Chemoprophylaxis is the use of an antimicrobial agent to prevent infection. Chemoprophylaxis is often administered after exposure to a virulent pathogen or before a procedure associated with risk of infection. Chronic chemoprophylaxis is sometimes administered to persons with underlying conditions that predispose to recurrent or severe infection. Antibiotics can also be used as pre-emptive therapy (sometimes referred to as secondary prophylaxis) to prevent clinical disease in persons infected with a microorganism such as Mycobacterium tuberculosis. Immunization, another excellent means of preventing infection, is discussed in Chapter 115. For information on prophylaxis of bacterial endocarditis, see Chapter 37, Endocarditis of natural and prosthetic valves: treatment and prophylaxis; for information on prophylaxis in persons infected with the human immunodeficiency virus (HIV), see Chapter 102, Prophylaxis of opportunistic infections in HIV disease; for malaria prophylaxis, see Chapter 200, Malaria; for prophylaxis related to transplant recipients and neutropenic patients, see Chapter 89, Infections in transplant recipients, and Chapter 85, Infections in the neutropenic patient; and for surgical prophylaxis, see Chapter 114, Surgical prophylaxis.
Several concepts are important in determining whether chemoprophylaxis is appropriate for a particular situation. In general, prophylaxis is recommended when the risk of infection is high or the consequences significant. The nature of the pathogen, type of exposure, and immunocompetence of the host are important determinants of the need for prophylaxis. The antimicrobial agent should eliminate or reduce the probability of infection or, if infection occurs, reduce the associated morbidity. The ideal agent is inexpensive, orally administered in most circumstances, and has few adverse effects. The ability to alter the normal microbiota and select for antimicrobial resistance should be limited. The emerging crisis of antibiotic-resistant bacteria underscores the importance of rational and not indiscriminate use of antimicrobial agents.