PROTOZOA
These are parasitic unicellular organisms that have an outer cell membrane, with membrane covering nuclei and organelles. Their sizes span from 1µm to 50µm. Below are a few examples:
Plasmodium species
The plasmodium species are protozoa that include: P. falciparum; P. ovale; P. malariae; P. vivax. They are the cause of malaria, and their transmission is by bite of female Anopheles mosquito. P. falciparum causes severe malaria.
The life cycle of plasmodium spp.: Sporozoites in blood (after female Anopheles’ mosquito bite); merozoites formation and release (in hepatocytes); hypnozoites formation and latent infection in hepatocytes (especially by P. vivax & P. ovale); merozoites (enter into RBC); trophozoites (stage 1 in RBC); schizont (stage 2 in RBC); merozoites or gametocytes (stage 3 in RBC); gametocytes (infects mosquitoes).
P. ovale/vivax/malariae cause mild anemia; mild parasitemia; splenic rupture; nephrotic syndrome.
P. falciparum causes severe parasitemia; severe anemia; cerebral malaria; pulmonary edema; hepatosplenomegaly; death.
Microscopic examination of blood smears is essential for diagnosis.
Babesia microti
This is a parasitic protozoan that cause babesiosis. Transmission occurs from white-footed mouse to humans by deer tick bite. It infects red blood cells and transmission can occur through blood transfusion.
Clinical manifestations: erythrophagocytosis; anemia; jaundice; hypoxia; hepatic & acute renal tubular necrosis; adult respiratory distress syndrome; visceral hemorrhages.
Leishmania species (Spp.)
These are obligate intracellular parasitic kinetoplastid protozoa, that cause leishmaniasis. Transmission occurs from dogs, rodents, or foxes to humans by sandflies bite. Lifecycle include: promastigote (in sandfly’s salivary gland & host’s tissue); amastigote (in host’s macrophage).
Clinical implications: fever; weight loss; indurated ulcerative skin lesion; ulcerative or papular nasopharyngeal lesions; pancytopenia; hepatosplenomegaly; extensive nodular skin lesions.
Trypanosoma brucei rhodesiense, Trypanosoma b. gambiense
These are extracellular kinetoplastid protozoa that cause African Trypanosomiasis. Transmission occurs from domestic, wild animals, or humans to humans by tsetse flies’ bite. Trypomastigote leaves tsetse flies salivary gland to infect blood plasma of humans.
Clinical implications: fever; cachexia; lymphadenopathy; panencephalitis; sleeping sickness; splenomegaly; death.
Trypanosoma cruzi
This is an intracellular kinetoplastid protozoan that causes the American Trypanosomiasis (Chagas disease). Transmission occurs from rodents, cats, dogs to humans through the feces of infected kissing bugs on mucous membranes or damaged skin.
Lifecycle: Trypomastigote (from mucous membrane, enters macrophage); amastigote (intracellular stage in host).
Clinical implications: splenomegaly, lymphadenopathy; acute myocarditis; cardiomegaly; dilated cardiomyopathy; mural thrombi.
FUNGI
Fungi are eukaryotes with polysaccharide (chitin) cell wall, and lipid (ergosterol) cell membrane. They may exist as rounded yeast cells, or mold, a short threadlike filament (hyphae). Hyphae commonly produce conidia (sexual spores), and can exist either as septate or aseptate. Their sizes span from 2µm to 200µm. Below are a few examples:
Candida albicans
These are yeast cells that causes candidiasis, & are the most frequent cause of human fungal infections. They are normal flora found in mouth; G.I tract; vagina; skin.
Clinical implications: oral thrush (gray-white oral pseudo membrane); esophagitis; onychomycosis (nail infection); paronychia; Diaper rash; vaginitis (itchy & thick-white vaginal discharge); balanitis (penile cutaneous infection); endocarditis; myocardial abscess; liver abscess; meningitis; brain abscess; endophthalmitis.
Cryptococcus neoformans
This is a yeast cell that causes cryptococcosis. It is present in soil and pigeon droppings, and transmission occurs by inhalation.
In people with defunct immunity, including AIDS, leukemia, lymphoma patients, it causes an opportunistic infection leading to meningoencephalitis.
Aspergillus fumigatus
This is a mold, that causes aspergillosis, and whose transmission occurs by inhalation. It is common everywhere & is the cause of allergic bronchopulmonary aspergillosis (coughing, wheezing, hemoptysis) in immunocompetent people.
