Diseases and Human Evolution

18: Transoceanic Hitchhikers: Yellow Fever and Its Dengue Cousin

CHAPTER 18 Transoceanic Hitchhikers Yellow Fever and Its Dengue Cousin Yellow fever and dengue fever belong to a large group of viruses called arboviruses carried by blood-sucking mosquitoes, ticks, sandflies, and biting midges capable of infecting vertebrates. Over five hundred of these arboviruses, representing several groups of different but closely related viruses, have been recognized around the world (Lederberg et al. 1992). Every part of the globe has its own set of insect vectors and arboviruses interacting with local fauna. Many of them infect primarily lower vertebrates and migratory birds without causing any harm to their natural hosts (Ray 1984). Human beings become accidental hosts to some of the arboviruses whenever they intrude into insect vector territory and disrupt the normal animal host–vector cycle of infection. Over a hundred arboviruses have spilled over into human populations. Most of these viruses produce few, if any, ill effects in human hosts. Some, like yellow fever and dengue fever, can generate acute infections with varying degrees of illness, depending on the type of virus and the host’s immune reaction. Symptoms can range from influenza-like illness with or without joint pains, to more severe illnesses, such as encephalitis, and hemorrhagic disease (Benenson 1995). 299 Most of the arboviruses causing severe disease have been restricted to remote areas of the world and have had negligible impact on major human populations. However, as transoceanic commerce has expanded, some of these viruses have managed to break out of their geographical isolation to find new habitats and vector-host cycles to their liking. Outbreaks of disease from some of these arbovirus infections have had major impacts on human settlements and commerce wherever they have been introduced. Yellow Fever The mosquito-transmitted arbovirus that causes yellow fever escaped from confinement in the jungles of west Africa with expansion of the slave trade in the seventeenth century. Along with malaria, slave ships carried this disease to the New World where it quickly established itself in a reign of terror over European immigrants and native peoples alike. The disease became known as “Yellow Jack” during its early history. The nickname derived from the requirement of ships carrying the infection to hoist a yellow flag when they approached ports (McNeill 1976). Unlike malaria infection, yellow fever was not carried to the New World by the African slaves themselves. The infection lasts only a short time and would not have survived the long sea voyages in the blood of the slaves. The only way the virus could have been shipped out of its homeland was through vector mosquitoes traveling as stowaways on board the slave ships, laying their eggs in the ships’ water casks, and attacking crewmen and human cargo alike (McNeill 1976). The mosquitoes would have found it difficult to survive without some form of nectar or other plant juice feedings. Without these feedings, the adults could only have survived for a few days, not long enough for females to produce the next generation of eggs (Nasci and Miller 1996). The ships’ crews must have provided some form of plant juice or sugar-laden feedings for its winged stowaways. Once they landed in the Caribbean, the mosquitoes quickly adapted to the tropical climate, so similar to their native habitat. The mosquito responsible for spreading yellow fever outside of west Africa proved to be Aedes aegypti. This particular mosquito evolved a close relationship with humans as they settled down to farm and build settlements. The mosquitoes primarily became attracted to the water containers people made, finding them ideal for egg laying. As day feeders, the females in their 300 CHAPTER 18 search for blood meals also grew attracted to the lactic acid oxidation products in human sweat. Their most active times coincide with human activities in the early morning and late afternoon, and they hunt their prey between 50 cm (2 inches) and 1 m (about 3 feet) above the ground (Ribeiro 1996). Wild Aedes females choose natural small containers of clean water, such as rot holes in trees or leaf axils of plants, in which to lay their eggs. They avoid water with dirt or sand bottoms. Each female lays about seventy eggs around the damp zone just above the water’s edge, and the eggs hatch in installments over time. Several females may lay hundreds of eggs in the same container, forming a large mass of eggs above the water line...