Free-living ameba of the genera Naegleria, Acanthamoeba, and Balamuthia have the potential to infect vertebrates and cause diseases in humans. The morphological characteristics of these amebas include a nucleus with large karyosome, a lack of chromatin granules at the nuclear membrane (see Entamoeba). Trophozoites: Naegleria fowleri (15–30 lm) with wide pseudopods, produces flagellated stages in water; Acanthamoeba spp. (24–56 lm) with fingerlike protrusions (“filopods”); Balamuthia mandrillaris (12–60 lm) has irregularly branched pseudopods. All of these genera produce cysts.
The causative agent of primary amebic meningoencephalitis (PAM) is Naegleria fowleri. This species occurs worldwide in bodies of freshwater, especially warm water in swimming pools, storage containers or thermally polluted lakes and rivers, etc. Infection of humans occurs by the nasal route with water containing trophozoites, i.e., during a swim or shower. The amebas migrate from the olfactory epithelium along the nerve tracts into the CNS and cause, after an incubation period of two to seven (rarely as long as 15) days a hyperacute to acute meningoencephalitis that usually has a lethal outcome. The infection occurs mainly in children and youths. Sporadic occurrence is reported from all continents. Treatment with amphotericin B has been successful in a small number of cases.
Potential human pathogens in this genus include Acanthamoeba Culbertson and several other species. These amebas occur worldwide in soil, sand, dust (also house dust), air, and water (also in tap water). The cysts can survive for several years in a dry state and disseminate with dust. Inhalation of cysts with dust is considered to be one of the main transmission routes. Acanthamoeba are frequent colonizers of human nasal mucosa and have been isolated from the oral mucosa, skin lesions, and the cornea. From the portal of entry, they can spread hematogenously to the CNS and other organs. Acanthamoeba infections in humans frequently take an asymptomatic course.
Keratitis is observed occasionally, especially in contact lens wearers. Diagnosis: culturing of amebas from conjunctival and contact lens rinsing liquids. Prevention: use only sterile lens rinsing liquid. In rare cases of generalized infection, Acanthamoeba can cause granulomatous amebic encephalitis (GAE) as well as granulomatous lesions in the lungs and other organs, especially in immunodeficient patients. Balamuthia mandrillaris can also cause GAE.
Toxoplasma gondii is the causative agent of a zoonosis that occurs worldwide with high prevalences (up to 80% depending on region and age). Humans are infected by ingesting oocysts excreted by the definitive hosts (cats) or by eating unprocessed meat containing Toxoplasma cysts. If a women contracts toxoplasmosis for the first time during pregnancy, diaplacental transmission of the pathogen to the fetus is possible with potential severe consequences (for example malformations, eye damage, clinical symptoms during childhood).
There is, however, no risk to the fetus from mothers who had been infected before their first pregnancy and have produced serum antibodies (about 35–45%). Latentinfections can be activated by immunodeficiencies (e.g., in AIDS patients) and may result in cerebral or generalized symptomatic toxoplasmosis. Serological surveillance in pregnant women is important to prevent prenatal infections.
Tachyzoites also multiply in experimental animals and cell cultures. This stage is highly labile outside of a host and usually does not survive the stomach passage following ingestion.