What Are Protozoans?
In the taxonomic hierarchy, the kingdom Protista stands unique as its diverse families, although resembling, are neither plants, animals, or fungi. Protozoans are considered a sub-kingdom with more than 50,000 described species and are diversely free-living or parasitic. Fossil records place the organism even in the Precambrian era (4.6 billion years ago to 538.8 million years ago). All protozoans are holozoic in nature, meaning they feed on organic compounds. Protozoans exhibit complex life cycles with multiple hosts and multiple vectors. Some species cause mild infections while others might be life-threatening, yet other species might be opportunistic, affecting only immunocompromised individuals.
What Are the Most Common Protozoal Diseases in Humans?
The most common protozoal diseases in humans are
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Malaria.
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Giardiasis.
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Amoebiasis.
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Leishmaniasis.
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Sleeping sickness.
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Trichomoniasis.
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Toxoplasmosis.
How Does Malaria Occur?
Malaria is one of the most devastating protozoal infections in developing and economically backward countries. The disease also puts a tonne of weight on the health care system of the said countries. It is caused by the protozoan family of Plasmodium. The species responsible for malaria in humans are Plasmodium falciparum, P. vivax, P. malariae, and P. ovale, while P. knowlesi is etiologic to zoonotic malaria. Malaria accounts for 500 million cases, with 1.5 to 2.7 million deaths worldwide.
The protozoal parasite is transmitted through the mosquito vector (female anopheles) among humans, incubates for several days (varies for each plasmodium species), and creates the classic malaria paroxysm of rigors, fevers, diaphoresis (sweating), and drop in core body temperature. The infection can be detected by a rapid diagnostic test kit, microbiologic evaluation of peripheral blood smear, or PCR (polymerase chain reaction) test.
The patients are treated via pharmacotherapy for presenting symptoms along with antimalarial drugs like Chloroquine, Hydroxychloroquine, Primaquine, Artemisinin-based combination therapy (ACT), and Atovaquone-Proguanil. Malaria may rapidly progress, requiring hospitalization, and may prove fatal.
How Does Giardiasis Occur?
Giardiasis is a protozoal infection affecting the small bowel, mostly seen in developing countries or in travelers returning from third-world countries. It is caused by the protozoal species - Giardia lamblia (also known as G. intestinalis or G. duodenalis). The infection spreads through the fecal-oral route, primarily through ingesting contaminated food or water or even from person-to-person or animal-to-person contact.
Giardia trophozoites adhere to the intestinal epithelium, disrupting the intestinal cell junctions and brush border enzymes and the individual shedding close to ten billion cysts per day in the feces. The patients present with abdominal pain, fever (uncommon), nausea, flatulence, large volume watery, foul-smelling, greasy stools, and weight loss. Giardiasis is detected by stool antigen detection assays and nucleic acid amplification tests (NAAT). The infection is initially managed by oral rehydration or intravenous fluid resuscitation. The pharmacotherapeutic approach includes Metronidazole (250 to 300 milligrams, thrice daily for up to ten days), Tinidazole, Nitazoxanide, Mebendazole, Albendazole, and Paromomycin. Paromomycin can be safely prescribed during the first trimester of pregnancy.
How Does Amoebiasis Occur?
Amebiasis, or amoebic dysentery, is a common enteral protozoal infection caused by Entamoeba histolytica. The infected cysts spread via the oral-fecal route through contaminated food and water. Rarely, sexual transmission has also been reported. Amoebiasis globally infects 50 million individuals racking a death toll of well over a million annually. Traveling to endemic or underdeveloped countries or being hospitalized or immunocompromised makes one susceptible to this protozoan. Amebiasis is the leading cause of parasitic death in humans.
The organism forms pseudopods and causes proteolysis and tissue lysis. Excystation of the mature cyst occurs within the small intestine, where the trophozoites are released, travel to the large bowel, and defecate out. The cysts can survive for several days to weeks in the wild. The organism incubates for two to four weeks and creates symptoms like abdominal cramps, watery diarrhea, severe colitis, and bloody diarrhea with mucus. The extraintestinal presentation includes liver abscess, which may rupture into the pleural cavity or the pericardium. Other rare state includes the involvement of the heart, brain, kidneys, spleen, and skin causing proctocolitis, toxic megacolon, peritonitis, brain abscess, and pericarditis.
The condition is detected in 30 percent of individuals via direct microscopy of stool and rectal swabs. The organism can be detected via ELISA (enzyme-linked immunosorbent assay) and PCR. The best diagnostic method is by loop-mediated isothermal amplification assay. Extraintestinal presentation is assessed by ultrasound and CT (computed tomography) scans. Other tests include liver aspiration, colonoscopy, blood studies, and fecal or rectal biopsy. Treatment is done with Metronidazole and Tinidazole. Luminal agents such as Paromomycin and Diloxanide furoate are also used. Liver abscesses can be aspirated with CT guidance.
