Schistosomiasis in U.S. Peace Corps Volunteers -- Malawi, 1992

Schistosomiasis (i.e., "snail fever" or "bilharzia") is a parasitic infection caused by trematodes (flukes) and is endemic in 74 countries in Africa, South America, the Caribbean, and Asia. U.S. residents who work or travel in these countries may be at risk for schistosomiasis. During 1992, two U.S. Peace Corps volunteers (PCVs) were evacuated from Africa because of Schistosoma hematobium infection of the central nervous system (CNS). Both were exposed to fresh water while vacationing at Cape Maclear, a popular resort area on Lake Malawi (Figure 1), in December 1991. This report summarizes the investigation of these two cases and a follow-up investigation of expatriates residing in Malawi. Patient 1

In March 1992, a 30-year-old PCV was evacuated from Namibia and evaluated at a U.S. medical center because of a 2-week history of headaches, unilateral (left) vision loss, and one episode of loss of consciousness consistent with a seizure. The patient had been a PCV for 2 years in Tunisia and was serving an additional 2 years in Namibia. He had no history of recreational freshwater exposure in Tunisia or Namibia. However, in December 1991, he had snorkeled during a 2-day period at Cape Maclear in Lake Malawi.

A physical examination and white blood cell count (including eosinophil count) were normal. However, computed tomography (CT) and magnetic resonance imaging (MRI) scans detected an enhancing left parietal lesion with extensive edema. Because the lesion was initially presumed to be a meningioma, an open brain biopsy was performed. The biopsy specimen demonstrated a granulomatous abscess containing distorted eggs that had peripheral spines consistent with schistosome eggs. S. hematobium eggs were identified in his urinary sediment but not in his stool. Schistosomiasis serology using the Falcon assay screening test-enzyme-linked immunosorbent assay (FAST-ELISA) was positive at CDC (1). A confirmatory immunoblot was positive for S. hematobium antibody but not S. mansoni antibody.

The patient was treated with praziquantel (60 mg per kg body weight per day, orally in three divided doses) for schistosomiasis, phenytoin for his seizure disorder, and dexamethasone for the cerebral edema. In June, his symptoms had resolved, and a CT scan documented continuing improvement. He returned to Namibia to complete his Peace Corps service. Patient 2

In January 1992, a 26-year-old PCV had onset of urinary frequency. In April 1992 she was evaluated in Botswana by a Peace Corps medical officer. Although a urinalysis was normal, she was treated in Botswana with antibiotics for a presumed bacterial cystitis. The patient had been stationed in an arid area of southern Botswana since 1990 and reported no recreational freshwater exposure in that country. However, in December 1991 she had snorkeled during a 7-day period at Cape Maclear, Lake Malawi.

Because of progressive symptoms, including incontinence, lower extremity pain, and difficulty walking, in August 1992 she was referred to a medical center in the Republic of South Africa for further evaluation, where an MRI scan revealed a mass in the conus medullaris of her spinal cord. A cauda equina tumor was suspected, and she was evacuated on August 25 to the United States for neurosurgical consultation.

On admission to the hospital in the United States, her general physical and neurologic examinations, complete blood count (including eosinophil count), urinalysis, and liver function tests were normal. On September 1, an exploratory laminectomy revealed that the area of the spinal cord opposite the body of the T11 vertebra was swollen, hyperemic, and firm to the touch. Examination of a biopsy specimen was negative for a neoplasm or other definitive diagnoses. She was treated with dexamethasone to reduce the spinal cord inflammation. A schistosomiasis FAST-ELISA was positive at CDC, and an immunoblot confirmed the presence of antibody to S. hematobium but not S. mansoni. Routine stool and urine examinations, a 24-hour filtered urine examination, and a rectal biopsy specimen were all negative for schistosome eggs.

