|Classification and external resources|
Rash from chikungunya on the right foot
Chikungunya (// CHI-kən-GUUN-yə; Makonde for "that which bends up") is an infection caused by the Chikungunya virus. It features sudden onset fever usually lasting two to seven days, and joint pains typically lasting weeks or months but sometimes years.
The virus is passed to humans by two species of mosquito of the genus Aedes: A. albopictus and A. aegypti. The strain of chikungunya spreading to the US from the Caribbean is most easily spread by A. aegypti. Concern exists that this strain of chikungunya could mutate to make the A. albopictus vector more efficient. If this mutation were to occur, chikungunya would be more of a public health concern to the US because the A. albopictus or Asian tiger mosquito is more widespread in the US and is more aggressive than the A. aegypti. Animal reservoirs of the virus include monkeys, birds, cattle, and rodents. This is in contrast to dengue, for which only humans and primates are hosts.
The best means of prevention is overall mosquito control and the avoidance of bites by any infected mosquitoes. No specific treatment is known, but medications can be used to reduce symptoms. Rest and fluids may also be useful.
Signs and symptoms
The incubation period of chikungunya disease ranges from two to twelve days, typically three to seven. 72-97% of those infected will develop symptoms. Symptoms include sudden onset, sometimes biphasic fever typically lasting from a few days to a week, sometimes up to ten days, usually above 39 °C (102 °F) and sometimes reaching 41 °C (104 °F), and strong joint pain or stiffness usually lasting weeks or months but sometimes lasting years. Rash (usually maculopapular), muscle pain, headache, fatigue, nausea or vomiting may also be present. Inflammation of the eyes may present as iridocyclitis, or uveitis, and retina lesions may occur.
Observations during recent epidemics have suggested chikungunya may cause long-term symptoms following acute infection. During the La Reunion outbreak in 2006, more than 50% of subjects over the age of 45 reported long-term musculoskeletal pain with up to 60% of people reporting prolonged arthralgia three years following initial infection. A study of imported cases in France reported that 59% of people still suffered from arthralgia two years after acute infection. Following a local epidemic of chikungunya in Italy, 66% of people reported muscles pains, joint pains, or asthenia at one year after acute infection. Long-term symptoms are not an entirely new observation; long-term arthritis was observed following an outbreak in 1979. Common predictors of prolonged symptoms are increased age and prior rheumatological disease. The cause of these chronic symptoms is currently not fully known. Markers of autoimmune or rheumatoid disease have not been found in people reporting chronic symptoms. However, some evidence from humans and animal models suggests chikungunya may be able to establish chronic infections within the host. Viral antigen was detected in a muscle biopsy of a people suffering a recurrent episode of disease three months after initial onset. Additionally, viral antigen and RNA were found in synovial macrophages of a person during a relapse of musculoskeletal disease 18 months after initial infection. Several animal models have also suggested chikungunya virus may establish persistent infections. In a mouse model, viral RNA was detected specifically in joint-associated tissue for at least 16 weeks after inoculation, and was associated with chronic synovitis. Similarly, another study reported detection of a viral reporter gene in joint tissue of mice for weeks after inoculation. In a nonhuman primate model, chikungunya virus was found to persist in the spleen for at least six weeks.
|Cryoelectron microscopy reconstruction of chikungunya virus. From EMDB entry |
|Group:||Group IV ((+)ssRNA)|
Chikungunya virus is an alphavirus with a positive-sense single-stranded RNA genome of about 11.6kb. It is a member of the Semliki Forest virus complex and is closely related to Ross River virus, O'nyong'nyong virus, and Semliki Forest virus. In the United States, it is classified as a category C priority pathogen and work requires biosafety level III precautions.
Human epithelial and endothelial cells, primary fibroblasts, and monocyte-derived macrophages are permissive for chikungunya virus in vitro, and viral replication is highly cytopathic, but susceptible to type-I and -II interferon. In vivo, chikungunya virus appears to replicate in fibroblasts, skeletal muscle progenitor cells, and myofibers.
Chikungunya is generally spread through bites from A. aegypti mosquitoes, but recent research by the Pasteur Institute in Paris has suggested chikungunya virus strains in the 2005-2006 Reunion Island outbreak incurred a mutation that facilitated transmission by the Asian tiger mosquito (A. albopictus).
Chikungunya virus infection of A. albopictus was caused by a point mutation in one of the viral envelope genes (E1). Enhanced transmission of chikungunya virus by A. albopictus could mean an increased risk for outbreaks in other areas where the Asian tiger mosquito is present. A recent epidemic in Italy was likely perpetuated by A. albopictus. In Africa, chikungunya is spread by a sylvatic cycle in which the virus largely resides in other primates between human outbreaks.
