Visceral Larva Migrans and Other Uncommon Helminth Infections

Most helminths that infect humans are relatively host specific to humans, undergo characteristic migration and development, and are found in typical anatomic locations. However, these helminths sometimes undergo atypical or aborted migrations and cause symptoms or signs because of their unusual or ectopic location. A good example of this is the deposition of schistosomal ova and the subsequent granulomatous inflammatory lesions in the spinal cord or brain. In addition, some helminths of animals can also infect humans. Examples are Echinococcus granulosus and Trichinella spiralis, which commonly infect humans, migrate and develop normally, and reside in locations similar to those in the animal host. In contrast, other helminths of animals are unable to develop or migrate normally. Commonly, they undergo prolonged aberrant migrations or locate abnormally in the tissues as underdeveloped larvae and incite an eosinophilic inflammation that is responsible for many of the symptoms and signs of these infections. Although a large number of animal parasites may infect humans, most do so rarely. In contrast, some helminths of animals infect humans more commonly and cause distinctive clinical syndromes ( Table 290.1 ), sometimes associated with characteristic epidemiology, exposure history, and geographic locations. More often than not, similar clinical syndromes are caused by a group of related parasites. The diagnosis is suggested on clinical and epidemiologic grounds. Although pathologic examination of tissue can sometimes establish the diagnosis, the detection of larvae is commonly unrewarding. Serologic tests are sometimes helpful (see “ Visceral Larva Migrans [Toxocariasis] ”) but usually are not fully evaluated, are experimental, or are unavailable.

TABLE 290.1

Clinical Syndromes Associated With Unusual Helminth Infections in Humans

CLINICAL SYNDROME	PARASITE	USUAL HOST
Visceral larva migrans	Toxocara canis	Canines
	Toxocara cati	Felines
	Baylisascaris procyonis	Raccoons
Eosinophilic gastroenteritis	Anisakis spp.	Sea mammals
	Phocanema spp.	Sea mammals
	Ancylostoma caninum	Canines
Cutaneous larva migrans	Ancylostoma braziliense	Canines, felines
	Ancylostoma caninum	Canines, felines
	Uncinaria stenocephala	Canines, felines
Eosinophilic meningitis	Angiostrongylus cantonensis	Rats
	Gnathostoma spinigerum	Felines, other mammals
Pulmonary or cutaneous nodules	Dirofilaria spp.	Canines, other mammals
Abdominal angiostrongyliasis	Angiostrongylus costaricensis	Cotton rats
Capillariasis	Capillaria philippinensis	Birds
Swimmer's itch	Trichobilharzia spp.	Birds

The diagnostic procedures used to detect infections differ for each parasite, so a clear idea of the potential causes is essential. The physician must understand the sensitivity of the diagnostic procedures and the abilities of the laboratory personnel performing them.

Visceral Larva Migrans (Toxocariasis)

Visceral larva migrans (VLM) is a syndrome characterized in its most florid state by eosinophilia, fever, and hepatomegaly. It is caused primarily by infection with Toxocara canis but also, less frequently, infection with Toxocara cati and other helminths.

Life Cycle in the Dog

T. canis infects dogs and related mammals by a number of mechanisms. Most commonly, ingested eggs hatch in the small intestine, and the resulting larvae migrate to the liver, lung, and eventually the mucus layer of trachea. They are then swallowed and mature in the lumen of the small intestine, where eggs are shed. Other larvae migrate to and remain dormant in the muscles but are capable of development years after the primary infection, particularly in pregnant bitches. During pregnancy, larvae again develop and infect the puppies transplacentally and transmammarily. Infective larvae are commonly found in the feces of the puppies. Eggs are not infectious when passed in the feces and take 3 to 4 weeks to develop. They are hardy and often remain viable for months. Large numbers of viable eggs contaminate the environment because of the high prevalence of infection in dogs and the ability of eggs to survive relatively harsh environmental conditions. Humans become infected mostly from ingesting viable ova that contaminate the soil where dogs defecate. Ingestion of raw organs containing larvae, which is common in some regions, is another means of infection.

Infection in Humans

Prevalence

Toxocariasis is prevalent wherever dogs or cats are found, and Toxocara eggs are able to survive. The prevalence of infection or disease in humans is not known, but seroepidemiologic studies show wide differences in prevalence, depending on the population tested. In the United States, seropositivity ranged from 2.8% in an unselected population, to 23.1% in a kindergarten population in the southern United States, to 54% in a selected rural community. None of the seropositive persons had recognizable disease. A review of reported seroprevalence of toxocariasis in North America reported an overall prevalence rate of 13.9% in the United States, with higher rates in Mexico.

