YELLOW FEVER DISEASE
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Yellow Fever is one of the Flavivirus spp. (also called arbovirus) that occurs in Africa and tropical America. It is absent from other world regions even though potential vectors exist in many of those areas. The disease is an infection of simians in forests that can also be transmitted to humans. It is one of the most virulent of human diseases causing jaundice, hemorrhages and loss of vigor. The incubation period in humans is generally 3-7 days, while in monekys it is longer (9-14 days). Experiments have shown that an infected mosquito may be able transmit the virus for 118 days. In the 1850's just prior to the United States Civil War, Yellow Fever claimed the lives of thousands of people living in wet areas of the southern United States. Mosquitoes had not yet been implicated in its spread, and it was believed that the disease could be spread merely by human contact. The sick and dying were banished to inhospitable regions such as offshore islands where there was no medical treatment available.
Service (2008) noted that in Africa simians of the genera Colobus, Cercopithecus and Galago are primary reservoir hosts, and that the virus circulates among these primates with mosquito vectors such as Aedes africanus that breed in tree holes. Matheson (1950) provides a long list of monkey species that harbor the virus in nature. Mosquito activity is highest after sunset in the forest canopy when the hosts have settled down for the night. The reservoir hosts show only slight symptoms of the disease, it being rarely fatal. Imported exotic plants such as banana and pineapple provide a new habitat for the disease to circulate as monkeys leave the forest canopy to feed on their fruit. In such places different species of Aedes that are active during daytime vector the virus. Humans are more apt to be bitten in these forest peripheral habitats, and subsequently by traveling about they can spread the disease to other areas and in towns where different mosquitoes will serve as vectors, such as Aedes aegypti. Service (2008) thus distinguished the sylvatic from the rural cycle for the disease.
There are further complications with yellow fever transmission. For example, sometimes the virus may be circulating among monkey populations but rarely reach humans because the vector mosquitoes are not strongly attracted to them. The virus may also be transmitted transovarial among the Aedes species, which is more common in ticks. There is also a venereal transmission when virus-infected male mosquitoes pass the virus to female mosquitoes during mating.
In the Americas the yellow fever cycle is similar but differs in the kind of monkeys serving as reservoir hosts (Matheson 1950 & Service 2008). The principal vectors are in the forest-dwelling genus Haemagogus. Humans become affected in the forest habitat and then carry the virus to surrounding areas where Aedes aegypti becomes the principal vector species.
Matheson (1950) provides a list of 40 mosquito species that have been found capable of transmitting the virus (See: Yellow Fever Vectors). There are also other blood sucking arthropods that in experiments have been found to transmit yellow fever virus, including stable flies and assassin bugs.
There is an effective vaccine for yellow fever, which is important to receive before traveling to areas where the disease exists.
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