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Incidence of HIV-1 and HIV-2 infections in a rural community in southern Senegal

Diop, Ousmane M.a; Pison, Gillesb; Diouf, Ibrahimac; Enel, Catherineb; Lagarde, Emmanueld


aInstitut Pasteur, Dakar, Senegal; bInstitute National d'Etudes Démographique et Laboratoire de Dynamique des Populations Humaines, Muséum National d'Histoire Naturelle/Centre National de La Recherche Scientifique, Paris, France; cMinistère de la Santé, Dakar, Sénégal; and dUnité 88, Institut National de la Santé et de la Recherche Médicale, Saint-Maurice, France.

Received: 1 March 2000; accepted: 8 March 2000.

Since the beginning of the epidemic in sub-Saharan Africa, Senegal showed particularly low levels of HIV infection when compared with surrounding countries such as Mali, Burkina-Faso or Ivory Coast [1]. Sentinel surveillance surveys conducted among pregnant women in urban areas found a prevalence of HIV infection of less than 1% in 1997, with highest levels in the south of the country [2]. Very few population-based data on HIV levels and incidence are, however, available, especially from rural areas where 60% of the population live.

We conducted two exhaustive surveys of HIV infection in 1990 and 1995 in the population of a rural community of Senegal. The study area is located in the region of Ziguinchor, in the south-west of Senegal. Demographic follow-up of the population of this area has been underway since 1985. After an initial census, all new vital events (births, deaths, marriages and migrations) have been recorded annually. In 1995, 7243 inhabitants lived in the area.

After informed consent, all adults aged 20 years or older in 1990 [3,4] and all adults aged 18 years or older in 1995 were invited to give blood samples for HIV testing.

In 1990, blood samples were provided by 3230 adults aged 20 years or more, that is 95% of the population of this age group. Among them, 25 individuals (0.8%) were HIV-2 seropositive and two individuals (0.06%) were HIV-1 seropositive. In 1995, 2410 adults aged 18 years or more, that is 69% of the population of this age group, were tested. No participant less than 20 years of age was seropositive. Considering only the 2179 participants aged 20 years or more, 12 (0.6%) were HIV-2 seropositive and seven (0.3%) were HIV-1 seropositive.

Among the 1434 participants who were seronegative in 1990 and who were tested again in 1995, six were seropositive in 1995 (four HIV-1 and two HIV-2). This corresponds to an annual incidence rate of HIV infection of 0.8 per one thousand adults during the period 1990–1995 (95% confidence interval 0.71–0.97).

Participation refusal rates were higher in 1995 than in 1990. As a consequence, the comparisons between the two surveys might be biased if those who did not participate in the 1995 survey were more infected, or less infected, than those who did. To examine the potential bias, the 3230 participants who were previously tested in 1990 were divided into four groups: (i) those who were present in 1995 but did not attend the 1995 blood sampling session (n = 704); (ii) those who had left the area between 1990 and 1995 (n = 891); (iii) those who had died in the interval (n = 191); and (iv) those who were tested again in 1995 (n = 1444). Seroprevalence in 1990 among these four groups was 0.6, 1.1, 1.6 and 0.7%, respectively, when combining HIV-1 and HIV-2. Differences between groups are small, particularly between the first group, who refused the test in 1995, and the last group, who accepted it, an indication that there is no important bias. Power analysis showed that it can be stated with a confidence of 86% that the ‘actual’ difference of prevalence between groups 1 and 4 would not exceed 2%.

The relative proportion of those infected with HIV-1 among HIV-infected people increased significantly between the two surveys, from 4% (one out of 27) in 1990 to 37% (seven out of 19) in 1995 (P = 0.006; Fisher exact test).

In summary, the present study showed that the prevalence of HIV infection among adults had not changed during the period 1990–1995 in this rural area of southern Senegal, and that the prevalence of HIV-1 infection, although still very low, has increased recently. For HIV-1 and HIV-2 infections combined, the annual incidence rate among adults was 0.8 per thousand.

Ousmane M. Diopa

Gilles Pisonb

Ibrahima Dioufc

Catherine Enelb

Emmanuel Lagarded

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1. Meda N, Ndoye I, M'Boup S. et al. Low and stable HIV infection rates in Senegal: natural course of the epidemic or evidence for success of prevention? AIDS 1999, 13: 1397 –1405.
2. Groupe de Surveillance Séro Epidémiologique. Comité National de Prévention du SIDA, Sénégal. Bulletin Epidémiologique HIV no. 7. June 1999.
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© 2000 Lippincott Williams & Wilkins, Inc.