As the name imply, waterborne diseases, such as waterborne parasitic diseases, are transmitted by contaminated water. It is important to mention that some bacteria and viruses are also susceptible to be transmitted by contaminated water. Waterborne diseases, mainly diarrhoeal diseases, are a big issue worldwide, but it is more preoccupying in developing countries where access to treated drinking water could be an issue. They are also a big concern after natural disasters when access to safe drinking water might be compromised.
The main modes of contamination related to water are by drinking, swimming, bathing or eating fresh food that had been washed in contaminated water. In the case of parasites, the water is generally contaminated by protozoan parasites’ cysts or by helminths’ eggs or larvae rather than by the adult form of the parasite. Eggs or larvae can also be present in microscopic freshwater insects susceptible to be ingested with the water. The source of the contamination of water by parasites is usually caused by the presence of infected human’s or animal’s faeces nearby. In some cases, the contamination could also come from contaminated soil in contact with the source of water. Despite some exceptions, the majority of the waterborne parasites are considered as intestinal parasites and cause gastrointestinal symptoms.
There are many examples of waterborne parasitic diseases. For example, Giardia is one of the most common intestinal parasites commonly acquired by drinking contaminated water, like infections with Cryptosporidium and Cyclospora cayetanensis. These three parasites cause usually a mild intestinal disease susceptible to resolve by itself without any specific treatment. Another known waterborne parasite is Entamoeba histolytica. This parasite is susceptible to cause an invasive intestinal amoebiasis involving dysentery with bloody diarrhoea or an invasive extra-intestinal amoebiasis affecting other organs not related to the gastrointestinal track.
The majority of helminths are either waterborne or foodborne parasites. Consequently, they are mainly transmitted by the ingestion of contaminated water or food. One notable exception is Schistosoma, which is a waterborne parasite susceptible to be acquired by bathing or swimming into contaminated water. This parasite is able to enter directly the human body by penetrating through the skin. Another interesting case that is at the limit between waterborne and foodborne parasite is Fasciola hepatica. This parasite is transmitted by the ingestion of aquatic or semi-aquatic plants, such as watercress, that had been grown in contaminated water.
Finally, some extremely rare but deadly infections with amoeba are also transmitted by contaminated water. However, these parasites cause a deadly neurological disease rather than an intestinal illness. It is the case of Acanthamoeba, Balamuthia mandrillaris and Naegleria fowleri. The former two cause a very rare but deadly disease called granulomatous amoebic encephalitis, while the latter one cause the primary amoebic meningoencephalitis. It is interesting to note that Naegleria fowleri has a very unusual way to penetrate the human body, as the contaminated water has to enter the body through the nose in order to produce an infection, meaning that drinking contaminated water with this specific parasite does not make someone sick.
Sources: Centers for Disease Control and Prevention; World Health Organization