7.1  Dengue fever/Dengue haemorrhagic fever

Dengue fever (DF) and dengue haemorrhagic fever (DHF) exist throughout most of the tropics and subtropics. There has been a dramatic increase in transmission and cases in recent years with epidemics in tropical South America, the Caribbean and SE Asia and increased cases imported into the UK, from the Caribbean and Thai islands especially.

The four dengue viruses (flaviviruses) are transmitted to man by aedes mosquitoes. The disease may be subclinical or non-specific or have a sudden onset of fever (one to five days), severe headache, joint and muscle aches ('breakbone fever'). A transient early generalised rash may be replaced later by petechiae. Nausea and vomiting may occur.

DF in travellers is usually self-limiting although a return to complete health can sometimes be slow. Immunity is to the type encountered but it is believed that infection with a second type (usually within two years of the first) may result in the more severe DHF which carries a high mortality (particularly in local children) and has occurred in travellers.

There is no specific therapy. Prevention is by reduction of mosquito bites during the day, especially just after dawn and just before dusk (see 7.5).

No vaccine is currently available but several candidate vaccines are under development.

7.2  Japanese encephalitis

Japanese encephalitis (JE) exists only in Asia, from India (and a small area in Pakistan) eastwards across Thailand and China to Korea and Japan and down through south east Asia. It has recently reached the Torres Straight islands between Papua New Guinea and northern Australia.

The flavivirus is transmitted by various species of culicene mosquito from agricultural animals (often pigs) and birds to man. The mosquitoes most commonly breed in rice fields.

The risk season corresponds with the hotter, wetter seasons in the northern part of the endemic zone (usually May-October) whilst it tends to be year round in Malaysia, Indonesia and the Philippines.

The infection is asymptomatic in over 99 per cent of cases. However, when encephalitis develops there is a 30 per cent mortality rate and about 50 per cent of the survivors are left with neurological sequelae.

The disease is extremely rare in travellers, the risk estimated to be less than 0.1 per 100,000 in tourists and business people. It is increased for those staying in rural, especially agricultural, areas within the endemic zone and in the transmission season. Vaccine should be considered for those who will be at this increased risk for at least a month. Prevention for all travellers to rural areas is by reducing the chance of being bitten by these predominantly dusk to dawn biting mosquitoes (see 7.5).

Vaccine (see also Immunisation against Infectious Disease and table in Chapter 8)

The unlicensed, inactivated, mouse brain derived vaccine can be administered on a named doctor/named patient basis to those considered at sufficient risk. Possible adverse events include delayed allergic reactions and so the course should be completed at least ten (and preferably 14) days before travel. Vaccinees should be observed for 30 minutes after each dose. Those with a history of urticaria or multiple allergies are considered at higher risk of allergic reactions. Rare neurological reactions also occur.

7.3  Yellow fever

Yellow fever exists within two endemic zones - a belt across Africa and the tropical part of South America reaching as far north as Panama (see maps inside back cover). The risks within these zones will vary according to mosquito activity.

The flavivirus is transmitted by species of aedes and haemagogus mosquitoes in a jungle cycle which includes non-human primates (and occasional humans in the forest) and an urban cycle involving humans.

The disease can be mild, flu-like or hepatitis-like or a severe viral haemorrhagic fever with a 50 to 60 per cent mortality in non-immune travellers.

Prevention is by reducing the chance of mosquito bites from these day biting mosquitoes, especially after dawn and late afternoon (see 7.5) and by vaccine.

Immunisation is advised for all travellers to endemic zones unless travel is restricted to urban areas at high altitude (whether or not it is a mandatory requirement for entry).

Immunisation is available only from designated centres (see pages 3-4).

Vaccine (see also Immunisation against Infectious Disease and table in Chapter 8)

The live attenuated 17D strain vaccine is highly effective with a very low rate of serious adverse events.

An International Certificate of Vaccination against yellow fever is required for entry to some countries (see 8.2.3).

7.4  Tick-borne encephalitis

Tick-borne encephalitis (TBE) exists in Scandinavia, across Central and Eastern Europe and the Western part of the former USSR. The flavivirus is transmitted by the vector tick Ixodes ricinus. A different tick Ixodes persulcatus transmits the closely related Russian spring summer encephalitis across the former USSR, north of Mongolia to the Pacific coast and to parts of China (far north east), Korea and Japan. The countries with areas most affected by TBE are Austria, Belarus, Croatia, Czech Republic, Estonia, Germany, Hungary, Latvia, Lithuania, Poland, Russia, Slovakia and Ukraine.

Areas with lower prevalence or where sporadic cases have been reported include Albania, Bulgaria, Denmark (Bornholm Island), SW coast of Finland, France, Greece, Italy, Norway, Romania, Serbia, the Baltic coast of Southern Sweden and Switzerland.

The infection is asymptomatic in 90 per cent of cases especially in children. Those who develop flu-like symptoms may recover but ten per cent of them suffer a relapse with encephalitis with possible neurological sequelae or fatal outcome. The outlook is worse with increasing age.

The risk is mainly to those who are working, walking or camping in rural areas where ticks are prevalent. It is greatest from April through to August and sometimes October. It can extend outside those seasons in the warmer south of the area. The disease is occasionally transmitted by eating or drinking unpasteurised dairy products.

Prevention is by reduction of tick bites, avoidance of consumption of unpasteurised dairy products and by vaccine. The general measures to prevent ticks getting on to skin are described below. Those in tick areas should check their skin for attached ticks, which is easier to do with a partner. Ticks should be removed as soon as possible with tweezers (or fingers covered by tissue paper if no tweezers are available) as close to the skin attachment as possible, by steady pulling without jerking or twisting. Only one to two per cent of ticks are likely to be infected although occasionally up to ten per cent are. Medical advice should be sought locally as specific immunoglobulin may be available and advised within 48 hours (manufacturers state 96 hours) of a tick bite. However its efficacy has been questioned. Immunoglobulin is unlicensed in the UK but can be obtained on a named doctor/named patient basis where it is believed to be beneficial.

Vaccine (see also Immunisation against Infectious Disease and table in Chapter 8)

Inactivated vaccines are available in the UK for those considered at risk. Ideally immunisation should be completed at least a month before travel. It is considered to be effective against both strains of the disease. The specific immunoglobulin may on occasion be considered for those at high risk and travelling at short notice, although it is unlicensed in the UK.

Experience with TBE vaccine in the UK is limited. Adverse reactions including tenderness and swelling at the injection site with regional lymph gland swelling are reported, with some more generalised malaise, limb aches and pyrexia in some cases. Neuritis is rarely reported.

7.5  Physical methods of protection against mosquito and tick-borne diseases

For the prevention of bites from night time (dusk-dawn) biting mosquitoes see paragraph 6.4. For day time biting mosquitoes this advice applies dawn to dusk. In practice this will often include sleeping time.

Tick bites are reduced by preventing vegetation from brushing against bare skin, which should therefore be covered eg long trousers tucked into socks. Open sandals should not be worn. DEET based repellents have some action against ticks and can be used on skin or to spray clothing. Permethrin insecticide spray can also be used on clothes. (See previous page for removal of ticks).