What are opportunistic infections?
An opportunistic infection is an infection caused by harmful infectious agents, or pathogens (bacteria, viruses, fungi, parasites or protozoa), that usually do not cause disease in a healthy person, i.e. one with an immune system whose function is not impaired. Opportunistic infections observed in PLHIV include a wide range of diseases, from minor ailments like chronic skin itching to severe diseases such as tuberculosis (TB).
You may be asking yourself why PLHIV are so susceptible to opportunistic infections compared to an uninfected person. Remember from Study Session 20 that the immune system in HIV-infected people becomes progressively weakened. A weak immune system presents an 'opportunity' for pathogens to cause an infection. Thus, PLHIV can be infected by pathogens causing opportunistic infections more easily than HIV-negative immune-competent people, because HIV damages the function of the immune system.
An immune-competent person is an individual whose immune system is healthy and functions normally.
There are several important issues that we need to consider here before we describe the possible opportunistic infections that PLHIV may present (Section 21.4).
First, serious opportunistic infections usually develop 5 to 10 years following infection with HIV. You should expect a person who is infected with HIV to show minor and/or mild episodes of opportunistic infections during the early period of the disease. However, untreated PLHIV will increasingly acquire more serious infections as they progress to AIDS, mainly due to the gradual deterioration of their immune system.
Based on your previous study of the role of the immune system in the progression to AIDS, why do you think serious opportunistic infections do not develop until later in the course of the disease?
As you learnt in Section 20.2, during the first few years of HIV infection, a person's immune system is weakened, but it still functions quite well. Thus, it is able to fight off most infectious agents for some time.
Indeed, an HIV-infected person may have no symptoms during the first stages of HIV infection. Sometimes minor symptoms like skin diseases, a little loss of weight, or repeated sinusitis (inflammation of the nasal sinuses and nasal passages) may be present, and this is indicative of a slightly weakened immune system. The most important information for you to remember here is that the immune system has to be compromised beyond a certain level for serious opportunistic infections to arise in PLHIV.
Nasal means associated with the nose and its related structures.
Secondly, the onset of opportunistic infections will be different for each person living with HIV, and will depend on many factors such as nutritional status, exposure to pathogens, individual level of immunity, etc. These factors differ from person to person. Hence, in some cases PLHIV may progress to AIDS rapidly, while in others it may take longer for serious opportunistic infections to arise.
Thirdly, not all PLHIV develop the same opportunistic infections. The opportunistic infections developed by a person living with HIV depend primarily on the pathogens they have been exposed to. Although progression to AIDS from HIV infection follows a somewhat stereotyped series of different clinical stages, each individual patient has a unique pattern of progression through them (i.e. they present with different opportunistic infections at each stage).
Finally, we also need to briefly consider opportunistic infections in the context of HIV-infected children (Study Session 28). Indeed, opportunistic infections appear earlier in children than in adults. In the absence of treatment, around half of HIV-infected children die by the age of two years, due to serious opportunistic infections and diseases. This is because children have immature immune systems, so their immune system becomes weakened faster than in adults, who already have a well-developed immune system before they get HIV.
In addition, other factors that influence the onset of opportunistic infections, such as exposure to pathogens and malnutrition, are more common problems in children than in adults. You will be learning about HIV in children in detail in Study Session 28. Here, we will be focusing on opportunistic infections occurring in adults.