Parasitology Notes

Phlebotomus papatasi

  • Acts as a vector for cutaneous leishmaniasis.

Leishmania tropica minor

  • Causes Anthroponous cutaneous leishmaniasis.
  • Geographic distribution: Southern Europe, Middle East, Africa, India, Pakistan, Uzbekistan, Turkmenistan.

Leishmania tropica major

  • Causes Zoonotic cutaneous leishmaniasis of the Old World.
  • Geographic distribution: Southern Europe, Middle East, Africa, India, Pakistan, Uzbekistan, Turkmenistan.

Leishmania mexicana and L. peruviana

  • Cause New World skin and mucosal leishmaniasis.
  • Multiple sources of infestations: rodents, wild and domestic animals.
  • Can cause severe deformities of the nose, auricles, nasopharynx, larynx, and external genitalia.
  • The course of the infection is long, and the prognosis is unfavorable.

L. braziliensis

  • Associated with New World skin and mucosal leishmaniasis, known as «ухо чиклеро».

Visceral Leishmaniasis (Kala-Azar)

  • Affected organs: spleen, liver, bone marrow, lymph nodes.
  • Metabolic processes lead to the formation of toxic substances, causing general intoxication of the body.
  • Vector: Phlebotomus argentipes
  • Causative agent: Leishmania donovani
  • Geographic distribution: Africa and South Asian countries.

Leishmania donovani

  • Causative agent of Visceral Leishmaniasis.

Morphological features of Leishmania

  • Amastigotes (leishmaniform forms):
    • Oval body, immobile.
    • Parasitize cells of the bone marrow, spleen, liver, and skin cells.
  • Promastigotes (leptomonas forms):
    • Mobile, have 1 flagellum.
    • Divide longitudinally, develop in the mosquito's body.

Life cycle of Leishmania

  • Specific carriers are mosquitoes.
  • When bitten, a mosquito injects flagellated forms (promastigotes) into the body of vertebrates and humans.
  • Promastigotes penetrate into the cells of internal organs, transform into amastigotes, and start multiplying.
  • The leishmanial form (amastigote) is the main form of existence of Leishmania.

Cutaneous Leishmaniasis

  • Vectors: Phlebotomus mosquitoes
  • Causative agent: Leishmania tropica.
  • L. tropica major and L. tropica minor
  • Symptoms:
    • Small erythematous bumps appear on the skin 2-6 weeks after a mosquito bite.
    • Later, a lump forms above the skin, with an ulcer in the center (leishmanioma).

New World skin and mucosal leishmaniasis

  • Causative agents: L. brasiliensis, L. mexicana, L. peruviana
  • Symptoms:
    • Ulcers that increase in size and destroy all tissues.
    • Overgrowth of tissues of the nose, lips, pharynx, larynx.

Visceral Leishmaniasis (Kala-Azar)

  • Causative agents: L. donovani, L. infantum
  • Symptoms:
    • Fever of the wrong type, weakness, headache, intoxication, exhaustion, skin pigmentation, rash.
    • Enlarged liver and spleen, anemia.

Trichomoniasis

  • Causative agent: Trichomonas vaginalis, a flagellated single-cell eukaryote.
  • Exists only in the trophozoite form and divides by binary fission in the human host.
  • No cyst form, and no known animal or environmental reservoirs.
  • The only truly sexually transmitted parasitic infection in humans.
  • Found in the lower urinary and reproductive tracts of men and women.
    • In women: superficial infection of the vagina and urethra, occasionally ascending to cause cystitis.
    • In males: infection largely confined to the urethra but can ascend into the prostate.
  • Characteristic symptoms: itching, burning, green discharge with an unpleasant odor.
  • Diagnosis: detection of trophozoite in discharge smears. The parasite has a long spike at the posterior end and five flagella.

