VI.

Parasites of Humans

Humans are subjected to numerous protozoan, worm, and insect-related parasites. Two of the most damaging human parasites are the protozoan Plasmodium that causes malaria and the flatworm Schistosoma that causes schistosomiasis. There are an estimated 400 million to 600 million cases of malaria each year and 200 million cases of schistosomiasis worldwide.

In malaria, the infective larval stage of the Plasmodium protozoan is transmitted to humans by the bite of a female Anopheles mosquito. The larvae undergo asexual reproduction in the liver producing a cyst that releases new larval stages into the blood stream. Larvae invade red blood cells and reproduce, eventually rupturing the blood cells. Upon rupturing, a toxin is released that causes the chills and fever that are the characteristic symptoms of malaria patients. Drugs such as chloroquine can be used to prevent infection in the blood. Mosquito control by use of repellents and pesticides is also helpful in preventing spread of the parasite.

Humans are infected with Schistosoma when they enter water containing infected snails. The larval stages of this flatworm develop in the tissues of infected snails and eventually release fork-tailed cercariae into the water. The cercariae penetrate human skin, lose their forked tails, enter the blood, and migrate to major veins in the liver, intestine, or urinary bladder. Within about six weeks of infection, the juvenile worms develop into sexually mature adults measuring 1 to 2 cm (0.4 to 0.8 in) in length. The males and females mate and produce microscopic eggs, some of which migrate to the liver and cause a condition known as cirrhosis. Other eggs move into the intestine and are passed out in the feces. When untreated human sewage enters waters containing the snail hosts, the eggs hatch and start a new cycle.

Preventive measures include the use of boots and gloves or special ointments to block penetration of the larvae into the skin. Molluscicides (drugs that kill snails) are used to kill infected snails but they often kill other important fish and invertebrate species. The drug praziquantel has proved effective in killing Schistosoma in humans, although some people experience adverse side effects.