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Neural Larva Migrans

NEURAL LARVA MIGRANS (NLM):  Baylisascaris procyonis, Toxocara cati, T. canis(Tissue Nematodes)
Cause of Neural Larva Migrans (NLM), Visceral Larva Migrans (VLM) and Ocular Migrans (OLM)

Baylisascaris procyonis is an ascarid normally found in raccoons (Procyon lotor), has a normal ascarid‑like life cycle, causes a very serious zoonotic disease in humans, and is most often reported from North America.  This syndrome can also be caused by the migration of larvae of Toxocara canis, T. cati, and some other animal helminths.

Life Cycle:
Humans acquire the infection by ingesting infective eggs of the raccoon nematode Baylisascaris procyonis or the dog (primarily) or cat ascarid T. canis or T. cati. After the eggs are accidentally ingested by a human, the larvae hatch in the small intestine, penetrating the intestinal mucosa, and migrating to the liver. Migratory routes include the lungs and/or other parts of the body, or the larvae may remain in the liver. During this migration, the larvae do not mature, even if they make their way back into the intestine.

Infection in humans is acquired through ingestion of eggs in contaminated soil.

Although not all raccoons are infected with Baylisascaris, infected animals shed millions of eggs over a short time frame; these eggs are very environmentally resistant.  Given that (i) Toxocara worms are commonly found in dogs and cats, (ii) puppies and kittens are infected early in life, and (iii) pets and children are often found in the same household, it is not surprising that a combination of small children playing in contaminated soil and pets passing large numbers of infective eggs leads to VLM and/or OLM. However, it is important to remember that this disease can also occur in adults. The eggs will become infective in about 3 weeks and will remain viable in the soil for months.

Clinical Features:
After human ingestion of infective eggs (Baylisascaris procyonis/raccoon round worm; Toxocara cati, T. canis/dog and cat roundworms), larvae hatch and migrate to multiple organs and tissues including the musculature, connective tissues, viscera, eyes, and central nervous system. Neural larva migrans (NLM) often presents as acute eosinophilic meningoencephalitis. Signs and symptoms may develop within 2 to 4 weeks after ingestion of large numbers of infective eggs and include weakness, incoordination, ataxia, irritability, weakness, seizures, altered mental status, stupor, and/or coma. Once symptoms and signs of neurologic disease are detected, significant pathology generally is already present.

The first confirmed cases of human B. procyonis infection were described in the 1980s. Risk factors have been identified as contact with infected raccoons, their feces, or a contaminated environment. Geophagia or pica has also been identified as a potential risk, which is often seen in children younger than 2 years. Tissue damage is caused by the actual larval migration, as well as an intense inflammatory reaction. Unlike other helminth larvae that cause VLM, the larvae of B. procyonis continue to grow during the migratory phase of the life cycle and can reach lengths of 2 mm. In addition to continued growth, the larvae tend to exhibit very vigorous migratory behavior and remain viable for long periods. They tend to invade the eyes, causing ocular larva migrans (OLM), which has also been found in immunocompetent adults, and the spinal cord and brain, causing NLM, found primarily in infants and young children.

Massive larval invasion of the CNS is characteristic, with estimates of more than 3,200 larvae being isolated from the brain of a single case (3 larvae/g of tissue) (40). This neural form, NLM, may present with symptoms ranging from mild neuropsychological problems to seizure, convulsions, ataxia, coma, and death. Patients may exhibit sudden lethargy, irritability, loss of muscle coordination, decreased head control, spasmodic contractions of the neck muscles, stupor, nystagmus, obtundation, coma, hypotonia, and hyperreflexia. Infants who have survived meningoencephalitis demonstrate sequelae including hemiparesis, inability to sit or stand, ocular muscle paralysis, cortical blindness, and severely delayed development. Seizures often occur, and these symptoms tend to be difficult to control. The patients may deteriorate rapidly, progressing to stupor, coma, and death.  Survivors are left in a persistent vegetative state or with severe neurologic deficits, including blindness, all of which can require extensive supportive care.

Although a rare complication of toxocariasis, CNS involvement (neurotoxocariasis) can cause seizures, neuropsychiatric symptoms, or encephalopathy.  Due to improved diagnosis, an increasing number of clinical NLM cases due to larval invasion of the brain or spinal cord have been recorded during the last thirty plus years.  Neurological damage and epilepsy, neuropsychological deficits, eosinophilic meningoencephalitis, myelitis, and cerebral vasculitis have been reported.  Toxocara has been described as “America’s most common neglected infection of poverty in the United States and a disease with global importance”.   Unfortunately, cases of NLM will tend to be underestimated due to nonspecific clinical signs and the lack of relevant testing thus leading to possible under-diagnosis.

Clinical Specimen:
Serum:  Acute and convalescent titers demonstrate several-fold increases in both serum and CSF antibody levels.  Testing is available through CDC.
Tissue:  Larvae can often be identified via histology; CDC would also be an excellent reference source.

Albendazole is the treatment of choice.  Although mebendazole is poorly absorbed outside the gastrointestinal tract, it has been used with some success.

The following preventive measures should be emphasized:  control of raccoons around property (do not feed them, do not leave dog food out, restrict infants from playing in potentially contaminated soil),regular deworming of dogs and cats, beginning at 2 weeks of age; removing animal feces in places adjacent to homes and children’s playgrounds; keeping children’s sand boxes covered when not being used; regular hand washing after handling soil and before eating; and teaching children not to put dirty objects into their mouths.

Garcia, L.S. 2015.  Diagnostic Medical Parasitology, 6th ed., ASM Press, Washington, D.C.