Parasitology
1. What this chapter covers, and how NEET PG actually tests it
Parasitology carries substantial weight in an Indian examination because so many of these infections remain endemic here.
The life cycle is the organising tool, because it predicts nearly everything the exam asks.
Where the parasite enters predicts how infection is acquired and therefore how it is prevented.
Where it lives predicts which organ is damaged and which specimen will contain it.
When it releases its diagnostic stage predicts when to collect that specimen, which is why timing questions exist at all.
| Question type | What the life cycle tells you |
|---|---|
| Mode of acquisition | Where the infective stage enters |
| Clinical features | Where the adult or larva resides |
| Diagnostic specimen | Where the diagnostic stage is shed |
| Timing of sampling | When that stage appears |
| Prevention | Which link is easiest to break |
Definitive host harbours the sexual stage; intermediate host harbours the asexual stage. For malaria the mosquito is therefore the definitive host and the human the intermediate one, which surprises candidates and is examined for that reason.
2. Malaria
2.1 The life cycle and what it explains
Sporozoites are injected by a female Anopheles, travel to the liver, and multiply there in the exo-erythrocytic stage before releasing merozoites into the blood.
Plasmodium vivax and ovale leave dormant hypnozoites in the liver, which is why only these two relapse and why primaquine is required to prevent it.
Blood-stage schizogony causes the cyclical fever, and its periodicity reflects the synchrony of rupture: 48 hours in vivax and ovale, 72 in malariae, and irregular in falciparum.
Gametocytes are the stage taken up by the mosquito, so they matter for transmission rather than for symptoms.
2.2 Why falciparum is different
Falciparum infects red cells of all ages, whereas vivax prefers reticulocytes and malariae prefers older cells.
That single difference allows falciparum to reach far higher parasite densities, which is the root of its severity.
Falciparum-infected cells also express knob proteins that adhere to endothelium, causing sequestration in deep capillaries.
Sequestration explains cerebral malaria, and it also explains why peripheral blood films can underestimate the true parasite burden.
Severe features include cerebral involvement, hypoglycaemia, acidosis, renal failure, and blackwater fever from massive intravascular haemolysis.
Duffy antigen negativity confers resistance to vivax specifically, because the parasite requires that receptor to enter the red cell.
2.3 Genetic protection and what it demonstrates
Several inherited red cell disorders persist at high frequency in malarial regions because they confer partial protection, which is textbook balanced polymorphism.
Sickle cell trait, thalassaemia trait, glucose-6-phosphate dehydrogenase deficiency and hereditary ovalocytosis all reduce falciparum severity.
The protection works because the parasite depends on a normal red cell environment, and each of these disorders degrades that environment in a different way.
Sickle trait cells sickle preferentially when parasitised, and the abnormal cells are cleared by the spleen along with the parasite inside them.
G6PD-deficient cells cannot handle the oxidative stress the growing parasite generates, so infected cells are destroyed early.
The clinical consequence is that these conditions cluster in exactly the populations where malaria is endemic, which is why G6PD testing before primaquine matters most precisely where primaquine is most needed.
2.4 Diagnosis and treatment
Peripheral smear examination remains the reference standard, with thick films for detecting parasites and thin films for speciation and counting.
Rapid diagnostic tests detect histidine-rich protein 2 for falciparum and lactate dehydrogenase for other species, and HRP2 can remain positive for weeks after successful treatment.
Artesunate is the treatment of choice in severe falciparum malaria, and artemisinin combination therapy is used for uncomplicated disease.
Primaquine is required for radical cure of vivax and ovale, and glucose-6-phosphate dehydrogenase status must be checked first.
3. Intestinal protozoa and other protozoal disease
3.1 The intestinal protozoa
| Organism | Feature | Diagnosis |
|---|---|---|
| Entamoeba histolytica | Flask-shaped ulcers, liver abscess | Stool, serology, antigen |
| Giardia lamblia | Fat malabsorption, no blood | Stool, duodenal aspirate, antigen |
| Cryptosporidium | Chronic diarrhoea in HIV | Modified acid-fast stain |
| Cyclospora | Prolonged diarrhoea, berries | Modified acid-fast, autofluorescence |
Amoebic liver abscess produces anchovy-sauce pus that is usually sterile on culture, because the material is liquefied hepatocyte rather than pus in the bacterial sense, and trophozoites sit at the abscess margin rather than in the centre.
