Parasitology

Zoology (B.Sc) · Applied zoology · free study notes for exams, placements and interviews

In short

Parasitology is the study of organisms that live on or in another species (the host), drawing nourishment at the host's expense, and of the disease, transmission and control that follow. For a B.Sc Zoology student it is the applied core of the syllabus: morphology, life cycles, pathogenicity, diagnosis and control of protozoan, helminth and arthropod parasites of humans and livestock. It matters because India still carries a very large share of the world's lymphatic filariasis, soil-transmitted helminth and visceral leishmaniasis burden, and runs national programmes (NCVBDC, formerly NVBDCP; National Deworming Day) built on exactly this knowledge. It is examined in university practicals and theory, in CSIR-UGC NET and GATE (XL - Life Sciences, Animal Sciences section), in ICMR/ICAR and State Public Service entrance tests, and in interviews for medical-laboratory, veterinary-diagnostic, vector-surveillance and pharma-research roles.

Explained

Definitions, types and the host-parasite relationship

Symbiosis is any close living-together; mutualism benefits both, commensalism benefits one without harming the other, parasitism benefits one at the other's cost.
Ectoparasite: lives on the body surface (lice, ticks, fleas, mites). Endoparasite: lives inside - intracellular (*Plasmodium*, *Leishmania*) or intercellular/luminal (*Entamoeba*, *Ascaris*).
Obligate parasite: cannot complete life cycle without a host (*Plasmodium*). Facultative: normally free-living but can parasitise (some *Naegleria*, flesh-fly larvae). Accidental/incidental: in an unusual host. Erratic: in an unusual site in the right host.
Temporary (intermittent) parasite: visits only to feed (mosquito, bedbug). Permanent: stays for life (*Taenia*).
Hyperparasite: a parasite of a parasite. Monoxenous: one host; heteroxenous/digenetic: two or more hosts.
Definitive (primary) host: harbours the adult or sexual stage. Intermediate host: harbours larval/asexual stages. In malaria the mosquito is the definitive host (sexual stages occur there) and humans the intermediate host - the usual exam trap.
Paratenic (transport) host: carries the larva without development. Reservoir host: maintains infection in nature and is a source for humans (dog for *Leishmania* in some foci, cattle for *Fasciola*).
Vector: transmits the parasite - biological (parasite develops in it: anopheline mosquito, sandfly) or mechanical (housefly carrying *Entamoeba* cysts on its body).
Zoonosis: animal disease transmissible to humans (hydatid disease, trichinellosis, toxoplasmosis); anthroponosis is human-to-human.
Terms: infection = endoparasite; infestation = ectoparasite. Incubation period = infection to symptoms; prepatent period = infection to detectable stage in excreta/blood. Autoinfection = reinfecting oneself (*Enterobius*, *Strongyloides*, *Taenia solium* causing cysticercosis).
Parasitic adaptations: loss of locomotory organs and sense organs, reduced gut (absent in cestodes - absorb through tegument), anaerobic metabolism, cuticle/tegument resistant to host enzymes, hooks-suckers-adhesive organs, antigenic variation and immune evasion, enormous reproductive output (*Ascaris* female: about 2,00,000 eggs/day), hermaphroditism and self-fertilisation, resistant cysts and eggs.

