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Snails, Parasites, Water!




You may have heard of schistosomiasis, but what exactly is it? Schistosomiasis is a neglected tropical disease (NTD) endemic to Africa, Southeast Asia, and South America (Secor, William Evan, 2014). Second to malaria, it is the disease with the highest economic and livelihood impact (The Carter Center, 2018). To truly understand the linkage between the burden of disease from schistosoma flukes and water, a basic understanding of its life cycle is needed. Infection of the human occurs when a cercariae penetrates the skin (CDC). A cercariae can best be described as a swimming larval schistosoma released by snails in the water (Secor, W.E., 2014). Prior to cercariae release, the aquatic snails are infected by miracidia, which are the free swimming larvae hatched from eggs. The head of the cercariae burrows into the human body for the next four to six weeks, traveling through various organs and eventually ending up in the veins near the bladder or intestines. At the time the worms reach sexual maturity, the adult worms lay eggs in the mesenteric venule in the host’s bowel or rectum. When the human host urinates or passes stool, the eggs are released and re-enter water sources, beginning its life cycle once again. The gist of it is that schistosoma has two intermediate stages (either a cercariae or miracidia depending on the hosts), two different hosts, and a diagnostic stage in water. If any of that was confusing, there’s a detailed diagram below from the CDC that outlines the schistosoma life cycle.


Figure 1: Schistosoma life cycle Source: CDC https://www.cdc.gov/dpdx/schistosomiasis/index.html

Schistosomiasis rarely results in mortality, but often causes great disability in the infected individual and has a high burden of disease due to its impact on organs critical for nutrient uptake (Secor, W.E., 2014). Individuals inflicted by schistosomiasis can suffer from severe diarrhea, bloody stool, and abdominal pain. Although an effective treatment exists, praziquantel does not prevent reinfection or guarantee 100% eradication of the worm from the body.

Image 1: Man receiving pills for schistosomiasis Source: https://www.sightsavers.org/protecting-sight/ntds/what-is-schistosomiasis/
Furthermore, praziquantel is not readily accessible for most people around the world. In 2016, the WHO reported that of the 206.4 million infected individuals, only 89 million received the necessary treatment (WHO, 2018). Thus, reducing exposure to contaminated water sources is one of the most effective solutions in combatting schistosomiasis. But more importantly, limiting dependence on outdoor freshwater sources such as lakes and ponds is key (Barkia H., Barkia A., Nhammi H., Belghyti H, 2011).

Image 2: Intermediate hosts of flukes
Source:https://www.merckgroup.com/en/stories/eliminating_schistosomiasis.html

Take Morocco for example. Since the early 90’s, Morocco implemented an expansive plan to eradicate schistosomiasis from its borders. Initially, Morocco worked to ensure access to clean drinking water for its people by expanding its water services and constructing hydraulic dams. Afterwards, the government began targeting specific aspects of the schistosoma life cycle. First, they concentrated on sanitation conditions in Morocco by building more latrines to prevent fecal matter and urine containing schistosome eggs from entering water sources (Suzanne Sokolow, 2015).  In addition, the government began spraying molluscicides in standing water to eliminate potential hosts. Unfortunately, the chemicals utilized were nonspecific and killed a lot of aquatic organisms in the process. Expectedly, pesticide use has been met with opposition and alternative methods of biocontrol are currently being used in Morocco. Lastly, the government implemented education programs about water, sanitation, and hygiene. This included educating communities about using boiled water for laundry, drinking water, and showering. Through these combined efforts, Morocco was officially recognized as a schistosoma free region in 2004 by the World Health Organization (Barkia H., Barkia A., Nhammi H., Belghyti H, 2011).

Schistosomiasis is intimately tied to WASH conditions and evidently, a lot can be done by providing individuals with access to safe water. The question then becomes - is there enough political will from governments and the international community to enact the necessary changes that will provide safe water for its people?


References:
Secor, William Evan. (2014). Water-based interventions for schistosomiasis control. Pathology Global Health, Vol. 108 Issue 5, 246 - 254.


The Carter Center. (2018). Schistosomiasis Control Program. https://www.cartercenter.org/health/schisto somiasis/index.html.
World Health Organization. (2018). Schistosomiasis. http://www.who.int/news-room/fact-sheets/detail /schistosomiasis.


Barkia H., Barkia A., Nhammi H., Belghyti H. (2011). Schistosomiasis in Morocco: from delivery to after elimination. East Mediterranean Journal, Vol. 17, Issue 3, 250 - 256.

Sokolow, Suzanne. (2015). Morocco Schistosomiasis. Stanford Woodrow Institute.

Comments

  1. Richard taylorNovember 13, 2018 at 4:25 PM

    This is an interesting and well-written blogpost - thanks. Having taken praziquantel myself, I can tell you that the cure is (nearly) as nasty as the disease. One needs a good two days of rest after taking the drug.

    ReplyDelete
    Replies
    1. UnknownNovember 19, 2018 at 6:25 AM

      Wow! That definitely must have been a troublesome experience. Where did you contract schistosomiasis?

      Delete

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