S. hematobium infection of the spinal cord was presumptively diagnosed based on the clinical presentation, exposure history, and positive serology. She was treated with praziquantel (60 mg per kg body weight per day, orally in three divided doses) and discharged from the hospital on September 9. By October 7, her leg pains and gait disturbance had improved. However, she has remained incontinent of urine and requires oxybutynin chloride and periodic self-catheterizations. Follow-Up Investigation

These two cases prompted an investigation of the occurrence of and risk factors for schistosomiasis among expatriates in Malawi by CDC in collaboration with the Malawian Ministry of Health, the U.S. Department of State (DOS) (Malawi), the U.S. Peace Corps (Malawi), and the U.S. Agency for International Development (Malawi). In March 1993, a total of 995 resident expatriates in Malawi were surveyed to determine the prevalence of schistosomal antibody and to examine the seroprevalence in relation to recreational water exposure at Lake Malawi. In addition, the southwestern shoreline of the lake was searched for vector snails (Figure 1).

Of the 917 persons serologically tested, 302 (33%) had schistosomal antibody detectable by immunoblot; of these, 293 (97%) had antibody to S. hematobium. In addition, the seroprevalence was 33% among the 427 persons whose only reported recreational water exposure was at Lake Malawi (i.e., these persons reported no other recreational water contact in any country in which schistosomiasis is endemic).

Infected Bulinus globosus snails (an intermediate host of S. hematobium) were identified in protected coves adjacent to resort areas along the southern shore of Lake Malawi. This species of snail and B. nyassanus were found also on aquatic vegetation at Cape Maclear.

Reported by: M Wolfe, MD, D Parenti, MD, J Pollner, MD, A Kobrine, MD, A Schwartz, MD, George Washington Univ Medical Center, Washington, DC. Office of Medical Svcs, US Peace Corps, Washington, DC. US Agency for International Development, Lilongwe, Malawi. Malawian Ministry of Health, Lilongwe. Parasitic Diseases Br, Div of Parasitic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Worldwide, an estimated 200 million persons are infected with and more than 600 million are at risk for schistosomiasis (2). S. mansoni and S. japonicum primarily affect the genitourinary tract; chronic infection can lead to hepato- splenomegaly, variceal bleeding, and cirrhosis. S. hematobium primarily affects the genitourinary tract; chronic infection can lead to persistent cystitis, pyelonephritis, obstructive renal disease, and increased incidence of bladder cancer.

Infection is acquired by exposure to cercariae that penetrate the skin of persons in contact with fresh water containing infected snails. In the human host, cercariae mature into adult worms that mate and deposit eggs (Figure 2). Adult worm pairs are generally located in the venous plexi surrounding the intestines (S. mansoni) or bladder (S. hematobium). Migration of either adult worm pairs or ova may result in the dissemination of eggs to ectopic locations such as the CNS. Schistosome ova have been found in a variety of host tissues (2-5), but the factors influencing ectopic migration are not understood.

Most reported cases of acute cerebral schistosomiasis are caused by S. japonicum (4,6), and most cases of schistosomal transverse myelitis, by S. mansoni (4,6). CNS disease caused by S. hematobium is rare. Based on autopsy studies of patients with urinary schistosomiasis, S. hematobium ova may involve the brain in 30%-50% of those infected. However, CNS sequelae are uncommon (3,5), and the pathogenesis and risk factors for development of CNS disease are not understood.

Both S. hematobium and S. mansoni are endemic throughout much of Africa, including Malawi, although Lake Malawi has been widely regarded as "risk-free" for transmission of schistosomiasis (7). However, in addition to the two cases described in this report, CNS schistosomiasis in two British expatriates was associated with antecedent recreational freshwater exposure in Lake Malawi (one of these two persons acknowledged freshwater exposure elsewhere in Africa {Lake Tanganyika}) (8,9). The combination of the four case reports, the survey results documenting a high sero-prevalence of schistosomiasis in expatriates, and the detection of infected snails strongly suggest Lake Malawi as a source of transmission of schistosomiasis.

