Optic neuritis in cerebral toxocariasis

Case series of neuroretinitis in Korea

BACKGROUND

To present the clinical characteristics of neuroretinitis in Korea.

METHODS

Twelve patients with neuroretinitis between January 2009 and September 2020 were retrospectively reviewed. Neuroretinitis was diagnosed based on fundus findings, optical coherence tomography, and fluorescein angiography. The serological findings of each patient were reviewed.

RESULTS

Fifteen eyes of 12 patients (9 male and 3 female), with a mean age of 46.0 ± 10.7 years were included. Of the nine patients who underwent serological testing for Toxocara antibodies, six (66.6%) were positive. One patient had high titers of Toxoplasma immunoglobulins M and G. One patient diagnosed with dengue fever was suspected to have neuroretinitis in both eyes. There were no related abnormalities in the serological findings in four patients (33.3%) out of 12 patients. There were no suspected cases of cat-scratch disease. The six patients who were positive for Toxocara antibodies were older (mean age: 54.5 ± 9.1 years) than the others (mean age: 37.5 ± 4.4 years, p = 0.004). The four patients without any abnormal serological findings were relatively younger (mean age: 35.7 ± 3.0 years) than the other 8 patients (mean age: 51.1 ± 10.1 years, p = 0.008).

CONCLUSIONS

Two-thirds of neuroretinitis patients were seropositive for Toxocara in the current cohort from Koreans. Causative factors in cases of neuroretinitis may vary according to age and region.

Clinical spectrum of symptoms in cerebral Toxocariasis (Review)

Human helminth zoonosis is one of the most encountered helminthiases worldwide. Representative diseases include Toxocara canis and Toxocara cati, which are common nematodes prevalent in dogs and cats. The infiltration of these roundworms in the human body through contaminated food or nematode eggs could lead to central nervous system injury as the roundworms can cross the blood-brain barrier leading to neurotoxocariasis. Among the neurological and neuropsychological disturbances produced by Toxocara infection, in humans, the most representative are meningitis, encephalitis, myelitis and cerebral vasculitis, but asymptomatic central nervous system infection is probably the most prevalent. The present review examines the clinical symptomatology of neurotoxocariasis in case reports in the literature in the last 7 decades (1950-2020). The available evidence was retrieved from PubMed and Medline electronic databases. The present review reports the most prevalent clinical symptomatology in the cases of detected and diagnosed Toxocara infection with neuroinvasion. Thus, the present review aims to raise the awareness of neurological cases of Toxocara infection with the potential to at least establish differential diagnosis of neurotoxocariasis.

Toxocariasis of the Nervous System

PURPOSE

Toxocariasis is a helminthozoonosis caused by the infection of a human host by the larva of Toxocara spp., predominately involving Toxocara canis and Toxocara cati, which are common nematodes in dogs and cats, respectively. Human transmission occurs through contact with animals or by consumption of food contaminated with parasite's eggs. The purpose of this article is to review the current knowledge regarding human neurotoxocariasis.

METHODS

We conducted a systematic review of the existing literature concerning toxocariasis of the nervous system.

RESULTS

Clinical spectrum of human toxocariasis varies widely from a subclinical course to significant organ morbidity. Clinical course depends on parasitic load, the migration route of the larvae and host response. Human neurotoxocariasis is a relatively rare entity yet associated with severe sequelae. Manifestations include meningitis (usually eosinophilic), encephalitis, myelitis, cerebellar vasculitis, space-occupying lesion, behavioral abnormalities, and optic neuritis. Even though valid diagnostic criteria are lacking, neurotoxocariasis should be suspected in patients with neurologic symptoms and cerebrospinal fluid (CSF) pleocytosis with eosinophilia, positive serology for anti-Toxocara antibodies, in serum and/or CSF, sterile CSF and clinical improvement after antihelminthic treatment. Neurotoxocariasis is treated by benzimidazole components, most frequently albendazole, corticosteroids, or diethylcarbamazine.

CONCLUSION

Parasite larvae migrate through tissues and are able to reach the nervous system causing neurotoxocariasis. Its clinical spectrum varies and includes myelitis, meningoencephalitis, brain abscess, and vasculitis. Neurotoxocariasis should always be suspected in patients with neurologic symptoms accompanied by eosinophilia in blood and/or CSF. Early diagnosis and treatment could prevent long-term neurologic impairment.

