Baylisascaris procyonis

Authors: Helmut Albrecht, M.D., Michael Martin M.D., MPH, DTM&H

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Parasitology

Baylisascaris procyonis, an intestinal roundworm of raccoons, causes visceral larva migrans in more than 40 species of wild and domestic mammals and birds (14, 17).

Epidemiology

The prevalence of B. procyonis infection in raccoons of the midwestern and northeastern United States reaches 70-82% (17). More than two-thirds of raccoons examined in the northern Main region of Germany were also found to be infected with B. procyonis (18). Humans become infected after ingestion of B. procyonis eggs from areas contaminated with raccoon feces. Children 1 to 4 years old are at the greatest risk of heavy infection.

Clinical Manifestations

Experimental infection of non-human primates has led to extensive larva migrans and fatal central nervous system (CNS) disease (17). B. procyonis has been reported to cause asymptomatic infections (5), visceral and ocular larva migrans (16), diffuse unilateral subacute neuroretinitis (18), and eosinophilic meningoencephalitis in humans (5, 7, 11).

Laboratory Diagnosis

Because neither eggs nor larvae can be detected in fecal or blood samples of infected humans the diagnosis of B. procyonis depends on the exposure history, clinical findings, and serologic test (17). Enzyme immunoassays (EIA) and immunoblot methods have been developed to test for B. procyonis larval excretory-secretory or structural proteins (3).

Pathogenesis

Larvae hatch in the small intestine and migrate via first the portal circulation and then the systemic circulation to multiple organ systems, including the liver, lungs, heart, eyes, and brain. Larval migration occurs rapidly, as demonstrated in murine studies, which detected larvae in the eyes, brain, and somatic tissues within 3 days. In subhuman primates, they were observed in the eyes as early as 7 days after infection (14,15,16). In addition to causing traumatic damage and necrosis, the larvae incite a potent predominantly eosinophilic inflammatory reaction. Larval migration is halted when the immune system overtakes the larvae and encapsulates them within eosinophilic granulomas.

SUSCEPTIBILITY IN VITRO AND IN VIVO

There are no reliable in vitro susceptibility studies for this organism. Pyrantel embonate (20 mg base/ kg x 1 dose), ivermectin (1 mg/ kg x 1 dose), moxidectin (1 mg/ kg x 1 dose), albendazole (50 mg/ kg x 3 doses), fenbendazole (50 mg/ kg x 3 doses), and ubendazole (22 mg/kg x 3 doses) are all effective against luminal stages of Baylisascaris procyonis in naturally infected raccoons (2). Despite their efficacy against intestinal parasites these anthelmintics are less effective against tissue larvae (5, 7, 17). Albendazole concentrations in CSF and brain tissue are 40% to 50% of that in plasma (6,12,23). Experimentally, mice treated with albendazole (25-50 mg/ kg) or diethylcarbamazine (100 mg/kg) were protected from CNS disease when the drugs were given on days 1 to 10 or days 3 to 10 post infection (8,17,19). Given the fact that the clinical incubation period in mice exceeds 10 to 20 days these observations may not be applicable to clinical cases (22).

ANTIPARASITIC THERAPY

General

The appropriate therapy for B. procyonis infection remains unclear. Case reports describe the use of anthelmintics but no large, randomized, controlled trails have been possible because of the relatively low number of diagnosed clinical cases. Five well-documented cases of eosinophilic meningoencephalitis due to B. procyonis have been reported in young children. A 10 month-old boy presented with inability to crawl and decreased head control (11). CSF examination revealed 92 white blood cells (WBCs)/mm3, with 68% eosinophils. The child became increasingly lethargic and eventually unresponsive. He died 14 months later with no neurologic improvement. Eosinophilic granulomas containing B. procyonis were found in the brain at autopsy. An 18 month-old child presented with lethargy, vertical nystagmus, and hypertonia of the right arm (7). CSF examination showed 123 WBCs/mm3, with 50% eosinophils. He was treated with thiabendazole but died on his 6th hospital day, 3 days after starting thiabendazole. Larvae ofB. procyonis were identified in sections of the cerebrum, cerebellum, and spinal cord. A 13 month-old child was admitted because he refused to walk, had a right torticollis, and a right gaze preference (5). The CSF contained 125 WBCs/mm3, with 60% eosinophils. Serum and CSF samples were examined for antibodies to B. procyonis. EIA demonstrated titers of >1:10 240 for serum and 1:40-1:80 for CSF. Immunoblot analysis of serum and CSF were strongly reactive to several protein bands (3). The child was treated initially with thiabendazole and prednisone. After 1 week there was no improvement in his neurologic status and ivermectin was given. The child's appetite and activity gradually improved, but at 6 months he had persistent cortical blindness and right-sided hemiparesis. Another 13 month-old child developed eosinophilic meningoencephalitis, retinitis, and a protracted encephalopathy. The patient was treated with various antibiotics and steroids but no anthelmintics. Brain biopsy later confirmed B. procyonis encephalitis. At 7 years follow-up, the child continues to display severe residual deficits. He was functionally blind, was incontinent, uttered only a few syllables, suffered from refractory seizures and was largely confined to wheelchair use (21). An 11 month-old boy with extensive raccoon exposure presented with irritability and behavioral regression. Encephalitis and unilateral subacute neuroretinitis developed. Baylisascaris IFT was positive at 1:1024. He was treated with albendazole (40 mg/kg/d) for 28 days and methylprednisolone (20mg/kg/day). Almost two years later he has significant residual neurologic deficits including incomplete seizure control and manifests a developmental delay (20).

