Schistosomiasis; sometimes known as snail fever or bilharzia, is an acute and chronic disease caused by a group of parasitic worms called schistosomes or blood flukes. The main three species of schistosome worms that are known to infect humans and cause schistosomiasis disease are: S. haematobium, S. mansoni and S. japonicum. Schistosomes; predominantly found in rural areas upholding agriculture and inland fisheries, are mainly concentrated across Africa, Asia and South America. Following malaria, Schistosomiasis is the most prevalent parasitic infection; affecting an approximate 200 million people worldwide, and in sub-Saharan Africa alone, it is the primary cause of over 200,000 deaths each year. Schistosomiasis can be categorized into two main formats of the disease; intestinal and urogenital.
The cause of infection:
Schistosomes have a very intricate and complex lifecycle revolving around a list of hosts that includes: humans, snails and freshwater sources such as lakes, ponds and reservoirs. Schistosomes have five key developmental stages: eggs, miracidia, sporocysts, cercariae and adult worms. The lifecycle of Schistosomes begins when an infected individual releases Schistosoma eggs into a freshwater environment through urine, faeces or other bodily secretions. Upon coming into direct contact with a freshwater environment, The Schistosoma eggs hatch, producing free living and ciliated forms of their earlier selves; named miracidium. Using unique ciliary movements, Miracidium can freely swim towards their target through propelling-like motion and penetrate the soft tissue of a suitable snail intermediate host; in which they develop into mother sporocysts. The mother sporocysts then reproduce asexually to produce daughter sporocysts which travel to and grow in the hepatic and gonadal tissue of the infected freshwater snail. After a period of time, daughter sporocysts undergo metamorphosis; a process of development by which sporocysts grow into adult forms known as cercariae. Eventually, sunlight stimulates the emergence of free-swimming, fork-tail cercariae from the mollusk of the infected snail into the contaminated freshwater. When an individual gets into direct contact with the contaminated water, the immature and infective cercariae penetrate the skin of the human host via mechanical activity and the action of proteolytic enzymes. During the process of penetration, the cercariae lose their tail; developing into schistosomules (adult worms) allowing easier entry through the epidermis and dermis before entering the blood or lymphatic vessels. Once in the bloodstream, schistosmules begin to reproduce in the blood vessels where they remain in a set state of copulation throughout their adulthood. Blood vessels act as a strategic pathway for reproduction as newly formed eggs can easily migrate to a wide selection of organs, on which they can act. Upon deposition in organs and soft tissues, eggs may cause inflammation, chronic pain, scarring and in some cases, renal failure. A small fraction of other eggs are transported into the gut and are eventually excreted from the body via the individuals faeces, reinjecting the water source, and hence, restarting the entire cycle.
There are two main factors determining the impact of schistosomiasis: The type of schistosomiasis (intestinal or urogenital) and the stage of infection. Schistosoma mansoni is largely responsible for inducing the intestinal form of the disease, while S. haematobium is directly linked to the development of urogenital schistosomiasis. Intestinal schistosomiasis can result in abdominal pain, malnutrition and diarrhoea. In some more serious and chronic cases, liver enlargement can be frequent and is strongly linked with the accumulation of fluid in the abdomen and therefore hypertension of the abdominal blood vessels. The typical sign of urogenital schistosomiasis is haematuria (presence of red blood cells in the urine).Haematuria can therefore directly lead to anaemia. In more serious cases, fibrosis of the bladder and ureter may occur, possibly causing the rise of other complications, including bladder cancer. In women, urogenital schistosomiasis may lead to genital lesions, vaginal bleeding and pain during sexual intercourse. In men, urogenital schistosomiasis may bring about infection of the seminal vesicles, prostate and other organs. An even more serious and frightening consequence of this disease is infertility.
Diagnosis of schistosomiasis is implemented using parasitological (Detection of parasite eggs in stool or urine specimens using microscopical methods) or immunological techniques (Detection of antibodies and/or antigens in blood samples). For urogenital schistosomiasis, a simple filtration technique using materials as basic as paper filters can be used to detect infection thro ugh the presence of blood in urine (indicating haematuria); this can also be detected by chemical reagent strips. For intestinal schistosomiasis, the diagnosis is carried out by examination of stool samples using a method called Kato- katz thick smear method.This method can provide both qualitative and quantitative examinations of intestinal schistosomes. The main advantages of the Kato-katz method is that it is highly specific, cheap, and to some extent simple. However, it was shown by enough evidence that this method lacks the ability of detecting low worm burdens.
In most cases, schistosomiasis can be treated with a short course on a drug named Praziquantel; an acylated quinoline-pyrazine efficacious against all schistosome species parasitizing humans. Even though Praziquantel has been used as the Primary drug for treating and controlling schistosomiasis for many years, its exact mechanism of action remains unclear. The drug acts within one hour of ingestion and effectively kills the targeted schistosome by paralysing the worm and severely damaging its tegument. Side-effects are relatively mild and may include nausea, vomiting, malaise, and abdominal pain. However, in heavy infections, a strong and sudden renal pain (known as acute colic) with bloody diarrhoea can occur shortly after treatment, most likely provoked by large worm shifts and antigen release. However, like with any disease, being proactive is always better than being reactive. And so, its recommended to avoid swimming, paddling, washing or drinking in fresh water that is suspected to be infected. Additionally, it is always advised to wear waterproof clothing if there is a possibility of being near infected areas.
In summary, schistosomiasis is by no means a disease that can be taken lightly, if left untreated it can be extremely potent in both its urogenital and intestinal form; as illustrated in some of its severe symptoms. The well adapted nature of schistosomes have made the disease the second most prevalent parasitic infection but a short course of Praziquantel can prove to be effective against schistosome’s.