Human immunodeficiency virus type-2 (HIV-2) is a close relative of the prototype acquired immunodeficiency syndrome (AIDS) virus, HIV-1. HIV-2 is biologically similar to HIV-1, but information is lacking concerning clinical outcomes of HIV-2-infected individuals. From 1985 to 1993, a prospective clinical study was conducted in women with HIV-2 and HIV-1 infection to determine and compare rates of disease development. HIV-1-infected women had a 67% probability of AIDS-free survival 5 years after seroconversion in contrast with 100% for HIV-2-infected women. In addition to having significantly less HIV-related disease outcome in HIV-2 enrollees compared to HIV-1 enrollees, the rate of developing abnormal CD4+ lymphocyte counts with HIV-2 infection was also significantly reduced. This natural history study demonstrates that HIV-2 has a reduced virulence compared to HIV-1.

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Human immunodeficiency virus type 2 (HIV-2) is less pathogenic than HIV type 1 (HIV-1), but the mechanisms underlying this difference have not been defined. We developed an internally controlled quantitative reverse transcriptase-polymerase chain reaction to measure HIV-2 viral load and determined levels of plasma virus in a cohort of registered commercial sex workers in Dakar, Senegal. The assay has a lower limit of detection of 100 copies/mL and is linear over 4 logs. HIV-2 viral RNA was detectable in 56% of all samples tested; the median load was 141 copies/mL. Levels of viral RNA in the plasma were inversely related to CD4+ cell counts. HIV-2 and HIV-1 viral loads were compared among the seroincident women in the cohort; the median viral load was 30x lower in the HIV-2-infected women (P<.001, Wilcoxon rank sum test), irrespective of the length of time infected. This suggests that plasma viremia is linked to the differences in the pathogenicity of the 2 viruses.

Significant differences have been observed in the rates of transmission and disease development in human immunodeficiency virus (HIV) types 1 and 2. Because many HIV-2-infected people remain asymptomatic for prolonged periods, the hypothesis that HIV-2 might protect against subsequent infection by HIV-1 was considered. During a 9-year period in Dakar, Senegal, the seroincidence of both HIV types was measured in a cohort of commercial sex workers. Despite a higher incidence of other sexually transmitted diseases (STDs), HIV-2-infected women had a lower incidence of HIV-1 than did HIV-seronegative women, with a relative risk of 0.32 (P = 0.008). An understanding of the cross-protective mechanisms involved may be directly relevant to HIV-1 vaccine development.

Antimicrobial resistance can have 2 effects on the outcome of infection: there can be an accompanying change in the virulence of the organism, and there can be a poorer response to treatment because of the empiric choice of an antimicrobial to which the organism is resistant. We have reviewed published studies relating antimicrobial resistance to the outcomes of infection caused by enteric pathogens. The data for Salmonella and Campylobacter infections suggest that antimicrobial-resistant strains are somewhat more virulent than susceptible strains-that is, they cause more prolonged or more severe illness than do antimicrobial-susceptible strains. However, not all studies corrected for possible differences in age and underlying diseases between patients infected by antimicrobial-resistant and -susceptible strains of Salmonella. Two studies of Campylobacter infection suggest that poorer outcomes with antimicrobial-resistant pathogens could be related to the initial choice of an ineffective antimicrobial for treatment. Estimates from various sources indicate that fluoroquinolone resistance, likely acquired from the administration of antimicrobials to food animals, leads to >400,000 excess days of diarrhea in the United States per year compared with the duration that would occur if all of the isolates were susceptible. Antimicrobial resistance also could account for an extra 8677 days of hospitalization for nontyphoidal salmonellosis, mainly arising from food animals.