direct or indirect meaning that it can use but does not necessarily require an intermediate host (see: susceptible animals) to complete its development. The raccoon is the definitive host i.e. the animal in which egg-producing adult worms live. An intermediate or accidental host which becomes infected by larvae but in which no further development occurs is called a paratenic host and they can act as vectors.
Raccoons and other procyonidae are definitive hosts in which larvae develop into egg-producing adults, but in the USA dogs have increasingly been implicated in the lifecycle, acting as definitive hosts. Infected animals can excrete millions of eggs daily and these can survive for years in suitably damp environments. In the eggs, which are invisible to the naked eye, the larvae develop into 2nd stage larvae and these so-called embryonated eggs represent the infectious stage. At least 100 species of birds and small mammals are known to be susceptible to infection with embryonated eggs of Baylisascaris procyonis. Ingestion of these eggs may occur whilst foraging in areas contaminated by raccoon faeces which, as they often contain undigested seeds, represent a convenient food source for some species.
After ingestion the larvae leave the egg and penetrate the gut wall, migrating to various locations in the body including the central nervous system and the eye (this process is known as larval migrans) where they become encysted (Maas et al. 2014). Clinical signs depend on the number and size of the encysted larvae; particularly in the brain their presence can cause severe problems and even death.
Raccoons become infected in one of two ways; either through the ingestion of embryonated eggs or through ingestion of encysted larvae in the tissue of intermediate hosts. In raccoons infection does not typically cause clinical signs.
People can become infected via ingestion of embryonated eggs through contact with raccoon feces or contact contaminated fur.
Disposable gloves should be worn by people coming into contact with raccoons (including cadavers). Raccoon latrines should be avoided and it should be remembered that eggs in the environment can remain infectious for years.
