Abstract

4.5 Yellow fever and Aedes

A highly effective prophylaxis exists for yellow fever but no specific therapy as the course of the disease is too fast. The pathogen is a virus with a spheric nucleoid surrounded by a nucleocapsid covered by a peplos in which the antigenic specifity is located. Sytematically, such a virus was formerly named an arbovirus (arthropod-borne virus) but now they are classified as flaviridae, which are transmitted by mosquitoes or hard ticks.

Disease and virus life cycle: After an incubation of 4 – 6 days, the initial phase starts with sudden high fever and icterus attributable to an onslaught on the reticulo-endothelial system, lymph nodes, bone marrow and liver. Viraemia lasts throughout the first to third day of sickness (transferent phase). Thereafter, follows a short remission of the fever for one or two days and one third of the cases recovers completely acquiring an occult protective immunity. Two thirds of the cases suffer once again increasing fever, which represents the start of the phase of systemic organ damage characterized by oliguria to anuria, visceral haemorrhagiae, "vomito negro" (vomitting of coagulated blood), and the disease ends fatally in 4 of 5 cases. Therapy is limited to symptomatically supportive measures (fig. 4.13, page 229).

Vaccines provide secure protection for at least 10 years. Vaccine 17 D (Rockefeller institute) needs a cooling chain and is administered subcutaneously. Dakar (Institute Pasteur) is thermo-resistant and is administered by scarification. Because of the risk of encephalitis during childhood, only persons of 14 and older are vaccinated. For antibody proof, test serum and virus are injected simultaneously into mice, which survive when the test is positive.

Development in the vector, Aedes aegypti. The virus passes the intestinal epithelium and propagates in haemolymph cells for 10 – 12 days. Celation occurs in wild mosquitoes at common tropical outside temperatures. Thereafter, the mosquito is infective lifelong. Aedes aegypti is fully active at 13 °C, motionless at 17 °C and dies at 6 °C. Celation lasts only 4 days at 34 °C but 30 days at 18 °C. The virus is transmitted by the saliva at each bite.

Biology of Aedes spec. The mosquitos breed in small to minute ponds of water even if these are of limited duration, e.g. empty snail shells or puddles in the shafts of elevators. The eggs are deposited there even under dry conditions and undergo embryogenesis in the open air. Thus, the first larva is ready to hatch immediately at the onset of rain (fig. 4.18, page 232). In rain forests, epiphytes in the crown region serve as breeding places, whereas these are provided in plantations by the leaf axil e.g. of banana plants. Aedes spec. breeding in these types of regions pass through silvatic cycles in arboretic monkeys (animal reservoir). They represent the source of rural epidemics of yellow fever and, subsequently, human traffic results in urban epidemics (fig. 4.19, page 233). The disease is a viral zoonosis. Successful control is achieved by organized surveillance: weekly information by the World Health Organisation (WHO, table 4.2, page 229) and strict regulations for vaccination with respect to international travel. In Europe, exclusively authorities exclusively administer vaccinations. Confusion of yellow fever with hepatitis must be avoided.

Biology of the aquatic stages of mosquitoes or culicidae . -- Eggs are laid singly with swimming bodies (Anopheles spec.) or united into layers, which are free-swimming (Culex spec.) or submerged adhered to plants (Mansonia spec.). – Larvae may dive actively and breathe by the closable paired stigma of the eighth abdominal segment without (Anopheles spec.) or with a long (Culex spec.) or a short (Aedes spec.) sipho (fig. 4.16 and 4.17, page 231). Mansonia spec. pierces the air tubes of water-plants by means of a cuticle saw on the stump sipho (fig. 4.16 b and c). All the larvae feed selectively on detritus or small crustaceae obtained by a self-produced local circulation of water or collected from the water surface (Anopheles spec.) (fig. 4.18, page 232). – The diving pupae swim by means of their mobile abdomen. Its paired mesothoracic stigma is provided with a closable tube. – The imago is winged and the external male and female genitalia of Aedes aegypti are used for identification ( fig. 4.14 and 4.15, page 230). For the biology and biting apparatus, see chapter 2.1.2 Biology of culicids.

Recent epidemics have occurred in populations as a result of uncontrolled urbanisation, the growth in the numbers and density of populations, the selective logging of large trees in tropical forests and the construction of roads. In Western Africa south of Sahara, the border of the enzootic zone runs between the grass-bush-savannah and the forested savannah. During occasion local epidemics in 1986/7, a total of 5345 cases were registered, of which 1515 ended fatally. At Brasilia, the population at risk (17 million people) is immunisized (80 – 100 %) by the17D vaccine. Only single cases are observed. Asia and Australia are free of yellow fever. For its recent geographical distribution, see fig. 4.20, page 234.

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