Chikungunya
Chikungunya is an infection caused by the chikungunya virus. The most common symptoms are fever and joint pain, typically occurring four to eight days after the bite of an infected mosquito; however some people may be infected without showing any symptoms. Other symptoms may include headache, muscle pain, joint swelling, and a rash. Symptoms usually improve within a week; however, occasionally the joint pain may last for months or years. The very young, old, and those with other health problems are at risk of more severe disease.
The virus is spread between people by two species of mosquito in the genus Aedes: Aedes albopictus and Aedes aegypti, which mainly bite during the day, particularly around dawn and in the late afternoon. The virus may circulate within a number of animals, including birds and rodents. Diagnosis is done by testing the blood for either viral RNA or antibodies to the virus. The symptoms can be mistaken for those of dengue fever and Zika fever, which are spread by the same mosquitoes. It is believed most people become immune after a single infection.
The best means of prevention are overall mosquito control and the avoidance of bites in areas where the disease is common. This may be partly achieved by decreasing mosquitoes' access to water, as well as the use of insect repellent and mosquito nets. Chikungunya vaccines have been approved for use in the United States and in the European Union. No specific treatment for chikungunya is available.
The chikungunya virus is widespread in tropical and subtropical regions where warm climates and abundant populations of its mosquito vectors facilitate its transmission. The disease was first identified in 1952 in Tanzania and named based on the Makonde words for "to become contorted". The disease has spread widely since the 2000s with outbreaks reported in many tropical and some temperate areas. It is considered to be endemic in many parts of the world, affecting millions of people every year. Chikungunya has become a global health concern due to its rapid geographic expansion, recurrent outbreaks, the lack of effective antiviral treatments, and potential to cause severe symptoms and death.
Signs and symptoms
Chikungunya can be asymptomatic, with estimates of between 17% and 40% of infections showing no symptoms. For those experiencing symptoms, they typically begin with a sudden high fever above around 3 to 7 days after the bite of an infected mosquito. The fever is often accompanied by severe muscle and joint pain, which affects multiple joints in the arms and legs and is often symmetric – i.e. if one elbow is affected, the other is as well. People with chikungunya also frequently experience headaches, back pain, nausea, and fatigue. Around half of those affected develop a rash, with reddening and sometimes small bumps on the palms, foot soles, torso, and face.For some, the rash remains constrained to a small part of the body; for others, the rash can be extensive, covering more than 90% of the skin. Some people experience gastrointestinal issues, with abdominal pain and vomiting. Others experience eye problems, namely sensitivity to light, conjunctivitis, and pain behind the eye. This first set of symptoms – called the "acute phase" of chikungunya – lasts around a week, after which most symptoms resolve on their own.
For those with severe symptoms, approximately 30% to 40% continue to have symptoms after the "acute phase" resolves. The lasting symptoms tend to be joint pains: arthritis, tenosynovitis, and/or bursitis. If the affected person has pre-existing joint issues, these tend to worsen. Overuse of a joint can result in painful swelling, stiffness, nerve damage, and neuropathic pain. Typically the joint pain improves with time; however, the chronic stage can last anywhere from a few months to several years.
Almost all symptomatic cases feature joint pain, generally in more than one joint. Pain most commonly occurs in peripheral joints, such as the wrists, ankles, and joints of the hands and feet as well as some of the larger joints, typically the shoulders, elbows and knees. Joints are more likely to be affected if they have previously been damaged by disorders such as arthritis. Pain may also occur in the muscles or ligaments. In more than half of cases, normal activity is limited by significant fatigue and pain. Infrequently, inflammation of the eyes may occur in the form of iridocyclitis, or uveitis, and retinal lesions may occur. Temporary damage to the liver may occur.
People with chikungunya occasionally develop long term neurologic disorders, most frequently swelling or degeneration of the brain, inflammation or degeneration of the myelin sheaths around neurons, Guillain–Barré syndrome, acute disseminated encephalomyelitis, hypotonia, and issues with visual processing.
