Porth's Essentials of Pathophysiology, 4e

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Disorders of the Immune Response

C h a p t e r 1 6

Opportunistic Infections Opportunistic infections involve common organisms that do not typically produce infection unless there is impaired immune function. As the number of CD4 + T cells declines, the risk of these infections increases. In addition, the baseline HIV RNA level contributes and serves as an independent risk factor. 2,3 Opportunistic infections are most often catego- rized by the type of organism (e.g., fungal, proto- zoal, bacterial and mycobacterial, viral). Bacterial and mycobacterial opportunistic infections include bacterial pneumonia, salmonellosis, bartonellosis, Mycobacterium tuberculosis (TB), and Mycobacterium avium complex (MAC). Fungal opportunistic infec- tions include candidiasis, coccidioidomycosis, cryptococcosis, histoplasmosis, penicilliosis, and pneu- mocystosis. Protozoal opportunistic infections include cryptosporidiosis, microsporidiosis, isosporiasis, and toxoplasmosis. Viral infections include those caused by cytomegalovirus (CMV), herpes simplex and zoster viruses, human papillomavirus, and JC virus, a virus that is the causative agent of progressive multifocal leukoencephalopathy (PML). RespiratoryTract Infections. The most common causes of respiratory disease in persons with HIV infection are bacterial pneumonia, Pneumocystis jiroveci pneumo- nia, and pulmonary tuberculosis. Other organisms that cause opportunistic pulmonary infections in persons with AIDS include CMV, MAC, Toxoplasma gondii, and Cryptococcus neoformans. 2 Pneumonia also may be caused by more common bacterial pulmonary patho- gens, including S. pneumoniae, Pseudomonas aeru- ginosa, and H. influenzae. Some persons may become infected with multiple organisms, causing a polymicro- bial infection. Kaposi sarcoma (to be discussed) also can occur in the lungs. P. jiroveci (formerly known as P. carinii ) pneu- monia (PCP) was the most common presenting manifestation of AIDS during the first decade of the epidemic. P. jiroveci is an organism common in soil, houses, and many other places in the environment, and in healthy persons does not cause infection or disease. In persons with HIV infection, P. jiroveci can multiply quickly in the lungs and cause pneumonia. As the disease progresses, the alveoli become filled with a foamy exudate that forms cup-shaped cyst walls within the exudate (Fig. 16-10). Since highly active antiretroviral therapy (HAART) and prophy- laxis for PCP were instituted, the incidence of PCP has decreased. 61 P. jiroveci (formerly known as P. carinii ) pneumonia still is common in people unaware of their HIV-infected status, in those who choose not to treat their HIV infection or take prophylaxis, and in those with poor access to health care. The best pre- dictor of PCP is a CD4 + cell count below 200 cells/ μ L, 61 and it is at this point that antimicrobial pro- phylaxis with trimethoprim-sulfamethoxazole or an alternative agent (in the case of adverse reactions to

sulfa drugs) is strongly recommended. The symptoms of P. jiroveci pneumonia may be acute or gradually progressive. Patients may present with complaints of a mild cough, fever, shortness of breath, and weight loss. Diagnosis is made by identifying the organism in pulmonary secretions. Tuberculosis (TB) is the leading cause of death for people with HIV infection worldwide, and is often the first manifestation of HIV infection. In 2011, 23% of those with TB tested positive for HIV. 44 In the United States, the number of TB cases decreased from the 1950s to 1985; then, in 1986, the number began to increase (see Chapter 22). 62 Several factors contributed to this increase, but the most profound factor was HIV infection. The lungs are the most common site of M. tuberculosis infection, but extrapulmonary infection of the kidney, bone marrow, and other organs also occurs in people with HIV infection. Whether a person has pulmonary or extrapulmonary TB, most persons pres- ent with fever, night sweats, cough, and weight loss. Persons infected with both HIV and TB are more likely to have a rapidly progressive form of TB, and usually have an increase in viral load, which decreases the suc- cess of TB therapy. They also have an increased num- ber of other opportunistic infections and an increased mortality rate. Since the late 1960s, most persons with TB have responded well to therapy. However, in 1991, there were outbreaks of multidrug-resistant (MDR) TB. Since the original outbreak of MDR TB in the early 1990s, new cases of MDR TB have declined, largely because of improved infection control practices and the expansion of directly observed therapy programs. Gastrointestinal Manifestations. Diseases of the gastrointestinal tract are some of the most frequent complications of HIV infection and AIDS. Esophageal candidiasis, CMV infection, and herpes simplex virus infection commonly cause esophagitis in people with FIGURE 16-10. Pneumocystis jiroveci pneumonia. Histopathology of lung shows characteristic cysts with cup forms and dotlike wall thickening (methenamine silver stain). (From the Centers for Disease Control and Prevention Public Health Image Library. No. 960. Courtesy of Edwin P. Ewing, Jr.)