Chikungunya is a mosquito-borne disease first described during an outbreak in the southern part of the United Republic of Tanzania in 1952. The name chikungunya derives from a root verb in the Kimakonde language meaning "to become contorted" and it describes the stooped appearance of sufferers with arthralgia. The causative agent is an alphavirus of the family Togaviridae. Since the first description of chikungunya, there have been numerous outbreaks in Africa, India and South-East Asia, the principal vectors being Aedes mosquitoes, of which Aedes albopictus and A. aegypti are the most important. Chikungunya often produces a mild illness that may be confused with dengue, since symptoms include fever, headache, arthralgia, myalgia and rash. Although serious complications are uncommon, arthralgia can be debilitating and may persist for months or even years after infection; in elderly people, chikungunya can be a contributing factor to death. In immunologically naive populations, where mosquito vectors are numerous, epidemic outbreaks may occur affecting many thousands of people. Outside of epidemics and wherever serological surveillance is lacking, the diagnosis of chikungunya may easily be missed. Chikungunya circulated in West and East Africa at relatively low levels until 1999-2000 when around 50 000 people became infected during an outbreak in the Democratic Republic of the Congo. Evidence from India suggested that outbreaks during the 1960s, were followed by relatively little transmission after 1973. Chikungunya outbreaks are often separated by periods of [greater than or equal to] 10 years when infection is not apparent. For example, in Indonesia, chikungunya occurred sporadically until 1985 after which there were no reports until a series of outbreaks between 2001 and 2007.
Recent outbreaks, 2001-2007
Starting in February 2005, a major outbreak occurred among islands in the western part of the Indian Ocean, affecting the Comoros, Madagascar, Mayotte, Mauritius, La Reunion and the Seychelles (Map 1). In La Reunion, by June 2006 there had been an estimated 266 000 cases, accounting for roughly one third of the population. Chikungunya continued to circulate in La Reunion in 2007, albeit at a much reduced level. The major vector in La Reunion is A. albopictus, which appears to have displaced A. aegypti on much of the island as it has also in some other regions. During this period, there were an estimated 9000 cases of chikungunya in the Seychelles, 7290 in Mayotte and about 6000 in Mauritius. Associated with this outbreak in the Indian Ocean islands, there were a large number of imported cases of chikungunya in Europe, occurring in returning tourists and visitors, particularly those returning from La Reunion. Metropolitan France experienced the largest number of cases: between April 2005 and August 2006, there were 808 imported cases of chikungunya confirmed by serology. The temporal change in numbers reflected the rise and decline of the epidemic in La Reunion, with a peak of 178 cases in March 2006. Elsewhere in Europe, cases were reported from Germany, Italy, Norway and Spain.
A. albopictus has been inadvertently introduced into several European countries during the past 30 years; these countries include Albania, Belgium, Bosnia, Croatia, France, Greece, Italy, Montenegro, the Netherlands, Serbia, Slovenia, Spain and Switzerland. Although A. albopictus occurs in the departments of Alpes-Maritimes and Var in France, where cases of chikungunya that originated in La Reunion have been diagnosed, there has been no evidence of local transmission.
During 2006, there was a large outbreak of chikungunya in India, with 1.39 million officially reported cases spread over 16 states; attack rates were estimated at 45% in some areas. The outbreak was first noticed in Andhra Pradesh and it subsequently spread to Tamil Nadu. Thereafter, Kerala and Karnataka were affected; subsequently, the outbreak spread northwards as far as Delhi. During 2007, up until 12 October, a further 37 683 cases had been reported by national authorities, largely in areas not involved in the previous year's epidemic. In India, the persistent and disabling arthralgia following infection has been of particular concern. In some areas, the outbreak was associated with high densities of A. albopictus, but A. aegypti is thought to be the major vector elsewhere. During 2006, there were also outbreaks in the Andaman and Nicobar islands, Malaysia, and by November 2006, chikungunya had appeared in Sri Lanka. Between January 2001 and April 2007, Indonesia reported 15 207 chikungunya cases from 7 provinces, with a peak in 2003.
During 2006, there were 9 imported cases of chikungunya in the Caribbean, involving Martinique, French Guyana and Guadeloupe; these mainly occurred in travellers from the Indian Ocean islands. The Caribbean is an area of active dengue transmission, the vectors of which are also vectors of chikungunya, hence the region is vulnerable to local transmission. In 2006, in the United States, 37 cases of …