Protozoan parasites

Entamoeba histolytica (Phylum Amoebozoa) - amoebiasis. Faeco-oral transmission by the quadrinucleate cyst; trophozoite invades the colon, producing flask-shaped ulcers, and may spread via the portal vein to cause amoebic liver abscess ("anchovy-sauce" pus). Trophozoites with ingested RBCs in dysenteric stool are diagnostic; cysts in formed stool. Treatment: metronidazole/tinidazole plus a luminal amoebicide (diloxanide furoate or paromomycin). Distinguish from the harmless *E. dispar* and from *E. coli* (8-nucleate cyst).
Giardia lamblia - giardiasis. Flagellate trophozoite with a ventral sucking disc; sticks to duodenal mucosa causing fat malabsorption and steatorrhoea; "string test"/duodenal aspirate or stool ELISA; metronidazole.
Plasmodium - malaria. Four human species in India (*P. vivax*, *P. falciparum*, *P. malariae*, *P. ovale*; *P. knowlesi* zoonotically). *P. vivax* and *P. falciparum* dominate; *falciparum* causes cerebral malaria, and *vivax* forms dormant liver hypnozoites causing relapse.
Malaria life cycle: sporozoites from the saliva of an infected female *Anopheles* reach the liver → exo-erythrocytic schizogony → merozoites → erythrocytic schizogony (ring → trophozoite → schizont → merozoites; rupture causes the paroxysm) → some become gametocytes → taken up by the mosquito → gametogony and fertilisation in the gut → ookinete → oocyst on the gut wall → sporogony → sporozoites to the salivary glands. Fever periodicity: 48 h (tertian, *vivax*/*ovale*), 72 h (quartan, *malariae*), irregular in *falciparum*.
Diagnosis and drugs: thick and thin Giemsa-stained blood smear is the gold standard; RDT detects HRP-2 (*falciparum*) and pLDH. India's National Drug Policy on Malaria uses ACT-SP (artesunate + sulfadoxine-pyrimethamine) for *P. falciparum* across most of the country, AL (artemether-lumefantrine) in the North-Eastern states, chloroquine for *P. vivax*, and primaquine as radical cure (14 days for *vivax*; single low dose as gametocytocide in *falciparum*) - check G6PD status. India targets malaria elimination by 2030 under the NFME/NSP framework; treatment schedules are revised periodically, so confirm the current NCVBDC guideline.
Leishmania donovani - kala-azar (visceral leishmaniasis), transmitted by the female sandfly *Phlebotomus argentipes*; amastigote (LD body) in macrophages of spleen, liver and bone marrow, promastigote in the vector. Endemic in Bihar, Jharkhand, West Bengal and eastern Uttar Pradesh; PKDL (post-kala-azar dermal leishmaniasis) is the human reservoir. Diagnosis: rK39 rapid test, splenic/bone-marrow aspirate. Treatment: single-dose liposomal amphotericin B (miltefosine as an option); elimination target is under 1 case per 10,000 population at block level, and India has reported reaching that threshold - status is updated annually.
Trypanosoma - African sleeping sickness (tsetse, *Glossina*) and Chagas disease (*T. cruzi*, reduviid bug) - not endemic in India but syllabus-standard for comparison; *T. evansi* causes surra in Indian camels, horses and cattle.
Trichomonas vaginalis - urogenital, sexually transmitted, no cyst stage. Toxoplasma gondii - cat is the definitive host; congenital toxoplasmosis. Balantidium coli - only ciliate human parasite; pig reservoir.

Helminth parasites (Platyhelminthes and Nematoda)