Toxocara infection in the differential diagnosis of multiple sclerosis in the Middle East

A critical step in the diagnosis of multiple sclerosis is to rule out a heterogeneous variety of multiple sclerosis mimickers, which is crucial in the era of powerful immune-modulator treatments. In this review, we discuss the background of toxocariasis in general, present central nervous system Toxocara infection as one of the multiple sclerosis mimickers in the Middle East, and share our experience about the diagnosis and management of this condition. This entity seems very relevant in a region such as the Middle East, where displacement of populations and conflict can result in non-hygienic food and water management bundles. The diagnosis should be entertained, especially when assessing patients with myelopathy. The presence of a single lesion in the spinal cord with inflammatory features should prompt serological testing for Toxocara IgG and IgM in serum and the cerebrospinal fluid. This infection is treatable, with the regimen of high-dose albendazole being one of the most accepted treatments. Although most cases exhibit a good prognosis, some have residual deficits localized to the affected spinal cord level.

Clinical Features of Toxocara-Seropositive Optic Neuritis in Korea

Purpose: This study was undertaken to analyze the characteristics of optic neuritis in Korean patients seropositive for Toxocara. Methods: We retrospectively reviewed data from patients diagnosed with optic neuritis and followed up for at least one month between 2012 and 2016. Patients were grouped according to Toxocara serological testing outcomes (positive or negative) and clinical characteristics were compared. Results: The seropositive and seronegative groups comprised 13 and 12 patients, respectively. The seropositive patients were older (56.8 years versus 34.5 years), reported ocular pain less frequently (30.8% versus 91.7%), and showed more frequent asymmetric optic disc swelling (72.7% versus 22.2%). During follow-up, visual acuity of all seronegative patients improved to 20/40 or better, compared with 38.5% of the seropositive group. Conclusion: Atypical features such as painless, older age, or asymmetric disc swelling in optic neuritis may be related to seropositivity for Toxocara, suggesting the possibility of undiagnosed Toxocara optic neuropathy.

Clinical and Magnetic Resonance Imaging Findings of Neurotoxocariasis

Human toxocariasis is one of the most prevalent helminthiases worldwide. Toxocara canis larvae can cross the blood-brain barrier leading to the neurotoxocariasis. The clinical presentation consists of a wide spectrum of neurological manifestations, but asymptomatic infection is probably common. Neurotoxocariasis is not a frequent diagnosis probably due to the non-specific nature of its symptoms as well as the lack of confirmatory diagnostic tests. Diagnosis of neurotoxocariasis is based on the presence of a high titer of anti-Toxocara antibody in the cerebrospinal fluid or in the serum, presence of eosinophilia in the serum or cerebrospinal fluid, and clinical and radiological improvement after anthelmintic therapy; however, universally accepted diagnostic criteria are lacking. Magnetic resonance imaging (MRI) findings include single or multiple, subcortical, cortical or white matter hyperintense lesions, best visualized on FLAIR and T2-weighted imaging, and usually isointense or hypointense on T1. These imaging findings are suggestive but not specific to neurotoxocariasis. Definitive diagnosis is made by histological confirmation, but it is rarely followed. This review provides an overview of the clinical manifestations, management options, and MRI findings of neurotoxocariasis.

Neurotoxocariasis: a systematic literature review

PURPOSE

Toxocariasis is a widespread zoonosis, which may result in central nervous system injury.

METHODS

We conducted a systematic literature review in MEDLINE, SciELO, ScienceDirect and Google Scholar up to April 2015 using a combination of the following search terms: "neurotoxocariasis" or "neurotoxocarosis", "toxocariasis" or "toxocarosis" and "cerebral" or "neurologic".

RESULTS

One hundred cases of neurotoxocariasis were identified in literature. The majority of patients were male (58 %), with a median age of 42 years. The predominant clinical pictures were myelitis (60 %), encephalitis (47 %) and/or meningitis (29 %). Fever was inconstant (23 %). The suspected mode of transmission, mentioned in only 49 % of cases, was mainly contact with dogs and/or cats (67 %) and ingestion of contaminated food (31 %). Diagnostic imaging examinations found hypodense lesions in cerebral scanner sequences and hyperintense lesions in cerebral MRI T2-weighted sequences in 65 and 57 % of encephalitis cases respectively, and in 92 % of myelitis cases in medullary MRI T2-weighted sequences. The detection of antibodies against Toxocara spp. was almost constant in blood and cerebrospinal fluid (CSF), 99 and 93 %, respectively. The two most commonly used drugs were corticosteroids (72 %) and/or albendazole (68 %) for a period of at least 3 weeks, which often needed to be repeated. Despite a low mortality rate (6 %), complete remission was observed in only 40 % of cases.

