|In the 'schistosomes' (Schistosoma and related genera), the definitive host is infected when free-swimming cercariae penetrate the host's skin. If the schistosome is a species that normally infects humans, a cercaria penetrates the skin rapidly and transforms into a schistosomule, and the schistosomule enters the circulatory system and leaves the skin. Penetration of the skin by cercariae of human schistosomes may result no symptoms or in a mild inflammation of the skin (dermatitis), especially in hosts that have been exposed previously to cercariae. |
Schistosome cercariae are unable to differentiate the skin of different animals. If cercariae of non-human schistosomes come in contact with human skin, they will penetrate it just as if it was the skin of their normal host. When this occurs the schistosomules can cause a dramatic inflammatory response, especially in hosts that have been exposed previously, because (1) humans are an abnormal host and (2) the schistosomules remain in the skin for an extended period of time and eventually die there. The inflammatory response that results when non-human schistosome cercariae penetrate the skin of humans is called 'swimmer's itch' or schistosome cercarial dermatitis.
The cercariae of bovine and avian 'schistosomes' are probably responsible for a majority of cases of swimmer's itch, and infections have been reported throughout the world. Assuming that one swims, bathes, plays, etc., in a natural body of water in which these cercariae are found, there is virtually no way to prevent an infection. The risk of infection can be lowered by spending less time in the water or by drying off completely after getting out of the water. (Some people mention that covering your body with petroleum jelly will also decrease the risk of infection, but most people would find this less than appealing.) Schistosome cercarial dermatitis can be extremely uncomfortable and annoying, to the point of ruining your vacation, but the schistosomules will not develop into adult worms, and the infection is not life threatening.
The schistosomes are unusual trematodes in that the sexes are separate (they are dioecious), they reside in the blood vessels of the definitive host, and there are no second intermediate hosts in their life cycles. There are a number of species of schistosomes that can infect humans, but most human infections are caused by one of the three following species: Schistosoma mansoni; S. haematobium; S. japonicum. Considering the distributions of all three species, schistosomiasis is distributed throughout almost all of Africa, parts of southeast Asia, parts of northwest South America, and some islands in the Caribbean Sea. It is estimated that approximately 200,000,000 million people are infected with schistosomes, resulting in 1,000,000 deaths each year. The approximate geographic distributions of S. mansoni and S. japonicum are shown here.
The life cycles of the three primary species of human schistosomes are similar. The male and female worms average about 10 mm in length and live in the veins of the abdominal cavity. Here they mate and the females produce eggs. The adult worms can live 20-30 years and, depending on the species, and each female can produce several hundred eggs each day. The eggs escape from the body by penetrating the walls of the veins and small intestine or urinary bladder, and they are passed in the feces or urine. The eggs hatch in water, the first intermediate host (a snail) is infected, and cercariae are liberated from the snails. When humans come in contact with water containing cercariae, the cercariae penetrate their skin and they become infected. This occurs when the humans swim, bath, wash clothes, etc., in rivers and streams. After the cercariae penetrate the skin the immature worms enter the circulatory system and migrate to the veins of the abdominal cavity, and in about six weeks they reach sexual maturity (view diagram of the life cycle).
As the eggs of the schistosomes penetrate the walls of the veins and the small intestine or urinary bladder, they cause a significant amount of damage to the tissues. The tissues hemorrhage, so blood often appears in the urine or feces. As the infection progresses the tissues become inflamed and fibrotic and unable to function normally. Many of the eggs produced by the female worms do not escape from the veins, but are swept up in the circulatory system and deposited in the host's liver. The liver responds to the presence of the eggs by encapsulating them in a fibrous granuloma. The damage to the small intestine (or urinary bladder) and liver accumulate over time and result in a chronic, disabling disease that can be fatal.
As with most trematode infections, diagnosis most often depends on finding the parasite's eggs. In the case of S. haematobium, eggs are most often recovered in the urine; eggs of the other two species are most often recovered in the feces.