Parasitic Infections

Abstract

The wording “parasite” originates from the ancient greek “parasitos” (pará  =  besides, sítos  =  eating). Parasites are organisms living in (endoparasites, intracellular parasites) or on (ectoparasites) other species (host). The host provides nutrients for the parasite and is injured by the parasite. The relationship between host and parasite is “antagonistic” in favor of the parasite. Detrimental effects to the host include malnutrition, organ injury, inflammation, toxicity, mass effects, and others. Typically, the host is undermined but not killed. Parasitism has been extremely successful during evolution. It has been suggested that approximately 50 % of all organisms are parasitic permanently or have a parasitic period in life. This is supported by the fact that nearly all animals host a number of different parasites, each specifically adapted to their host. Stationary parasites are in permanent contact with the host, whereas temporary parasites attack the host only for feeding (e.g., mosquitoes). The cycle of life of most parasites is very complex including changes between different hosts (intermediate and definite) as well as sexual and non-sexual reproduction. There are different ways of parasite transmission leading to infection of the host: direct surface contact; active percutaneous; oral; feco-oral; cyclic-alimentary (e.g., malaria: while feeding the anopheles mosquitoes transmit parasites between individuals); sexual and diaplacental (e.g., in fetal toxoplasmosis). This chapter addresses the most relevant CNS manifestations of human pathogen parasites including imaging features and differential diagnoses. These are the protozoans Toxoplasma gondii (toxoplasmosis), Plasmodium sp. (malaria), and Entameba histolytica (amebiasis), as well as the Helminthes (worms), Taenia solium (neurocysticercosis), Echinococcus granulosus and Alveolaris (echinococcosis), Trichinella spiralis (trichinosis), and Schistosoma sp. (schistosomiasis). In general, MRI is superior to CT in diagnostic neuroimaging of parasitic CNS infections.