Mem Inst Oswaldo Cruz, Rio de Janeiro, 94(4) Jul/Ago 1999
Development of Dirofilaria immitis (Leidy) in Aedes aegypti (L.) and Culex quinquefasciatus (Say) from Maceió, Alagoas, Brazil
Departamento de Patologia, Centro de Ciências Biológicas, Universidade Federal de Alagoas, Praça Afrânio Jorge s/no, 57010-020 Maceió, AL, Brasil
*Departamento de Entomologia, Centro de Pesquisas Aggeu Magalhães-Fiocruz, Recife, PE, Brasil
Dirofilaria immitis (Leidy, 1856) is a nematode that can be transmitted mainly by mosquitoes of the genera Aedes, Culexand Anopheles (KW Ludlam et al. 1970 JAVMA 157: 1354-1359). The susceptibility of mosquitoes to support development of D. immitis not only differs according to the vector species but also with regard to strains of a particular species (BM Christensen et al. 1984 J Invertebr Pathol 44: 267-274, CSB Apperson et al. 1989 J Am Mosq Control Assoc 5: 377-382). A recent survey carried out in Maceió, State of Alagoas, Brazil, showed a prevalence rate of 3% of canine filariasis (unpublished data), but the local vector of D. immitis has not yet been determined. The occurrence ofAe. scapularis, Cx. nigripalpus and Mansonia titillans mosquitoes has been reported in Maceió (CML Calheiros 1996Transmissores em Potencial e Naturalmente Infectados pela Wuchereria bancrofti (Cobbold, 1877) em Maceió- Alagoas, MSc Thesis, UFMG, 147 pp.). However, Cx. quinquefasciatus and Ae. aegypti,the local vectors of bancroftian filariasis and dengue fever respectively, are the most abundant mosquito species in Maceió. Results of experiments carried out to demonstrate the susceptibility of these species to infection with D. immitis and their vector efficiency are reported here.
Both mosquitos populations were originated from Maceió. Therefore, Ae. aegypti were reared at the insectary of Fiocruz, Recife, PE and the strain of Cx. quinquefasciatus was maintained at the insectary of the Universidade Federal de Alagoas in Maceió. Mosquitoes eggs were hatched in cubes containing dechlorinated water, larvae were reared with cat food (WhiskasÒ) and adults were kept at 27±1ºC and 70±10% R.H. A 4 years-old mixed-breed male dog naturally infected with D. immitis was used as the microfilariae (mf) donor (80 mf/20 ml blood) and one uninfected animal was used as control. Females 3-to 7-day-old of both mosquito species were allowed to feed on blood with anticoagulant via an artificial apparatus (LC Rutledge et al. 1964 Mosq News 24: 407-419) or directly on microfilaraemic and amicrofilaraemic dogs. Four different experiments were carried out and both mosquito species were fed simultaneously on the dog and via artificial apparatus between 4 p.m. and 5 p.m., during 30 min. Mosquitoes were dissected daily andD. immitis larvae were recorded. The developmental stages of the parasite were identified according to AER Taylor (1960 J Helminthol 34: 27-39). Mortality was recorded daily and the vector efficiency of mosquito species was determined as described by L Kartman (1954 Exp Parasitol 2: 27-78).
Survival rates recorded with a total of 1,573 Cx. quinquefasciatus and 1,588 Ae. aegypti females fed on the microfilaraemic dog, were 30.6% and 20% respectively. For mosquitoes fed on the uninfected dog the values were 85% for the former and 46.4% for the latter species. The death of infected mosquitoes could be caused by the movement of large numbers of mf from the midgut to the Malpighian tubules as suggested by DR Hamilton and RE Bradley (1979 J Med Entomol 3: 305-306). The sausage form was observed on the 4th day and from days 8 to 9, the larvae grew longer and more narrow (L2) reaching the infective larval stage (L3) on the 10th day for Ae. aegypti and four days later forCx. quinquefasciatus. Shorter developmental periods of D. immitis in a vector, as observed in Ae. aegypti, would favour disease transmission (KM Loftin et al. 1995 J Am Mosq Control Assoc 11: 90-93). Although both mosquito species ingested similar number of mf, the number of L3 recorded per female was significantly higher in Ae. aegypti. In fact, no L3 were recorded in Cx. quinquefasciatus which fed directly on the microfilaraemic dog and only one L3 was recorded in females which fed using the apparatus. In both species a large number of L3 parasite larvae was produced when females were fed through the apparatus (Table). The overall vector efficiency of Ae. aegypti was seven times higher than that ofCx. quinquefasciatus. According to Loftin et al. (1995 loc. cit.), the vector efficiency in Cx. quinquefasciatus was 2.7% compared with 20.4% of Ae. vexans. RC Lowrie (1991 J Am Mosq Control Assoc 7: 30-36) observed that the efficiency of Ae. taeniorhynchus was 65 times higher than that of Cx. quinquefasciatus. In conclusion, our results demonstrate that local populations of both mosquito species are susceptible to D. immitis allowing larval evolution until the L3 stage and that Ae. aegypti has clearly better potential than Cx. quinquefasciatus as a vector of canine heartworm in Maceió. In this context, Cx. quin-quefasciatus has been considered as a poor vectorof D. immits in different regions (N Labarthe et al. 1998 Mem Inst Oswaldo Cruz 93: 425-432, Lowrie 1991 loc. cit.). Investigations are being conducted to identify mosquitoes naturally infected among species from Maceió, by polymerase chain reaction using primers species-specific for D. immitis.