Trematode Zoonosis - SCHISTOSOMIASIS

Humans are the reservoir for S. mansoni and S. haematobium. S. japonicum infects cattle, water buffalo, horses, dogs, cats, rodents and monkeys. Intermediate hosts are species of snails (Biomphalaria and Bulinus). S. mansoni occurs in Africa, South America and some Caribbean islands (including Puerto Rico); S. haematobium in Africa and the Middle East; and S. japonicum in China, Japan, the Philippines, and South East Asia.

TRANSMISSION:

Cercariae in contaminated water penetrate human skin, especially in irrigated fields or rivers. In the body the parasite migrates via the liver to the superior mesenteric vein where maturation takes place in about 6 weeks. Eggs are disseminated throughout the body via the blood, released into the intestinal lumen and excreted. In water miracidia develop and penetrate the snail, which in turn excretes cercariae into the water.

DISEASE IN ANIMALS:

Abdominal pain, diarrhea, anemia, and emaciation occur. Cattle have hematuria.

DISEASE IN HUMANS:

Penetration of larvae through the skin causes an itchy rash. With heavy infection, penetration of the parasite through the skin gives rise to local dermatitis and pruritus followed by pneumonitis when the parasites reach the lung. The deposition of ova provokes the growth of small multiple granulomata throughout the body. Eventually intestinal and hepatic fibroses develop. With S. japonicum, acute symptoms include fever, abdominal pain, cough, weight loss, diarrhea and dysentery. Chronic infection may result in symptoms months to years later, with enlarged liver and spleen, cirrhosis, ascites, and fits due to cerebral involvement.

DIAGNOSIS:

Definitive diagnosis is made by finding the characteristic eggs in excreta or by mucosal or liver biopsy. Screening for infection is possible by skin or serologic tests, but neither is sufficiently sensitive or specific to justify treatment.

TREATMENT:

Praziquantel, metrifonate, or oxamniquine.

PREVENTION/CONTROL:

Dispose of feces and urine so that viable eggs will not reach bodies of fresh water containing intermediate snail hosts. Reduce snail habitats by removing vegetation or by draining and filling. Treat snail-breeding sites with molluscicides. Prevent exposure to contaminated water (e.g., wear rubber boots and gloves). To minimize cercarial penetration, towel dry, vigorously and completely, skin surfaces wet with suspected water. Apply 70% alcohol immediately to the skin to kill surface cercariae. Provide water for drinking, bathing, and washing clothes from sources free of cercariae. Rapid treatment of patients to prevent disease progression and to reduce transmission by reducing egg passage.

Schistosomiasis