Toxoplasma gondii was first described by Nicolle in 1909 in a North African rodent, Ctenodactylus gondii. The pathogen derives its species name from this. It is a unicellular parasite that is very widespread in nature and can infect many animal species. It is localised and replicates in practically all nucleated cells. The parasite has 3 principal forms: (1) banana-shaped trophozoites (hence the generic name “toxon” = bow) which reproduce asexually in nucleated cells, (2) intracellular dormant cysts with bradyzoites. In the cells the parasite forms cysts that are held in check by the immune system. The parasite thus remains present in the body, especially in muscle cells and in the brain, throughout the animal’s life. (3) oocysts that are the result of the parasite’s sexual cycle in the intestine of the cat. These oocysts can survive in the outside world for several months. If tissue cysts are present the parasite can become active again due to diminished activity of the immune system.

People are infected primarily via cat faeces or by eating contaminated and insufficiently cooked meat. In adults with an intact immune system Toxoplasma usually causes few problems, apart from occasional lymph node swelling and/or hepatitis. Reactivated parasites can attack the brain and retina in immunodepressed persons. Headache, neurological problems, blindness and focal cerebral lesions can occur. Such focal lesions can lead to epileptic fits. A favourable clinical response to anti-toxoplasmosis therapy is suggestive of cerebral toxoplasmosis (a good clinical response usually occurs within 2 weeks). Cerebral biopsy and PCR (Polymerase chain reaction) can be performed if there is doubt about the diagnosis. Toxoplamosis is an infection that can be prevented by prophylactic therapy. Cerebral localisations of lymphomas as well as tuberculomas can resemble cerebral toxoplasmosis.

Cryptococcosis is a cosmopolitan infectious disease caused by a yeast, Cryptococcus neoformans. There are two varieties: C. neoformans var. neoformans and C. neoformans var. gattii. These differ in their geographical distribution, ecology and biochemical characteristics. The antigenic specificity of the polysaccharide capsule determines four serotypes: A, B, C and D. Serotypes A and D are found worldwide in bird droppings (avian excreta), especially of pigeons. Serotypes B and C are geographically associated with certain Eucalyptus trees. C. n. neoformans occurs worldwide, whereas C. n. gattii is restricted to the tropics and subtropics. In Australia Eucalyptus camadulensis trees form the natural habitat for the var. gattii. The typical vegetative form of C. neoformans is a yeast with a diameter of 2.5 to 10 µm. The organism can also reproduce sexually. As it is a basidiomycete (Filobasidiella neoformans) it forms sexual spores: “basidiospores”. [Basidiospore: Gr: “basidon”: small base and “sporon”: seed. This indicates the morphology: a club-shaped cell with the haploid spores at the far end.] Infection can probably occur as a result of inhalation of either dehydrated yeast form or basidiospores. Cryptococcus neoformans primarily causes a chronic meningitis. Systemic infections, inflammation of the lungs and cutaneous lesions also occur as a result of these yeasts. The demonstration of cryptococcal meningitis in a patient is at present considered proof of HIV infection and AIDS.

The clinical picture can be very atypical. Mild fever, headache, confusion and emaciation can be found in patients with cryptococcal meningitis. Neck stiffness is present in less than half of infected persons. Photophobia occurs in 25%. Confusion and coma occur later. Focal signs are seldom observed. Blindness can occur (possibly due to concomitant arachnoiditis), although this is less frequent in HIV patients than in immunocompetent individuals with cryptococcal meningitis. The diagnosis of a cryptococcal meningitis is made by centrifuging a few ml of cerebrospinal fluid and mixing the sediment with an equal quantity of East Indian ink. The yeasts are recognized quite easily as round organisms with a thick capsule. The saccharide capsule can be detected via an antigen detection test (latex agglutination test for serum, cerebrospinal fluid and urine). The encapsulated organism can likewise be detected in tissue biopsies.