Infections in the Secondary Immunocompromised Host

Deficiency in host defences allows infections to occur with greater frequency. Their nature and presentation may be unusual and hence diagnosis may be difficult. The type, severity and duration of immunocompromise determine the infections to which the host is vulnerable. Unlike in primary immunocompromised patients, the susceptibility of secondary immunocompromised patients often changes during the course of their illness as their immune function may vary with the use of immunosuppressive drugs or highly active anti-retroviral agents prescribed for those with human immunodeficiency virus. There is an ever-increasing number of new agents that modify the host immune response. These include biologic therapies that target specific cytokines and are used in various inflammatory disorders. The growing numbers of immunocompromised patients make this a key area for those involved in managing infections. A number of strategies are employed to combat the infectious threat ranging from prophylaxis to pre-emptive, empirical and definitive treatment. Key Concepts Immunocompromised patients are susceptible to a range of opportunistic infections due to deficiencies in their immune defences. Opportunistic infections occur in immunocompromised but not immunocompetent hosts. Specific immune defects may predispose to a restricted range of opportunistic infections. Immune defects may involve innate (antigen non-specific) or adaptive (antigen specific) responses. Clinical features may be modified or less apparent in the presence of immunodeficiency. The diagnosis of an infection requires consideration of infectious exposures, details of immunodeficiency and all clinical features and investigation results. Clinical illness may be prevented by screening and treating latent infection, prophylaxis or pre-emptive treatment in the subclinical phase if markers of infection progression are available. Keywords: immunocompromised host; opportunistic infection; neutropaenia; transplantation; malignancy; human immunodeficiency virus (HIV); biologic therapies; latent screening; chemoprophylaxis; pre-emptive treatment