Luke Versten received his B.S. in Biological Sciences from The University of Chicago and is currently pursuing an MPH in Epidemiology of Microbial Disease at Yale University.
Parasitic trematode flatworms cause a neglected tropical disease called Fasciolosis. The disease is a plant-borne zoonosis, and is specifically caused by the liver flukes Fasciola hepatica and Fasciola gigantica, which infect about 2.4 million people worldwide. The severe hepatic infections caused by these parasites frequently and significantly affect the world’s most impoverished people, typically in resource-limited settings. Despite the high prevalence of infection, Fascioliasis receives very little attention from government agencies or sources of research funding. This is surprising for two reasons: aside from the obvious public health burden this neglected tropical disease presents to the world’s poorest populations, both its life cycle and transmission patterns are puzzling phenomena worthy of deeper investigation. The developmental biology of this parasite is astoundingly complex, as it must undergo several forms in different hosts and environmental settings before causing liver infection in humans. A greater understanding of how it completes the various stages of its development process would be useful to inform public health prevention, treatment, and control measures.
Despite the high prevalence of infection, humans are in fact merely accidental hosts of Fasciola sub-species. These Trematodes are true zoonoses, since parasites are predominantly maintained by infected cattle, and are only incidentally transmitted to humans. In fact, humans do not significantly contribute to the parasites’ life-cycle, as infection occurs typically after failure to observe basic hygiene measures. For this reason, a risk factor significantly associated with Fasciolosis is age: infections are most prevalent in children in developing countries who may either avoid or be ignorant to proper sanitary behaviors. Fasciola is most prevalent in South American countries such as Peru and Bolivia, with substantial disease prevalence also occurring in African and Asian countries.
As mentioned previously, a significant number of steps in the Fasciola life cycle must occur prior to infection taking place in an incidental human host. The Fasciola infectious lifecycle begins when feces of an infected zoonotic host containing parasite eggs are excreted into water, where the eggs enter the so-called ciliated miracidia stage. The freshwater snail Galba truncatula is a prominent intermediate host for Fasciola disease transmission, and infection of this organism by the miracidia is necessary to enter the next stage of the parasite’s life cycle. The parasite proliferates in the snail host, and eventually is excreted as flagellated cercariae. The cercariae form cysts on aquatic plants, developing into the metacercarial stage, and at this point can infect either definitive zoonotic hosts such as cattle or accidental hosts such as humans through ingestion of contaminated aquatic vegetation or by drinking contaminated water. Upon entering the host, the metacercariae puncture the intestinal wall, enter the peritoneum, and eventually take root in the hepatic biliary tract, which is where the parasite matures into a fluke that releases eggs that are ultimately excreted in feces, thereby completing the life-cycle.
Acute (symptomatic) fasciolosis occurs a few weeks after initial infection, with symptoms including fever, swelling of the liver, and elevated white blood cell count. The acute phase (wherein the metacercariae mature into adult flukes) may last for as long as 2-4 months, during which time the maturing parasites can grow anywhere between 30 to 75 mm long. A chronic-latent infection is characterized by hyperplasia of the epithelium and hepatic fibrosis as a consequence of the extended period of inflammation, which may last for years if gone untreated. Furthermore, significant mechanical obstruction of the biliary tree ducts can occur not only from the astounding size the parasites can grow to, but also from cholangitis and cholecystitis as a result of liver fibrosis. The effects this parasite can have of quality of life for populations living in tropical and subtropical regions of the globe are significant. It is critical to bear in mind that humans are only an accidental host to this parasite, and the socioeconomic costs and burden of infection can be greatly mitigated if proper sanitation infrastructure were implemented. While infection with this parasite is highly treatable, the disease’s morbidity can be greatly reduced if proper prevention and control measures are taken to formally acknowledge the potential burden Fasciola sub-species can have as a neglected tropical disease.