Schistosomiasis Projections

Possible Impacts of Eutrophication and Climate Change on Schistosomiasis

Thomas W. Elston

Baylor University

Schistosomiasis, also known as bilharzia, is a disease is a diseases caused by parasitic trematode worms (typically Schistosoma mansoni, S. haematobium, or S. japonicum) (“Parasites – schistosomiasis,” 2012). Currently, approximately 200 million people worldwide are infected. The disease, a predominantly tropical one, is considered a Neglected Tropical Disease, impacting over a billion people globally. Symptoms of schistosomiasis include impaired physical and cognitive development. Schistosomiasis’ global distribution and primary mode of infection relative to region is depicted in the CDC infographic below.


Geographic distribution of schistosomiasis with primary modes of infection by region. Source: the Yellow Book, chapter 3: Infectious Diseases Related To Travel (2011).

Schistosomiasis moves through a variety of vector organisms, most notably fresh water snails before infecting humans. The generalized schistosomiasis life cycle in humans sequentially consists of: 1) eggs are excreted from the human host via stool 2) eggs hatch and release miracidia, 3) the miracidia swim and penetrate fresh water snail vectors, 4) two generations of sporocysts occur within the snail, 5) cercariae, a free swimming larval stage in which the fluke passes from one intermediate host to another, are developed and released. 6) the cercariae swim and penetrate the skin of a human host, 7) shedding their forked tail, becoming schistosomulae. 8) The schistosomulae are then circulated and 9) migrate to the portal blood in liver and mature into adults. 10) Adults mate-pairs migrate to the mesenteric venules of the bowel and rectum, laying eggs which are shed in stools. This life cycle is summarized in the below CDC infographic.

Fig. B. Life cycle of schistosomiasis in humans. Source: U.S. Center for Disease Control and Prevention (2012).

Fig. B. Life cycle of schistosomiasis in humans. Source: U.S. Center for Disease Control and Prevention (2012).

A 2007 study (Johnson et al.) investigated the impact of eutrophication, process by which a body of water acquires a high concentration of nutrients (i.e. nitrogen and phosophorous), typically via agricultural fertilizers, on a disease system which parallels the vector of schistosomiasis. The investigators found that eutrophication significantly elevated the presence of the free-floating cercariae phase of the disease (see Fig. B. above). Their results demonstrated that the increased nitrogen and phosphorous content led to biomagnification of the parasite Ribeiroia ondatrae in the controlled trophic systems of their experimental design. This is significant because both R. ondatrae and the various trematodes leading to schistosomiasis 1) utilize a snail vector and 2) progress from the snail vector to free floating cercariae which penetrate/infect humans. The authors found that as more nutrients were introduced to the system 1) alga populations, which feed the vector-snails, increased, 2) the alga-feeding snail population increased, and 3) a particular amphibian, the terminal host of R. ondatrae, increased in population. The investigators found that eutrophication led to 1) increased density of infected snails, 2) greater per-snail production of parasites (i.e. cercariae-phase organisms), and 3) higher rates of amphibian infection. Thus, eutrophication directly drove up the infection and prevalence rates.

Because of the similarities between the R. ondatrae diseases system in amphibians and the schistosomiasis system in humans, it is conceivable that eutrophication could as well elevate the infection and prevalence of schistosomiasis in humans. The implications of the Johnson et al. (2007) study for schistosomiasis in humans are that as we [humans] continue to rely on and increase the amounts of fertilizer used for agriculture we also greatly increase our risk of infection. Further, because schistosomiasis is found predominantly in the developing third-world, which has not fully implemented fertilizer as we have in the developed first-world, humans run the risk of exponentially increasing the density of infection per capita as they begin to implement fertilizer-driven agricultural programs. Thus, it seems that, in the interest of public health, an alternative to eutrophicizing fertilizers must be found – perhaps hybrid strains of mycorrhizae fungi or other biotechnology.

In light of global warming, the distribution of schistosomiasis is likely to invade the US and its heavily eutrophicized waters with implications of mass infections rates as the temperature increases and the organisms’ viability range extends northward. This would postulate that we in the US must soon find alternatives to eutrophicizing fertilizers as well in the near future.


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