The monsoon has ceased to be a pleasure for urban India. It brings a host of woes including mosquito-borne diseases. Dengue fever exploded in contemporary urban India in 1996 with more than 10,000 cases and 400 plus deaths reported from Delhi alone. A decade later, chikungunya appeared in epidemic form across the country, certainly in its urban areas. While not a fatal disease like dengue hemorrhagic fever, chikungunya acquired notoriety on account of excruciating joint pain that lasted up to months.
Chikungunya was first described during an outbreak in southern Tanzania in 1952; the name is derived from the local Kimakonde language and means “to become contorted”, evoking the stooped appearance of patients suffering acute joint pain. The Indian Ocean epidemic peaked in 2006-2007 and the disease is reported from nearly all of south and south-east Asia as well as a number of African countries.
Large numbers of cases have also been reported from the Caribbean islands, Mexico and Latin America. This year about 10,000 cases have been reported in India, with the most from southern states. The US reported its first laboratory confirmed case of locally acquired chikungunya virus in Texas this year. Another significant development is the emergence of congenital chikungunya virus infection in two pregnant women and significant perinatal infections in the newborn in Brazil —the much-dreaded Zika-like situation.
Two species of mosquitoes, Aedes aegypti and Aedes albopictus are responsible for large outbreaks across the world. Mosquitoes acquire the infection by biting infected humans or animals. Weather conditions affect their breeding and survival. Ae. Albopictus (not the dominant species in India) thrives in a wider range of water-filled breeding sites than Ae. Aegypti that include coconut husks, bamboo stumps and rock pools, as well as artificial containers such as vehicle tyres and saucers beneath plant pots. Ae. aegypti is associated with human habitation and uses indoor breeding sites, including flower vases, water storage vessels, and overflow trays of refrigerators and concrete water tanks, as well as the same artificial outdoor habitats as Ae. albopictus.
Symptoms appear four to seven days after the bite of an infected mosquito. It was believed to be rarely fatal, but mortality may occur in the elderly. Some clinical features are common between chikungunya and dengue; not infrequently, misdiagnosis may occur in areas where both are endemic.
Vector transmission is reduced through the use or combination of three methods: Environmental management, chemical control and biological control. Some man-made habitats produce large numbers of adult mosquitoes and control measures in resource constrained settings need to target habitats that are most productive and therefore epidemiologically more relevant. Aedes species are day biters; clothing that minimises skin exposure during daylight hours affords some protection from bites. Mosquito nets afford good protection for those who sleep during the day (infants, the bedridden and night-shift workers). Environmental management ought to be the mainstay of vector control and involves destroying, altering, removing or recycling non-essential containers that provide egg, larval or pupal habitats.
Equally crucial is installation of reliable piped water supply to communities to reduce the need for storage. Legislation and regulation can significantly change planning and construction of buildings. Biological control is based on the introduction of organisms that prey upon, compete with or reduce populations of the target species.
Singapore has one of the highest burdens of infections transmitted by Aedes mosquitoes; dengue hemorrhagic fever appeared in the 1960s and became a major cause of child deaths. It achieved significant control through integrated vector management that entailed: Advocacy, social mobilisation and legislation; collaboration within the health sector and across other sectors; evidence based decision-making and capacity building of providers and communities. It has come down heavily on the construction sector for not complying with vector control guidelines.
Trials of chikungunya vaccine are under way but it is difficult to speculate when it will be available.