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Delhi has been hit by a chikungunya epidemic – what is this disease?
Chikungunya is not a name that rolls easily off the tongue. It is a Swahili word meaning “that which bends up”. It describes the symptoms of this crippling arthritic disease, a disease which is currently gripping Delhi in India.
Chikungunya is a viral disease that is transmitted by mosquitoes. People who are unlucky enough to contract the disease tend to suffer from a fever and severe joint pain, causing them to have a painful and contorted posture. Other symptoms include headaches, nausea, fatigue, rash, swollen joints and muscle pain. These are similar symptoms to those infected with dengue virus which, although historically attributed to higher mortality rates, is said to be less painful and shorter lived than chikungunya infections.
Symptoms usually begin around three to seven days following a bite from an infected mosquito. As with other mosquito-borne viruses, chikungunya is classed as an emerging or re-emerging pathogen in many countries. Epidemics occur in cycles which can have many intervening years. However, the past couple of decades have seen an increase in the number of large outbreaks.
The widening geographical scope of the virus is due to the expanding range of the mosquitoes that carry it. Increasing global temperatures, international travel and population densities all contribute to allowing mosquitoes to take up residence in previously unoccupied areas.
Avoiding mosquitoes is the best defence
There is no cure for chikunguya infections, so medical treatment involves relieving symptoms. Most sufferers are expected to make a full recovery, although this may take up to a year or more. There is also no vaccine for preventing infections either, so the best defence is to avoid mosquito bites.
Unlike the mosquitoes that transmit malaria, which are active at night, chikungunya is spread by species that feed during the day – which makes bite prevention more challenging. This requires measures in addition to sleeping under mosquito nets. For instance wearing long-sleeve clothing, and the use of mosquito repellents and deterrents. The virus persists in the blood for five to seven days so it is also important to prevent mosquitoes feeding on an infected person during this time to reduce the risk of human to mosquito transmission.
Virus carrying mosquitoes are found across tropical and subtropical regions. They have adapted well to living in populated areas and can breed in small pools of stagnant water. Small amounts of water in discarded tyres, flowerpots and food containers can be used by female mosquitoes to lay batches of around 200 eggs. Preventing access to these water sources is essential for reducing disease transmission through population control.
Chikunguya, dengue and Zika viruses are spread by the same mosquito species and exist in the same areas. Due to the similarity of the symptoms of all three infections, many historical epidemics that were reported as dengue virus could have been caused by chikunguya. Similarly, the number of Zika infections early in the current epidemic could have been under reported and attributed to dengue or chikungunya viruses.
Originally recorded in Tanzania in 1952, minor outbreaks of chikungunya virus were documented in Asia from the early 1960s. The first major epidemic was in 2004 on the coast of Kenya after which the virus spread to many islands of the Indian Ocean, India and South-East Asia. Recently other Asian, European and North American countries have recorded imported cases. A few of these countries have developed virus transmission in local mosquito populations.
The continuation of human, viral and environmental factors which promote the emergence and spread of chikungunya and other mosquito-borne diseases suggest that the pattern of outbreaks is likely to continue for some time. As the virus spreads, population immunity will eventually reduce the frequency and range of these outbreaks since humans develop lifelong immunity following infection. Isolated populations, such as some Pacific and Caribbean islands, are able to successfully reduce the rate of infection after a couple of transmission cycles. But in areas of large population densities, such as Delhi, this may take significantly longer.
Claire Donald receives funding from the Medical Research Council of the UK.