Period of communicability As long as there are infected mosquitoes continuing to bite people. Mosquitoes can also become infected by feeding on a clinical case any time during the illness.
Occurrence and distribution Serological surveys indicate that most people living in endemic areas contract sub-clinical infection before the age of 15 years. However, young children and adults, who have not been infected as children (including visitors), may get clinical disease and possibly severe disease, with 20-30% mortality. There are about 50,000 cases and 10,000 deaths reported annually.
The endemic area is South and Southeast Asia, particularly Cambodia, Laos, Vietnam, Thailand, Malaysia, Myanmar, Indonesia, Philippines, the Indian subcontinent, Russia and a decreasing incidence in China, Japan and Korea. A small outbreak in Torres Strait islands is a worrying sign that the infection might spread into northern Australia. Risk within any of these countries is greatest during the rice-growing season and when an epidemic is ongoing.
Control and prevention Agricultural methods, such as drying out rice fields when no crop is growing or decreasing the number of crops, can reduce the period of risk. Personal protection with long sleeved clothing, the wearing of trousers and use of repellents can reduce mosquito biting. The mosquito bites during the daytime, so babies and young children should be made to sleep under insecticide-treated mosquito nets. The main method of prevention is the use of vaccination to all children in endemic areas, after the age of 1 year, so as not to interfere with remaining maternal antibodies. However, allergic reactions to the vaccine are reasonably frequent, so caution should be exercised especially in allergic persons. Repeat booster doses are necessary as the period of protection has not been fully worked out and there is some suggestion of a shift in the age group contracting the disease, in areas where vaccination has been used for some years.
Treatment There is no treatment.
Surveillance Notification of cases should be reported to WHO, so that neighbouring countries and visitors can take precautions.
Organism Dengue virus has four serotypes (1, 2, 3 and 4).
Clinical features Dengue presents as a sudden onset of fever, retro-orbital headache, joint and muscle pains. A maculo-papular or scarlatina-form rash usually appears after 3-4 days. Depression and prolonged fatigue often occur following the acute manifestations. Dengue haemorrhagic fever (DHF), in which there is profound bleeding into skin and tissues, is now a serious feature of many epidemics.
DHF is probably due to a sensitization with a previous dengue serotype, either acquired at birth or from a previous infection, type 2 being the most potent and types 3, 4 and 1 being responsible in decreasing importance. Differential effect on racial groups suggests that host factors may also have a role, as well as the geographical origin of the dengue strain.
Diagnosis Virus can be isolated from the blood in acute cases or a rising antibody level may assist in diagnosis.
Transmission Mosquitoes of the Aedes group, especially Ae. aegypti, Ae. albopictus or a member of the Ae. scutellaris group are responsible for transmission. These mosquitoes are more easily identifiable than most by their black colour, with distinctive white markings (Fig. 15.2). They like to breed close to humans, taking advantage of any water containers, old tyres, empty tins or other small collections of water in which they can breed. They are daytime biters and can be found in large numbers in urban and peri-urban areas. Ae. albopic-tus has comparatively recently become established in the USA, Central America and the Caribbean due to the trade in used tyres.
Virus is maintained in a human/mosquito cycle in many parts of the world, but in Africa and Southeast Asia, a monkey/ mosquito cycle is involved.
Incubation period 3-15 days (commonly 4-6 days).
Period of communicability The mosquito is able to transmit infection for 8-12 days after taking an infective blood meal and remains so for the rest of its life. Humans and monkeys are infectious during and just before the febrile period.
Occurrence and distribution Dengue is now endemic in many parts of the world, South and Central America, sub-Saharan Africa, South and Southeast Asia. In more isolated communities, large epidemics have occurred, especially in the island countries of the Caribbean and Pacific with devastating effect. The epidemic can be so massive as to immobilize large segments of the population, disrupt the work force and cause a breakdown in organization. The development of DHF has been variable, producing a number of deaths. It is estimated that there are about 50 million cases of dengue, 0.5 million cases of DHF and 12,000 deaths due to dengue every year. Children are the main sufferers of both dengue and DHF.
Control and prevention The main method of control is to reduce mosquito breeding, especially of the Aedes mosquito, by depriving it of collections of water or covering them so that mosquitoes cannot enter. All water tanks, pots or other containers must be covered at all times, a recent improvement being to treat these covers with insecticides as it is often difficult to get a perfect fit.
Fig. 15.2. Aedes, the black and white mosquitoes.
Fig. 15.2. Aedes, the black and white mosquitoes.
Guttering around the roof can also allow pools of water to collect, so these should be of sufficient slope and cleaned out regularly so that water cannot collect. Old tyres should have holes cut in them or removed altogether (one answer to the disposal problem is to weight them and bury them at sea to form artificial reefs).
People should check their gardens and immediate vicinity at regular intervals to remove any cans, coconut shells or other temporary collections of water. Children are very effective at doing this and can be encouraged with a marks or reward scheme.
Screening of houses and mosquito nets are of little use because people are often outside their houses when the mosquito bites, but are of value for young children. ULV spraying, either by fogging or by aircraft, is of value in the presence of an epidemic, but only adult mosquitoes are killed and are soon replaced by young adults unless simultaneous larval control is also in operation.
Treatment There is no specific treatment, but hypovolaemic shock must be treated with rapid fluid replacement and oxygen therapy.
Surveillance Regular checks should be made on mosquito breeding, especially of Ae. aegypti. Samples should be taken and the number of larvae breeding counted to give an indication of the risk of transmission. Further details will be found under yellow fever.
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Remember to prepare for everyone in the home. When you are putting together a plan to prepare in the case of an emergency, it is very important to remember to plan for not only yourself and your children, but also for your family pets and any guests who could potentially be with you at the time of the emergency.