CONCLUSIONS

Neurotoxocariasis, a completely preventable zoonosis, could lead to severe sequelae failing prompt diagnosis. A compatible clinical picture, presence of risk factors, blood eosinophilia and high titers of antibodies against Toxocara spp. in CSF should alert physicians.

Infectious optic neuropathies: a clinical update

Different forms of optic neuropathy causing visual impairment of varying severity have been reported in association with a wide variety of infectious agents. Proper clinical diagnosis of any of these infectious conditions is based on epidemiological data, history, systemic symptoms and signs, and the pattern of ocular findings. Diagnosis is confirmed by serologic testing and polymerase chain reaction in selected cases. Treatment of infectious optic neuropathies involves the use of specific anti-infectious drugs and corticosteroids to suppress the associated inflammatory reaction. The visual prognosis is generally good, but persistent severe vision loss with optic atrophy can occur. This review presents optic neuropathies caused by specific viral, bacterial, parasitic, and fungal diseases.

Expect the unexpected: a case of isolated eosinophilic meningitis in toxocariasis

We present the case of a young police officer suffering from headache without other neurological symptoms caused by isolated eosinophilic meningitis, which resulted from an infection with Toxocara cati, along with a discussion of the differential diagnosis.

Central nervous system infectious diseases mimicking multiple sclerosis: recognizing distinguishable features using MRI

The current diagnostic criteria for multiple sclerosis (MS) confirm the relevant role of magnetic resonance imaging (MRI), supporting the possibility of characterizing the dissemination in space (DIS) and the dissemination in time (DIT) in a single scan. To maintain the specificity of these criteria, it is necessary to determine whether T2/FLAIR visible lesions and the gadolinium enhancement can be attributed to diseases that mimic MS. Several diseases are included in the MS differential diagnosis list, including diseases with exacerbation, remitting periods and numerous treatable infectious diseases, which can mimic the MRI features of MS. We discuss the most relevant imaging features in several infectious diseases that resemble MS and examine the primary spatial distributions of lesions and the gadolinium enhancement patterns related to MS. Recognizing imaging "red flags" can be useful for the proper diagnostic evaluation of suspected cases of MS, facilitating the correct differential diagnosis by assessing the combined clinical, laboratory and MR imaging information.

Therapeutic battle against larval toxocariasis: are we still far behind?

Toxocariasis is a widespread zoonotic helminthic disease. Human infection is acquired by ingestion of embryonated eggs of Toxocara which reach the environment via stools of dogs and cats. In paratenic hosts, such as humans and mice, the hatched larvae migrate systematically in the body and could reach critical sites such as the eye and the central nervous system. The clinical expression of toxocariasis includes two main forms of varying intensity: visceral and ocular, as well as several organ-specific clinical manifestations. The issue of treatment is a matter of great controversy not only because of limited efficacy of anthelmintic drugs in terms of parasite eradication, but also because of doubts about the benefits of therapy as most Toxocara infections are self-limiting. Many animal studies were conducted employing different drugs and regimens, yet the results were variable and inconsistent. On the other hand, experience in human therapy is unfortunately limited due to paucity of clinical trials. Herein, the problems and pitfalls of treatment of toxocariasis are addressed, and new trends in therapy are discussed.

[Toxocara canis cerebral vasculitis revealed by iterative strokes]

INTRODUCTION

Cerebral vasculitis is a rare and severe condition, posing problems for diagnosis and treatment. Toxocara canis cerebral vasculitis is exceptionally rare, with only 4 cases having been reported. We report an additional case revealed by iterative strokes.

OBSERVATION

A 49-years-old Laotian man presented with right ACA infarction associated with contrast enhancement of cerebrospinal fluid, and multiple segmental stenoses in small and medium caliber encephalic arteries, in a context of hypereosinophilia and chronic headaches. Laboratory tests showed lymphocytic meningitis and T. canis antibody IgE in the blood and CSF. The diagnosis of T. canis cerebral vasculitis was retained. During follow-up, the patient presented again with left pontine hemorrhagic stroke. Conventional cerebral angiography confirmed progression of vasculitis despite treatment.

CONCLUSION

This case-report illustrates the diagnostic and therapeutic difficulties associated with vasculitis.