People with defunct immunity present in addition with severe sinusitis, necrotizing pneumonia, neutropenia.
Zygomycetes (e.g. mucor, rhizopus)
These are bread mold fungi that cause zygomycosis, and whose asexual spores infect humans by inhalation, ingestion, & damaged skin. They are found everywhere, and cause disease especially in people with immunosuppression.
Clinical implications: sinusitis, meningoencephalitis; cerebral infarction; hemorrhagic pneumonia.
HELMINTHS
Helminths are multicellular organisms, that may reproduce sexually in a definitive host or multiply asexually in an intermediary host or vector. They span from 3mm to 10m. Below are a few examples:
Strongyloides stercoralis
This is a parasitic helminth that causes strongyloidiasis. Transmission occurs when larvae present in soil penetrates host’s skin. Larvae enter venous circulation to lungs from where it ascends up for ingestion into small intestine.
Adult females in intestinal mucosa reproduce eggs asexually, which form larvae that is excreted into soil.
Clinical implications: diarrhea; nausea; bloating, malabsorption.
Taenia solium, Echinococcus granulosus
These are parasitic helminths of the class cestodes, that cause cysticercosis and hydatid disease respectively.
T. solium Infection in human begins after eating cysticerci (larva cysts) from undercooked pork meat. cysticerci develop into adult tape worms in the small intestine and shed eggs in faeces. Adult tapeworms cause nausea, vomiting, stomach ache.
By fecal-oral route humans ingest eggs which hatch into larvae that penetrate intestinal wall. Larvae invade organs including brain, skeletal muscle, skin, heart where they encyst (cysticerci). Cysticerci in the brain would increase intracranial pressure, also cause convulsion & neurological disturbances.
E. granulosus infection occurs when humans ingest eggs in food or water shed by dogs or foxes. Eggs hatch into larvae that penetrate intestinal wall. Larvae invade organs including liver, lungs, or bones where they encyst (cysticerci). Clinical manifestations may include: jaundice; nausea; vomiting; abdominal pain.
Trichinella spiralis
This is a parasitic nematode that causes trichinosis. Infection in humans begins after ingesting larvae from undercooked meat from pigs, horses, boars. Larvae develop into adult male and female that also mate to reproduce larvae that penetrate gut to other tissues.
T. spiralis larvae mainly encyst in skeletal muscles. They cause: fever; myalgia; periorbital edema; dyspnea; encephalitis; cardiac failure.
Schistosoma mansoni & S. japonicum, S. haematobium
These are helminthic parasitic blood-flukes, that cause schistosomiasis. Transmission occurs from swimming in lakes or rivers containing freshwater snails from which infectious larvae (cercariae) develop from.
In S. mansoni and S. japonicum, cercariae penetrates skin and then through lungs, migrate to hepatic capillaries (sinusoids) and mature into adult male-female worms that mate. The females produce many eggs in liver parenchyma that cause: acute hepatitis and chronic liver fibrosis.
S. haematobium adult male-female worms that mature in hepatic capillaries, migrate to pelvic venous system. The females lay eggs that cause: cystitis; bladder & ureteral fibrosis; hydronephrosis; chronic pyelonephritis; squamous cell bladder carcinoma.
Wuchereria bancrofti, Brugia species
These are parasitic helminthic nematodes, that cause lymphatic filariasis. Infection in humans occurs through mosquito bite (e.g. culex or mansonia mosquito). Mosquito bite releases infective larvae into circulation that develop into adult male-female filariae within lymphatic vessels.
The adult parasites mate & reproduce microfilariae, & also cause destruction of lymphatic vessels.
Clinical manifestations: asymptomatic microfilaremia; tropical pulmonary eosinophilia; elephantiasis (subcutaneous fibrosis; epithelial hyperkeratosis; genital & extremity lymphedema).
Onchocerca volvulus
This is a parasitic nematode that causes onchocerciasis. Infection in humans begins after blackfly bite. The blackfly injects microfilariae (larvae) into subcutaneous tissue.
In the dermis, larvae mature into adult male-female parasites that mate to reproduce microfilariae. Microfilariae cause subcutaneous inflammation and also migrate to eye chambers through the blood circulation.
Clinical implications: itchy dermatitis; lizard skin (focal darkening or loss of skin pigment); keratitis; iridocyclitis; glaucoma; loss of vision.
ECTOPARASITES
These are arthropods that cause damage to the host or serve as vectors for infectious agent transmission. They include: lice; bedbugs; fleas; mites; ticks; spiders.