🧭 Framework & Terms
| Protozoa | Metazoa (helminths) | |
|---|---|---|
| Organization | Single cell | Multicellular |
| Symptom onset | Days to weeks | Often over a month |
| Diagnostic forms | Trophozoites, cysts, oocysts | Eggs, larvae, adult worms |
| Typical white-cell response | Neutrophils | Eosinophils |
Protozoa groups
- Amebae
- Entamoeba, Naegleria, Acanthamoeba
- Flagellates
- Luminal: Giardia, Trichomonas
- Blood and tissue: Trypanosoma, Leishmania
- Apicomplexa
- Blood and tissue: Plasmodium, Toxoplasma, Babesia
- Intestinal: Cryptosporidium, Cystoisospora, Cyclospora
Helminth groups
- Nematodes = roundworms
- Platyhelminths
- Trematodes = flukes
- Cestodes = tapeworms
- Arthropods
- Serve as vectors or intermediate hosts
Definitions
- Definitive host = adult or sexual stage lives here
- Intermediate host = larval or asexual stage lives here
- Mechanical vector = carries parasite without development
- Biologic vector = parasite completes part of its cycle inside
- Infective stage = cysts (protozoa), eggs or larvae (helminths)
💧 Gut, Urogenital & Free-Living Protozoa
| Organism | Disease features | Transmission | Diagnosis | Treatment |
|---|---|---|---|---|
| Entamoeba histolytica | Bloody, mucoid dysentery; flask-shaped colonic ulcers; liver abscess; rare lung, brain, heart spread | Cysts, fecal-oral (water, produce) | Trophozoites or cysts in stool; nucleus with central karyosome and spoke-like chromatin; serology | Metronidazole, then paromomycin |
| Giardia lamblia | Foul, fatty diarrhea; malabsorption (duodenum, jejunum); sucking disk adheres to mucosa | Cysts; water, day care, wild animals, oral-anal sex | Pear-shaped binucleate trophozoites; cysts; stool antigen; "falling leaf" motility | Metronidazole |
| Cryptosporidium parvum | Self-limited diarrhea; severe in AIDS | Oocysts in water (chlorine-resistant), undercooked food | Acid-fast oocysts in stool | Nitazoxanide (no fully effective drug) |
| Cystoisospora belli | Giardia-like diarrhea in AIDS | Fecal-oral oocysts | Acid-fast elliptical oocysts | TMP-SMX |
| Cyclospora cayetanensis | Prolonged diarrhea, worse in AIDS | Imported produce, water | Acid-fast spherical oocysts; UV fluorescence | TMP-SMX |
| Microsporidia | Chronic diarrhea in AIDS; other species cause hepatitis, neurologic disease | Spores ingested | Gram-positive, acid-fast spores in stool or biopsy | No proven therapy (albendazole used) |
| Trichomonas vaginalis | Often silent; frothy vaginal discharge; strawberry cervix | Sexual, trophozoite only | Motile trophozoites on wet mount, corkscrew motion | Metronidazole (treat partner) |
Free-living amebae
| Organism | Syndrome | Exposure | Diagnosis | Treatment |
|---|---|---|---|---|
| Naegleria | Primary amebic meningoencephalitis: severe frontal headache, fever, nausea, smell disturbance; usually fatal | Diving or swimming in warm fresh water | Motile trophozoites in CSF; culture trails on bacteria-seeded agar | Amphotericin B (rarely successful) |
| Acanthamoeba | Keratitis; granulomatous amebic encephalitis in immunocompromised (slow, fatal) | Contaminated contact-lens solution; soil or water | Star-shaped cysts on biopsy; culture | Keratitis: topical miconazole and propamidine; encephalitis: azoles (poor outcome) |
Exam traps
- Acid-fast oocysts in an AIDS patient with diarrhea point to Cryptosporidium, Cystoisospora or Cyclospora.