Toxocara canis myelitis: clinical features, magnetic resonance imaging (MRI) findings, and treatment outcome in 17 patients

Toxocara myelitis is a rare disease. Few cases have been reported in the literature. Patients present with myelopathy, occasional eosinophilia in blood and cerebrospinal fluid (CSF), with abnormal signals on magnetic resonance imaging (MRI). In the current study we report 17 cases of isolated Toxocara myelitis from a single tertiary referral center in Lebanon, with description of the clinical presentation, laboratory data, MRI findings, and response to antihelminthic treatment. Clinical and laboratory data were collected for 17 patients who presented with evidence of spinal cord disease. The clinical presentation included sensory, motor, and autonomic dysfunction, predominantly in the lower extremities. Patients exhibited a subacute or chronic course; this was either slowly progressive or remitting-relapsing with mild to moderate disability. The patients underwent extensive blood and CSF workup as well as MRI of the spinal cord and brain. Only 2 patients had a high eosinophil count in the CSF, although blood eosinophilia was seen in 6 patients. All patients tested positive for Toxocara canis antibodies in the blood and CSF. MRI of the spinal cord revealed a single characteristic lesion in the spinal cord with fusiform enlargement that was isointense on T1-weighted images and hyperintense on T2-weighted images. Nodular enhancement was seen after gadolinium injection. Treatment with albendazole, with or without steroids, resulted in marked neurologic improvement and normalization of the MRI in all patients.The finding of a single inflammatory MRI lesion in the spinal cord with positive Toxocara canis serology in the blood and CSF in cases of subacute or chronic myelitis suggests the diagnosis of Toxocara myelitis, irrespective of the presence of eosinophilia. Antihelminthic treatment is associated with a good outcome.

MRI findings of spinal visceral larva migrans of Toxocara canis

PURPOSE

The purpose of this study is to investigate the MRI findings of visceral larva migrans (VLS) of Toxocara canis in spinal cord.

MATERIALS AND METHODS

We retrospectively reviewed spinal MRI findings in eight patients with serologically proven Toxocara canis between 2005 and 2008. We evaluated the location, length, extent and migration of the lesion, MR signal intensity (SI), enhancement pattern, and swelling of the spinal cord. We evaluated clinical features including presenting symptoms and signs and treatment response.

RESULTS

Total 8 patients (M=8; age range 36-79 years) were included. The lesions were located in the cervical or thoracic spinal cord in all patients. All lesions showed high SI and minimal or mild swelling of involved spinal cord on T2WI and focal nodular enhancement on posterior or posterolateral segment of spinal cord. The length of involved lesion was relatively short in most patients. There was a migration of lesion in one patient. In spite of albendazole or steroid treatment, neurological symptoms or signs were not significantly improved in all patients.

CONCLUSION

Although all lesions show non-specific imaging findings like non-tumorous myelopathy mimicking transverse myelitis, single lesion, focal nodular enhancement on posterior or posterolateral segment of spinal cord, relatively short segmental involvement and migration of lesion may be characteristic findings of spinal VLM of Toxocara canis. In addition, the reluctant response to the treatment may be characteristic of spinal VLM of Toxocara canis.

Recurrent cerebral larva migrans: A case report and review of literature

Cerebral larva migrans (CLM) is an uncommon and rarely diagnosed entity. We report here a histopathologically proven case of CLM in a 13 year-old girl, who presented with bilateral focal seizures, right hemiparesis, and lapsed into altered sensorium prior to admission. Her CT scan was suggestive of a granulomatous lesion which eventually turned out to be CLM (toxocariasis). Interestingly, the lesion recurred after excision, with a confusing clinical picture. Following the second surgery, the pathology was reconfirmed and cure was provided for the disease. There has been no further intracranial recurrence during the past ten years although she developed cutaneous lesions.

Neurotoxocarosis

Infection of humans with embryonated eggs of Toxocara canis (larva migrans) remains asymptomatic, or results in covert or common toxocarosis, visceral larva migrans syndrome, or ophthalmologic and neurologic impairment. Though neurological manifestations of Toxocara canis larvae are rare, toxocarosis remains an important differential diagnosis of various neurological disorders. Manifestations of the central nervous system are dementia, meningo-encephalitis, myelitis, cerebral vasculitis, epilepsy, or optic neuritis. Manifestations of the peripheral nervous system comprise radiculitis, affection of cranial nerves, or musculo-skeletal involvement. If toxocarosis is neglected, ignored, or refused as a differential of these abnormalities, it may be easily overlooked for years. Early recognition and treatment of the infection is, however, of paramount importance since it reduces morbidity and mortality and the risk of secondary superinfection. Like the visceral manifestations, neurological manifestations of toxocarosis are treated by benzimidazole components, most frequently albendazole, corticosteroids, or diethylcarbamazine. If detected and treated early, the prognosis of neurological manifestations of toxocarosis is favourable.

