Aedes (stegomyia) albopictus (skuse), a potential new dengue vector in Southern Cameroon. (Dispatches).

Aedes albopictus, a mosquito vector of Dengue virus, has been recorded for the first time in Cameroon. Entomologic surveys in 2000 demonstrated that it is widespread in southern Cameroon, colonizing a wide variety of breeding sites and biting humans in every district surveyed. The presence of this vector increases the risk for emergence of dengue in Cameroon.

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Aedes albopictus is among the most important arbovirus vectors in the world, particularly for Dengue virus (DV) (1). The microhabitats of its larvae are mainly tree holes and a wide variety of containers. The eggs can survive desiccation for several months. The adult biology of Ae. albopictus is similar to that of the urban population of Ae. aegypti, a dengue and yellow fever vector (2). The characteristics of its eggs, its close association with humans, and increasing intercontinental travel have favored the expanding global distribution of this Asian species (3).

Ae. albopictus was recorded in North America as early as 1972. Established populations were detected in 1985, imported from Asia in used tires (4). Its presence was reported in Brazil in 1986, then in the Pacific islands and the Caribbean islands, and more recently in Europe (Albania, Italy, and France) (5,6).

In Africa, this vector was observed for the frst time in 1989 in South Africa. After its eggs were introduced in tires from Japan (7), Ae. albopictus was recorded in Nigeria in 1991 (8), where it has become widespread. To date, this vector has not been observed in other sub-Saharan countries.

Surveys of Ae. aegypti distribution conducted from 1950 to 1995 in several regions in Cameroon did not record Ae. albopictus. A large trial conducted in 1976 in 84 locations recorded 1,112 Ae. aegypti-positive larval development sites but none positive for Ae. albopictus (9). Moreover, entomologic investigations during two yellow fever epidemics in 1990 and 1995 in North Cameroon recorded only Ae. aegypti (10; unpub, report: Enquete entomo-epidemiologique sur deux cas mortels de fievre jaune survenus dans la ville de Ngaoundere [Province de l'Adamaoua, Cameroun], ORS-TOM laboratory, Centre Pasteur of Cameroon, 1995). DV has never been isolated in Cameroon.

In October 1999, one of the authors captured biting Ae. albopictus females, which prompted a thorough investigation to monitor the presence, distribution, and biology of this species in southern Cameroon.

Materials and Methods

Study Sites

Surveys were conducted in the two main cities of Cameroon: Douala, pop. 1,400,000 (4 [degrees] 00'N, 9 [degrees] 45'E), commercial harbor and largest city in Cameroon, and Yaounde, pop. 1,300,000 (3 [degrees] 4150'N, 11 [degrees] 30'E) the capital city, located at an altitude of 800 m. Entomologic studies were also conducted in Campo (2 [degrees] 30'N, 9 [degrees] 50'E; pop. 4,000), Edea (3 [degrees] 45'N, 10 [degrees] 10'E; pop. 100,000), and Bafia (4 [degrees] 45'N, 11 [degrees] 15'E; pop. 50,000).

Larvae and Adult Mosquito Collections

Larval development sites of mosquitoes were investigated in four districts in Yaounde (Gare, Cite Verte, Brasseries, and Biyemassi), four districts in Douala (Dibom, New Bell, Bonaberi, and Makepe), and three districts in Edea and Bafia. Approximately 20 potential breeding sites containing water were sampled in each district in Yaounde and Douala; an average of seven breeding sites were sampled in each district in Edea and Bafia. A breeding site was recorded as positive when it contained mosquito larvae or pupae, whatever the species.

Biting behavior of mosquitoes was checked by five adult volunteers in the districts of Yaounde, Douala, and Campo. These volunteers collected mosquitoes landing on their arms or legs from 5:00 to 6:30 p.m. All surveys were conducted in October and November 2000, at the end of the long rainy season.

Larvae and adults were identified by the morphologic identification keys and morphologic descriptions of African Aedes species (11-13). Male genitalia were dissected and examined under a microscope.

Results

Ae. albopictus was present in all five towns and in every district sampled. Species identification was confirmed on larvae and adult males and females. Of the positive larval development sites sampled, 75% of 36 in Yaounde and 45% of 53 in Douala contained Ae. albopictus larvae. Ae. albopictus was found in five breeding sites in Edea and seven in Bafia (Table).

The volume of water in Ae. albopictus-positive breeding sites ranged from 50 mL to 100 L. Species found together in the same sites were Ae. aegypti, Anopheles gambiae s.s., Culex gr. decens, Cx. quinquefasciatus, Cx. poicilipes, Cx. duttoni, Cx. (Culiciomyia) sp., Cx. (Lutzia) tigripes, and Eretmapodites quinquevittatus. Of breeding sites positive for Ae. albopictus or Ae. aegypti, both species were found together in 68% of sites in Yaounde, 50% in Douala, 33% in Edea, and 38% in Bafia.

