Protozoan Diseases

NCERT grounding

NCERT Class 12, Chapter 7 (Section 7.1 — Common Diseases in Humans) places protozoan diseases immediately after the bacterial and viral entries. Two illnesses receive detailed treatment in the textbook: malaria (caused by Plasmodium spp.) and amoebiasis (caused by Entamoeba histolytica). NCERT walks through the Plasmodium life cycle in Figure 7.1 and names three species — P. vivax, P. malariae and P. falciparum — singling out P. falciparum as the cause of malignant, potentially fatal malaria.

NIOS Biology Chapter 29 supplements the NCERT account with a structured pathogen–transmission–symptom–cure table for malaria and amoebic dysentery, and lists incubation period (~12 days for malaria) along with public-health control measures. Kala-azar (visceral leishmaniasis) is the third protozoan disease that has recurred in NEET (NEET 2024, Q.161 explicitly matched leishmaniasis with protozoa) and is best learnt as a sandfly-borne extension of this cluster.

Malaria, amoebiasis and kala-azar

Protozoans are unicellular eukaryotes — larger than bacteria, structurally more complex, and capable of multiple morphological stages within a single life cycle. Of the four main parasitic protozoan groups (apicomplexans, amoebozoans, kinetoplastids and flagellates), NEET concentrates on three pathogens: Plasmodium (apicomplexan, intracellular blood parasite), Entamoeba histolytica (amoebozoan, gut-lumen parasite), and Leishmania donovani (kinetoplastid, intracellular macrophage parasite). Each illustrates a different transmission strategy — biological vector, faecal–oral, and a different biological vector again — and a different anatomical target — blood and liver, large intestine, and reticulo-endothelial system respectively.

Malaria — Plasmodium life cycle in two hosts

Malaria is the textbook example of a vector-borne disease with an obligate two-host life cycle. The protozoan parasite alternates between a human host (where it undergoes asexual multiplication, called schizogony) and a Female Anopheles mosquito host (where it completes sexual development, called sporogony). The sporozoite is the only stage infectious to humans; gametocytes are the only stage infectious to mosquitoes. Mixing up these two stages is one of NEET's favourite traps.

When an infected Female Anopheles mosquito bites a person, sporozoites stored in its salivary glands are injected into the bloodstream. They travel quickly to the liver, where they invade hepatocytes and multiply (exo-erythrocytic schizogony). The progeny — merozoites — exit the liver and invade red blood cells, where another round of schizogony occurs. When the infected RBCs rupture synchronously, they release fresh merozoites along with a toxic substance called haemozoin. NCERT states this in one sentence: "The rupture of RBCs is associated with release of a toxic substance, haemozoin, which is responsible for the chill and high fever recurring every three to four days."

Why the fever recurs in pulses

The periodic fever of malaria is a direct consequence of synchronous schizogony inside red blood cells. After the liver phase, merozoites enter RBCs and replicate on a fixed clock — about 48 hours for P. vivax and P. falciparum, and about 72 hours for P. malariae. When the cohort of infected cells matures together, they burst together, releasing merozoites and haemozoin in a single pulse. Haemozoin is taken up by macrophages and triggers a brisk cytokine response — chills, then fever, then sweating — which subsides until the next cohort matures. This periodicity is the diagnostic clinical fingerprint of malaria.

Amoebiasis — Entamoeba histolytica and the faecal–oral route

Entamoeba histolytica is a protozoan parasite that lives in the lumen of the human large intestine. It causes amoebiasis, also known as amoebic dysentery. The clinical picture NCERT lists is distinctive: constipation, abdominal pain and cramps, stools with excess mucus and blood clots. Note that NCERT actually names constipation rather than diarrhoea — examiners have built one-mark items on this counter-intuitive symptom. The trophozoite invades the colonic mucosa, producing the classical flask-shaped ulcers that NIOS Chapter 29 lists as the first symptom.

Transmission is faecal–oral. Cysts shed in the faeces of an infected person contaminate water or food; another person ingests the cysts, the cysts excyst in the small intestine, and the trophozoites colonise the large intestine. NCERT highlights one specific mechanical link: "Houseflies act as mechanical carriers and serve to transmit the parasite from faeces of infected person to food and food products, thereby contaminating them." The housefly is not a biological vector — the parasite does not develop inside it — and this distinction is heavily tested.

Kala-azar — Leishmania donovani and the sandfly

Kala-azar, also called visceral leishmaniasis, is caused by Leishmania donovani, a kinetoplastid protozoan, and is transmitted by the bite of a female sandfly (genus Phlebotomus). The parasite resides in the salivary apparatus of the sandfly as a flagellated promastigote; in the human host it transforms into a non-flagellated amastigote that multiplies inside macrophages of the spleen, liver and bone marrow. Classical features include prolonged fever, weight loss, massive splenomegaly and anaemia. NCERT does not detail kala-azar, but NEET 2024 (Q.161) explicitly matched leishmaniasis with protozoa, making it a required name in the protozoan cluster.

From an exam-engineering standpoint, kala-azar pairs with malaria on two parameters — both are biological-vector-borne and both are blood/reticuloendothelial parasites — but differs on the vector (sandfly versus Female Anopheles) and the pathogen (Leishmania versus Plasmodium). Memorise the four-row line: Pathogen — Leishmania donovani · Vector — sandfly · Body system — spleen, liver, bone marrow · Disease — kala-azar / visceral leishmaniasis.

Treatment principle and prevention

Antimalarial chemotherapy targets distinct stages of the Plasmodium life cycle: chloroquine and artemisinin-based combination therapies (ACTs) act on erythrocytic stages, whereas primaquine clears liver hypnozoites of P. vivax and P. ovale. Quinine, derived from the bark of Cinchona, was the first effective drug and remains useful for severe malaria. Public-health control attacks the vector — eliminating stagnant water that breeds Anopheles larvae, fitting mesh on doors and windows, using mosquito nets, and applying repellents. For amoebiasis, metronidazole is the treatment of choice; prevention is hygienic — proper sanitation, washing fruits and vegetables, and protecting food from houseflies. Kala-azar is treated with amphotericin B and miltefosine; sandfly control and case-finding remain key public-health steps.

Worked examples

Common confusion & NEET traps