Science Wiki

Chikungunya is a mosquito-borne viral disease caused by chikungunya virus (CHIKV; species Alphavirus chikungunya), an enveloped, positive-sense, single-stranded RNA virus of the genus Alphavirus, family Togaviridae. It is transmitted mainly in an urban cycle between people and day-biting female Aedes mosquitoes, principally Aedes aegypti and the cold-tolerant Aedes albopictus, with forest-dwelling African mosquitoes maintaining a separate cycle among wild primates.

The name comes from the Kimakonde language of southern Tanzania and northern Mozambique and means roughly "that which bends up", describing the stooped posture of patients suffering the severe joint pain that characterizes the illness.

Chikungunya differs from the other major urban arboviruses in four respects:

1. Almost everyone infected becomes ill. Whereas most dengue and Zika infections cause no symptoms, the great majority of chikungunya infections do. Combined with very high levels of virus in the blood and an unusually short interval before mosquitoes become infectious, often only two to four days, this allows the virus to sweep through a population with little prior immunity within a single season.
2. Joint pain often persists. Death is uncommon, but a substantial proportion of patients have continuing or relapsing joint pain, tendon inflammation, and in some cases a destructive arthritis resembling rheumatoid arthritis, lasting months or years. Estimates of how many are affected vary widely with the definition used and the length of follow-up.
3. The virus adapted to a new mosquito. During the Indian Ocean epidemic of 2005 and 2006, a single amino acid change in an envelope protein greatly improved the virus's ability to infect Aedes albopictus, turning a secondary vector into the main one across the Indian Ocean, Asia, and Europe.
4. It transmits in temperate climates. Because Aedes albopictus survives cold winters as diapausing eggs and transmits the virus efficiently at summer temperatures, locally acquired outbreaks have occurred repeatedly in southern and western Europe.

Alphavirus Taxonomy and Sylvatic Origins

Chikungunya virus belongs to the genus Alphavirus in the family Togaviridae. The genus divides broadly into two groups:

• New World alphaviruses, including the equine encephalitis viruses of the Americas, which infect the central nervous system.
• Old World alphaviruses, which cause joint disease. These include chikungunya virus, its closest relative O'nyong'nyong virus (unusual in being transmitted by Anopheles mosquitoes), Mayaro virus, Ross River virus, and Sindbis virus.

The virus originated in the forests and savannas of sub-Saharan Africa. It was first isolated in 1952 and 1953 during an outbreak of fever and joint pain on the Makonde Plateau in southern Tanzania, by Marion Robinson, William Lumsden, and colleagues.

In Africa it circulates in a forest cycle between wild primates, chiefly baboons and guenons, and canopy-dwelling mosquitoes including Aedes africanus, Ae. furcifer, and Ae. luteocephalus, which breed in tree holes and bite people working at the forest margin, causing occasional rural outbreaks.

Global Lineages and Serotype Conservation

All strains belong to a single serotype: antibodies raised against one lineage neutralize all others. Four lineages are recognized.

Lineages of chikungunya virus

Adaptive Evolution of the E1-A226V Substitution and Epistatic Constraints

The rise of the Indian Ocean lineage is one of the best-documented examples of a virus adapting to a new vector, worked out by Isabelle Schuffenecker, Konstantin Tsetsarkin, Scott Weaver, and others.

In 2004 an ECSA strain carrying the ancestral alanine at position 226 of the E1 protein caused an outbreak on the Kenyan coast during a drought, spread to the Comoros, and reached Réunion in early 2005. On Réunion, Aedes aegypti was almost absent, while the invasive Aedes albopictus was abundant everywhere.

• The early, modest wave on Réunion was caused by viruses carrying the ancestral residue.
• By late 2005 a single nucleotide change had substituted valine for alanine at this position, giving E1-A226V.
• Within a few months the new variant had almost entirely replaced the old one, coinciding with an explosive epidemic that infected roughly a third of the island's population. The same change subsequently arose independently in India, Sri Lanka, Cameroon, Gabon, and Papua New Guinea wherever ECSA strains met dense Aedes albopictus populations.

