Infections in the Immunocompromised Host

Abstract

An understanding of the main aspects and functions of the immune system is important, i.e. physical barriers, innate, humoral, and cell-mediated immunity (see Chapter 6, Basic Immunology), when caring for the immunocompromised patient. In adults, secondary immunodeficiency is much more common than primary, and is most often due to iatrogenic immunosuppression with drugs, e.g. corticosteroids, chemotherapy agents, immunosuppressive agents, ‘biological’ therapies. For example, treatment with corticosteroids for more than one month is enough to increase the risk of some fungal infections such as Candida and Pneumocystis jirovecii, such that PCP prophylaxis should be considered in patients receiving ≤ 20mg/day prednisolone for four or more weeks. Chemotherapy and immunosuppressive agents may cause profound immunosuppression. The degree and duration of immunosuppression following a transplant, and the conditioning regimen used before the transplant varies with respect to the type of transplant: heart and lung transplant recipients typically receive more significant immunosuppression, and so are at increased risk of opportunistic infection compared to other solid-organ transplant recipients. Infections (e.g. HIV), cancer, and autoimmune disorders and the treatment of these conditions can also affect the immune system. Other diseases are also considered immunosuppressive although the exact nature of this is less well defined, for example, poorly controlled diabetes mellitus increases the risk of candidal infections and common bacterial infections. Cirrhosis is also considered to be a relatively immunosuppressed state. Understanding the nature of immune defects in both primary and secondary immunodeficiency allows more accurate prediction of overall infection risk and risk of specific pathogens, allowing a rational approach to infection prevention and investigation when patients become unwell. The initial assessment of the immunocompromised host should be to identify why the patient is immunocompromised, how long they have been immunocompromised (is it a congenital or acquired immunodeficiency?), and whether there is potential for immune recovery. Clearly, a person with a congenital immunodeficiency will have lifelong susceptibility to specific infections, unlike an acquired deficiency due to chemotherapy or transplantation which may be transient. If the immunosuppression is due to a drug, is it possible to reduce or change the immunosuppression? If an infection is suspected, pre-immunosuppression infection screening results can help identify whether the current presentation represents reactivation of a latent infection or primary infection.