Fasciola hepatica / F. gigantica (liver fluke, digenean): adult in bile ducts of sheep, goat and cattle. Egg → miracidium → snail (*Lymnaea*) → sporocyst → redia → cercaria → encysts on water plants as metacercaria → eaten. Causes "liver rot" and major losses in Indian dairy herds; triclabendazole.
Schistosoma (blood fluke): separate sexes, male carries female in the gynaecophoric canal; cercaria penetrates skin directly - no metacercaria. Human schistosomiasis in India is limited to a focus in Gimvi, Maharashtra; cercarial dermatitis occurs more widely.
Taenia solium (pork tapeworm): scolex with four suckers and a rostellum with hooks; Cysticercus cellulosae in pig muscle. Eating undercooked pork → intestinal taeniasis; swallowing eggs → cysticercosis, and neurocysticercosis is a leading cause of adult-onset epilepsy in India. *T. saginata* (beef, unarmed scolex, *Cysticercus bovis*) does not cause human cysticercosis.
Echinococcus granulosus: dog is the definitive host, sheep/cattle/human the intermediate; hydatid cyst in liver and lung; cyst rupture risks anaphylaxis - surgery with PAIR technique plus albendazole.
Hymenolepis nana - dwarf tapeworm, commonest cestode of Indian children, direct cycle with autoinfection. Diphyllobothrium latum - two intermediate hosts (copepod, fish), causes B₁₂ deficiency anaemia.
Ascaris lumbricoides (roundworm): soil-transmitted; eggs with mammillated albuminous coat embryonate in soil; larvae hatch in intestine, undergo the hepato-tracheal (Loeffler's) migration - gut → liver → heart → lungs → trachea → swallowed → adult in small intestine. Heavy loads cause obstruction; larval migration causes Loeffler's eosinophilic pneumonitis.
Ancylostoma duodenale / Necator americanus (hookworm): filariform larva penetrates skin (ground itch), same lung migration, adult sucks blood - the major cause of helminth iron-deficiency anaemia. Diagnosis by egg count; Harada-Mori culture distinguishes species.
Enterobius vermicularis (pinworm): perianal egg-laying, nocturnal pruritus, NIH swab / cellophane tape test, autoinfection and family clusters.
Trichuris trichiura (whipworm): barrel-shaped bile-stained eggs with mucoid plugs; heavy load causes rectal prolapse. Strongyloides stercoralis: autoinfection, hyperinfection in immunosuppression. Trichinella spiralis: encysted larva in muscle, pork-borne.
Wuchereria bancrofti (and *Brugia malayi*): lymphatic filariasis, transmitted by *Culex quinquefasciatus* (and *Mansonia* for *Brugia*); sheathed microfilariae show nocturnal periodicity (peak about 10 pm-2 am - time the blood smear accordingly); adults in lymphatics cause lymphoedema, elephantiasis and hydrocele. Control: MDA with DEC + albendazole, and IDA (ivermectin + DEC + albendazole) in selected districts; diagnosis by night blood smear, membrane filtration or filarial antigen (ICT) card.
Dracunculus medinensis (guinea worm) - eradicated from India, certified in 2000; cycle through *Cyclops*.

Pathogenicity, immunity and chemotherapy

Mechanisms of damage: mechanical obstruction (*Ascaris* bolus, hydatid pressure), tissue destruction and lysis (*E. histolytica* amoebapores and cysteine proteases), blood loss (hookworm, roughly 0.03-0.26 mL per worm per day), nutrient competition (B₁₂ by *D. latum*), toxic and allergic reaction (hydatid fluid), immunopathology (lymphatic damage in filariasis), and carcinogenesis (*Clonorchis*, *Schistosoma haematobium*).
Host response: Th2-type immunity - IgE, mast cells, eosinophilia and ADCC against helminths; Th1/cell-mediated response against intracellular protozoa; premunition (partial, non-sterile immunity) in malaria.
Immune evasion: antigenic variation (*Trypanosoma* VSG, *Plasmodium* var/PfEMP1), intracellular hiding, host-antigen masking (*Schistosoma*), immunosuppression, anatomical sequestration.
Drug classes: benzimidazoles (albendazole, mebendazole - bind β-tubulin), ivermectin (glutamate-gated Cl⁻ channels), praziquantel (Ca²⁺ influx in flukes and tapeworms), DEC (microfilaricide), nitroimidazoles (metronidazole - anaerobic protozoa), artemisinins and 8-aminoquinolines (malaria).
Drug resistance is a live concern: chloroquine-resistant *P. falciparum*, artemisinin partial resistance in South-East Asia, and benzimidazole resistance in livestock nematodes - monitored by the FECRT (faecal egg count reduction test).

Diagnosis and laboratory methods

Stool examination: direct saline and iodine wet mounts; concentration by formol-ether (Ritchie) sedimentation or zinc-sulphate/saturated-salt flotation; Kato-Katz thick smear for quantitative egg counting (template delivers 41.7 mg of faeces, so EPG = eggs counted × 24); McMaster slide for veterinary EPG.
Blood: thick smear (parasite detection, higher sensitivity) and thin smear (species identification); Giemsa or JSB stain; RDT; concentration methods and membrane filtration for microfilariae.
Tissue and serology: splenic/bone-marrow aspirate for LD bodies, skin snip, rK39 and ICT cards, ELISA, IFA, PCR and LAMP for species confirmation and low parasitaemia; imaging (CT/MRI) for neurocysticercosis and hydatid.
Intensity classes (WHO, soil-transmitted helminths): *Ascaris* light 1-4,999 EPG, moderate 5,000-49,999, heavy ≥50,000; hookworm light 1-1,999, moderate 2,000-3,999, heavy ≥4,000; *Trichuris* light 1-999, moderate 1,000-9,999, heavy ≥10,000.
Preservation and lab practice: 10% formalin or PVA for stool, 70% ethanol or AFA for helminth specimens; carmine/haematoxylin whole-mount staining of flukes and proglottids; biosafety, sharps and biomedical-waste segregation under the Bio-Medical Waste Management Rules, 2016 (as amended).