Upon infection with chikungunya, the host's fibroblasts produce type-1 (alpha and beta) interferon. Mice that lack the interferon alpha receptor die in two to three days upon being exposed to 102 chikungunya PFUs, while wild-type mice survive even when exposed to as many as 106 PFUs of the virus. At the same time, mice that are partially type-1 deficient (IFN α/β +/−) are mildly affected and experience symptoms such as muscle weakness and lethargy. Partidos et al. 2011 saw similar results with the live attenuated strain CHIKV181/25. However, rather than dying, the type-1 interferon-deficient (IFN α/β −/−) mice were temporarily disabled and the partially type-1 interferon-deficient mice did not have any problems.
Several studies have attempted to find the upstream components of the type-1 interferon pathway involved in the host's response to chikungunya infection. So far, no one knows the chikungunya-specific pathogen associated molecular pattern. Nonetheless, IPS-1—also known as Cardif, MAVS, and VISA—has been found to be an important factor. In 2011, White et al. found that interfering with IPS-1 decreased the phosphorylation of interferon regulatory factor 3 (IRF3) and the production of IFN-β. Other studies have found that IRF3 and IRF7 are important in an age-dependent manner. Adult mice that lack both of these regulatory factors die upon infection with chikungunya. Neonates, on the other hand, succumb to the virus if they are deficient in one of these factors.
Chikungunya counters the type-I interferon response by producing NS2, a nonstructural protein that degrades Rpb and turns off the host cell's ability to transcribe DNA. NS2 interferes with the JAK-STAT signaling pathway and prevents STAT from becoming phosphorylated.
Common laboratory tests for chikungunya include RT-PCR, virus isolation, and serological tests.
- Virus isolation provides the most definitive diagnosis, but takes one to two weeks for completion and must be carried out in biosafety level III laboratories. The technique involves exposing specific cell lines to samples from whole blood and identifying chikungunya virus-specific responses.
- RT-PCR using nested primer pairs is used to amplify several chikungunya-specific genes from whole blood. Results can be determined in one to two days.
- Serological diagnosis requires a larger amount of blood than the other methods, and uses an ELISA assay to measure chikungunya-specific IgM levels. Results require two to three days, and false positives can occur with infection via other related viruses, such as o'nyong'nyong virus and Semliki Forest virus.
The differential diagnosis may include infection with other mosquito-borne viruses, such as dengue and influenza. Chronic recurrent polyarthralgia occurs in at least 20% of chikungunya patients one year after infection, whereas such symptoms are uncommon in dengue.
The most effective means of prevention are protection against contact with the disease-carrying mosquitoes and mosquito control. These include using insect repellents with substances such as DEET (N,N-diethyl-meta-toluamide; also known as N,N'-diethyl-3-methylbenzamide or NNDB), icaridin (also known as picaridin and KBR3023), PMD (p-menthane-3,8-diol, a substance derived from the lemon eucalyptus tree), or IR3535. Wearing bite-proof long sleeves and trousers also offers protection.
In addition, garments can be treated with pyrethroids, a class of insecticides that often has repellent properties. Vaporized pyrethroids (for example in mosquito coils) are also insect repellents. Securing screens on windows and doors will help to keep mosquitoes out of the house. In the case of the day-active A. aegypti and A. albopictus, however, this will have only a limited effect, since many contacts between the mosquitoes and humans occur outside.
Currently, no approved vaccines are available. A phase-II vaccine trial used a live, attenuated virus, to develop viral resistance in 98% of those tested after 28 days and 85% still showed resistance after one year. However, 8% of people reported transient joint pain, and attenuation was found to be due to only two mutations in the E2 glycoprotein. Alternative vaccine strategies have been developed, and show efficacy in mouse models, but have so far not reached clinical trials. In August 2014 it was revealed researchers at the National Institute of Allergy and Infectious Diseases were testing an experimental vaccine. If a vaccine becomes available, public health officials will then have to decide who should be vaccinated and under what conditions a vaccination is necessary. Chikungunya will be difficult to control with a vaccine alone.
Currently, no specific treatment is available. Attempts to relieve the symptoms include the use of NSAIDs such as naproxen or paracetamol (acetaminophen) and fluids. Aspirin is not recommended.
In those who have more than two weeks of arthritis, ribavirin may be useful. The effect of chloroquine is not clear. It does not appear to help acute disease, but tentative evidence indicates it might help those with chronic arthritis. Steroids do not appear useful, either.
Chikungunya is mostly present in the developing world.
The epidemiology of chikungunya is related to mosquitoes, their environments, and human behavior. The adaptation of mosquitoes to the changing climate of North Africa around 5,000 years ago made them seek out environments where humans stored water. Human habitation and the mosquitoes’ environments were then very closely connected. During periods of epidemics humans are the reservoir of the virus. During other times, monkey, birds and other vertebrates have served as reservoirs.