- Naegleria enters through the cribriform plate; Acanthamoeba is the contact-lens organism.
🦟 Malaria (Plasmodium)
Life cycle
Anopheles bite injects sporozoites→Liver schizont→Merozoites enter RBC→Ring → trophozoite → schizont→RBC lysis, fever
- Some merozoites become gametocytes
- Taken up by mosquito
- Sexual phase and sporogony occur in the mosquito
- Human host: asexual phase (schizogony) in liver and red cells
- Hypnozoites persist in liver of P. vivax and P. ovale
Pathogenesis
- Hemoglobin consumed, infected cells lysed → hemolytic anemia
- Synchronized lysis → chills, fever spike, sweats (paroxysm)
- Splenomegaly
- P. falciparum infected cells stick to vessels → cerebral malaria
- Host resistance
- Sickle trait protects against falciparum
- Duffy antigen is the receptor for P. vivax
- Thalassemia and other hemoglobin variants impair growth
| Species | Fever pattern | Blood smear | Liver stage | Treatment |
|---|---|---|---|---|
| P. vivax | Every 48 h | Enlarged RBCs, ameboid trophozoites | Hypnozoites → relapse | Chloroquine, then primaquine |
| P. ovale | Every 48 h | Oval, fimbriated RBCs | Hypnozoites → relapse | Chloroquine, then primaquine |
| P. malariae | Every 72 h | Band forms, rosette schizonts | None (recrudescence) | Chloroquine |
| P. falciparum | Irregular, severe | Multiple rings per cell, crescent gametocytes | None (recrudescence) | Chloroquine if sensitive; atovaquone-proguanil, artemether-lumefantrine or mefloquine if resistant; IV artesunate (or quinine/quinidine) for severe |
| P. knowlesi | Every 24 h | Resembles P. malariae | None | Chloroquine |
Relapse vs recrudescence
- Relapse = clinical return from liver hypnozoites
- Recrudescence = return from low-level red-cell parasites
Treatment goals
- Suppressive prophylaxis
- Therapeutic clearance of red-cell forms
- Radical cure of hypnozoites (primaquine)
- Gametocide to block transmission
Clinical warnings
- Check G6PD before primaquine because hemolysis can occur.
- Falciparum with altered mentation or high parasitemia is an emergency.
🩸 Hemoflagellates
Trypanosomes
- Blood: trypomastigote (flagellum, undulating membrane)
- Tissue: amastigote (oval, no flagellum)
Leishmania
- Always amastigotes inside macrophages
- Sandfly vector
| Organism | Disease | Vector / route | Reservoir | Diagnosis | Treatment |
|---|---|---|---|---|---|
| Trypanosoma cruzi | Acute: periorbital swelling (Romaña sign), myocarditis, meningoencephalitis. Chronic: dilated cardiomyopathy, megacolon, achalasia | Reduviid (kissing) bug feces rubbed into bite or mucosa; transfusion risk | Dogs, cats, armadillos, opossums | Trypomastigotes on blood film | Benznidazole or nifurtimox |
| T. brucei gambiense / rhodesiense | African sleeping sickness; antigenic variation | Tsetse fly bite | Humans, wild animals | Trypomastigotes in blood or CSF; high CSF IgM | Early: suramin; CNS: melarsoprol |
| Leishmania donovani complex | Visceral disease (liver, spleen, marrow) | Sandfly | Humans; rodents, canines | Amastigotes in macrophages of marrow, liver, spleen | Stibogluconate (amphotericin B alternative) |
| Leishmania (cutaneous species) | Skin ulcer (Oriental sore) | Sandfly | Humans, rodents | Amastigotes in lesion | Stibogluconate |
| Leishmania braziliensis complex | Mucocutaneous destruction | Sandfly | Rodents, wild animals | Amastigotes in lesion | Stibogluconate |
High-yield
- Chagas heart and gut disease reflects destruction of cardiac and enteric neurons.
