McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 1 2  Antiprotozoal agents

Environment (contaminated food, water, hands)

Ingested by host (humans, primates, other mammals)

Cysts

Metronidazole works here

Fatal to trophozoites

Faeces of host

Fatal to host

Enters intestinal tract (produced daughter cells— trophozoites—and cysts)

FIGURE 12.3  Life cycle of Entamoeba histolytica and the sites of action of metronidazole and chloroquine, which are used to treat amoebiasis. Cysts ingested by the host enter the intestinal tract and produce trophozoites. Trophozoites enter the bloodstream to reach tissue. Trophozoites enter the liver, lungs, heart, brain and spleen, which can be fatal to the host. Trophozoites are excreted in the stool and die. Cysts excreted in the stool contaminate water and can be ingested by the host.

Metronidazole works here

Metronidazole, chloroquine works here

Bloodstream and tissue

Liver, lungs, heart, brain and spleen

The disease is transmitted while the protozoan is in the cystic stage in faecal matter, from which it can enter water and the ground. It can be passed to other humans who drink this water or eat food that has been grown in this ground. The cysts are swallowed and pass, unaffected by gastric acid, into the intestine. Some of these cysts are passed in faecal matter, and some of them become trophozoites that grow and reproduce. The tro- phozoites migrate into the mucosa of the colon, where they penetrate into the intestinal wall, forming erosions. These forms of Entamoeba release a chemical that dis- solves mucosal cells, and eventually they eat away tissue until they reach the vascular system, which carries them throughout the body. The trophozoites lodge in the liver, lungs, heart, brain, and so on. Early signs of amoebiasis include mild to fulmi- nate diarrhoea. In the worst cases, if the protozoan is able to invade extraintestinal tissue, it can dissolve the tissue and eventually cause the death of the host. Some

individuals can become carriers of the disease without having any overt signs or symptoms. These people seem to be resistant to the intestinal invasion but pass the cysts on in the stool. Leishmaniasis Leishmaniasis is a disease caused by a protozoan that is passed from sand flies to humans. The sand fly injects an asexual form of this flagellated protozoan, called a pro- mastigote, into the body of a human, where it is rapidly attacked and digested by human macrophages. Inside the macrophages, the promastigote divides, develop­ ing many new forms called amastigotes, which keep dividing and eventually kill the macrophage, releas- ing the amastigotes into the system to be devoured by more macrophages. Thus, a cyclic pattern of infection is established. These amastigotes can cause serious lesions in the skin, the viscera or the mucous membranes of the host.