CULICIDAE (Mosquitoes)
(Figure 5-44 and 5-45)
Mosquitoes are well known pests of humans and animals (Fig. 5-44). This group is divided into three subfamilies, the Culicinae, the Anophelinae, and the Toxorhynchitinae. Only the Culicinae and the Anophelinae are important as parasites, the Toxorhynchitinae have predatory larvae and feed on plant fluids as adults. The distinction between culicine and anopheline mosquitoes is especially imprtant in human medicine and in mosquito control operations because the anopheline mosquitoes are vectors of the Plasmodium spp. which cause malaria in humans. In feline medicine, mosquitoes are important as vectors of heartworms and as the cause of hypersensitivity.
In the life cycle of a mosquito, the female lays eggs in or near water or in areas that are likely to have water when the water level rises. Culicine mosquitoes often lay their eggs in rafts, and the eggs of anopheline mosquitoes often bear later projections or wings that add buoyancy causing the eggs to float better. From the eggs, a larval stage hatches. The larvae of the culicine mosquitoes have a posterior breathing tube on the end of the body (Fig. 5-45); this breathing tube is lacking in anopheline larvae. The larvae tend to be filter feeders; culince larvae tend to hang from the surface film of the water from the breathing tube, while the anopheline lies parallel to the wurface film with the aid of palmate hairs that are present on the latero-ventral surfaces of the body segments. The larvae undergo four instars before pupation. The pupae are comma-shaped and have a pair of breathing tubes on the dorsal surface from which they hang in calm water. If disturbed the pupae will use its tail with terminal paddles to again force its way to the surface. After about one to 5 days as a pupa, the adult is ready to emerge. A split appears along the midline of the dorsal surface of the pupa, and the adult head and thorax emerges, next the wings come out and ungold, and finally, the legs are freed and stretch out. The adult is ready to fly in just a few minutes after the emrgence is complete. Males swarm and the females fly into the swarms where they pair with a male and copulate, typically one to two days after emergence. Female mosquitoes tend to mate only once during their life, and the males tend to live only for a few days, dying soon after copulation. Some females live for weeks to months in nature, and a few are capable of overwintering in cold or dry weather. For mosquitoes to be successful vectors of a pathogen, they must either take more than one blood meal as adults or be infected transovarily with a pathogen that was ingested by their mother. In feline medicine, the parasites that are transmitted by mosquitoes are those that are obtained from a prior bood meal.
Many different genera and species of mosquito are capable of serving as vectors of heartworms, Dirofilariaimmitis. One important vector of Dirofilariaimmitis is Culexpipiens which is worldwide in distribution. Culexpipiens breeds around human habitation, is willing to enter houses, and feeds at night. The mosquito vectors of canine dirofilariasis include species of Culex, Aedes, and Anopheles. There have been approximately 70 species of mosquito incriminated as potential vectors of canine dirofilariasis although not all are considered to be principle vectors in nature (Otto & Jachowski, 1981). At the same time, it is not clear how many of these mosquitos that are potential or principle vectors of canine heartworm are willing or prefer to feed on cats. Work by Genchi et al. (1992) has revealed that in northern Italy Culexpipiens was found to feed on cats at night while both Aedescaspius and Culexpipiens fed on dogs. Less total mosquitoes were recovered each night from cats then from dogs. This is obviously a field that requires more study iin order to understand the dynamics of feline dirofilariasis transmission.
A direct pathogenic effect of mosquitoes to the feline host is manifested as eosinophilic dermatitis (Mason and Evans, 1991). This condition was first described by workers in Australia as a seasonal and intermittent condition that usually resolved in the winter (Wilkinson and Bate, 1984). The disease has also been observed in the United States. The condition presents as papular eruptions, erosions, and depigmentation of the sin over the bridge of the nose, papular eruptions on the pinnae of the ears, and sometimes lesions are seen on the pads of the paws. These cats will sometimes have positive skin tests to extracts of mosquito antigens (Mason and Evans, 1991). Culexorbostiensis and Aedesmultiplex have been collected from affected cats in Australia (Mason and Evans, 1991), and potential vectors in New Zealand included Aedes notoscriptus and Culex pervigilans (Johnstone et al., 1992).
REFERENCES:
Genchi C, Sacco BD, Cancrini G. 1992. Epizootiology of canine and feline heartwrom infection in norther Italy: possible mosquito vectors. Proc Heartworm Symp '92, Austin, TX, 39-46.
Johnstone AC, Graham DG, Andersen HJ. 1992. A seasonal eosinophilic dermatitis in cats. New Zeal Vet J 40:168-172.
Mason KV, Evans AG. 1991. Mosquito bite-caused eosinophilic dermatitis in cats. JAVMA 198:2086-2088.
Otto GF, Jachowski LA. 1981. Mosquitoes and canine heartworm disease. Proc Am Heartworm Symp '80, Dallas, TX. 17-32.
Wilkinson GT, Bate MJ. 1984. A possible further clinical manifestation of the feine eosinophilic granuloma complex. J Am An Hosp Assoc 20:325-331.
Figure 5-44. Aedesaegypti. Adult mosquito probing on a human arm for a feeding site. This mosquito is missing one of its legs.
Figure 5-45. Aedesaegypti. Larval stage that is found in water.