The treatment of patients presenting with signs and symptoms of B. procyonis infection is problematic. Anthelmintics that are effective against intestinal parasites are often less effective against tissue larvae (5,7,17). In addition, patients with CNS disease are likely to have extensive larval invasion of the brain. Effective antihelminthic therapy that leads to the death of these larvae may elicit an inflammatory response that could worsen preexisting neurologic damage (5,17).

To date, the administration of various anthelmintics for the treatment of neural larval migrans, including albendazole, mebendazole, thiabendazole, fenbendazole, levamisole, and ivermectin, have been recommended and used to treat animals and humans with B. procyonis infections. None of these have clearly prevented a poor outcome in any of the documented or suspected human cases. In some cases living larvae were subsequently recovered on autopsy from the brains of treated animals and humans (1,5,7,17). Ivermectin has been used with success in patients with strongyloidiasis, onchocerciasis, and ocular toxocariasis (5,24). Ivermectin, however, does not cross the blood brain barrier well in animals (4) and was not found in CNS sampling of a patient with B. procyonis infection and meningoencephalitis 12 hours after dosing (5). Theoretically, very early treatment of suspected infections based on exposure history and serologic examination should improve outcome by halting progression to CNS disease, which is uniformly associated with a poor outcome. Nevertheless, if treatment were to be initiated, we would consider albendazole as the first option with the caveat that efficacy may well be marginal.

Ocular Infection

Diffuse unilateral subacute neuroretinitis (DUSN), a syndrome characterized by progressive unilateral visual loss, vitritis, papillitis, gray-white retinal lesions, and diffuse retinal pigment epithelial degeneration, was described in 1978 (9) and attributed to Toxocara canis infection (10). Since 1978 several authors have provided evidence that B. procyonis causes ocular larva migrans and DUSN (15,16,18).Laser photocoagulation therapy has been used successfully to destroy intraretinal larvae of Baylisascaris.

ADJUNCTIVE THERAPY

Considering the possibility of inciting additional inflammation from anthelmintic therapy adjunctive corticosteroid therapy has been recommended for the treatment of eye and CNS infections due to prevent or lessen an adverse inflammatory response (1). Concomitant administration of steroids, specifically dexamethasone, may also enhance albendazole plasma concentration by 50% (13).

ENDPOINTS FOR MONITORING THERAPY

The endpoints of monitoring therapy are unknown.

VACCINES

No vaccines are available for B. procyonis.

PREVENTION

General

Because antihelminthic therapy does not provide satisfactory outcomes, prevention of B. procyonis infection is a priority. Raccoons in captivity should be periodically examined and treated for this parasitic infection (7). Infected raccoons may shed millions of eggs per day and these eggs can remain viable for months to years (17). Thus, contaminated areas can become long-term potential sources of infection for both humans and animals. If these sites cannot be decontaminated they should be avoided, particularly by young children exhibiting pica, especially geophagia (5). At least in the mouse model postexposure prophylaxis with albendazole (25-50mg/kg) or diethylcarbamazine (100 mg/kg) was able to prevent progressive CNS disease (8,17,19). The relevance of this observation for human exposures is currently unknown.

CAVEATS

B. procyonis is a common infection of raccoons and a rare cause of devastating CNS disease in humans, especially young children. Effective antihelminthic therapy for established CNS infections is not yet available. At this point it is not even clear whether treatment should be directed against the larvae, the immune response or both. Early treatment of suspected infections based on exposure history and/or serologic examination could potentially prevent progressive CNS disease. Awareness of the risk factors for acquisition and the clinical presentation of this disease may therefore be the most critical determinants of an early and therefore most likely successful intervention. As long as therapy of this disease is of such limited benefit, prevention of infection of young children should be a public health priority.

REFERENCES

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