commensalism describes a relationship in which one organism benefits while the other is neither helped nor harmed. for example, certain bacteria live on human skin and feed on dead cells without affecting the host's health.
the definitive host is the organism in which a parasite reaches sexual maturity and reproduces. in tapeworms, for instance, the adult worm develops and produces eggs only after inhabiting its definitive host.
echinococcus granulosus has a two-host life cycle. dogs (and other canids) serve as the definitive hosts where adult tapeworms live in the intestine and release eggs into the environment via feces.
schistosoma haematobium is transmitted by freshwater snails; it is not spread by mosquitoes. infective cercariae emerge from snails and penetrate human skin in contaminated water.
schistosomiasis is acquired when free-swimming cercariae released by infected freshwater snails penetrate intact human skin. once inside, they migrate through the bloodstream to mature in the mesenteric or vesical veins.
trypanosoma brucei is the protozoan parasite responsible for african sleeping sickness. it is transmitted to humans by the tsetse fly (genus glossina) during blood meals.
a kinetoplast is a specialized mitochondrial DNA-containing organelle found in certain flagellates; trypanosoma cruzi, the agent of american trypanosomiasis, possesses this structure.
the statement that african trypanosome causes chagas disease is incorrect. african trypanosomiasis is caused by trypanosoma brucei, whereas chagas disease results from trypanosoma cruzi.
trypanosoma brucei evades the host immune system through antigenic variation of its variant surface glycoprotein (vsg) coat. by periodically switching vsg gene expression, it avoids antibody-mediated clearance.
ascaris lumbricoides, the large intestinal roundworm, is transmitted via ingestion of embryonated eggs in contaminated food or water; it does not require an arthropod vector.
dracunculus medinensis causes guinea worm disease, not onchocerciasis. onchocerciasis (river blindness) is caused by onchocerca volvulus and transmitted by blackflies.
wuchereria bancrofti is a filarial nematode that causes lymphatic filariasis. adult worms lodge in lymphatic vessels, leading to inflammation, lymphedema, and elephantiasis.
the incorrect statement that wuchereria bancrofti infection spreads through a snail is false; transmission occurs via infected mosquito bites (anopheles or culex species), not snails.
echinococcus is a cestode (tapeworm), not a nematode (roundworm). its small adult form infects canids, while hydatid cysts develop in intermediate hosts like sheep or humans.
in the schistosoma life cycle, the cercaria is the infective stage for humans. cercariae emerge from snails, swim in freshwater, and penetrate human skin to continue development.
the incorrect assertion that plasmodium species use glossina (tsetse flies) as vectors is false. malaria parasites are transmitted by anopheles mosquitoes, not tsetse flies.
in the plasmodium falciparum life cycle, hypnozoites remain dormant in hepatocytes; this is actually characteristic of plasmodium vivax and ovale. falciparum does not form hypnozoites.
american trypanosomiasis (chagas disease) can present with a chagoma—an inflammatory nodule at the site of parasite entry—or romana's sign (unilateral periorbital swelling).
nematodes do not have trophozoite and cyst stages; those are protozoan life stages. nematodes exhibit egg, larval (several molts), and adult stages.
echinococcus granulosus is a cestode (class cestoda), characterized by a segmented body (proglottids) and absence of a digestive tract.
• parasitism: one organism (the parasite) benefits at the expense of the other (the host), as seen with plasmodium infecting red blood cells and causing malaria.
• commensalism: one organism benefits while the other is unaffected, such as barnacles attaching to a whale's skin without harming it.
• mutualism: both organisms benefit, for example, gut bacteria that synthesize vitamins for their host and in turn receive nutrients and habitat.
• host: any organism that harbors a parasite; it may provide habitat or nutrients.
• definitive host: where the parasite undergoes sexual reproduction (e.g., dog for echinococcus).
• intermediate host: harbors larval or asexual stages (e.g., sheep for echinococcus hydatid cysts).
• vector: an arthropod or other animal that transmits the parasite between hosts without undergoing parasite development (e.g., mosquito transmitting plasmodium).
