Amplification of the Sylvatic Cycle of Dengue Virus Type 2, Senegal, 1999-2000: Entomologic Findings and Epidemiologic Considerations

Mawlouth Diallo, Yamar Ba, Amadou A. Sall, Ousmane M. Diop, Jacques A. Ndione, Mireille Mondo, Lang Girault, Christian Mathiot

Disclosures

Emerging Infectious Diseases. 2003;9(3) 

In This Article

Results

A total of 24,747 mosquitoes belonging to six genera and 55 species were collected by using all sampling methods in 1999. Table 1 lists the number of pools and infection rates of mosquito species infected with DENV-2. Sixty-four DENV-2 strains were isolated and distributed as follows: Ae. furcifer(35 strains), Ae. taylori (11 strains), Ae. l uteocephalus (16 strains), and Ae. aegypti (2 strains) captured in October and November. From mosquitoes collected in June, no DENV-2 strain was isolated.

The highest mean infection rates were obtained from Ae. aegypti (2.74%), followed by Ae. taylori (1.28%). However, mean infection rates of species did not differ significantly (p=0.34). Infection rates showed temporal and spatial variations. Most virus strains were isolated from mosquitoes captured in November. Infection rates observed during that month were often higher than those obtained in October, but only Ae. furcifer showed a significant difference between these 2 months (p<0.001). Table 2 shows the spatial distribution of biting, infection, and inoculation rates for mosquito species associated with DENV-2. In the forest gallery, Ae. furcifer, Ae. taylori,and Ae. luteocephalus were very aggressive, whereas Ae. aegypti displayed weak biting activity. The highest biting rate was obtained from Ae. furcifer (average 4.16 bites per person per hour). The lowest biting rate was obtained from Ae. aegypti (0.08 bite per person per hour). The maximum rate for Ae. furcifer was obtained in October (5.87 bites per person per hour). For Ae. luteocephalus and Ae. taylori, the maximum rate occurred in June (3.28 bites per person per hour) and November (1.64 bite per person per hour), respectively. In the villages, only Ae. furcifer had significant activity, with a maximum of 8.75 bites per person per hour in Ngari. When collections from Silling, Bandafassi, and Ngari were compared, the highest biting rates of the species regularly collected in the villages were obtained in October.

Our results ( Table 2 ) show that DENV-2 circulated in the forest gallery and all the sampled villages. Indeed, among the 64 DENV-2 strains isolated, 58 were isolated from mosquitoes caught in the forest gallery and 6 from those caught in the villages: Ngari (2 strains), Silling (1 strain), Bandafassi (2 strains), and Kenioto (1 strain). In the forest gallery, except for Ae. furcifer, which exhibited significantly higher infection rates in November than in October (p<0.001), infection rates for all species were comparable in October and November. In the Kedougou area, the highest infection rate was obtained from Ae. aegypti (17%). However, the highest inoculation rate was obtained from Ae. furcifer, which was estimated to be responsible for at least two infected bites per person per week in November. Ae. furcifer was also the only mosquito infected with DENV-2 in the villages. The highest entomologic inoculation rate was obtained in Bandafassi, where a person might receive at least two infectious bites each evening.

A total of 2,423 adult mosquitoes belonging to the genus Aedes and the subgenera Albuginosus, Stegomyia, Aedimorphus, Diceromyia, and Finlaya emerged from the tree hole samples collected during the dry season in 2000. Ae. aegypti, Ae. bomeliae, and Ae. luteocephalus were the most common species. Out of 68 pools, no DENV-2 was isolated ( Table 3 ).

During the 2000 rainy season, out of 31,521 mosquitoes collected in the same locations, nine DENV-2 strains were isolated, all associated with circulation of yellow fever virus. Isolations were obtained from female mosquitoes including Ae. furcifer (one strain), Ae. taylori (two strains), Ae. luteocephalus (two strains), Ae. aegypti (one strain), Ae. vittatus (two strains); isolations were obtained from males of Ae. furcifer (two strains) captured in the forest gallery. Isolations were obtained in August (three strains), October (five strains), and November (one strain).

DENV-2 IgG was detected in nonhuman primate blood samples, but no evidence of recent infection (IgM antibody) was obtained. Of a total of 17 African green monkeys (Chlorocebus sabaeus) collected, 8 juveniles (<4 years of age) and 9 adults were captured in a forest gallery near Ngari. In all samples, serologic test results were negative for DENV-2 IgM antibody. A seroprevalence of 58% for DENV-2 IgG antibodies was detected; no significant difference (p=0.09) was detected according to age (77% in adult and 37% in juvenile monkeys).

The years 1999-2000 were characterized by a rainfall surplus compared to the 1960-1999 seasonal mean rainfall. Retrospective analysis showed no clear relationship between dengue emergence and rainfall anomalies. DENV-2 amplifications were detected during periods of heavy rainfall as well as during periods of low precipitation (Figure 2).

Seasonal rainfall anomalies in Kedougou, Senegal, 1972-1999.

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