Chapter 325. Baylisascariasis

Baylisascariasis is a potentially serious form of larva migrans that is caused by larvae of the raccoon ascarid, Baylisascaris procyonis, a nematode found in 50% to 80% of raccoons in North America. It also occurs in raccoons in other countries, including Germany and Japan. B procyonis is a well-known cause of larva migrans in animals and is usually associated with central nervous system disease. Larvae of B procyonis have produced fatal or severe central nervous system disease in over 100 species of mammals and birds. Other Baylisascaris species, including B melis of badgers and B columnaris of skunks, are also potential causes of human disease.1

Adult female Baylisascaris organisms are 12 to 23 cm long and reside in the raccoon’s small intestine (eFig. 325.1).2 Adult males are about half as large. Adult females produce huge numbers of eggs, with estimates as high as 179,000 eggs per worm per day.3 Infected raccoons can thus shed millions of eggs per day, leading to heavy environmental contamination. Eggs shed with the raccoon’s feces become infective in 3 to 4 weeks. Young raccoons become infected by ingesting infective eggs, whereas older raccoons become infected by ingesting larvae in the tissues of intermediate hosts, including rodents, rabbits, and birds (eFig. 325.2).

Humans become infected with Baylisascaris by accidentally ingesting infective eggs (eFig. 325.3) from objects contaminated with the feces of wild or pet raccoons. In the wild, raccoons defecate in communal sites, called latrines. In rural areas, these sites can be found in tree stumps or limbs or rock piles. However, in suburban and urban areas, they are often found in attics, chimneys, flat roofs or patios, creating potentially long-standing sources of infection. Baylisascaris eggs in the soil can remain infective for years. Larvae emerge from ingested eggs and migrate to many tissues, including lung, skeletal muscles, eye, and brain. Approximately 5% to 7% of ingested larvae enter the brain, where they can produce extensive damage before they are walled off. Migrating larvae cause mechanical damage and incite vigorous host inflammatory reactions, producing eosinophilic granulomas in many tissues.

The risk of human infection with Baylisascaris is greatest in children younger than 4 years of age because of hygienic habits and propensity for pica and geophagia. B procyonis can produce visceral larva migrans (VLM), ocular larva migrans (OLM), and neural larva migrans (NLM), with eosinophilic meningoencephalitis, which usually presents in patients with concurrent VLM.4 Subclinical infection probably is most common, followed by clinical OLM, VLM, and NLM. Fatal or severe Baylisascaris NLM has been documented primarily in children, although the disease remains rare.

The diagnosis of infection with Baylisascaris is based primarily on clinical findings, history, and serology. Clinical findings vary from mild illness to severe central nervous system (CNS) disease depending on the level and frequency of infection and the degree of CNS involvement. Signs of visceral larva migrans, including hepatomegaly, eosinophilia, and elevated isohemagglutinin titers, may be present and are similar to those of toxocariasis. Because B procyonis demonstrates neurotropism, larvae tend to concentrate in the head, neck, and thoracic regions. If enough Baylisascaris eggs are ingested, severe central nervous system disease can develop within 2 to 4 weeks. Clinical signs in such cases include sudden lethargy, loss of muscle coordination, decreased head control, torticollis, hemiparesis, ocular muscle paralysis, cortical blindness, and nystagmus, which may quickly progress to coma and death.5 An important diagnostic finding in baylisascariasis is eosinophilic pleocytosis of the cerebrospinal fluid, especially in a patient with concurrent peripheral eosinophilia and progressive central nervous system disease. However, other parasitic pathogens, notably Angiostrongylus cantonensis, may also produce eosinophilic meningitis. Computed tomography and magnetic resonance imaging findings may include marked periventricular contrast enhancement of the brain and diffuse cerebral and cerebellar atrophy. Brain biopsies have shown mixed eosinophilic inflammation and occasionally larvae.

Ocular larva migrans typically is seen in older individuals without other symptoms, and it manifests as unilateral loss of vision.6 Ophthalmoscopy reveals inflammation, migration tracks, and/or granulomas in the retina or choroid, or evidence of diffuse unilateral subacute neuroretinitis. If visualized, larvae are 3 to 5 times larger than Toxocara and measure 1.5 to 2.0 mm long.

Serologic methods (eg, indirect immunofluorescence, enzyme-linked immunosorbent assay, Western blotting) were developed for Baylisascaris and are useful in a limited number of documented cases, although patients with ocular larva migrans are frequently seronegative or only weakly seropositive.

Information on effective anthelminthic treatment of this infection is limited, and the treatment of neural larva migrans is not promising. Anthelminthics that show the greatest promise include albendazole, thiabendazole, and levamisole. Ivermectin does not cross the blood-brain barrier well and has not proved successful in clinical cases. Even if effective drugs were available, clinical neural larva migrans caused by Baylisascaris has a poor prognosis because the diagnosis is usually delayed and extensive central nervous system damage has often occurred by the time diagnosis is made. In these cases, control of inflammation and supportive maintenance of the patient are both important until larvae are killed by anthelminthics or are walled off in granulomas. Ocular baylisascariasis has been successfully treated using laser photocoagulation.

Prevention is important because treatment is so inadequate. Keeping raccoons as pets should be discouraged, especially in households with young children. Parents should prevent access by children to known or potential areas that are contaminated with raccoon feces. Because raccoons commonly defecate on fallen timber, caution should be exercised in using it for firewood.