• reservoir: a species or environment where the parasite normally lives and multiplies, maintaining infection risk for other hosts (e.g., rodents for leishmania).
metazoa are multicellular, eukaryotic animals that include helminths such as nematodes and cestodes. protozoa are single-celled eukaryotes, often motile, such as giardia, trypanosoma, and plasmodium.
a kinetoplast is a dense region of mitochondrial DNA found in kinetoplastid protozoa; it contains maxicircles and minicircles of DNA. organisms possessing a kinetoplast include trypanosoma brucei, trypanosoma cruzi, and leishmania species.
american trypanosomiasis (chagas disease) is caused by trypanosoma cruzi. transmission occurs when triatomine bugs (reduviid or "kissing" bugs) defecate infected feces near a bite site; the parasite enters through the wound or mucous membranes.
Trypanosoma cruzi has a complex life cycle involving three main morphological stages. infection begins when a reduviid bug takes a blood meal and defecates on the skin. metacyclic trypomastigotes in the feces enter through the bite wound or mucous membranes. inside the human host, they invade various cells—especially cardiac and smooth muscle—transforming into intracellular amastigotes that multiply by binary fission. released trypomastigotes circulate in the bloodstream until taken up by another bug. in the vector's midgut, they differentiate into epimastigotes, proliferate, then transform into infective metacyclic trypomastigotes in the hindgut, completing the cycle .
american trypanosomiasis progresses through three clinical stages. the acute phase (weeks to months) features non-specific febrile illness, chagoma or romana's sign (periorbital swelling), and lymphadenopathy. the indeterminate phase (years to decades) is largely asymptomatic but with persistent low-level parasitemia. the chronic phase can cause cardiomyopathy—arrhythmias, heart failure—megaesophagus, and megacolon due to destruction of autonomic ganglia .
african trypanosomiasis ("sleeping sickness") is caused by Trypanosoma brucei gambiense in west/central africa or T. b. rhodesiense in east/southern africa. the parasite is transmitted to humans via the bite of an infected tsetse fly (Glossina spp.), which injects metacyclic trypomastigotes into the skin during feeding .
in Trypanosoma brucei's life cycle, metacyclic trypomastigotes enter the human host via a tsetse fly bite and transform into bloodstream trypomastigotes. these multiply by binary fission in blood and lymph. when a tsetse takes another blood meal, trypomastigotes enter its midgut and become procyclic trypomastigotes. they migrate to the salivary glands, differentiate into epimastigotes, then into infective metacyclic trypomastigotes, ready to infect the next human host .
stage 1 (hemolymphatic) of sleeping sickness involves parasite multiplication in subcutaneous tissues, blood and lymph, causing fever, lymphadenopathy (notably Winterbottom's sign in gambiense infection), and malaise. stage 2 (meningoencephalitic) occurs when trypanosomes cross the blood–brain barrier, leading to neurological symptoms: confusion, sleep disturbances, tremors, and eventual coma .
variant surface glycoprotein (VSG) is a dense coat on Trypanosoma brucei that undergoes antigenic variation. by periodically switching VSG gene expression, the parasite evades host antibody responses and sustains chronic infection .
trophozoites are the active, feeding, motile stages of protozoan parasites, generally found in host tissues or fluids. cysts are dormant, resistant forms with a protective wall, allowing survival in harsh environments and transmission between hosts .
matching statements:
• Entamoeba histolytica: uses pseudopods (E), trophozoites bind mucosa (I), can invade via bloodstream causing liver abscesses (H)
• Giardia lamblia: flagellated protozoan (F), cyst contains four nuclei (A), often asymptomatic (G)
• Toxoplasma gondii: worldwide prevalence (B), poses particular risk in pregnancy (C), no independent locomotion, exclusively parasitic (D) .
nematodes are unsegmented, bilaterally symmetrical, with a pseudocoelom and tubular digestive tract. they shed a cuticle via ecdysis and are usually dioecious. examples include Ascaris lumbricoides, Trichuris trichiura, and Ancylostoma duodenale .
the Guinea worm (Dracunculus medinensis) life cycle begins when humans drink water containing infected copepods. larvae are released in the stomach, penetrate the intestinal wall, and mature in subcutaneous tissues. about a year later, the gravid female migrates to the skin surface, forms a painful blister, and emerges when the host enters water, releasing larvae to infect new copepods .
dracunculiasis is characterized by intense pain and ulceration at the worm emergence site, secondary bacterial infections, and debilitating skin lesions. adult females reach lengths up to 1 meter .