Neuroparasitic infections: nematodes

Globalization has produced an increase in the number of people at risk for contracting parasitic infection. Central nervous system infection by nematodal parasites can be devastating. Early recognition and treatment of infection can significantly decrease morbidity of the parasitic infection, as well as the risk of secondary superinfection. The clinical presentation, diagnosis, and treatment for five of the more common nematodal infections of the nervous system--Angiostrongylus spp., Baylisacaris procyonis, Gnathostoma spinigerum, Strongyloides stercoralis, and Toxocara spp.--is reviewed.

Eosinophilic meningomyelitis in toxocariasis: case report and review of the literature

Toxocariasis is a worldwide-occurring parasitic infection leading to tissue damage in various organs due to wandering Toxocara larvae (visceral larva migrans). More than 40 cases of CNS involvement in children and immunocompetent adults have been documented in detail to date. Here, we present evidence of eosinophilic meningomyelitis in an adult without known risk factors and with positive Toxocara antibody response in CSF, but not in blood. Toxocariasis has to remain among the differential diagnosis in patients with eosinophilic CNS infection even if serological tests in blood are negative. Adult cases seem to be more frequent than previously thought (about 60%).

Ocular Toxocariasis in Schoolchildren

Ocular toxocariasis in humans is typically a unilateral disease caused by second-stage larvae of the Toxocara species. Serological evidence of widespread infection in humans provides little information on clinical disease. There is only a single previous estimate of the prevalence of ocular toxocariasis (from Alabama). The present survey examined the extent of consultant-diagnosed toxocaral eye disease among a population of schoolchildren. More than 120,000 participants were surveyed by questionnaire and follow-up. Two sets of control subjects from the same school and from the same county were compared with persons who had ocular toxocariasis. The prevalence of consultant-diagnosed toxocaral eye disease was 6.6 cases per 100,000 persons when only cases regarded as definite by the consultant ophthalmologist were included. This increased to 9.7 cases per 100,000 persons when both definite and strongly suspected cases were included. Geophagia and a history of convulsion were associated with toxocaral eye disease in both of the case-control studies.

Evidence for the involvement of the optic nerve as a migration route for larvae in ocular toxocariasis of Mongolian gerbils

Although Toxocara canis, an important pathogen of ocular disease, tends to migrate to the eye, the precise migratory route has yet to be determined experimentally. Mongolian gerbils, Meriones unguiculatus, known as a useful animal model for human toxocariasis, were used to investigate the migration route toward the eyes. Infective larvae of T. canis were directly inoculated into the intracranial region. Haemorrhagic lesions or larvae were observed in 56.3% of cases. Histopathologically, a larva was observed in the optic nerve of gerbils 6 days after inoculation, and two larvae were found in the optic chiasma in the gerbils having a haemorrhage in the retina 9 days after inoculation. These results indicate that T. canis migrates from the brain to the eye through the optic nerve. Considering these data and previous studies showing that the ocular changes appear as early as 3 days of infection in the oral-administrated gerbils, there are two phases in the migration to the retina: a haematogenous early phase and an optic nerve route late phase.

CT and MR imaging findings in cerebral toxocaral disease

We report serial MR findings in a 54-year-old woman with eosinophilic meningoencephalitis due to Toxocara canis infection, a parasitic disease contracted through exposure with soil contaminated by the eggs of the roundworm. MR imaging revealed several enhancing subcortical and white matter lesions in both lobes. Antihelminthic chemotherapy yielded marked improvement of the neurologic deficits and cerebral lesions. The specific MR findings-low signal intensity on T1-weighted images, high signal intensity on T2-weighted images, and contrast enhancement-and the clinical and epidemiologic features of CNS involvement are herein reviewed.