That is why aspirate examination is often negative and why serology is more useful.
Giardia coats the duodenal mucosa and interferes with fat absorption, producing greasy, foul-smelling stools without blood or fever, which distinguishes it from invasive causes.
Entamoeba histolytica must be distinguished from the harmless Entamoeba dispar, which is morphologically identical, so molecular or antigen testing is required for certainty.
Seeing trophozoites containing ingested red cells is the one microscopic finding that establishes histolytica, since only the pathogenic species is invasive enough to consume erythrocytes.
Amoebic and bacillary dysentery are separated by the stool findings: amoebic stool contains few neutrophils because the trophozoite destroys them, while bacillary dysentery produces abundant pus cells.
3.2 Blood and tissue protozoa
Leishmania donovani causes visceral leishmaniasis, or kala-azar, transmitted by the sandfly and endemic in Bihar and neighbouring states.
It presents with prolonged fever, massive splenomegaly, pancytopenia and hypergammaglobulinaemia, and amastigotes are seen in splenic or bone marrow aspirate.
Post-kala-azar dermal leishmaniasis appears after apparently successful treatment and is important because those patients act as a human reservoir, which is why elimination programmes track it.
Toxoplasma gondii has the cat as definitive host, causes congenital infection with the classic triad of chorioretinitis, hydrocephalus and intracranial calcification, and causes ring-enhancing brain lesions in advanced HIV.
Trypanosoma species cause African sleeping sickness and American Chagas disease, the latter producing megaoesophagus, megacolon and cardiomyopathy.
Trichomonas vaginalis is a flagellate causing frothy discharge with a strawberry cervix, diagnosed on wet mount by its motility.
3.3 The free-living amoebae
These are environmental organisms that cause disease only accidentally, and each is defined by its route of entry.
Naegleria fowleri enters through the cribriform plate after freshwater swimming and causes primary amoebic meningoencephalitis, a fulminant illness killing within days.
The route explains the presentation: the organism travels along the olfactory nerve directly into the frontal lobes, so early loss of smell and rapid deterioration are characteristic.
Cerebrospinal fluid resembles bacterial meningitis but contains motile trophozoites on a wet mount, and that motility is the diagnostic finding.
Acanthamoeba causes a much slower granulomatous encephalitis in the immunocompromised, and keratitis in contact lens wearers who use tap water.
The contrast is instructive: the same class of organism causes a fulminant illness in the healthy by one route and an indolent one in the immunosuppressed by another.
4. Helminths
4.1 Nematodes
| Worm | Acquisition | Distinctive feature |
|---|---|---|
| Ascaris lumbricoides | Ingested eggs | Largest, biliary and intestinal obstruction |
| Hookworm | Skin penetration | Iron deficiency anaemia |
| Strongyloides | Skin penetration | Autoinfection, hyperinfection on steroids |
| Enterobius | Ingested eggs | Perianal itch, cellophane tape test |
| Trichuris | Ingested eggs | Rectal prolapse in heavy infection |
| Wuchereria bancrofti | Mosquito | Lymphatic filariasis, nocturnal periodicity |
Strongyloides is the only common nematode capable of autoinfection, completing its cycle within one host, which is why infection can persist for decades and why corticosteroids can precipitate lethal hyperinfection.
That single property is the reason Strongyloides screening is recommended before immunosuppression in endemic areas.
Hookworm larvae penetrate the skin, migrate through the lungs, are swallowed, and attach to the small intestine where they feed on blood, which is the mechanism of the anaemia.
Ascaris follows a similar migratory route, which is why it can produce transient pulmonary eosinophilia, called Loeffler syndrome, before intestinal symptoms appear.
Wuchereria microfilariae show nocturnal periodicity in India, appearing in peripheral blood around midnight, which is precisely why blood must be collected at night.
That periodicity matches the biting habit of the vector mosquito, which is the point of it: the parasite is in the blood exactly when something is likely to take it away.
Chronic filariasis causes lymphoedema and elephantiasis through repeated lymphatic inflammation, and by that stage the microfilariae have usually disappeared.
A patient with established elephantiasis therefore often has a negative night blood film, which catches candidates who expect the diagnostic test to be positive when the disease is most obvious.
Tropical pulmonary eosinophilia is a distinct filarial presentation with nocturnal wheeze, very high IgE and extreme eosinophilia, and no microfilariae in blood because they are trapped in the lungs.