- Antigenic variation lets sleeping-sickness trypanosomes evade antibody.
🐈 Toxoplasma & Babesia
Toxoplasma gondii
- Cat = definitive host
- Other animals = intermediate hosts
- Acquisition
- Undercooked meat (pork common)
- Cat feces contact
- Transplacental
- Diagnosis: serology (high or rising IgM = acute)
- Treatment: pyrimethamine plus sulfadiazine
Babesia microti
- Ixodes tick (same range as Lyme disease)
- Coinfection with Borrelia possible
- Hemolytic, malaria-like illness
- Smear: small rings, tetrad "Maltese cross" form
- Treatment: clindamycin plus quinine
Toxoplasmosis syndromes
| Host | Presentation |
|---|---|
| Healthy | Silent or mononucleosis-like (heterophile-negative) with nodes; latent bradyzoite cysts persist |
| Pregnancy, early transmission | Severe congenital disease: intracranial calcifications, chorioretinitis, hydrocephalus or microcephaly, seizures |
| Pregnancy, late transmission | Often silent at birth; vision loss may appear years later |
| Prior maternal immunity | Protects the fetus |
| AIDS | Reactivation; leading cause of focal brain lesions; ring-enhancing masses; prophylaxis needed |
🪱 Nematodes (Roundworms)
- Cylindrical, unsegmented
- Three entry routes
- Egg ingestion: Enterobius, Ascaris, Trichuris
- Larval ingestion in meat: Trichinella
- Skin penetration: Necator, Ancylostoma, Strongyloides
- Insect bite: Wuchereria, Brugia, Loa loa, Onchocerca
Egg-transmitted
| Worm | Features | Diagnosis | Treatment |
|---|---|---|---|
| Enterobius vermicularis (pinworm) | Most common US helminth; night perianal itch; autoinfection | Cellophane tape prep; flattened-side eggs | Albendazole or pyrantel; treat household |
| Trichuris trichiura (whipworm) | Cecal infection; rectal prolapse | Barrel-shaped eggs with polar plugs | Albendazole |
| Ascaris lumbricoides | Most common helminth worldwide; lung migration then small-bowel adults 15–30 cm; obstruction, biliary blockage | Bile-stained, knobby eggs | Albendazole; supportive for pneumonitis; surgery for ectopic worms |
| Toxocara canis / cati | Visceral larva migrans; larvae wander and inflame tissue; pica, puppies | Clinical picture, serology | Albendazole; often self-limited |
Larva-transmitted
| Worm | Features | Diagnosis | Treatment |
|---|---|---|---|
| Necator americanus (hookworm) | Larvae through bare feet → lung → gut; blood loss → iron-deficiency anemia | Eggs in stool; occult blood | Albendazole plus iron |
| Ancylostoma braziliense / caninum | Cutaneous larva migrans: itchy serpiginous track; larvae cannot mature in humans | Exposure history | Albendazole or ivermectin; topical steroids for itch |
| Strongyloides stercoralis | Skin entry; pneumonitis, abdominal pain, diarrhea; autoinfection lasts for decades; hyperinfection in immunosuppression | Larvae in stool, serology | Ivermectin |
| Trichinella spiralis | Larvae in undercooked pork or game encyst in muscle; fever, myalgia, periorbital edema, splinter hemorrhages, eosinophilia | Muscle biopsy, serology | Albendazole; steroids if severe |
Filarial (vector-borne)
| Worm | Disease | Vector | Diagnosis | Treatment |
|---|---|---|---|---|
| Wuchereria bancrofti, Brugia malayi | Lymphatic obstruction, elephantiasis; tropical pulmonary eosinophilia | Mosquito | Microfilariae in blood; eosinophilia | Diethylcarbamazine (± ivermectin) |
| Loa loa | Itching, transient Calabar swellings; worm crosses the eye | Deer fly (Chrysops) | Microfilariae in blood | Diethylcarbamazine; surgical worm removal |
| Onchocerca volvulus | River blindness; itchy skin rash | Blackfly | Skin snips | Ivermectin |
| Dracunculus medinensis (guinea worm) | Painful skin ulcer where worm emerges | Water containing infected copepods | Visible worm; raised IgE | Slow extraction on a stick; wound care |
Exam traps
- Steroids in an unrecognized Strongyloides carrier can trigger fatal hyperinfection.