Eosinophilic meningitis

Eosinophilic meningitis is a rare clinical entity that can be useful in narrowing the differential diagnosis of central nervous system disease. It is defined by the presence of 10 or more eosinophils/microL in the cerebrospinal fluid (CSF) or a CSF eosinophilia of at least 10%. The most common cause is invasion of the central nervous system by helminthic parasites, particularly Angiostrongylus cantonensis, but other infections as well as noninfectious conditions may also be associated. This review describes the etiologies of eosinophilic meningitis, focusing primarily on the helminths that cause CSF eosinophilia.

Highlights of human toxocariasis

Human toxocariasis is a helminthozoonosis due to the migration of Toxocara species larvae through human organism. Humans become infected by ingesting either embryonated eggs from soil (geophagia, pica), dirty hands or raw vegetables, or larvae from undercooked giblets. The diagnosis relies upon sensitive immunological methods (ELISA or western-blot) which use Toxocara excretory-secretory antigens. Seroprevalence is high in developed countries, especially in rural areas, and also in some tropical islands. The clinical spectrum of the disease comprises four syndromes, namely visceral larva migrans, ocular larva migrans, and the more recently recognized "common" (in adults) and "covert" (in children) pictures. Therapy of ocular toxocariasis is primarily based upon corticosteroids use, when visceral larva migrans and few cases of common or covert toxocariasis can be treated by anthelmintics whose the most efficient appeared to be diethylcarbamazine. When diagnosed, all of these syndromes require thorough prevention of recontamination (especially by deworming pets) and sanitary education.

Eosinophilic pleocytosis and myelitis related to Toxocara canis infection

Toxocara canis causes the visceral larva migrans syndrome in which central nervous involvement is rare. We report the case of a 40-year-old woman presenting with a subacute weakness of the right leg and dysaesthesiae in the right Th8-Th10 dermatomas. Spinal magnetic resonance imaging examination showed abnormal hyperintensity within the spinal cord. Cerebrospinal fluid analysis revealed eosinophilic pleocytosis. Antibody titres to Toxocara canis were higher in the cerebrospinal fluid than in the serum. Treatment using mebendazole led to a complete clinical recovery, normalization of cerebrospinal fluid parameters and improvement in spinal magnetic resonance imaging abnormalities.

The contribution of magnetic resonance imaging in the differential diagnosis of optic nerve damage

In this paper we review the findings of magnetic resonance imaging (MRI) in optic neuritis and visual dysfunction due to other optic neuropathies. With advances in MRI technology, it has become possible to visualise optic nerve pathology. STIR and RARE sequences, contrast-enhanced sequences, and phased array surface coils are technical developments that provide fine anatomical detail and that are sensitive to pathological changes. MRI can offer information in the differential diagnosis of optic neuropathies, the monitoring of their treatment, and in some instances should provide new insights into the underlying pathophysiological mechanisms.

Current views on geographic distribution and modes of infection of neurohelminthic diseases

Neurohelminthiases are more prevalent in geographic areas where environmental factors and poor sanitary conditions favor the parasitism between man and animals. In recent years, population shifts and rapid transport have facilitated the spread of certain helminthic diseases from endemic to non-endemic areas. Although many helminthic parasites are known to cause various human diseases afflicting many millions of people in the world, neurohelminthiases are often not diagnosed because they have been unrecognized by clinicians or confirmatory diagnostic tests are not easily available. Paragonimiasis and schistosomiasis (fluke diseases) are endemic in Asia, Africa and Central America; lesions in the central nervous system (CNS) due to ectopic parasitism of the preadult and adult flukes produce various clinical features that often mimic other diseases. In most cestodiasis (tapeworm disease), the adult worm that lodges in the alimentary tract does not involve the CNS; however, the larvae often enter the nervous system by migration or by metastasis via the systemic circulation, where they cause cystic lesions. Cysticercosis is the most common CNS helminthic infection especially in endemic areas where the parasitism between man and pigs is maintained. In other cestodiases, infections to man are often caused by ingestion of food or water contaminated with feces of the definitive hosts (mammals or man). Nematodes (roundworms) generally enter the CNS by ectopic migration of the infective larvae (larva migrans); the routes of infection to man vary with species of the nematodes, and the animal hosts they infest. Angiostrongylus cantonensis is a neurotropic nematode that requires the CNS of mammalian hosts for its growth; the third-stage larvae frequently invade skeletal muscles and the nervous system. Strongyloides, a gastrointestinal nematode, is known to cause CNS involvement in immunosuppressed patients. Recently, some nematodes heretofore unknown to cause human parasitism have been recognized as the causative agents of CNS infections.