4.2 Cestodes
Taenia solium is acquired as adult tapeworm from undercooked pork, but ingesting its eggs instead produces cysticercosis.
Humans therefore act as definitive host in one route and intermediate host in the other, and it is the egg route that causes neurocysticercosis, the commonest cause of acquired epilepsy in India.
Taenia saginata from beef causes only intestinal infection, since its eggs are not infective to humans.
Echinococcus granulosus produces hydatid cysts, with the dog as definitive host and sheep as intermediate host, and humans as accidental intermediate hosts.
Aspiration of a hydatid cyst risks anaphylaxis and seeding, which is why the PAIR technique uses a scolicidal agent and why albendazole cover is given.
Diphyllobothrium latum from raw fish competes for vitamin B12 and can cause megaloblastic anaemia.
Hymenolepis nana is the one tapeworm needing no intermediate host at all, which is why it spreads directly between children and is the commonest cestode worldwide.
That absence of an intermediate host also means internal autoinfection can occur, so infections can be heavy and persistent in the same way Strongyloides infections are.
Echinococcus multilocularis produces alveolar hydatid disease, which behaves like a malignancy by infiltrating rather than forming a discrete cyst, and carries a far worse prognosis than the granulosus form.
4.3 Trematodes
Schistosoma species penetrate skin from freshwater, with snails as intermediate hosts.
Schistosoma haematobium involves the urinary tract and is associated with squamous cell carcinoma of the bladder, while mansoni and japonicum involve the bowel and liver, producing periportal fibrosis and portal hypertension.
Fasciola hepatica affects the biliary tree, Clonorchis is associated with cholangiocarcinoma, and Paragonimus affects the lung and mimics tuberculosis.
Praziquantel treats almost all trematodes and most cestodes, while albendazole treats most nematodes and tissue cestodes.
4.4 When humans are the wrong host
Several syndromes occur because a parasite has entered a host it cannot complete its cycle in, and the resulting confusion is what causes the disease.
Cutaneous larva migrans follows skin penetration by animal hookworm larvae, which cannot reach the human intestine and instead wander in the epidermis, producing a serpiginous advancing track.
Visceral larva migrans follows ingestion of Toxocara eggs from dog or cat faeces, with larvae migrating through liver, lung and occasionally eye, producing marked eosinophilia in a child.
Ocular involvement can mimic retinoblastoma, which is a diagnosis with very different consequences, so serology matters.
Trichinella is acquired from undercooked pork and encysts in striated muscle, producing myositis, periorbital oedema and eosinophilia.
The unifying feature of these syndromes is a self-limiting infection with disproportionate symptoms, because the larvae are trapped and provoke intense inflammation while being unable to mature.
Hydatid disease belongs to the same category, since humans are accidental intermediate hosts in a cycle that normally runs between dogs and sheep.
5. Diagnosis and control
5.1 Choosing the right specimen and time
The life cycle determines both, and getting either wrong produces a false negative.
Nocturnal collection for Wuchereria, cellophane tape in the early morning for Enterobius, and repeated stool samples for intermittently shed protozoa all follow directly from shedding patterns.
Serology is preferred where the parasite is in tissue rather than lumen, as in amoebic liver abscess, hydatid disease and cysticercosis.
Concentration techniques increase yield for stool ova and cysts, and modified acid-fast staining is needed for Cryptospordium and Cyclospora, which ordinary stains miss.
Eosinophilia suggests tissue-invasive helminth infection and is characteristically absent in protozoal infection and in luminal worms that never invade.
5.2 Control programmes in India
Mass drug administration with diethylcarbamazine and albendazole has driven the lymphatic filariasis elimination programme.
Kala-azar elimination relies on case detection, single-dose liposomal amphotericin treatment and indoor residual spraying against the sandfly vector.
Deworming programmes give albendazole to school-age children periodically, because reinfection is inevitable where sanitation is inadequate and periodic treatment reduces worm burden even if it cannot eradicate infection.
Malaria control combines vector measures, insecticide-treated nets, prompt diagnosis with rapid tests and artemisinin combination therapy.
Each programme targets the weakest link in the relevant life cycle, which is why the interventions differ so much between diseases.
For filariasis the weakest link is the human reservoir of microfilariae, so mass drug administration is used; for kala-azar it is the sandfly and the small number of infected humans, so spraying plus case-finding works.