How Does Leishmaniasis Occur?
Leishmaniasis is a protozoal disorder caused by the organism Leishmania spread by sandflies. The infection is endemic to Asia, the Middle East, Northern Africa, the Mediterranean, and South and Central America (in 89 countries), with 1.5 to 2 million annual incidences contributing to 70,000 annual fatalities.
The protozoan enters the injured capillaries secondary to a female sandfly bite. The protozoa enter the phagolysosomes and, based on the subtype of the phagocytic cells. Leishmaniasis can occur in two subtypes- cutaneous and visceral. The disease presents itself as cutaneous, mucocutaneous, and visceral features after an incubation period of two to four weeks. The main symptoms are painless skin ulcers, oral ulcers, runny or stuffy nose, nosebleeds, dyspnea, weight loss, weakness, extended fevers, splenomegaly, hepatomegaly, decreased hemopoiesis, lymphadenopathy, and other infections.
The condition is evaluated with a histopathological examination, enzyme-linked immunosorbent assays, western blot, or direct agglutination. It is treated with liposomal Amphotericin B, Paromomycin, Sodium stibogluconate, and Miltefosine.
How Is Sleeping Sickness Caused?
Sleeping sickness or African trypanosomiasis is caused by Trypanosoma brucei protozoa and is transmitted by tsetse flu, especially in sub-Saharan Africa. There are two variants caused by two different species of Trypanosoma, causing a combined 20,000 patients with nearly 9100 deaths, as recorded in 2010. Trypanosomes multiply within the tsetse fly and are rapidly injected into the bloodstream and lymphatics after the fly bite.
The physical manifestations of Trypanosoma are induration at the bite site, generalized lymphadenopathy, fever, tachycardia (increases heart rate), edema, splenomegaly (enlarged spleen), disorientation, altered mental status, psychosis (disconnection from reality), stupor, and coma. The condition is diagnosed by blood smear, lymph node aspiration, and CT and MRI (magnetic resonance imaging) scan of the brain (imaging reveals massive cerebral edema). The infection can be treated with symptomatic management and Pentamidine, Suramin, Melarsoprol, Eflornithine, Nifurtimox, or Fexinidazole medication.
How Does Trichomoniasis Occur?
Trichomoniasis is a sexually transmitted protozoal infection caused by Trichomonas vaginalis, especially in women between 40 and 49 years. The pathogen has an incubation period between 5 to 28 days, residing within the lumen of the urogenital tract and then destroying the epithelial lining.
Women with the infection often complain of foul-smelling yellow or greenish vaginal discharge, dyspareunia (persistent or recurrent genital pain), urinary frequency, dysuria, and vulvar pruritus or erythema. Men are often asymptomatic or may present with urethritis (inflammation of the urethra ), prostatitis (prostate inflammation), and epididymitis (inflammation of the tube at the back of the testicle-epididymis). The condition is diagnosed with wet pref microscopy and NAATs (nucleic acid amplification tests). It is then treated with Metronidazole and Tinidazole.
How Does Toxoplasmosis Occur?
Toxoplasmosis occurs via the obligate intracellular parasite Toxoplasma gondii affecting up to 11 percent of individuals above six years of age. This is the leading cause of foodborne infections. The intracellular growth of tachyzoites causes direct cytopathic effects, cellular inflammation, and necrosis.
The disease is diagnosed with a biopsy of the lymph nodes, brain, and eyes. The tachyzoites or tissue cysts are seen closer to the blood vessels, indicative of hematogenous infection dissemination. Individuals acquire the parasite through domesticated animals and birds, which are the main host for this parasite, cat feces, contaminated food and water, unwashed fruits and veggies, contaminated kitchen tools, and infected blood or organ transplant. The symptoms include fever, lymphadenopathy, headaches, myopathies, rashes, ocular pain, poor vision, eye floaters, breathing issues, devices, cough, confusion, lack of coordination, muscle weakness, and seizures.
The protozoan can infect the fetus via placental transfer, causing congenital toxoplasmosis. This causes hydrocephalus, ocular infections, brain tissue irregularities, splenomegaly, hepatomegaly, mental kill dysfunction, motor dysfunction, hearing issues, seizures, heart disorders, jaundice, rashes, growth retardation, and early puberty. The infection is diagnosed with serological and molecular testing, radiological diagnosis, and biopsy. The treatment protocol includes Pyrimethamine, Sulfadiazine, folic acid supplementation, and Trimethoprim-Sulfamethoxazole.
Conclusion:
Protozoans are parasitic organisms that widely differ from bacteria and viruses. Most of the organisms follow a host-vector pathway for transmission and propagation. Often multiplying in billions, these parasites can be highly fatal. Hence, a good knowledge of this kingdom is essential to report to a physician early in the pathogenesis and receive treatment at the earliest.
