1st year DDS - Practical class 7

Plasmodium sp.

Kind of parasite: heteroxenous and monoxenous.

Host(s): humans as the intermediate host and mosquito (Anopheles) as the definitive host.

Infective stage: sporozoite.

Transmission by the injection of sporozoites with female mosquito saliva.

Site of infection: the parenchyma cells of the liver (exo-erythrocytic schizogony) and RBC (erythrocytic schizogony).

Diagnostic method(s):
• microscopic examination of thin or thick stained smears of peripheral blood;
• molecular methods.

Geographical distribution: in tropical, subtropical and temperate climate zones.

Remarks:

• the most important species parasitizing human beings are: P. falciparum (malignant tertian malaria), P. malariae (quartan malaria), P. vivax (benign tertian malaria), and P. ovale (tertian malaria);

• in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver and cause relapses by invading the bloodstream weeks, or even years later.

Toxoplasma gondii

Kind of parasite: heteroxenous and polyxenous.

Host(s):
• domestic cat and other members of the family Felidae as definitive hosts;
• many species of mammals (including humans) and birds as intermediate hosts.

Infective stage: cyst, oocyst (sporocysts), trophozoite.

Transmission: foodborne and waterborne, congenital, transfusion, transplantations.
• ingestion of cysts present in the tissues of infected host (raw meat);
• ingestion of oocysts (sporocysts) found in the environment;
• during pregnancy the tachyzoites may cross the placental barrier and infect the developing fetus.

Site of infection: tissue (intracellular).

Diagnostic method(s):
• immunological tests;
• demonstration of the parasite in biopsies (histological procedures);
• molecular methods.

Geographical distribution: cosmopolitan parasite.

Remarks:
• some authors believe that approximately 30% of humans are infected;
• toxoplasmosis in immunocompetent persons is, as a rule, asymptomatic;
• waterborne outbreaks of toxoplasmosis have recently been reported.

Naegleria fowleri

Kind of parasite: homoxenous and monoxenous.

Host(s): humans.

Infective stage: trophozoite.

Transmission: aspiration of trophozoites with water during swimming.

Site of infection: CNS.

Diagnostic method(s):
• microscopic identification of motile trophozoites in the sediment from fresh cerebrospinal fluid (CSF);
• microscopic identification of trophozoites in stained smears of the CSF sediment;
• culture of CSF sediment is necessary to confirm diagnosis.

Geographical distribution: cosmopolitan; N. fowleri occurs in a variety of habitats: swimming-pools, man-made warm water reservoirs, and waters polluted with hot water discharges.

Remarks:
• the causal agent of fatal primary amebic meningoencephalitis (PAM);
• no cases of PAM have ever been known to develop from drinking water containing amebae;
• up to now ~200 cases of PAM have been reported (mainly from the USA);
• primary free-living organism;
• the trophozoites occur in two reversible forms: an ameboid and temporarily flagellate form.

Acanthamoeba sp.

Kind of parasite: homoxenous and polyxenous.

Host(s): humans and mammals.

Infective stage: trophozoite and cyst.

Transmission:
• infective forms may enter the body via the respiratory tract, the cornea (contact lens), damaged or ulcerated skin and mucosa;
• the route of CNS penetration is via the blood;
• the infection may be acquired by introducing the parasites into the host with contaminated dust, air, soil, soft lenses.

Site of infection: CNS, skin, cornea, respiratory tract.

Diagnostic method(s):
• microscopic identification of the parasite in the sediment from the materials from corneal scraping, skin lesions, nasal sinuses, lungs and cerebrospinal fluid (CSF);
• stained smears confirming the presence of Acanthamoeba;
• culture techniques.

Geographical distribution: cosmopolitan.

Remarks:
• several Acanthamoeba species are the causal agents of granulomatous amebic encephalitis (GAE), keratitis, ad cutaneous, nosopharyngeal and disseminated infections;
• Acanthamoeba infection is limited to chronically ill, debilitated or immunocompromised individuals (except Acanthamoeba keratitis!);
• Acanthamoeba trophozoites and/or cysts have rarely been detected in the CSF;
• cysts are usually seen in sections of GAE patients (but never in those with PAM);
• primary free-living organisms.