Control, programmes and Indian regulation

Levels of control: source reduction (treating the human reservoir), transmission interruption (vector control), and host protection (hygiene, sanitation, cooking, vaccination where available).
Vector control: IRS (indoor residual spraying - DDT, malathion, synthetic pyrethroids as per susceptibility), LLINs (long-lasting insecticidal nets), larvicides (temephos, *Bti*), biological control with larvivorous fish (*Gambusia*, *Poecilia*), environmental management and the anti-larval provisions of municipal bye-laws. Insecticide-resistance monitoring guides the choice of compound.
Indian programmes: NCVBDC (National Center for Vector Borne Diseases Control, the renamed NVBDCP) covers malaria, lymphatic filariasis, kala-azar, dengue, chikungunya and Japanese encephalitis; National Deworming Day gives biannual albendazole to children and adolescents; MDA/IDA rounds for filariasis; Swachh Bharat Mission sanitation underpins STH control; ASHA-based fever surveillance and free diagnosis-and-treatment.
Veterinary and food side: strategic deworming calendars under state animal-husbandry departments and ICAR institutes; meat inspection for cysticercosis and hydatid at licensed slaughterhouses under FSSAI regulations (Food Safety and Standards Act, 2006 and the Licensing & Registration and Slaughter House regulations); drinking-water quality to IS 10500 - which requires water to be free from pathogenic organisms including protozoan cysts; the Prevention of Cruelty to Animals Act, 1960 and institutional ethics (CPCSEA/IAEC) govern the use of animals in parasitology research.
One Health is the current framing: human, animal and environmental surveillance together, relevant for zoonotic parasites and for antiparasitic resistance.

Key points

Parasitism - an association in which one organism gains at the metabolic expense of another, usually without killing it quickly.
Definitive host - harbours the adult/sexual stage; in malaria that is the *Anopheles* mosquito, not the human.
Heteroxenous cycle - needs two or more hosts; a snail is the first intermediate host of every digenean fluke.
Infective stage - the only stage that can establish infection: *Entamoeba* quadrinucleate cyst, *Ascaris* embryonated egg, hookworm filariform larva, *Plasmodium* sporozoite, *Taenia solium* cysticercus (taeniasis) or egg (cysticercosis).
Schizogony vs sporogony - asexual multiplication in the vertebrate vs sexual development completed in the mosquito.
Hypnozoite - dormant *P. vivax* liver stage causing relapse; needs primaquine for radical cure.
Nocturnal periodicity - *W. bancrofti* microfilariae peak in peripheral blood around 10 pm-2 am, so night smears are taken.
Cysticercosis - caused by swallowing *T. solium* eggs (not pork); neurocysticercosis is a major cause of epilepsy in India.
Hydatid cyst - larval *Echinococcus granulosus*; dog definitive host, sheep/human intermediate; rupture risks anaphylaxis.
Loeffler's migration - hepato-tracheal larval route of *Ascaris*; hookworm larvae make a similar heart-lung-trachea passage but bypass the liver, both causing transient eosinophilic pneumonitis.
Eosinophilia and IgE - hallmark Th2 response to tissue helminths; absent or mild in luminal protozoan infection.
Thick vs thin blood film - thick for detecting parasites, thin for identifying the species.
Kato-Katz - quantitative egg count; multiply eggs on the 41.7 mg template by 24 to get EPG.
MDA/IDA - mass drug administration (DEC + albendazole, or ivermectin + DEC + albendazole) is the filariasis elimination strategy.
rK39 and liposomal amphotericin B - the diagnostic test and first-line treatment of Indian kala-azar; PKDL cases are the human reservoir.
Vector control toolkit - IRS, LLINs, larvicides, larvivorous fish and source reduction, chosen by insecticide-susceptibility data.
Reservoir host - keeps the parasite circulating in nature and defeats control aimed only at humans.
FECRT - faecal egg count reduction test, the field measure of anthelmintic resistance (<95% reduction suggests resistance).

