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About yellow fever

Yellow fever is a viral illness which is spread by the bite of a mosquito. The species of mosquito that is responsible for spreading yellow fever is Aedes Aegypti and is a different species from the anopheles mosquito that transmits malaria.

Essentially yellow fever is a disease of monkeys living in tropical rain forests. The virus which causes the disease is one of a group of viruses known as arboviruses. Humans are infected by being bitten by rain forest mosquitos carrying the yellow fever virus.

As their name implies "arboviruses" are transmitted to humans by insects (arthropod borne virus). The mosquito in particular is well suited to the transport and spread of the infection due to its wide distribution throughout the tropics.

The geographical distribution of yellow fever is confined to Equatorial Africa and Central South America as shown on the maps as shown.

Strangely, yellow fever is unknown in Asia despite the presence of mosquitos capable of spreading the virus.

Arbovirus illnesses usually have two characteristic phases, the first when the virus is invading the host cells, and the second a few days later when the body's immune system is fighting the infection. The antibodies produced during the second phase of illness can cause damage to the blood vessels which explains why arboviruses often cause bleeding.

Many yellow fever infections are mild and go unrecognized but severe and life threatening illness is not uncommon. After an incubation period of about three to six days fever, headache, abdominal pain and vomiting develop. After a brief recovery period, shock, bleeding and signs of liver and kidney failure develop. Liver failure is associated with jaundice hence the name "yellow fever".

There is no drug available to cure yellow fever hence treatment is aimed at symptomatic relief. Overall about 5% of patients die. Those who recover do so completely and are immune thereafter.

Fortunately yellow fever is one of the few arboviruses for which a vaccination is available. A single injection of a live, weakened (and harmless) virus stimulates the body's immune defences and confers effective immunity for ten years.

In general, all travellers going to an endemic area require a yellow fever vaccination certificate and travellers going to some parts of Asia from an endemic region will also require a certificate.

Dengue Fever

This is an unusual arbovirus infection since no other animals except humans and mosquitoes play a significant part in perpetuating the infection. It is present in Africa, South East Asia, the Pacific area and northern South America.

The disease is spread from person to person by the bite of a mosquito (Aedes aegypti) and after about five days incubation period there is a sudden onset of fever, headache and severe joint and muscle pains. The initial fever resolves after about three to five days only to recur with the appearance of a rash consisting of small white spots which starts on the trunk and spreads to the limbs and face. Within a few days the fever subsides and recovery follows.

Although dengue is a very unpleasant illness, complications are uncommon and recovery is usually complete.

There is also a more severe and life threatening hemorrhagic form of the disease which has appeared with dramatic outbreaks. Fortunately this form occurs only rarely.

This is thought to be the result of a second infection where there is some remaining immunity from a first attack causing a vigorous immunological response in which severe blood vessel damage occurs.

Unfortunately, immunity to infection does not last long and subsequent attacks are possible. There is no vaccine available. Prevention is by avoiding mosquito bites.

Japanese B Encephalitis

This is a rare but serious arboviral infection with a 20% fatality rate. It occurs in most of the Far East and South East Asia. The endemic zone extends from India and Nepal across the whole of South East Asia to Japan and Korea in the Far East.

The risk of infection is greatest in long term visitors to rural areas, and the risk to short term visitors and visitors to major cities is small. Precautions against mosquito bites are essential.

Japanese B Encephalitis is transmitted by rice field breeding mosquitoes (of the Culex group) that become infected with Japanese encephalitis virus.

Mosquitoes become infected by feeding on domestic pigs and wild birds infected with the Japanese encephalitis virus. Infected mosquitoes then transmit the Japanese encephalitis virus to humans and animals during the feeding process.

The incubation period is normally between 5 and 15 days. The illness cannot be passed on from person to person. There is no specific treatment. Intensive supportive therapy is indicated.

Mild infections can sometimes occur without apparent symptoms other than mild fever with headache. More severe infection is marked by quick onset, headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic paralysis.

There is an effective vaccine available in the UK on a named patient basis. It should be considered by anyone travelling to Asia for more than a month or visiting rural areas.

West Nile Virus

West Nile Virus is a flavivirus commonly found in Africa, West Asia, the Middle East and the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

The principal route of human infection with West Nile virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus replicates in the mosquito and spreads to the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

At least 36 species of mosquito are known to be carriers of West Nile Virus. The most common carrier of West Nile in the United States is the Culex pipiens (Northern house) mosquito.

West Nile fever is usually a case of mild disease characterized by flu-like symptoms. It typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease due to a person being infected with this virus can be West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

The incubation period is usually 3 to 14 days. Symptoms of mild disease will generally last a few days. Symptoms of severe disease may last several weeks, although neurological effects may be permanent.

Many people who are infected with the West Nile virus will not have any type of illness. It is estimated that 20% of the people who become infected will develop West Nile fever: mild symptoms, including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.

The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease.

There is no specific treatment for West Nile virus infection. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.

When travelling to areas where the disease is endemic you can reduce the risk of becoming infected with the virus by employing preventive measures such as protecting yourself from mosquito bites:

  • Apply insect repellent to your skin and clothes when going outside.
  • When possible, wear long-sleeved shirts and long pants whenever you are outdoors.
  • Place mosquito netting over infant carriers when you are outdoors with infants.
  • Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.
  • Make sure any window and door screens are intact so that mosquitoes cannot get indoors.
  • Use mosquito nets around beds at night while asleep.

Many of the mosquitoes that carry the West Nile virus are especially likely to bite around dusk and dawn. If you are outdoors around these times of the day, it is important to apply repellent. But there are also mosquitoes that bite during the day, and these mosquitoes have also been found to carry the West Nile virus. The safest decision is to apply repellent whenever you are outdoors.

Apply insect repellent sparingly to exposed skin. The more DEET a repellent contains the longer time it can protect you from mosquito bites. A higher percentage of DEET in a repellent does not mean that your protection is better, just that it will last longer. DEET concentrations higher than 50% do not increase the length of protection. Choose a repellent that provides protection for the amount of time that you will be outdoors.