For soil-transmitted helminths the weakest link would be sanitation, but since that cannot be changed quickly, periodic deworming reduces morbidity while sanitation improves in parallel.
Guinea worm eradication succeeded because the cycle depended entirely on drinking water containing copepods, so filtering water alone broke it without any drug at all.
That case is the clearest demonstration of the chapter's principle: understanding the cycle identified an intervention that needed no medicine.
6. Worked examples
Example 1
A patient on long-term corticosteroids for asthma develops Gram-negative bacteraemia, abdominal pain and diffuse pulmonary infiltrates. He worked barefoot in rural fields decades ago.
The decades-old exposure is a deliberate clue, since most parasites would have died long ago.
Strongyloides completes its cycle within the host by autoinfection, so it can persist for the whole of a person's life.
Corticosteroids remove the immune restraint on autoinfection, producing hyperinfection with massive larval migration.
Migrating larvae carry gut bacteria with them, which is why Gram-negative sepsis accompanies the syndrome, and this is why screening precedes immunosuppression in endemic areas.
Example 2
A child from Bihar has three months of fever, massive splenomegaly and pancytopenia, with markedly raised globulins.
The geography, the duration and the size of the spleen together are highly suggestive.
Visceral leishmaniasis is transmitted by the sandfly and is endemic in Bihar and neighbouring states.
The pancytopenia reflects both splenic sequestration and marrow infiltration by amastigote-laden macrophages, and the hypergammaglobulinaemia reflects intense polyclonal B cell stimulation.
Splenic or bone marrow aspirate demonstrates amastigotes, and rK39 serology is the usual field test.
Example 3
A young man from a pork-eating community presents with a first seizure. Imaging shows a solitary ring-enhancing lesion with a central dot.
The central dot within a ring-enhancing lesion represents the scolex and is close to pathognomonic.
Neurocysticercosis follows ingestion of Taenia solium eggs rather than of cysts in pork.
Eating undercooked pork gives intestinal tapeworm; swallowing eggs from faecal contamination gives cysticercosis, so a patient with neurocysticercosis need not have eaten pork at all.
It is the commonest cause of acquired epilepsy in India, and treatment combines antiepileptics with albendazole and corticosteroid cover.
7. Traps the exam sets repeatedly
Assuming the human is the definitive host in malaria. The mosquito is, because it harbours the sexual stage.
Expecting hypnozoites in falciparum. Only vivax and ovale relapse, and only they need primaquine.
Collecting daytime blood for microfilariae. Wuchereria shows nocturnal periodicity in India.
Assuming neurocysticercosis requires eating pork. It follows ingestion of eggs, not of cysts.
Aspirating a hydatid cyst without precautions. Spillage risks anaphylaxis and seeding.
Expecting eosinophilia in protozoal infection. It indicates tissue-invasive helminths and is characteristically absent in protozoal disease.
Missing Strongyloides before starting steroids. Autoinfection means decades-old infection can still cause lethal hyperinfection.
Treating a serpiginous skin track as a fungal infection. Cutaneous larva migrans is an animal hookworm larva that cannot mature in humans.
Summary
The life cycle predicts acquisition, organ involvement, specimen, timing and the point of control.
The definitive host harbours the sexual stage, which makes the mosquito definitive for malaria.
Only vivax and ovale form hypnozoites, so only they relapse and only they need primaquine.
Falciparum infects red cells of all ages and sequesters through knob adhesion, which is why it is severe.
Amoebic liver abscess yields sterile anchovy-sauce material with trophozoites at the margin.
Strongyloides autoinfects, persists for decades, and causes hyperinfection when steroids are given.
Wuchereria microfilariae are nocturnal in India, which dictates when blood is taken.
Pork gives intestinal Taenia solium while its eggs give cysticercosis, the commonest cause of acquired epilepsy in India.
Eosinophilia indicates tissue-invasive helminths and is absent in protozoal and purely luminal infection.
Inherited red cell disorders persist in malarial regions because they degrade the environment the parasite depends on.
Naegleria enters through the cribriform plate and kills within days; Acanthamoeba causes indolent disease and contact lens keratitis.
Larva migrans syndromes occur because humans are the wrong host, so trapped larvae cause disproportionate inflammation.
Praziquantel covers trematodes and most cestodes; albendazole covers nematodes and tissue cestodes.
