Many diseases or their treatments adversely affect the immune system, including genetic immunodeficiencies, human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), cancer, stem cell and organ transplantation, and immunosuppressive drugs used to treat cancer, autoimmune diseases, and transplant patients. These patients are at significant risk of life-threatening infections from invasive endogenous infection from bacterial or fungal flora of the oropharynx, skin, and gastrointestinal tract, acquisition of exogenous infection from infected persons, and reactivation of latent infections until immune function recovers (Table 120-1).
The infections seen in immunocompromised persons canoften be predicted by which component of the immune system isabnormal, and the evaluation of fever in immunocompromisedhosts differs. Episodes of fever and neutropenia, defined as an absolute neutrophil count of less than 500/mm3 neutrophilsand bands, are especially common in cancer and transplant patients and increase risk for bacterial and fungal infections. The use of corticosteroids and potent immunosuppressive drugs that impair the activation of T lymphocytes increases the risk for pathogens normally controlled by T cell–mediated responses, such as Pneumocystis jiroveci and Toxoplasma gondii, and intracellular pathogens, such as Salmonella, Listeria, and Mycobacterium.
Staphylococcus aureus, Escherichia coli, Pseudomonasaeruginosa, Klebsiella pneumoniae, and coagulase-negativestaphylococci are the most commonly identified bacterialpathogens in immunocompromised persons. Central indwelling catheters often are associated with infections caused bycoagulase-negative staphylococci, S. aureus, gram-negative bacteria, Enterococcus, and Candida.
Fungal pathogens account for approximately 10% of all infections associated with childhood cancer. Candida causes 60% of all fungal infections, with Aspergillus as the second most common pathogen. Other risk factors for fungal infections include oropharyngeal and gastrointestinal mucositis
Clostridium difficile Candida spp. Cystoisospora belli Echoviruses
Enterobacter cloacae Fusarium spp. Septata intestinalis Human herpes virus 6
Chapter 120 u Infection in the Immunocompromised Person 391 facilitating systemic fungal invasion, presence of long-term indwelling intravascular catheters, extreme prematurity, and broad-spectrum antibacterial therapy that promotes fungal colonization as a precursor to infection.
Infection with T. gondii represents reactivation of quiescent infection facilitated by cancer-associated or therapy-associated cellular immunodeficiency. P. jirovecii primarily causes pneumonitis in patients with leukemia, lymphoma, HIV or those on long-term corticosteroids, but can also cause extrapulmonary disease (sinusitis, otitis media).