1. It reduces the virus's dependence on cholesterol. The E1 protein fuses the virus with the host membrane inside the mosquito gut, a process assisted by cholesterol in that membrane. Residue 226 lies in a loop next to the fusion peptide. Substituting valine alters the behaviour of this region in ways that make infection of Aedes albopictus cells far more efficient, by reported factors of several tens, and lower the dose needed to infect a mosquito by about a hundredfold, without impairing replication in Aedes aegypti.
2. Later changes added to the effect. As the lineage circulated in Asia it acquired further substitutions in the companion E2 protein, each adding modestly to its fitness in Aedes albopictus.
3. The Asian lineage could not follow. When the Asian lineage reached the Americas in 2013, it was feared that it would acquire the same change and spread into temperate North America. Reverse-genetics experiments by Tsetsarkin and Weaver showed why this did not happen: Asian strains carry a threonine at residue 98 of E1, and in that context the 226 change is harmful rather than beneficial. Adapting would require two changes at once, which is a far less likely evolutionary step.

Icosahedral Spike Shell and Nucleocapsid Core

The mature particle is about 70 nm across. Unlike the flaviviruses, which have a flat outer protein layer and a disordered core, chikungunya virus has strict icosahedral symmetry in both, resolved at near-atomic detail by Felix Rey, Michael Rossmann, Richard Kuhn, and others.

• The outer shell consists of 240 copies each of two envelope glycoproteins, E1 and E2, paired together and assembled into 80 three-bladed spikes projecting from the surface.
• E2 forms the outer petals of each spike. Its three domains include one at the tip that caps the E1 fusion peptide at neutral pH and carries the main targets of neutralizing antibodies, and one near the membrane whose tail binds a capsid protein beneath.
• E1 lies beneath, forming a lattice over the membrane. Its fusion peptide is tucked under the E2 cap until acidification releases it.
• E3 is a small protein cleaved from a precursor by host furin but remaining attached at acidic pH, preventing premature fusion during exit from the cell. 6K and a related protein produced by a shift in reading frame act as small membrane channels and assist budding.
• The lipid membrane is acquired as the particle buds through the host cell surface, and is rich in cholesterol and sphingomyelin.
• The core consists of 240 copies of the capsid protein around a single copy of the genome. The capsid's N-terminal region is disordered and basic, binding and condensing the RNA; its C-terminal region is a protease that cuts itself free from the growing protein chain and then blocks its own active site, so that it acts only once.

Two-Reading-Frame Genome and Non-Structural Proteins

The genome is a single RNA strand of about 11,800 bases with a cap at one end and a poly(A) tail at the other. Unlike the flaviviruses, which encode everything in one reading frame, chikungunya virus has two, separated by a promoter from which a second, shorter messenger RNA is made in large excess.

Proteins encoded by chikungunya virus

MXRA8 Receptor Attachment, Membrane Spherules, and Budding

1. Attachment. A CRISPR screen by Rong Zhang, Michael Diamond, and colleagues identified MXRA8 as a major receptor for chikungunya and related arthritis-causing alphaviruses. It is abundant on exactly the cells affected in the disease: synovial and dermal fibroblasts, cartilage cells, muscle, and bone-forming cells. Structures show a single MXRA8 molecule wedged into a cleft between adjacent protein pairs within a spike. MXRA8 is not the only route in, since cells lacking it can still be infected, and it is a vertebrate protein with no counterpart in mosquitoes. Other attachment factors include lectins and heparan sulfate.
2. Entry and fusion. The particle is taken into the cell and delivered to an acidifying compartment. Falling pH releases the E2 cap, the pairs come apart, and E1 inserts its fusion peptide into the host membrane and refolds into a trimer, pulling the two membranes together. Host ribosomes then strip the core, releasing the genome.
3. Replication in membrane pouches. Early after infection, the uncleaved polyprotein and the polymerase anchor to the inner face of the cell membrane and copy the genome into a negative-sense strand. The membrane bulges outward into a pouch 50 to 80 nm across, containing the double-stranded intermediate and gated by the nsP1 ring, which caps the RNAs leaving it and keeps the double-stranded RNA hidden from the cell's sensors.

As the viral proteins accumulate, the protease cuts them apart, and the mature machinery switches irreversibly to making new genomes and large quantities of the second messenger RNA.

1. Continued translation and budding. Infection triggers the cell to shut down its own protein synthesis. The viral structural messenger RNA circumvents this through a stable hairpin just after the start codon, which allows the ribosome to begin without the usual initiation factor, a mechanism first described in the related Sindbis virus. New cores then dock against the tails of the surface proteins at the cell membrane and bud out as complete particles.

Acute Febrile Phase and Polyarthralgia

After an incubation period of two to six days, the great majority of infected people become ill, in contrast to dengue and Zika, where most infections pass unnoticed.