Formulas and examples

Prevalence (%) = (number infected / number examined) × 100 - proportion of a population carrying the parasite at a point in time (dimensionless, %).
Incidence rate = new cases / (population at risk × time) - per 1,000 per year in programme reports.
Mean intensity = total parasites (or eggs) counted / number of hosts infected; Abundance = total parasites / all hosts examined (infected and uninfected).
EPG (Kato-Katz) = eggs counted on slide × 24 - 24 is the factor for the 41.7 mg template (1,000/41.7 ≈ 24); units: eggs per gram of faeces.
EPG (McMaster) = eggs counted in chamber(s) × dilution factor - with the standard 1:15 dilution (2 g faeces in 28 mL) and 0.15 mL per chamber, the factor is × 100 for one chamber and × 50 when both chambers (0.3 mL) are counted.
FECRT (%) = [(pre-treatment mean EPG − post-treatment mean EPG) / pre-treatment mean EPG] × 100 - resistance suspected below about 95% for benzimidazoles.
Parasitaemia (%) = (parasitised RBCs / total RBCs counted) × 100; parasites/µL = (parasites counted / WBCs counted) × patient WBC count/µL, or ≈ parasites per 200 WBC × 40 using an assumed 8,000 WBC/µL.
Vectorial capacity C = m·a²·pⁿ·b / (−ln p) - m = vectors per host, a = bites on humans per vector per day, p = daily survival probability, n = extrinsic incubation period in days, b = transmission efficiency.
Basic reproduction number R₀ = ma²bc·pⁿ / (r·(−ln p)) - r = human recovery rate per day, c = human-to-vector infectivity; transmission persists if R₀ > 1.
Annual Parasite Incidence API = (confirmed malaria cases in a year / population under surveillance) × 1,000 - cases per 1,000 per year; API < 1 is the elimination-programme benchmark.
Slide Positivity Rate SPR (%) = (positive slides / slides examined) × 100; Annual Blood Examination Rate ABER (%) = (slides examined / population) × 100 - ABER of about 10% indicates adequate surveillance.
Microfilaria rate (%) = (persons with mf in night blood / persons examined) × 100 - MDA may stop when mf rate falls below 1%.

Example 1 - Prevalence, intensity and abundance. A survey examines 250 schoolchildren in a block; 80 are positive for *Ascaris*. The total egg count across the 80 positives is 6,40,000 EPG.

Prevalence = 80/250 × 100 = 32%.
Mean intensity = 6,40,000/80 = 8,000 EPG - which falls in the WHO moderate class (5,000-49,999 EPG) for *Ascaris*.
Abundance = 6,40,000/250 = 2,560 EPG per child examined.
Interpretation: prevalence above 20% justifies annual community deworming (above 50%, biannual) under WHO/National Deworming Day criteria.

Example 2 - Kato-Katz to EPG. A Kato-Katz slide prepared with the standard 41.7 mg template shows 310 hookworm eggs.

EPG = 310 × 24 = 7,440 EPG.
Hookworm classes: light 1-1,999, moderate 2,000-3,999, heavy ≥4,000 → heavy infection.
If the laboratory used a 50 mg template instead, the factor would be 1,000/50 = 20, giving 310 × 20 = 6,200 EPG - so the template mass must always be stated.

Example 3 - FECRT in a goat flock. Pre-treatment mean EPG of 20 goats = 1,250; 14 days after albendazole the mean = 150.

FECRT = (1,250 − 150)/1,250 × 100 = 1,100/1,250 × 100 = 88%.
Below the 95% threshold → benzimidazole resistance suspected; rotate to a different class (levamisole or a macrocyclic lactone) and confirm with a larval-development or egg-hatch assay.