Three genotypes of this virus have been described: West African, East/Central/South African, and Asian genotypes. Explosive epidemics in Indian Ocean in 2005 and Pacific Islands in 2011, as well as now in the Americas, continue to change the distribution of genotypes. In a similar way to the 5 serotypes of dengue, an entry of a new genotype into a previously infected area causes new epidemics.
On 28 May 2009 in Changwat Trang of Thailand, where the virus is endemic, the provincial hospital decided to deliver by Caesarean section a male baby from his chikungunya-infected mother, Khwanruethai Sutmueang, 28, a Trang native, to prevent mother-fetus virus transmission. However, after delivering the baby, the physicians discovered the baby was already infected with the virus, and put him into intensive care because the infection had left the baby unable to breathe by himself or to drink milk. The physicians presumed the virus might be able to be transmitted from a mother to her fetus, but without laboratory confirmation.
In December 2013, chikungunya was confirmed on the Caribbean island of St. Martin with 66 confirmed cases and suspected cases of around 181. This outbreak is the first time in the Western Hemisphere that the disease has spread to humans from a population of infected mosquitoes. By January 2014, the Public Health Agency of Canada reported that cases were confirmed on the British Virgin Islands, Saint-Barthélemy, Guadeloupe, Dominica, Martinique, and French Guyana. In April 2014, chikungunya was also confirmed in the Dominican Republic by the Centers for Disease Control and Prevention (CDC). By the end of April, it had spread to 14 countries in all, including Jamaica, St. Lucia, St. Kitts and Nevis, and Haiti where an epidemic was declared.
By the end of May 2014, over ten imported cases of the virus had been reported in the United States by people traveling to Florida from areas where the virus is endemic.
On June 2014 six cases of the virus were confirmed in Brazil, two in the city of Campinas in the state of São Paulo. The six cases are Brazilian army soldiers who had recently returned from Haiti, where they were participating in the reconstruction efforts as members of the United Nations Stabilisation Mission in Haiti. The information was officially released by Campinas municipality, which considers that it has taken the appropriate actions.
On 16 June 2014, Florida had a cumulative total of 42 cases.
As of 11 September 2014, the number of reported cases in Puerto Rico for the year was 1,636. By 28 October, that number had increased to 2,974 confirmed cases with over 10,000 cases suspected.
On 17 June 2014, Department of Health officials in the U.S. state of Mississippi confirmed they are investigating the first potential case in a Mississippi resident who recently travelled to Haiti.
On 19 June 2014, the virus had spread to Georgia, USA.
On 17 July 2014, the first chikungunya case acquired in the United States was reported in Florida by the Centers for Disease Control and Prevention. Since 2006 over 200 cases have been reported in the United States but only in people who had traveled to other countries. This is the first time the virus was passed by mosquitoes to a person on the U.S. mainland.
On 2 September 2014, the Centers for Disease Control and Prevention reported that there had been 7 confirmed cases of Chikungunya in the United States in people who had acquired the disease locally.
Nov 2014: Brazil has reported a local transmission of a different strain (genotype) of chikungunya, that has never been documented in the Americas. This is an African genotype, but oddly fails to explain if its South African or West African. The new genotype (in the Americas) is more severe than the Asian genotype which is currently spreading through the Americas, and immunity to one genotype does not confer immunity to others. French Polynesia is among other regions experiencing ongoing outbreaks.
- Graph: Chikungunya cases development in Western Hemisphere from Dec 2013 (all Asian genotype except for 1 case in Brazil, data source PAHO since 2014, since 3/14 includes suspected case count -- similar diseases ruled out; Disclaimer: reports to PAHO are not synchronized, they vary by country by as much as 15 weeks, graph date reflects reported sum of collective known raw data, data are not processed to reflect actual date of transmission)
The word 'chikungunya' is thought to derive from a description in the Makonde language, meaning "that which bends up", of the contorted posture of people affected with the severe joint pain and arthritic symptoms associated with this disease. The disease was first described by Marion Robinson and W.H.R. Lumsden in 1955, following an outbreak in 1952 on the Makonde Plateau, along the border between Mozambique and Tanganyika (the mainland part of modern day Tanzania).
According to the initial 1955 report about the epidemiology of the disease, the term 'chikungunya' is derived from the Makonde root verb kungunyala, meaning to dry up or become contorted. In concurrent research, Robinson glossed the Makonde term more specifically as "that which bends up". Subsequent authors apparently overlooked the references to the Makonde language and assumed the term derived from Swahili, the lingua franca of the region. The erroneous attribution of the term as a Swahili word has been repeated in numerous print sources. Many erroneous spellings of the name of the disease are in common use.
Since its discovery in Tanganyika, Africa, in 1952, chikungunya virus outbreaks have occurred occasionally in Africa, South Asia, and Southeast Asia, but recent outbreaks have spread the disease over a wider range.
Society and culture
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