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[Baylisascaris procyonis]

Causal Agents

Human baylisascariasis is caused by larvae of Baylisascaris procyonis, an intestinal nematode of raccoons.

Life Cycle


Baylisascaris procyonis completes its life cycle in raccoons, with humans acquiring the infection as accidental hosts (dogs serve as alternate definitive hosts, as they can harbor patent and shed eggs). Unembryonated eggs are shed in the environment The Number 1, where they take 2-4 weeks to embryonate and become infective The Number 2. Raccoons can be infected by ingesting embryonated eggs from the environment The Number 3. Additionally, over 100 species of birds and mammals (especially rodents) can act as paratenic hosts The Number 4 for this parasite: eggs ingested by these hosts hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst The Number 5. The life cycle is completed when raccoons eat these hosts The Number 6. The larvae develop into egg-laying adult worms in the small intestine The Number 7 and eggs are eliminated in raccoon feces. Humans become accidentally infected when they ingest infective eggs from the environment; typically this occurs in young children playing in the dirt The Number 8. Migration of the larvae through a wide variety of tissues (liver, heart, lungs, brain, eyes) results in VLM and OLM syndromes, similar to toxocariasis The Number 9. In contrast to Toxocara larvae, Baylisascaris larvae continue to grow during their time in the human host. Tissue damage and the signs and symptoms of baylisascariasis are often severe because of the size of Baylisascaris larvae, their tendency to wander widely, and the fact that they do not readily die. Diagnosis is usually made by serology, or by identifying larvae in biopsy or autopsy specimens.

Geographic Distribution

Raccoons infected with Baylisascaris procyonis appear to be common in the Middle Atlantic, Midwest, and Northeast regions of the United States and are well documented in California and Georgia. Proven human cases have been reported in California, Oregon, New York, Pennsylvania, Illinois, Michigan, and Minnesota, with a suspected case in Missouri.

Clinical Presentation

Human infections can be asymptomatic. However, because these larvae continue to grow and wander in the human host, infections often result in severe disease manifestations. Much like toxocariasis, infection with Baylisascaris can result in visceral larva migrans (VLM) or ocular larva migrans (OLM) syndromes. The larvae of B. procyonis have a tendency to invade the spinal cord, brain, and eye of humans, resulting in permanent neurologic damage, blindness, or death. Human infection with Baylisascaris appears to be rare. To date, 13 well documented Baylisascaris encephalitis cases, and 1 suspected case in a young girl with CNS larva migrans, have been reported. The prevalence of subclinical cases is unknown. Because there is no widely available definitive diagnostic test for humans infected with this parasite, many cases are not diagnosed initially.

Baylisascaris procyonis eggs.


Baylisascaris procyonis eggs are 80-85 µm by 65-70 µm in size, thick-shelled, and usually slightly oval in shape. They have a similar morphology to fertile eggs of Ascaris lumbricoides, although eggs of A. lumbricoides are smaller (55-75 µm by 35-50 µm). The definitive host for B. procyonis is the raccoon, although dogs may also serve as definitive hosts.  As humans do not serve as definitive hosts for B. procyonis, eggs are not considered a diagnostic finding and are not excreted in human feces. The following images show larval development within the eggs, from a freshly-shed unembryonated egg to eggs containing L1 larvae. The images are all courtesy of Dr. Cheryl Davis, Western Kentucky University, KY.

Figure A: Unembryonated egg of B. procyonis

Figure B: Egg of B. procyonis. In this specimen, the developing embryo has started to divide.

Figure C: Eggs of B. procyonis in a further state of cleavage.

Figure D: Eggs of B. procyonis in a further state of cleavage.

Figure E: Embryonated eggs of B. procyonis, showing the developing larva inside.

Figure F: Embryonated eggs of B. procyonis, showing the developing larva inside.

Baylisascaris procyonis hatching larvae.

Figure A: Larva of B. procyonis hatching from an egg.

Viable B. procyonis egg in formalin, recovered from a raccoon.

Figure A: Egg of B. procyonis in formalin-fixed stool from a raccoon. Animated image contributed by the Oregon State Public Health Laboratory.

Larvae of Baylisascaris spp. in tissue.
B procyonis adults.

Figure A: Cross-sections of larvae of B. columnaris in the brain of a laboratory-infected mouse. The larval morphology and microscopic manifestations would be similar with B. procyonis in human tissue.Image taken at 400x magnification.

Figure B: Higher magnification (1000x oil) of a cross-section of B. columnaris from the same specimen as Figure A. Notice the prominent alae (green arrows), excretory columns (red arrows) and multinucleate intestinal cells (blue arrow).

Figure C: Cross-sections of larvae of B. columnaris in muscle of a laboratory-infected mouse. The larval morphology and microscopic manifestations would be similar with B. procyonis in human tissue. Image taken at 400x magnification.

B. procyonis adults.


Baylisascaris procyonis adults are found only in the definitive host, the raccoon. They have the typical form of ascarid worms. Males measure 60-80 mm in length and demonstrate the curved posterior end similar to Ascaris. Females measure 80-120 mm in length.

Figure A: Several adults of B. procyonis from a raccoon

Laboratory Diagnosis

Human infections are difficult to diagnose, and often the diagnosis is by exclusion of other causes. Results from complete blood count (CBC) and cerebrospinal fluid (CSF) examination would be consistent with parasitic infection, but tend to be nonspecific. Examination of tissue biopsies can be extremely helpful if a section of larva is contained, but removing an appropriate piece of tissue where the larva is actually present can be problematic. Ocular examinations revealing a migrating larva, larval tracks, or lesions consistent with a nematode larva are often the most significant clue to infection with Baylisascaris. Serologic testing can be extremely helpful in suspected cases; however, tests are not routinely in use nor widely available.

Treatment Information

Treatment information for baylisascaris can be found at:

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  • Page last reviewed: January 3, 2018
  • Page last updated: January 3, 2018
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