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Chikungunya is a mosquito-borne alpha virus that was first isolated after a 1952 outbreak in modern-day Tanzania.[1] The virus has circulated in forested regions of sub-Saharan African in cycles involving nonhuman primate hosts and arboreal mosquito vectors.[1] Phylogenetic studies indicate that the urban transmission cycle—the transmission of a pathogen between humans and mosquitoes that exist in urban environments—was established on multiple occasions from strains occurring on the eastern half of Africa in non-human primate hosts.[1] This emergence and spread beyond Africa may have started as early as the 18th century.[1] Currently, available data does not indicate whether the introduction of chikungunya into Asia occurred in the 19th century or more recently, but this epidemic Asian strain causes outbreaks in India and continues to circulate in Southeast Asia.[1]
A number of chikungunya outbreaks have occurred since 2005. However, As of the latest data available, developed countries have yet to report a confirmed indigenous case of chikungunya.[2] An analysis of the chikungunya virus's genetic code suggests that the increased severity of the 2005–present outbreak may be due to a change in the genetic sequence, altering the virus' viral coat protein, which potentially allows it to multiply more easily in mosquito cells.[3] The change allows the virus to use the Asian tiger mosquito (an invasive species) as a vector in addition to the more strictly tropical main vector, Aedes aegypti. In July 2006, a team analyzed the virus' RNA and determined the genetic changes that have occurred in various strains of the virus and identified those genetic sequences which led to the increased virulence of recent strains.[3] The virus, CHIKV, is a small, enveloped virus making it part of the alphavirus family Togaviridae.[2] This characteristic improves the viruses ability to enter into the body and impact those most affected such as individuals over 65 years of age and individuals with underlying medical conditions.[4] Individuals below the age of 30 are found to have a faster recovery time with the reasoning unknown at this time.[5]
Outbreaks of chikungunya, on average, have low mortality rates.[2] As it is generally a nonfatal disease, prevalence rates during most outbreaks are higher than incidence rates.[6] Recently, it was discovered that approximately 39% of the worldwide population resides in environments where the chikungunya virus is endemic.[7] The spikes of transmission have increased the worldwide fatal cases to 350 people per year as of October 2023 to 87 deaths in 2022.[8] Few studies have thoroughly investigated the risks to those living in medically insufficient areas, but some surveys suggest higher rates of chronic effects.[6] Challenges relating to staffing and financing in less-developed countries may contribute to the underreporting of cases.[9] Current data on the co-morbidities of chikungunya infection states that individuals with severe cases of chikungunya have an increased prevalence of cardiac conditions along with diabetes and respiratory difficulties.[10] With the exception of asthma, the risk of each concurrent condition with CHIKV infections increases with age.[10] While the long term effects still need to be investigated, on average, 40% individuals with the multiple chikungunya virus infections experience persistent disabilities after 6 months and 28% of the people still had it after 18 months.[5] Modern studies suggest a correlation between elevated CHIKV infections and risk factors including individuals that previously experienced joint-related pains and conditions, those aged 45 and above, and individuals of the female gender.[11]