Opportunistic Infections in HIV Infection

An infection is defined as opportunistic when it affects those with severe immunosuppression, i.e. takes a n opportunity to cause disease in a host with a weakened immune system. In people living with HIV it mainly affects those with a CD4 count < 200 although it is not impossible in those with CD4 count > 200. The CD4 percentage is also important as those with a CD4% < 14 are also more likely to have an OI. The lower the CD4 count the higher the risk of OIs, and some OIs are seen much more commonly with very low CD4 counts, e.g. cryptococcal meningitis in those with CD4 count of < 100. Before the introduction of antiretroviral therapy OIs were much more common than they are now, with previously up to 80% of those with AIDS having pneumocystis pneumonia (PCP). Since the introduction of antiretrovirals (ARVs) the rates of OIs has reduced greatly but unfortunately there are people who are still diagnosed late with an OI at diagnosis. Those with poor adherence or difficulty accessing ARVs are also more likely to be affected. In the UK in 2014, 40% of people diagnosed with HIV had a CD4 count of <350 which is defined as a late diagnosis (and 22% had a CD4 count of <200 which is defined as a very late diagnosis). In comparison to someone diagnosed with HIV early, those who are diagnosed late have a 10 times higher risk of dying in the year after they are diagnosed. This highlights the need for routine HIV testing so that people are diagnosed early to reduce the incidence of OIs further. The most common OIs seen in the UK are pneumocystis pneumonia (PCP), central nervous system (CNS) toxoplasmosis, cryptococcal meningitis, cytomegalovirus (CMV) retinitis, Mycobacterium avium intracellulare (MAI) infection and candidiasis. All those with HIV and a CD4 count ≤ 200, or with a CD4% < 14 should be given prophylaxis against PCP. Prophylaxis should also be recommended for those with oral candidiasis or a previous AIDs – defining illness. The options are co-trimoxazole 480mg od or 960mg 3x/week (960mg once daily can be given although does not confer any greater protection and has increased risk of side effects), dapsone 50mg once daily, or pentamidine nebulisers 300mg once every 4 weeks.