• Fever. Sudden onset of fever between 39 and 40.5 °C with chills, coinciding with very high levels of virus in the blood and a strong inflammatory response. Fever usually lasts three to seven days.
• Joint pain. Severe, symmetrical pain affecting most patients, beginning with or shortly after the fever. The small joints of the hands and feet, the wrists, and the ankles are worst affected, while the hips and spine are relatively spared, though previously damaged joints are often badly affected. Ultrasound shows inflammation of the joint lining and of the tendon sheaths, which frequently causes compression of nerves at the wrist and ankle.
• Rash and other features in roughly half of patients: a blotchy rash over the trunk and limbs, often itchy, sometimes preceded by facial flushing, together with headache, red eyes, swollen lymph nodes, mouth ulcers, and in infants blistering of the skin.
• Blood tests help distinguish chikungunya from dengue. Chikungunya characteristically causes a marked fall in lymphocytes with platelet counts that remain normal or nearly so, and no concentration of the blood. Dengue causes the opposite pattern: low platelets and a rising haematocrit as plasma leaks from the circulation.

Neonatal, Neurological, and Systemic Complications

A small proportion of acute cases, generally under a few per cent, become severe. Reported case-fatality rates are low, in the region of one in a thousand overall, but considerably higher among newborns and older adults with other illnesses.

Severe manifestations

Pathogenesis of Chronic Chikungunya Arthritis and Bone Erosion

After the acute illness settles, a substantial minority of patients do not recover and develop persistent joint disease lasting months or years. Published estimates range from roughly a tenth to more than half of patients, depending on how the condition is defined, how long patients are followed, and the population studied. Risk is higher in older patients, in women, and in those with severe acute illness. Associations with particular immune-system genetic types have been reported but rest on small studies.

Two patterns are distinguished:

• Persistent pain without joint damage, the more common form: continuing joint and tendon pain, frozen shoulder, heel pain, nerve compression, and fatigue, with normal X-rays.
• Inflammatory arthritis, in a smaller proportion: a genuine destructive arthritis meeting the criteria used for rheumatoid arthritis or related conditions, with prolonged morning stiffness, raised inflammatory markers, and progressive erosion of bone visible on imaging.

Studies of joint tissue from patients and from infected monkeys indicate three contributing processes:

1. Persistence of viral material. Viral RNA and proteins can be found in joint tissue, in macrophages and fibroblasts, for a year or more after the acute illness, long after infectious virus has disappeared from the blood. Viral interference with antigen presentation may help these cells escape clearance.
2. A self-sustaining inflammatory cycle. Persistent viral RNA continuously triggers the cell's sensors, driving release of inflammatory signals including interleukin 6 and the chemokine CCL2, which recruit a continuing influx of monocytes, natural killer cells, and T cells into the joint lining, which thickens and becomes invasive.
3. Bone erosion. The virus infects bone-forming cells, which then produce more of the signal that drives formation of bone-resorbing cells and less of its natural inhibitor. The resulting imbalance, together with enzymes released by joint fibroblasts, degrades cartilage and erodes bone around the joint.

Extrinsic Incubation Speed and Epidemic Dynamics

Transmission is described by the standard vector-borne model:

where m is mosquitoes per person, a the biting rate on humans, b and c the transmission probabilities in each direction, μ the mosquito death rate, n the time the virus needs to develop in the mosquito, and 1/_γ_ the period a person is infectious, about four to six days.

Comparing chikungunya with dengue and Zika in the same mosquitoes explains the difference in epidemic behaviour.

Chikungunya compared with dengue and Zika

Dual-Vector Thermal Envelopes and Temperate Outbreaks

Because chikungunya is transmitted efficiently by two mosquitoes with different thermal preferences, it occupies a wider temperature range than arboviruses dependent on one.

Temperature limits for transmission

1. Europe. Since Aedes albopictus became established across more than twenty European countries, hot summers have repeatedly allowed transmission to follow the arrival of an infected traveller.

• Emilia-Romagna, Italy, 2007. A traveller returning from India introduced the Indian Ocean lineage, and local mosquitoes transmitted it to several hundred people, the first such outbreak in modern Europe.
• Southern France, repeatedly since 2010, with clusters in the Var, Hérault, and Bouches-du-Rhône, and the first locally acquired cases near Paris in 2024.
• Lazio and Calabria, Italy, 2017, with several hundred cases caused by a strain lacking the E1-A226V change but carrying other substitutions, showing that adaptation to Aedes albopictus can occur by more than one route.

