Individuals

Ib C. Bygbjerg

The acquired immunodeficiency syndrome (AIDS) is the end-stage of a progressive infection of the human host's cellular immune system, in particular the T-helper (CD4+) lymphocytes, by the human immunodeficiency virus type 1 (HIV-1). Though the closely related HIV-2 may also cause immunodeficiency and thereby give rise to opportunistic parasitic infections, HIV-related parasites in the following section will focus on those affecting HIV-1 infected individuals.

HIV-infected individuals with progressive immune deficiency have an abnormally high susceptibility to infections with non-virulent and minimally pathogenic organisms. Life-threatening infections of the respiratory, central nervous, and gastrointestinal system arise by revival of dormant infections acquired many years previously, or by exposure to new pathogens, most commonly when the CD4+ counts decrease to 10-20% of normal levels. Thus, Pneumocystis carinii infections typically occur in patients with CD4+ counts below 200-300 mio./l; Toxoplasma gondii infections with CD4+ counts below 100 mio./l; and Cryptosporidium parvum may be fulminant in patients with CD4+ counts below 50mio./l, while it may be reversible if CD4+ counts are above 300-400 mio./l. Patients with Leishmania/HIV co-infection are virtually untreatable if CD4+ counts are low.

Besides contributing to the sufferings of HIV-infected individuals, parasitic infections are also important for the classification (stages) of HIV: several parasites are enlisted as AIDS defining events. In the Nordic countries, the most prevalent parasites in HIV-infected patients are P. carinii, T. gondii, C. parvum, and Microsporidia. Cyclospora cayetanensis and Isospora belli are less frequently diagnosed, while Leishmania infections and other (sub)-tropical parasitoses are very rare. Increasing population mobility may, however, broaden the spectrum of opportunistic infections encountered, whether in Scandinavians visiting endemic areas or immigrants and refugees succeeding in crossing the borders to the cool North. In the following, emphasis is on P. carinii, T. gondii, C. parvum, and Microsporidia. Effective chemoprophylaxis of P. carinii and T. gondii and more recently highly active anti-viral chemotherapy (HAART) of HIV have greatly affected the patterns of opportunistic infections, at least for those patients, who can afford it. Anti-parasitic prophylaxis may be stopped in HIV patients with CD4+ counts returning to levels >200mio./l (Schneider et al. 1999). Resistance to HAART, however, calls for cautiousness.

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