Arboviruses are viruses transmitted to humans by arthropods such as mosquitoes and ticks. The first two letters of the words arthropod' and borne, make up the 'arbo' that now designates this group of viruses as arthropod-borne.

ARthropod BOrne VIRUS

There are several arboviral diseases, the most famous including Yellow Fever and Dengue Fever.

Although spread by mosquitoes, Malaria does not fall into this category because the organism that causes malaria is not a virus but rather a protozoan.

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 peak biting times for many mosquito species is dusk to dawn. However, Aedes aegypti that transmits yellow fever virus, feeds during the daytime.

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 unrecognised 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.

For a list of clinics in the UK where you can obtain yellow fever & other vaccines:-

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 to humans 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.

With more than one-third of the world’s population living in areas at risk for infection, dengue virus is a leading cause of illness and death in the tropics and subtropics. As many as 400 million people are infected yearly.

Dengue is caused by any one of four related viruses transmitted by mosquitoes. There are not yet any vaccines to prevent infection with dengue virus and the most effective protective measures are those that avoid mosquito bites. When infected, early recognition and prompt supportive treatment can substantially lower the risk of medical complications and death.

There is no specific treatment for Dengue but in terms of management, sufferers are advised to take paracetamol and oral rehydration.

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

There is also a more severe and life threatening haemorrhagic 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.

Note: The Aedes mosquito responsible for transmitting Dengue Fever is a day time feeding species.

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.


Tick Borne Encephalitis

Ticks are blood feeding external parasites of mammals, birds, and reptiles throughout the world. Ticks are not insects like fleas, but arachnids like mites, spiders and scorpions.

Tick-borne encephalitis (TBE), is a viral disease affecting the central nervous system. The disease is most often manifest as meningitis or encephalitis or sometimes, even meningoencephalitis. It is caused by the tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae.

TBE is endemic in many European countries, the former Soviet Union, and Asia. It is found mainly in temperate regions. The natural hosts of the virus are small rodents and deer, with humans being accidental hosts. After attaching itself to the host, an infected tick transfers the virus to the host during feeding.

Map showing TBE endemic areas

In disease endemic areas, people with recreational or occupational exposure to rural or outdoor settings such as; hunters, hikers, campers, forest workers, farmers, etc. are potentially at risk of infection by contact with the infected ticks.

The incubation period of TBE is usually between 7 and 14 days.

A characteristic biphasic febrile illness follows, with an initial phase that lasts two to four days. It is non-specific with symptoms that may include fever, malaise, anorexia, muscle aches, headache, nausea, and/or vomiting.

After about eight days of remission, the second phase of the disease occurs in twenty to thirty percent of patients and involves the central nervous system with symptoms of meningitis (fever, headache, and a stiff neck) or encephalitis (drowsiness, confusion, sensory disturbances, and/or motor abnormalities such as paralysis) or meningoencephalitis. TBE is more severe in adults than in children.

Mortality is about two percent with deaths occurring five to seven days after the onset of neurological signs.

There is no specific treatment for TBE. Meningitis, encephalitis, or meningoencephalitis require hospitalisation and supportive care.

Like other tick-borne infectious diseases, TBEV infection can be prevented by using insect repellents and protective clothing to prevent tick bites.

A vaccine is available in some disease endemic areas but adverse vaccine reactions in children limit the use of the vaccine.

The vaccine involves two doses one month apart. If required a booster is given after a year.

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:-

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.

Chikungunya fever

Chikungunya fever is another viral disease transmitted to humans by the bite of infected mosquitoes. It is diagnosed based on symptoms, physical findings (e.g., joint swelling), laboratory testing, and the possibility of exposure to infected mosquitoes.

Chikungunya typically starts with one or more of the following symptoms - chills, fever, vomiting, nausea, head ache and joint pain. The attack is sudden and sometimes it is accompanied with rashes. Severe joint pain is the main and the most problematic symptom of Chikungunya.

Other less commonly seen symptoms include mouth ulcers, loss of taste and conjunctivitis. The fever usually subsides in a couple of days, but other symptoms may last for a longer duration. In some cases joint pain persisted for years.

There is no antiviral drug or medicine specifically for Chikungunya fever; care is based on symptoms. Chikungunya infection is not usually fatal and since chikungunya is cured by the immune system in almost all cases there is no need to worry.

Steps to prevent infection with chikungunya virus include use of insect repellent, protective clothing, and staying in areas with screens.

Chikungunya virus was first isolated in Tanzania in 1953, and has since been cited as the cause of numerous human epidemics in India, Indonesia, Malaysia, Singapore, and Sri Lanka.

Chikungunya Distribution and Global Map

The geographic range of chikungunya virus is mainly in Africa and Asia. Given the current large chikungunya virus epidemics and the worldwide distribution of Aedes aegypti and Aedes albopictus mosquitoes, there is a risk of importing chikungunya virus into new areas through infected travellers.

Zika Virus

If you’ve heard about the Zika virus and are wondering what it is, you’re not alone. Cases of this emerging infectious disease are soaring in the Americas and "spreading explosively," according to World Health Organization Director-General Margaret Chan, MD. Several states are reporting cases of U.S. travelers bringing the infection back with them, and health officials in Texas have confirmed a case of sexual transmission of the virus.

For most people, the Zika virus causes only a brief, mild flu-like illness. But in pregnant women it has been linked to an alarming increase in the rate of the birth defect known as microcephaly — a debilitatingly small head and brain size. The clusters of birth defects linked to the Zika virus are an international public health emergency, according to the WHO. Dr. Chan characterizes the situation as an "extraordinary event and a public health threat to other parts of the world."

The Centers for Disease Control and Prevention in the USA posted a travel alert advising pregnant women to delay travel to areas where Zika is active. The travel alert list continues to expand and now includes 28 countries or territories, most of them in the Americas.

The CDC's newest guidelines recommend that pregnant women coming back from these areas get tested for Zika if they have symptoms.

The World Health Organization expects Zika to spread to all but two countries in the Americas: Canada and Chile.

The most common symptoms of Zika virus disease are fever, rash, joint pain, and conjunctivitis. The illness is usually mild with symptoms lasting from several days to a week. Severe disease requiring hospitalization is uncommon.

Outbreaks of Zika have occurred in areas of Africa, Southeast Asia, the Pacific Islands, and the Americas. Because the Aedes species mosquitoes that spread Zika virus are found throughout the world, it is likely that outbreaks will spread to new countries.

In December 2015, Puerto Rico reported its first confirmed Zika virus case. Locally transmitted Zika has not been reported elsewhere in the United States, but cases of Zika have been reported in returning travelers.

There is no vaccine to prevent or medicine to treat Zika. Travellers can protect themselves from this disease by taking anti-mosquito measures: When traveling to countries where Zika virus has been reported, use insect repellents, wear long sleeves and pants, and stay in places with air conditioning or that have window and door screens.

Zika virus can be spread from a pregnant woman to her unborn baby. There have been reports of a serious birth defect of the brain called microcephaly and other poor pregnancy outcomes in babies of mothers who were infected with Zika virus while pregnant.

Knowledge of the link between Zika and these outcomes is evolving, but special precautions are recommended for the following groups:

• Women who are pregnant (in any trimester): Consider postponing travel to any area where Zika virus transmission is ongoing. If you must travel to one of these areas, talk to your doctor first and strictly follow steps to prevent mosquito bites during your trip.

• Women who are trying to become pregnant: Before you travel, talk to your doctor about your plans to become pregnant and the risk of Zika virus infection. Strictly follow steps to prevent mosquito bites during your trip.