ISSN 2398-2969      

Baylisascaris procyonis





  • Baylisascaris procyonis is a large nematode of the Order Ascaridida.
  • Closely related to Toxocara canis and Baylisascaris columnaris.


  • Baylisascaris from Baylis the man who discovered the clinical signs were due to the nematode worm, and ascaris from Ascarid, a descriptive term for worms in this order.
  • Nema meaning thread, Eidos meaning form and Osis meaning condition. Of Greek origin.

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Clinical Effects



  • Eggs are found predominantly in areas where raccoons defecate. 
  • These areas are known as latrines and can be in tree stumps, barns or stables or other sheltered areas within woodland habitats.


  • Eggs are usually ingested because feed stuffs are contaminated with raccoon feces.
  • The L2 larvae hatch from the eggs in the small intestine.
  • These then migrate into the intestinal wall where they develop into L3 larvae if they have been eaten by a raccoon.
  • The L3 larvae then re-emerge into the gut lumen and mature into adult worms.
  • If the eggs have been ingested by an intermediate host such as a rabbit, then the L2 larvae again migrate into the intestinal wall. However instead of maturing here they migrate via the portal circulation through the liver to the lungs.
  • They get into the left side of the heart through the pulmonary veins.
  • The systemic circulation then distributes them throughout the body.
  • A small number of larvae (approx 5%) will enter the central nervous system. 
  • The aggressive growth and migration of larvae causes an acute eosinophilic meningoencephalitis, which in turn causes clinical signs.


  • Ingestion of contaminated feces causes spread of infection to non-target species.
  • Raccoons can be contaminated by ingestion of contaminated feces or by ingestion of prey species which have L3 larvae encysted in their muscle.

Pathological effects

  • Larval migration within the central nervous system causes severe encephalopathy; this is specifically noted in the cerebrum, cerebellum, midbrain and medulla. 
  • Physical changes include multifocal areas of necrosis with associated aggregations of inflammatory cells (eosinophils, lymphocytes, plasma cells and macrophages); these in turn lead to the destruction of nervous tissue, which causes clinical signs.

Other Host Effects

  • Larval migration can affect liver, lung and cardiac function in addition to central nervous damage.
  • Visceral larval migrans can result in larvae encysted in muscle, accompanied by an intense eosinophilic or granulomatous inflammatory reaction.
  • Ocular larval migrans leads to damage of the structures within/around the eye and visual impairment.


Control via animal

  • Control of raccoon numbers and restricting access to potential latrine areas by pet rabbits. 
  • Control of raccoon access near house or farm buildings.

Control via chemotherapies

  • Baylisascaris procyonis infection can be diagnosed and successfully treated within captive raccoons using standard chemotherapeutic protocols, eg pyrantel   Pyrantel embonate, mebendazole Mebendazole and fenbendazole Fenbendazole.
  • Baylisascaris procyonis infection within an intermediate host such as the rabbit is not responsive to medical treatment, and is usually fatal. Albendazole Albendazole (with or without corticosteroids) has been used in humans, but with no patients recovering and remaining neurologically intact.

Control via environment

  • Restricting access to raccoon latrines, removal of raccoon feces. 
  • Eggs are very resistant to attempts to decontaminate the environment, and are also resistant to dehydration and extremes of temperature. 
  • Thorough environmental cleansing in association with heat treatment (flaming the area) is the most reliable way to control infection environmentally - 50:50 xylene ethanol mix after thorough cleaning and flaming should be effective.
  • Management of raccoon populations within a geographic area by trapping, baiting, etc.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Gavin P J, Kazacos K R & Shulman S T (2005) Baylisascariasis. Clin Microbiol Rev 18 (4), 703-718 PubMed.
  • Furuoka H, Sato H, Kubo M et al (2003) Neuropathological observations of rabbits (Oryctolagus cuniculus) affected with raccoon roundworm (Baylisascaris procyonis) larva migrans in Japan. J Vet Med Sci 65 (6), 695-699 PubMed.

Other sources of information

  • Samuels W M, Pybus M J & Kocan A A (2001) Parasitic Diseases of Wild Mammals. 2nd edn. Manson Publishing, UK.

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