Protozoan diseases

About fifteen genera of protozoans parasitize the human body. Four are covered here in depth:

Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, which lives in the human large intestine feeding on food particles and bacteria (Fig. 7.1). Its infective trophozoite stage penetrates the intestinal wall, secretes histolytic enzymes, and causes ulceration, bleeding, abdominal pain, and stools containing excess mucus — symptoms ranging from simple diarrhoea to full dysentery with blood and mucus. House flies act as a carrier, picking up the parasite from contaminated faeces and water and depositing it on food.

African sleeping sickness is caused by three species of Trypanosoma (Fig. 7.2), each with its own vector: T. gambiense (transmitted by the tsetse fly Glossina palpalis) causes Gambian/Central African sleeping sickness; T. rhodesiense (transmitted by Glossina morsitans) causes Rhodesian/East African sleeping sickness; and T. cruzi (transmitted by the bug Triatoma megista) causes Chagas disease (American trypanosomiasis).

Kala-azar (visceral leishmaniasis) is caused by Leishmania donovani, transmitted by the sandfly vector Phlebotomus. It infects the endothelial cells, bone marrow, liver, lymph glands and blood vessels of the spleen, producing weight loss, anaemia, fever, and enlargement of the spleen and liver.

Malaria is caused by several Plasmodium species — P. vivax, P. ovale, P. malariae and P. falciparum — each producing a distinct malaria type with its own erythrocytic-cycle duration (Table 7.3). The parasite is transmitted by the bite of an infected female Anopheles mosquito.

Life cycle of Plasmodium (Fig. 7.3): P. vivax is digenic, using humans as the secondary host and the female Anopheles mosquito as the primary host, and the cycle has three phases — schizogony, gamogony and sporogony. A feeding mosquito injects sporozoites into the human bloodstream; these enter liver (hepatic) cells and undergo asexual schizogony to form merozoites. Released merozoites invade RBCs, developing through early trophozoite and signet-ring stages into a schizont bearing yellow-brown Schuffner's granules; the schizont divides to release fresh merozoites and haemozoin toxin as the erythrocyte lyses, triggering the classic cycle of fever and chills roughly every 48–72 hours (depending on species). Some merozoites instead differentiate into macro- and micro-gametocytes; once ingested by a mosquito, these fertilize in its gut to form a diploid zygote (ookinete), which migrates to the gut wall, develops into an oocyst, and undergoes meiotic sporogony to produce sporozoites that travel to the salivary glands — completing the cycle when the mosquito next bites a human. Incubation is about 12 days; early symptoms are headache, nausea and muscular pain, followed by malarial paroxysms (shivering chills, high fever, sweating) driven partly by malarial toxins that trigger macrophages to release TNF-α and interleukin. …