Example 4 - Malaria parasite density and API. A thick film shows 160 parasites per 200 leucocytes; the patient's WBC count is 6,000/µL.

Density = (160/200) × 6,000 = 0.8 × 6,000 = 4,800 parasites/µL.
With a red cell count of 4.5 × 10⁶/µL, parasitaemia = 4,800/4.5 × 10⁶ × 100 = about 0.11%.
For the same district, 920 confirmed cases in a population of 4,60,000 gives API = 920/4,60,000 × 1,000 = 2.0 per 1,000 - above the API < 1 elimination benchmark, so the block stays under full IRS and surveillance.

Example 5 - Surveillance quality indicators. A PHC covering 50,000 people examined 4,500 blood slides in a year and found 135 positive.

ABER = 4,500/50,000 × 100 = 9% (near the recommended 10%).
SPR = 135/4,500 × 100 = 3%.
API = 135/50,000 × 1,000 = 2.7 per 1,000 - fever surveillance is adequate but transmission is still active.

Feature	Protozoa	Trematodes (flukes)	Cestodes (tapeworms)	Nematodes (roundworms)
Body	Unicellular	Leaf-like, unsegmented	Ribbon-like, segmented	Cylindrical, unsegmented
Gut	Food vacuoles	Blind, branched	Absent - absorbs via tegument	Complete
Sexes	Asexual + sexual	Hermaphrodite (except Schistosoma)	Hermaphrodite proglottids	Separate sexes
Body cavity	-	Acoelomate	Acoelomate	Pseudocoelomate
Multiplication in host	Yes (schizogony)	In snail only	Larval budding (hydatid)	No multiplication in host
Example	Plasmodium, Entamoeba	Fasciola, Schistosoma	Taenia, Echinococcus	Ascaris, Wuchereria
Drug of choice	Metronidazole, ACT	Praziquantel, triclabendazole	Praziquantel, albendazole	Albendazole, ivermectin, DEC

Parasite	Infective stage	Route	Intermediate host/vector	Diagnosis
E. histolytica	Quadrinucleate cyst	Faeco-oral	None (housefly mechanical)	Stool microscopy, antigen
P. falciparum	Sporozoite	Anopheles bite	Mosquito (definitive)	Giemsa smear, HRP-2 RDT
L. donovani	Promastigote	Sandfly bite	P. argentipes	rK39, aspirate for LD bodies
A. lumbricoides	Embryonated egg	Ingestion of soil-contaminated food	None	Stool egg (Kato-Katz)
Hookworm	Filariform larva	Skin penetration	None	Stool egg, Harada-Mori
W. bancrofti	Third-stage larva	Culex bite	Culex quinquefasciatus	Night blood smear, ICT
T. solium	Cysticercus / egg	Undercooked pork / egg ingestion	Pig	Proglottid, serology, CT/MRI
E. granulosus	Egg from dog faeces	Ingestion	Dog (definitive)	Imaging, serology

Where it is used

Public-health programmes: NCVBDC malaria, filariasis and kala-azar units, district vector-borne disease consultants, entomological surveillance (vector density, insecticide susceptibility), MDA and National Deworming Day coverage monitoring.
Diagnostic laboratories and hospitals: medical laboratory technologists and microbiology/parasitology technicians doing stool concentration, blood films, RDTs and serology in NABL-accredited labs and chains like SRL, Dr Lal PathLabs, Metropolis, and in AIIMS/PGI/CMC-type teaching hospitals.
Veterinary and livestock sector: state animal-husbandry departments, ICAR institutes (IVRI Izatnagar, NIVEDI Bengaluru), dairy cooperatives (Amul, NDDB) running strategic deworming and FECRT-based resistance monitoring; poultry coccidiosis management.
Aquaculture and fisheries: diagnosis of protozoan (*Ichthyophthirius*), monogenean and crustacean (*Argulus*, *Lernaea*) parasites in carp and shrimp farms - a routine fish-health service in Andhra Pradesh and West Bengal.
Pharma and research: antiparasitic screening and formulation at Indian firms making albendazole, DEC, artemisinin combinations and veterinary dewormers; ICMR institutes (NIMR Delhi, RMRIMS Patna, VCRC Puducherry) and molecular diagnostics (PCR/LAMP) development.
Food safety, quarantine and water testing: FSSAI-linked meat inspection for cysticercosis and hydatid, port and animal quarantine, and water-laboratory testing for protozoan cysts under IS 10500.