treatment involves gradual extraction of the emerging worm by winding it around a small stick over days to weeks, avoiding breakage. prevention relies on filtering drinking water, using larvicides to kill copepods, and educating communities to avoid entering water sources when lesions are present .
matching soil-transmitted helminths:
• Ascaris lumbricoides: adult worms (25–35 cm) in small intestine (F), lung migration causes Löffler's pneumonitis (H), light infections often asymptomatic (I)
• Trichuris trichiura: adult worms (3–5 cm) in large intestine (A), causes diarrhea, rectal prolapse, anemia, malnutrition, cognitive deficits (G, B)
• Ancylostoma duodenale: infective larvae penetrate skin (E), adult worms attach via buccal capsule to mucosa (D), cause mechanical GI blood loss, malnutrition, anemia (C) .
filarial worms (e.g., Wuchereria, Onchocerca) require insect vectors and have tissue or lymphatic localization, producing microfilariae circulating in blood or skin. gastrointestinal nematodes (e.g., Ascaris, Trichuris) inhabit the gut lumen and are transmitted via ingestion of eggs or skin penetration without vectors .
two types of filariasis:
• lymphatic filariasis (e.g., Wuchereria bancrofti, Brugia malayi): adult worms in lymphatics cause lymphedema and elephantiasis
• subcutaneous filariasis (e.g., Onchocerca volvulus, Loa loa): adult worms in skin or conjunctiva cause dermatitis, ocular lesions, or Calabar swellings .
vector associations:
Trypanosoma cruzi – reduviid bug (Triatoma infestans)
Trypanosoma brucei – tse-tse fly (Glossina)
Plasmodium falciparum – Anopheles mosquitoes
Wuchereria bancrofti – Culex mosquitoes
Onchocerca volvulus – Simulium (black flies)
Loa loa – Chrysops (deer flies) .
lymphatic filariasis is caused by Wuchereria bancrofti (and sometimes Brugia spp.). three clinical stages are: asymptomatic microfilaremia, acute inflammatory episodes (lymphangitis, fever), and chronic lymphedema/elephantiasis. pathology includes lymphatic obstruction, vessel dilation, and tissue hypertrophy .
cestodes (tapeworms) are dorso-ventrally flattened, segmented (proglottids), lack a digestive tract, and attach via a scolex with suckers/hooks. examples include Taenia solium, Taenia saginata, and Echinococcus granulosus .
echinococcosis is caused by Echinococcus granulosus. dogs are the definitive hosts harboring the adult tapeworm in their intestines. humans become accidental intermediate hosts by ingesting eggs from dog feces. disease manifests as hydatid cysts, chiefly in liver or lungs, causing mass effect or cyst rupture .
Echinococcus granulosus adults are small (3–6 mm) with a scolex bearing four suckers and a rostellum of hooks, a short unsegmented neck, and three proglottids: immature, mature (with reproductive organs), and gravid (filled with eggs) .
Plasmodium life cycle: sporozoites injected by Anopheles mosquito invade hepatocytes and form schizonts. schizonts release merozoites, infecting erythrocytes and developing through ring, trophozoite, and schizont stages. some merozoites differentiate into male/female gametocytes taken up by another mosquito, where they undergo sexual reproduction: gametes fuse to form ookinetes, develop into oocysts on the gut wall, and release sporozoites to the salivary glands .
tertian malaria involves fevers every 48 hours (P. vivax, P. ovale, P. falciparum sometimes), whereas quartan malaria has fevers every 72 hours (P. malariae) .
pfemp1 is a variant surface antigen expressed on P. falciparum–infected erythrocytes. it mediates cytoadherence to host endothelium, placental tissues, and uninfected erythrocytes, facilitating sequestration, immune evasion, and complicated malaria syndromes .
Schistosoma life cycle: eggs excreted in human urine or feces hatch in water, releasing miracidia that infect specific snail hosts. within snails, they develop into sporocysts and produce cercariae, which emerge and penetrate human skin, lose tails to become schistosomulae, enter circulation, mature in mesenteric or vesical veins, and produce eggs to continue the cycle .
Schistosoma evasion mechanisms include:
• tegumental antigen masking by host proteins to avoid detection
• rapid renewal of the surface membrane to shed bound antibodies
• secretion of proteases that degrade host immune molecules
• modulation of host cytokine responses through immunomodulatory excretory–secretory products .