Superinfection seen frequently in heterosexual couples in Zambia

Superinfection among heterosexual couples in sub-Saharan Africa may be surprisingly frequent, according to findings from Zambia presented on Tuesday at the 48^th Interscience Conference on Antimicrobial Agents and Chemotherapy in Washington DC.

Researchers from the Zambia Emory HIV Research Project presented evidence that 3 out of 34 people among heterosexual Zambian couples were superinfected during the time of the study. This represents a considerably more common prevalence of superinfection than has been shown in other studies.

Superinfection, or reinfection, occurs when an individual who is already HIV-positive becomes infected with a new, distinct strain of HIV in addition to their existing infection. This results in increased viral diversity within the same individual, which can be more difficult for the immune system to control, causing increases in viral load and possibly faster disease progression.

This study looked at cohabiting heterosexual couples in Zambia who were originally infected with genetically distinct strains of clade C HIV. These couples were drawn from the larger Zambia Emory HIV Research Project (ZEHRP), a cohort of HIV-discordant couples. Although HIV prevention efforts reduced HIV transmission amongst couples in the ZEHRP cohort, the uninfected partners still become HIV-positive at an annual incidence rate of 8%. Roughly 15% of these new infections come from someone other than the participant's cohabitating partner.

This superinfection sub-study looked at 17 couples in which both partners were HIV-infected, but with genotypically distinct viruses. Three of the 34 individuals in the study (nearly 9%) were confirmed as being superinfected. In one case, the male partner, who was already chronically infected, became superinfected from an outside partner. He was the source of his wife's initial infection; she then became superinfected from another outside partner. In a third case, a male partner became superinfected from his chronically infected spouse.

The cases of superinfection were identified by a two-phase genotypic analysis. First, a new assay (heteroduplex mobility assay, or HMA) screened for variations in the HIV gp41 protein. This was followed up by phylogenetic analysis of the HIV env gene. These analyses confirmed the emergence of distinct, genetically unrelated infections in each of the three people at time points after their original infection. The analyses also showed that the superinfections resulted in new, recombinant strains of HIV in all of the superinfected people. In two cases, superinfection was accompanied by a ten-fold increase in viral load.

The investigators concluded that, in this retrospective study of a small, specific population, "superinfection appears to be a frequent event," seen in 3 out of 34 individuals studied.

These findings were drawn from a particular group of heterosexual Zambian couples. There is no reason to assume that they would carry over to other populations, since so many factors might vary. (E.g., the HIV subtype C seen in all couples in this cohort is only one of many viral subtypes, which tend to be highly localised.) However, these results indicate that, in at least some populations, superinfection might be much more common than supposed.