**10. Yellow fever virus**

These human movements increase the risk of contamination of non-immune persons travel‐ ling in areas where contaminated vectors persist and, conversely, favour the introduction of

YF outbreaks are common in Africa despite the current knowledge of the disease transmis‐ sion and the availability of a vaccine. In Africa, YF cases are not uniformly distributed throughout the endemic area; rather, more cases are reported in West Africa compared to East and Central Africa. Genetic differences between genotypes of YF in Africa probably contribute to the observed distribution of YF outbreaks. Genetic and behavioral variation in mosquito vectors may also play a major role in the distribution of YF outbreaks. The other factors also contribute to the epidemiology of YF, including host genetic background, cli‐

Yellow fever virus is transmitted principally by insects (mosquitoes), but ticks (*Amblyomma variegatum*) may play a secondary and minor role in Africa. It was not until 1901 that yellow fever transmission to humans was associated with the blood-feeding by the *Ae. aegypti* mos‐ quito (Figure 2), which was a major breakthrough in understanding this dreadful disease. Dispatched to Cuba by the United States government to investigate the cause of YF, Walter Reed and colleagues confirmed that the primary mode of YF transmission to humans was the *Ae. aegypti* mosquito (Figure 2) and the in ground-breaking virologic studies demonstrat‐ ed that the disease was caused by an agent that could be filtered from the blood of infected individuals [49]. The reservoirof yellow fever virus is the susceptible vector mosquito spe‐ cies that remains infected throughout its life and can transmit the virus transovarially. Yel‐ low fever can persist as a zoonosis in the tropical areas of Africa and America, with nonhuman primates responsible for maintaining the infection. Man and monkey play the

mate, vaccination coverage, vertebrate hosts and movement of vertebrate hosts [48].

role of amplifiers of the amount of virus available for the infection of mosquitos [50].

**Figure 2.** *Aedes aegypti*, the primary disease vector for yellow fever (Photo by Muhammad Mahdi Kharim, published

the disease into previously YF free zones [47].

**8.6. Genetic and behavioral variation**

216 Encephalitis

**9. Yellow fever vectors**

under the GNU free documentation licences)

Ever since the causative agent of YF disease YFV, was first isolated in 1927 from a Ghanaian patient named Asibi [50], the Asibi YFV strain is still widely used by the scientists of today. YFV is the prototype member of the family Flaviviridae(from the Latin flavus, meaning yel‐ low), and genus Flavivirus, which get their name from the Latin word for yellow (flavus). The genome is a single-stranded, positive-sense RNA, 10,500 - 11,000 nucleotides in length. The genus Flavivirus contains approximately 70 viruses, and the major flavivirus diseases are yellow fever (YF), dengue, West Nile, Japanese encephalitis, and tick-borne encephalitis [51]. Unlike other mosquito-borne flaviviruses, YFV has a tropism for the liver and causes a viscerotropic disease whereas many other mosquito-borne flaviviruses have a tropism for the brain, or in the case of the DEN viruses they target cells of reticuloendothelial origin [52].

It was one of the earliest viruses to be identified and linked to human disease. Although substantial variation exists among strains, they can be grouped into monophyletic geo‐ graphical variants, called topotypes. African isolates are usually grouped into two topo‐ types, associated with East and West Africa [53,54], although some studies have argued for up to five [55]. Two more have been identified from South America, although one has not been recovered since 1974, suggesting that it may be extinct in the wild. There is no evidence for a difference in virulence between the topotypes [56]. YF activity often occurs in areas af‐ ter increases in temperature and rainfall that will favor increased biodiversity, including in‐ creased numbers of animals and arthropods while reduced rainfall limits mosquito vector density [49]. It has been known for over 50 years that increased temperatures are associated with enhanced transmission of YF virus [56] due to shortened extrinsic incubation period and increased biting by mosquitoes of vertebrate hosts [49].
