March 4, 2019
The families Heterophyidae and Echinostomatidae constitute the largest group of foodborne intestinal flukes, comprising at least 44 species belonging to a total of 19 genera. Approximately 40-50 million people are estimated infected, with the majority occurring in Southeast Asia. This figure however, may be grossly underestimated due to the difficulty of distinguishing eggs of the heterophyid flukes from those of the liver flukes.
Echinostomatid and heterophyid flukes are minute, measuring approximately 0.5-2 mm × 0.2 -10 mm and possess an indirect, three-host life cycle. A wide variety of fish-eating birds and mammals, including humans act as definitive hosts that contaminate natural rivers, lakes and ponds with eggs, which in turn infect the first intermediate host, prosobranchian molluscs. Cercariae shed by the snail infect a wide variety both freshwater and marine fish, which act as second intermediate hosts. Echinostomatids also use bivalve molluscs and tadpoles as second intermediate hosts.
In rural communities of Southeast Asia, the primary risk factors for infection in fish raised in nursery and grow-out ponds include the presence and density of infected snails, open defaecation practices, run-off and seepage of improperly treated effluent and the introduction of external natural water sources into aquaculture ponds. In humans, cultural practices of ingesting raw, fermented, or lightly cooked fish is the primary risk factor for infection.
Infection with heterophyid flukes are generally mild and transient unless heavily infected, immunologically naïve or immunocompromised. The most common symptoms relate to mild-to-moderate degrees of abdominal pain, dyspepsia, mucus-rich diarrhoea, anorexia, nausea, lethargy, and weight loss. Pathophysiological changes include mucosal ulceration, mucosal and sub-mucosal haemorrhages, fusion and shortening of villi, chronic inflammation, and fibrosis of the sub-mucosa. The severity of symptoms and pathological changes are generally more severe in echinostomatid infections. In a handful of cases, immunosuppression has predisposed to disseminate infection of heterophyid flukes to ectopic sites such as the brain, spinal cord and heart.
Praziquantel at a single oral dose of 10-20 mg/kg is the drug of choice for treating both heterophyid and echinostomatid infections. Sustainable control of infection remains challenging owing to a large non-human reservoir host population and entrenched traditional practices of eating undercooked fish. Nevertheless, an inter-sectorial approach involving preventive chemotherapy, improved sanitation, effluent treatment, education aimed at discouraging the ingestion of undercooked fish dishes and improvements in fish-production practices remains the mainstay of mitigating the risk of heterophyidiasis and echinostomatidiasis.