Interview questions

Q: In malaria, which is the definitive host and why? A: The female *Anopheles* mosquito, because the sexual phase - gametogony, fertilisation and sporogony - is completed in it; the human harbours only the asexual schizogony stages.
Q: How does a person get cysticercosis rather than taeniasis? A: Taeniasis follows eating undercooked pork containing cysticerci; cysticercosis follows swallowing *T. solium* eggs from contaminated food, water or autoinfection, so the human then behaves as an intermediate host.
Q: Why is blood for microfilariae collected at night? A: *W. bancrofti* microfilariae show nocturnal periodicity and appear in peripheral blood mainly between about 10 pm and 2 am, matching the biting habit of *Culex*; daytime smears are often falsely negative.
Q: Why is primaquine needed in *P. vivax* malaria? A: Chloroquine clears only blood stages; primaquine kills the dormant liver hypnozoites and prevents relapse - but it causes haemolysis in G6PD deficiency, so the status must be considered.
Q: What is the difference between a thick and a thin blood film? A: The thick film concentrates many layers of blood for sensitive detection of low parasitaemia; the thin film preserves cell morphology for species identification and parasitaemia counting.
Q: Why do cestodes have no digestive system? A: They live bathed in digested host nutrients and absorb them directly through a microtriche-covered tegument, so a gut would be redundant - a classic parasitic adaptation.
Q: Give two mechanisms by which parasites evade host immunity. A: Antigenic variation (trypanosome VSG, *Plasmodium* PfEMP1 switching) and intracellular or sequestered location; others include host-antigen masking and active immunosuppression.
Q: Why is eosinophilia typical of helminth but not luminal protozoan infection? A: Tissue-invading helminths trigger a Th2 response with IL-4/IL-5, IgE and eosinophil recruitment; luminal protozoa like *Giardia* rarely invade tissue, so that response is not mounted.
Q: How would you decide that a goat flock has anthelmintic resistance? A: Run a FECRT - compare mean EPG before and 10-14 days after treatment; a reduction below about 95% with wide confidence limits indicates resistance, confirmed by egg-hatch or larval-development assays.
Q: What makes PKDL important in kala-azar elimination? A: PKDL patients carry parasites in the skin and are infective to sandflies, so untreated PKDL sustains transmission even when visceral cases fall.
Q: How do you differentiate *Entamoeba histolytica* from *E. coli* on microscopy? A: The mature *E. histolytica* cyst has up to four nuclei with central karyosome and fine peripheral chromatin, while *E. coli* has up to eight nuclei with eccentric karyosome; the invasive trophozoite of *E. histolytica* often contains ingested RBCs.
Q: What does API below 1 mean for a district? A: Fewer than one confirmed malaria case per 1,000 population per year - the programme benchmark at which a district moves from control to elimination-phase strategies with case-based investigation.

Common mistakes

Calling the human the definitive host of *Plasmodium*, or the snail the definitive host of *Fasciola* - the definitive host carries the sexual/adult stage.
Writing that eating pork causes cysticercosis, or that *T. saginata* causes it at all.
Confusing cyst, trophozoite and infective stage in the exam answer; only the quadrinucleate cyst transmits amoebiasis.
Omitting the template mass or dilution factor in EPG calculations, or using × 24 for a non-41.7 mg Kato-Katz template.
Mixing up prevalence (per cent of hosts infected) with intensity (parasites per infected host) and abundance (per host examined).
Drawing the *Ascaris* life cycle without the hepato-tracheal migration, or hookworm without skin penetration.
Quoting withdrawn programme names and old treatment schedules - NVBDCP is now NCVBDC, and malaria drug policy, MDA regimens and kala-azar elimination status are revised periodically and must be checked against the current guideline.