All PCVs and most DOS employees in Malawi have been serologically screened for schistosomiasis. Clinical evaluation of seropositive expatriates in Malawi is planned to determine the rates of egg excretion and morbidity associated with infection. To prevent future morbidity associated with schistosomiasis, CDC recommends that expatriates and travelers with a history of freshwater exposure returning from areas with endemic schistosomiasis be serologically screened and that seropositive expatriates receive treatment with praziquantel following thorough clinical evaluation (i.e., quantitative stool and urine examinations for schistosome eggs).

All persons traveling in Africa should be advised of the risk for schistosomiasis associated with freshwater lakes, streams, and rivers throughout the continent, including Lake Malawi. The only completely effective method of prevention is avoiding contact with fresh water in areas with endemic disease. If contact with fresh water is unavoidable, water should be heated to 122 F (50 C) for 5 minutes or treated with iodine or chlorine. In addition, water can be strained with paper filters or allowed to stand for 3 days before use.

Physicians who treat travelers, expatriates, and immigrants should consider the possibility of neuroschistosomiasis in all patients who have a history of freshwater exposure in schistosomiasis-endemic areas and CNS abnormalities, even in the absence of classic signs and symptoms of acute schistosomiasis (e.g., fever, nausea, vomiting, abdominal pain, diarrhea, and hematuria). Neuroschistosomiasis can occur several months after exposure to infested water (10) and in low-intensity infections in which eggs may be undetectable or difficult to identify in urine or stool (10). Sensitive and specific serologic tests for diagnosing schistosomiasis (1) are available through CDC's Parasitic Diseases Branch, National Center for Infectious Diseases, telephone (404) 488-4050.

Treatment with a single dose of praziquantel (40-60 mg per kg body weight) is safe and effective therapy against the adult worms of the three major species of schistosomes infecting humans (S. hematobium, S. mansoni, and S. japonicum). Corticosteroids are often useful in neuroschistosomiasis to reduce edema and inflammation. Although CNS schistosomiasis is rare, substantial morbidity from this condition is preventable by early diagnosis and rapid treatment (9).

References

1. Tsang VCW, Wilkins PP. Immunodiagnosis of schistosomiasis: screen with FAST-ELISA and confirm with immunoblot. Clin Lab Med 1991;11:1029-39.

2. Chen MG, Mott KE. Progress in assessment of morbidity due to S. hematobium infection: a review of recent literature. In: Tropical diseases bulletin. Geneva: World Health Organization, Parasitic Diseases Program, 1989:R2. (Vol 86, no. 4).

3. Gelfand M. Schistosomal involvement of the brain and spinal cord. In: Schistosomiasis in South Central Africa: a clinico-pathological study. Cape Town, Cape Province: Post-Graduate Press, 1950:194-202.

4. Scrimgeour EM, Gajdusek DC. Involvement of the CNS in S. mansoni and S. hematobium infection. Brain 1985;108:1023-38.

5. Alves W. The distribution of Schistosoma eggs in human tissues. Bull World Health Org 1958;18:1092-7.

6. Marcial-Rojas RA, Fiol RE. Neurologic complications of schistosomiasis: review of the literature and report of two cases of transverse myelitis due to S. mansoni. Ann Intern Med 1963;59:215-30.

7. Blair DM. Bilharziasis survey in British West and East Africa, Nyassaland and the Rhodesias. Bull World Health Org 1956;15:203-73.

8. Blunt SB, Boulton J, Wise R. MRI in schistosomiasis of conus medullaris and lumbar spinal cord {Letter}. Lancet 1993;341:557.

9. Blanchard TJ, Milne LM, Pollok R, Cook GC. Early chemotherapy of imported neuroschistosomiasis {Letter}. Lancet 1993;341:959.

10. Istre GR, Fontaine RE, Tarr J, Hopkins RS. Acute schistosomiasis among Americans rafting the Omo River, Ethiopia. JAMA 1984;251:508-10.

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