- Pinworm eggs are collected from perianal skin, not stool.
🐌 Trematodes (Flukes)
- Leaf-shaped, flat
- First intermediate host = snail
- Hermaphroditic, except Schistosoma (separate sexes)
- Operculated eggs, except Schistosoma
- Drug of choice for nearly all: praziquantel
Schistosomes
Freshwater cercariae→Skin penetration, itch→Adults in veins→Egg granulomas
| Species | Vein site | Consequences |
|---|---|---|
| S. mansoni, S. japonicum | Mesenteric veins | Hepatic granulomas, portal pipestem fibrosis, hepatosplenomegaly, portal hypertension |
| S. haematobium | Bladder venous plexus | Hematuria, bladder fibrosis; squamous cell bladder cancer in Africa and Egypt |
| Nonhuman (bird) schistosomes | None; die in skin | Swimmer's itch dermatitis; treat symptoms |
Foodborne flukes
| Fluke | Acquired from | Disease |
|---|---|---|
| Clonorchis sinensis (Chinese liver fluke) | Raw freshwater fish | Biliary disease; cholangiocarcinoma risk |
| Fasciola hepatica (sheep liver fluke) | Watercress | Liver inflammation |
| Fasciolopsis buski (giant intestinal fluke) | Water chestnuts | Diarrhea, malabsorption |
| Paragonimus westermani (lung fluke) | Raw crab or crayfish | TB-like cough, hemoptysis; eggs in sputum |
High-yield
- Schistosoma eggs have a lateral or terminal spine and no operculum.
🎗️ Cestodes (Tapeworms)
Anatomy
- Scolex (head) with suckers or grooves, sometimes hooks
- Neck produces proglottids
- Strobila = neck plus segments
- Each proglottid is hermaphroditic; mature eggs in distal ones
Biology
- No gut; absorb host nutrients
- Diagnosis: eggs or proglottids in stool
- Larval disease (human as intermediate host) is more serious than adult intestinal infection
| Cestode | Humans are | Acquisition | Disease | Treatment |
|---|---|---|---|---|
| Taenia saginata (beef) | Definitive | Undercooked beef cysticerci | Mild or silent intestinal worm | Praziquantel |
| Taenia solium (pork), adult | Definitive | Undercooked pork cysticerci | Intestinal worm | Praziquantel |
| Taenia solium, larval | Intermediate | Ingested eggs (fecal-oral, autoinfection) | Cysticercosis: brain, eye, muscle; adult-onset seizures | Albendazole or praziquantel; surgery at some sites |
| Diphyllobothrium latum (fish) | Definitive | Raw freshwater fish | Very long worm; vitamin B12 deficiency, megaloblastic anemia | Praziquantel |
| Sparganosis (Spirometra larvae) | Intermediate | Copepod-contaminated water, snake or frog poultices | Subcutaneous migrating larva | Surgical removal |
| Echinococcus granulosus | Intermediate | Eggs from dog feces; sheep-raising regions | Hydatid cysts in liver, lung with brood capsules | Albendazole plus surgery |
| Echinococcus multilocularis | Intermediate | Eggs from fox or canine feces | Alveolar hydatid disease, tumor-like liver growth | Surgical resection |
Exam traps
- Cysticercosis comes from swallowing T. solium eggs, not from eating the cysts.
- Spilling a hydatid cyst can cause anaphylaxis and seeding.
Clinical pearl
- Eosinophilia points to a tissue-invading helminth rather than a protozoan.