Late afternoon captures of adults demonstrated that Ae. albopictus is anthropophilic. The average number of Ae. albopictus females collected per volunteer from 5:00 to 6:30 p.m. was 1.1 (range 0 to 8) in Douala and 3.0 (range 0 to 17) in Yaounde. Other species collected were Ae. aegypti, An. gambiae s.s., Cx. quinquefasciatus, Cx. antennatus, Cx. perfuscus, Cx. from neavei group, Cx. from decens group, Er. quinquevittatus, Mansonia uniformis, and Ma. africana. Ae. albopictus was the species most often captured, accounting for 35% of all the mosquitoes.

Conclusions

In 2000, Ae. albopictus was already widespread in South Cameroon. It was present in all the districts and towns sampled, in a wide variety of breeding sites, the most common being used tires, as described elsewhere (2). Used or retread tires are imported regularly from the United States, Nigeria, and South Africa, countries where Ae. albopictus is present (unpub. data, Ministry of Commerce, Cameroon). This observation strongly suggests that this species was introduced to Cameroon in this way, likely on multiple occasions in different regions.

The species is frequently associated with Ae. aegypti, as observed in other countries (14). Some observations from regions where Ae. albopictus was recently introduced suggest it tends to supplant Ae. aegypti (15). Such interspecific competition was experimentally observed in an insectary (16). The absence of Ae. albopictus in the lists of mosquito species observed in Cameroon before 1995 suggests that this species has colonized South Cameroon recently and that its diffusion has been rapid, as was the case in neighboring Nigeria and in America and Europe.

Ae. albopictus is a competent vector for DV. Because this disease is expanding in the world (17), data are needed on the actual distribution of Ae. albopictus throughout Cameroon and the potential risk for transmission of arbovirus. Surveillance of used tires, which seem to be its preferred breeding sites, can provide maximum information on species distribution at the lowest cost-effective rate. The presence of this vector, in association with Ae. aegypti, increases the risk for emergence of dengue in Cameroon.

Table. Breeding sites found positive for Aedes albopictus in 2000,
southern Cameroon

                          Number of positive /
Types of breeding sites   sites sampled of each     Percent
containing water                  type            Positive (%)

Used tire                         36/77                47
Plastic container                  7/27                26
Can and broken bottle              9/30                30
Plastic cup                        3/6                 50
200-L barrel                       0/7                  0
Abandoned car part                 6/35                17
Cement washtub                     0/4                  0
Flowerpot                          0/2                  0
Tree hole                          0/4                  0
Cow horn                           0/4                  0
Cocoa pod                          0/4                  0
Enameled plate                     1/6                 17
Snail shell                        1/3                 33

Acknowledgments

We thank Sue Gartland for help with the English translation.

This work was funded by l'Institut de Recherche pour le Developpement (IRD), France.

Dr. Fontenille is head of the medical entomology laboratory of OCEAC (Organisation de Coordination pour la lutte contre les Endemies en Afrique Centrale), Yaounde, Cameroon. For 20 years, he has conducted research in Africa on arboviruses and malaria vectors.

Mr. Toto is a technician in the medical entomology laboratory of OCEAC.

References

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(11.) Edwards FW. Mosquitoes of the Ethiopian region. III. Culicine adults and pupae. London: British Museum Natural History; 1941. p. 499.

(12.) Hopkins GHE. Mosquitoes of the Ethiopian region. I. Larval bionomics of mosquitoes and taxonomy of Culicine larvae. 2nd ed. London: British Museum Natural History; 1952. p. 355.

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(14.) Chan KL, Chan YC, Ho BC. Aedes aegypti (L.) and Aedes albopictus (Skuse) in Singapore city. Competition between species. Bull World Health Organ 1971;44:643-9.

(15.) Hobbs JH, Hughes EA, Eichold BH II. Replacement of Aedes aegypti by Aedes albopictus in Mobile, Alabama. J Am Mosq Control Assoc 1991;7:488-99.

(16.) Barrera R. Competition and resistance to starvation in larvae of container-inhabiting Aedes mosquitoes. Ecol Entomol 1996;21:117-27.

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Didier Fontenille * ([dagger]) and Jean Claude Toto *

* Organisation de Coordination pour la Lutte contre les Endemies en Afrique Centrale (OCEAC), Yaounde, Cameroon; and ([dagger]) Institut de Recherche pour le Developpement, Montpellier, France

Address for correspondence: Didier Fontenille, Laboratoire de Lutte contre les Insectes Nuisibles, Institute de Recherche pour le Developpement, 911 av. Agropolis, BP 5045, 34032 Montpellier Cedex 1, France; fax: [phone]; e-mail: didier, [e-mail]