Malaria

  • Malaria parasites are members of the Apicomplexa.
  • Four malaria species infect and cause disease in humans: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale.
  • Infection can be acquired congenitally, from blood transfusions, and from shared needles.
  • Most frequently initiated with the bite of an infected, female Anopheles mosquito, which injects the sporozoite stage of the parasite.
  • Human infection:
    • Sporozoites enter the bloodstream and are transferred to the liver.
    • They reproduce asexually, forming a tissue schizont, and merozoites come out of it.
    • This is the incubation period.
    • P. falciparum can complete this liver stage within 7 days; P. vivax, 6-8 days; P. malariae, 12-16 days; and P. ovale, 9 days.
  • Erythrocytic or blood stage:
    • Initiated when exo-erythrocytic merozoites from the liver invade red blood cells (RBCs).
    • Merozoites are released from hepatocytes, invade RBCs, and develop asexually over 2 or 3 days.
    • Stages of asexual development include the ring (early trophozoite), trophozoite, and schizont stages.
    • Diagnosis is made upon the identification of the parasites within erythrocytes on Giemsa-stained blood smears.
  • The mosquito larvae and pupae develop in water.
  • Erythrocyte phase:
    • Plasmodium malariae: the causative agent of four-day malaria characterized by the shape of a ribbon; seizures after 72 hours.
    • Plasmodium vivax, P.ovale: Seizures – after 48 hours
    • Plasmodium falciparum: the causative agent of tropical malaria is a form of the half-moon; paroxysms are acyclic, prolonged
  • Symptoms:
    • Incubation period: 10-14 days.
    • Fever, anemia, and circulatory disorders are typical.
    • The onset of the disease is acute.
    • The temperature rises to $40°C$, chills are replaced by intense heat. After a few hours, the temperature drops sharply to $35°C$.
    • After an attack, the patient feels very weak.
    • The number of seizures reaches 10-15, then they stop.
    • Complications: cerebral edema, malarial coma and acute renal failure, mental disorders.
  • Characteristic symptoms: Alternating bouts of fever after a certain time.
    • The attack lasts 6-12 hours and has 3 phases: chills, fever, and sweat.
  • Diagnosis: detection of the parasite in the blood. Blood is taken during an attack.

Toxoplasmosis (Toxoplasma gondii)

  • Localization: Cells of various human organs - the brain, heart and skeletal muscles, eye tissue, lungs and uterine walls, fetal membranes.
  • Distribution: Everywhere.
  • Morphology: At the stage of merozoites, toxoplasmas have the shape of an orange lobule or a crescent (their length is 4-7 microns, and their width is 2-4 microns).
    • The front end of the body is narrowed, the rear end is expanded and rounded.
    • At the front end is a conoid.
  • Parasite Life Cycle
    • Intermediate hosts: many wild and domestic animals, birds, and humans.
    • The main owner is cats.
    • Oocysts: Infective stages transmitted via cat feces after sexual development in cat.
    • Tachyzoites: Infective stages that infect macrophages and are carried throughout the human body via macrophages, causing pathology.
    • Tissue cysts (pseudocysts): Large cyst-like forms that become quiescent in response to host adaptive immune responses.
    • Bradyzoites: Slowly developing forms within tissue cyst.
  • Infection Cycle
    • Both oocysts and tissue cysts transform into tachyzoites shortly after ingestion.
    • Tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites.
    • If a pregnant woman becomes infected, tachyzoites can infect the fetus via the bloodstream.
  • Diagnostic Stage
    • Serological diagnosis
    • Direct identification of the parasite from peripheral blood, amniotic fluid, or in tissue sections.
  • T. gondii invades numerous organs, infecting a broad spectrum of cell types.
  • Tachyzoites infect macrophages and are disseminated through the blood to many organs, where they invade, asexually multiply, and cause cellular disruption, leading to cell death.
    • The resulting necrosis attracts inflammatory host cells, such as lymphocytes and monocytes.
  • Clinical Manifestations
    • Over 80-90% of primary infections produce no symptoms.
    • The incubation period for symptoms is 1 to 2 weeks.
    • Mild symptoms of primary infection include: localized, painless cervical or occipital lymphadenopathy, usually persisting 4-6 weeks, or nonspecific symptoms including myalgia, headache, rash or sore throat that persist for one month or longer.

Congenital toxoplasmosis

  • Caused by infection with Toxoplasma gondii in a pregnant woman.
    • Infants born to women who were infected before conception do not develop disease due to protection by maternal antibodies.
    • New infections with detectable maternal parasitemia are associated with up to a 50% transmission rate to the fetus.
    • Mothers who develop acute toxoplasmosis in the first trimester have a much lower fetal transmission rate than in the third trimester, but fetuses exposed early are at much higher risk for severe symptoms or death and spontaneous abortion.
  • Diagnosis relies on either indirect serological tests or direct detection of the organism.
    • Serologic tests, indicating recent or past infection, are most effective in immunocompetent adults who are able to mount a humoral response to the parasite.