Newborns, the elderly, and those with diabetes, heart disease, liver and kidney diseases, and human immunodeficiency virus infection tend to have more severe cases of chikungunya. Fewer than 1 in 1,000 people with symptomatic chikungunya die of the disease; generally these are people with pre-existing health conditions.
Transmission
Chikungunya is generally transmitted from mosquitoes to humans. Chikungunya is spread through bites from Aedes mosquitoes, specifically A. aegypti and A. albopictus. Because high amounts of virus are present in the blood during the first few days of infection, the virus can spread from an infected human to a mosquito, where it replicates without harming the mosquito. Subsequently, a bite from the infected mosquito will transmit the virus back to a human. The incubation period ranges from one to twelve days and is most typically three to seven.Rarely, the disease can be transmitted from mother to child during pregnancy or at birth, in women who become infected a few days before delivery.
Mechanism
Chikungunya virus is passed to humans when a bite from an infected mosquito breaks the skin and introduces the virus into the body. The virus initially replicates in cells near the location of the bite; from here it enters the lymphatic system and the bloodstream, enabling it to circulate to organs and tissues which become infected. Most frequently it reproduces in the lymphatic system and the spleen, as well as peripheral joints, muscles and tendons where symptoms frequently occur; it appears that the virus is able to penetrate and replicate in many different types of cells. In severe cases it can infect the brain and liver.During the acute phase of infection, large numbers of infectious virus particles are present in the bloodstream, making it very likely that an uninfected mosquito will pick up the virus if it bites the human host.
During the first few days of infection, the host's innate immune system is activated, producing type I interferons and inflammatory cytokines to fight the infection. This generates the fever and localised inflammation which is characteristic of the disease. It takes about a week before the host's adaptive immune system begins to develop antibodies which eventually clear the virus from the bloodstream. However the virus can persist within specific tissues, especially the joints, causing long term inflammation and pain in chronic cases.
The virus has mechanisms which help it to evade the immune response. Within an infected cell, the viral nonstructural protein 2 interferes with the JAK-STAT signalling pathway to hinder it from triggering an antiviral response. The virus can induce apoptosis in host cells; virus laden debris from apoptosis is engulfed by macrophages which in turn become infected. The virus also seems to be able to evade T lymphocytes which seek to target and destroy the virus particles.
Diagnosis
Diagnosing chikungunya can be difficult because its symptoms, such as sudden fever and joint pain, closely resemble other mosquito-borne illnesses like dengue fever and malaria. Chikungunya should be suspected if a patient with these symptoms either lives in an area where the virus is endemic, or if they have recently traveled to such an area.During the first week of illness, when virus is present in the bloodstream, it is possible to detect viral RNA in a blood sample using techniques such as reverse transcription-polymerase chain reaction or viral culture. After this time, the body develops antibodies and the virus is eliminated from the bloodstream. Antibodies in blood serum persist for between 3 and 12 months; they can be detected for up to a year after infection using enzyme-linked immunosorbent assay or indirect fluorescent antibody. All of these techniques are time consuming and costly, requiring sophisticated laboratory equipment which may not be available in resource poor settings.
Differential diagnosis
The Aedes mosquitoes which carry chikungunya virus can also carry other viruses such as dengue, zika, and yellow fever. Other infections which should be considered include malaria, leptospirosis, measles, mononucleosis and African tick bite fever, which are often endemic in the same areas and can have similar symptoms. It is possible for a patient to be infected by more than one virus simultaneously.Prevention
Although an approved vaccine exists, the most effective means of prevention is to avoid or prevent mosquito bites. The main strategies for this are: controlling mosquito populations by limiting their habitat; and protection against contact with disease-carrying mosquitoes. On a large scale, mosquito control focuses on eliminating standing water where mosquitoes lay eggs and develop as larvae. Individuals should use mosquito repellent, as well as barriers such as loose clothing that covers the arms and legs, mosquito nets and window and door screens.Once immunity against chikungunya has been acquired, whether as a result of infection or vaccination, it endures long term and may be lifelong.