Incubation period days

Period of communicability is from before the fever commences to 5 days after, so the patient should be nursed under a mosquito net to prevent new mosquitoes from becoming infected.

Occurrence and distribution Yellow fever nearly always presents as an epidemic in humans, affecting all ages and both sexes, although adults (particularly males) who go into the forest are likely to be the first to contract the disease. There are estimated to be 200,000 cases and 30,000 deaths annually from yellow fever, most of them in Africa. Most cases have been reported from Nigeria in recent times. Yellow fever is restricted to the areas of Africa and South America, and Panama in Central America, shown in Fig. 5.1.

Control and prevention The most important part of the complex mosquito transmission cycle is Ae. aegypti. With its proximity to humans, it is capable of infecting a large

AFRICA

AMERICA

Aedes africanus

Haemagogus sp. Sebethes sp. Aedes sp.

AFRICA

AMERICA

Aedes africanus

Fig. 15.3. Yellow fever transmission cycles in Africa and South America (including Panama).

number of people as well as being the most easy to control. It breeds in small collections of water near people's houses, so a careful search for larvae and the destruction of breeding places can do much to reduce the danger. Simple clearance is the most effective method of reducing the mosquito population (see under dengue above), but insecticides such as temephos (Abate) can be used where collections of water cannot be destroyed. In the event of an epidemic, emergency reduction by fogging or ULV spray from aircraft will rapidly destroy the adult population (but not the larvae).

One attack of yellow fever confers immunity for life if the person survives the disease. Inapparent infections can also occur. A very effective vaccine has been developed which provides immunity for at least 10 years and probably longer, so all those at risk in the known endemic areas should be vaccinated (Fig. 5.1). This has been attempted by offering vaccination at markets and meetings or systematically to school children. WHO has now recommended that yellow fever vaccination be included in the childhood vaccination programme in the 33 countries of Africa in the yellow fever zone, to be given at the same time as measles vaccine. In the event of an epidemic, ring vaccination can be performed; the epidemic is surrounded by a circle of vaccinated persons, progressively closing in on the centre of the outbreak. Areas of Africa and South America have been designated as yellow fever areas (Fig. 5.1) and all visitors to this zone require vaccination.

In these days of rapid air transport it has always been surprising that yellow fever has not been transported to Asia, where there are the vectors and conditions for transmission. A suggested reason is that there is some cross-immunity with other Group B viruses and the level of such induced immunity may be sufficient to prevent epidemic spread. A precaution is to spray all aircraft coming from a yellow fever area.

Treatment There is no specific treatment, but supportive therapy is given to combat shock and renal failure.

Surveillance All cases of suspected or confirmed yellow fever must be reported to WHO. The prevalence of the urban vector can be measured by the Ae. aegypti index. This is the number of houses found with Ae. aegypti breeding within a specified area of 100 houses. Alternatively, the Breteau index can be used, which is the number of containers in which larvae are found out of 100 samples. If these are kept below 5% or preferably 1%, then the danger of an epidemic is minimized.

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