Clinical Manifestations

VLM occurs most commonly in children younger than 6 years of age, frequently after ingestion of contaminated soil. Disease manifestations vary and range from asymptomatic infection to fulminant disease and death, but it is increasingly appreciated that most infections are asymptomatic. Individuals who come to medical attention most commonly present with cough, fever, wheezing, and other generalized symptoms. The liver is the organ most frequently involved, and hepatomegaly is a common finding, although almost any organ can be affected. Splenomegaly occurs in a small number of patients, and lymphadenopathy has been noted. Lung involvement with radiologic findings has been documented in 32% to 44% of patients, but respiratory distress occurs rarely. Skin lesions such as urticaria and nodules have also been described. Seizures have been noted to occur with increased frequency in VLM, but severe neurologic involvement is infrequent. Eye involvement in VLM is unusual but has been documented (see “ Ocular Larva Migrans ”). Eosinophilia, usually accompanied by leukocytosis, is the hallmark of VLM. Laboratory findings include hypergammaglobulinemia and elevated isohemagglutinin titers to A and B blood group antigens, which are caused by the host's immune response to cross-reacting antigens on the surface of T. canis or T. cati larvae. Serologically positive populations may be at increased risk for seizures.

Diagnosis

The diagnosis of VLM is usually suggested clinically by the presence of eosinophilia, leukocytosis, or both in a young child also presenting with hepatomegaly or signs and symptoms of other organ involvement. A history of pica and exposure to puppies is common. In the United States, patients are more commonly black and from rural areas.

The diagnosis is definitively confirmed by finding larvae in the affected tissues by histologic examination or by digestion of tissue; however, larvae are frequently not found. Computed tomography scans have demonstrated ill-defined hypodense round lesions in the liver of patients with VLM, the basis of which is unclear. Standard assays including enzyme-linked immunosorbent assay or Western blot using excretory-secretory products of T. canis larvae have been used to confirm the clinical diagnosis for decades. Newer assays using recombinant antigens in a Luminex Bead format have obviated the need for collection of infectious larvae and have led to assays with high sensitivity and less cross-reactivity with other helminths. Toxocara antibody titers in populations without clinically apparent VLM vary dramatically, and elevated titers cannot definitively establish the diagnosis.

Differential Diagnosis

Eosinophilia, fever, and hepatomegaly are frequently caused by helminths that migrate through the body. Baylisascaris procyonis (an ascarid of raccoons) is a recognized cause of larval migrans in the United States. Other helminth infections include acute schistosomiasis, Fasciola hepatica infections, Ascaris lumbricoides abscess of the liver, acute liver fluke infections (Clonorchis sinensis, Opisthorchis viverrini, Fasciola hepatica) , complications from Echinococcus infection of the liver, and Capillaria hepatica . Diseases not caused by parasitic infections should also be considered. Children with mild disease may manifest only eosinophilia.

Treatment and Management

Most patients recover without specific therapy. Treatment with antiinflammatory or anthelmintic drugs may be considered for patients with severe complications usually caused by involvement of the brain, lungs, or heart. There is no proven effective therapy, although albendazole, mebendazole, diethylcarbamazine, and other anthelmintics have been used. Injury to the parasite may provoke an intense inflammatory response, leading to worsening of the clinical picture. Corticosteroids are frequently used to control symptoms of infection or to suppress possible potentiation of symptoms caused by the host's immune responses to killed or injured larvae as a consequence of anthelminthic treatment.

Prevention

VLM can be easily prevented by a number of simple but effective measures that prevent T. canis or T. cati eggs from contaminating the environment and children from ingesting eggs and pica. Dogs, particularly puppies, should be periodically tested and treated for Toxocara and other worms. To prevent defecation by dogs and cats in sandboxes where young children play, they should be covered when not in use.

Ocular Larva Migrans

Ocular larva migrans is caused primarily by an infection of the eye with T. canis larvae. Although a present or past history of clinically recognized VLM has occasionally been noted, almost all patients present with unilateral eye visual loss without a past history or present systemic symptoms or signs. Presumably, a larva becomes entrapped in the eye by chance, resulting in an eosinophilic inflammatory mass. Children are most commonly affected and, on average, are older (mean, 8.6 years old in one study) than children with VLM. Although the most common lesion is a chorioretinal granuloma in the posterior pole, or occasionally more peripherally, diffuse panuveitis may also be seen. Retinal detachment may occur. This entity was first recognized in the examination of eyes enucleated for the treatment of presumed retinoblastoma, and it remains the most important distinction that ophthalmologists must make in children with subretinal lesions.

Eosinophilia, hepatomegaly, and other signs and symptoms of VLM are usually lacking. The diagnosis is established clinically. Although the serum titers to Toxocara larvae are higher than those of a control population, many patients with ocular larva migrans have low or negative titers. However, elevated vitreous and aqueous fluid titers to Toxocara larvae, compared with serum levels, appear useful for establishing the diagnosis. There is no specific therapy.

A characteristic clinically recognizable syndrome, diffuse unilateral subacute neuroretinitis is caused by infection with helminth larvae of B. procyonis and Toxocara spp. and other unidentified nematodes. Angiostrongylus cantonensis larvae can also involve the eye. A motile larva is commonly found in or below the retina. Photocoagulation is curative. Anthelmintic therapy such as albendazole may be effective.

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