1. The Americas. The Asian lineage caused the initial epidemic from 2013. The ECSA lineage introduced to Brazil in 2014 has since spread across South America and southward into the subtropics, with a large and unusually lethal epidemic in Paraguay in 2022 and 2023, and continued transmission in Brazil, Bolivia, and the Caribbean. Figures for the most recent years remain provisional.
2. Both drought and flood contribute. Drought interrupts piped water and leads households to store it indoors, favouring Aedes aegypti; heavy rain fills outdoor containers, favouring both species. The drought that preceded the Kenyan outbreak of 2004 began the entire Indian Ocean epidemic.
3. Higher elevations. Warming has lifted the lower limit for transmission several hundred metres, and locally acquired cases now occur well above previous altitude limits in the Andes and in highland Mexico and Ethiopia.

Stage-Specific Laboratory Diagnostics

Because chikungunya circulates alongside dengue, Zika, and other viruses spread by the same mosquitoes, and because treatment differs sharply, laboratory confirmation matters.

Diagnostic tests by stage

Acute Analgesia and Chronic Anti-Rheumatic Therapy

No antiviral drug is available, and management depends on the stage of illness.

1. Acute illness, the first two weeks.

• Paracetamol is used for fever and pain, with rest and fluids.
• Anti-inflammatory drugs must be withheld until dengue has been excluded. Aspirin, ibuprofen, and similar drugs impair platelet function and irritate the stomach, and can cause severe bleeding in a patient who actually has, or also has, dengue. Aspirin additionally risks Reye's syndrome in children.
• Corticosteroids are avoided during the first week, since they suppress the immune response to the acute infection and are followed by a flare of joint symptoms when stopped.

1. The following weeks. Once dengue has been excluded and the acute phase has passed, anti-inflammatory drugs become the mainstay for persistent joint and tendon inflammation. For severe or unresponsive symptoms, a short tapering course of oral corticosteroid, or injection into the affected joint, may be used alongside gentle physiotherapy.
2. Beyond three months. Patients with continuing inflammatory arthritis are usually referred to a rheumatologist. Methotrexate, sometimes with hydroxychloroquine, is the most commonly used drug, and observational studies and small trials suggest benefit, though the evidence is not strong. For patients who do not respond, the biological drugs used in rheumatoid arthritis, including inhibitors of tumour necrosis factor and of the interleukin 6 receptor, have been used.

Licensed Live-Attenuated and VLP Chikungunya Vaccines

Because all lineages share a single serotype and antibodies confer lasting protection, chikungunya is a comparatively straightforward target for vaccination, without the complications that antibody-dependent enhancement creates for dengue. Enhancement has been observed in laboratory systems for alphaviruses but is not thought to be clinically important.

Two single-dose vaccines were approved between late 2023 and early 2025.

Licensed chikungunya vaccines

Other approaches in trials include vaccines based on measles and adenovirus vectors, and an mRNA product encoding a neutralizing antibody directly, intended to give immediate protection to newborns or during pregnancy.

Integrated Vector Control and Wolbachia Suppression

Controlling chikungunya requires addressing both the indoor-resting Aedes aegypti and the outdoor Aedes albopictus.

1. Wolbachia. Introducing this bacterium into Aedes aegypti populations substantially reduces the amount of chikungunya virus reaching mosquito saliva, apparently by depriving the virus of the cholesterol its fusion protein requires and by raising the mosquito's baseline immune activity. Deployments in Niterói and Rio de Janeiro reported large reductions in chikungunya alongside reductions in dengue, though these were not randomized studies.
2. Targeted spraying and larval control. Spraying the lower indoor walls and furniture where Aedes aegypti rests, using insecticides other than pyrethroids given widespread resistance, combined with treatment of outdoor containers and traps that spread an insect growth regulator between breeding sites.
3. Protecting patients from being bitten. Because acute patients carry very high levels of virus for the first week while resting indoors, guidelines recommend that they sleep under nets or use repellents during the day, to reduce transmission to the rest of the household. This follows from the biology rather than from trial evidence.

• Alphavirus
• Aedes albopictus
• Mayaro virus
• Dengue fever
• Vector control

How did the E1-A226V mutation allow chikungunya virus to adapt to Aedes albopictus?

The E1 envelope glycoprotein mediates low-pH endosomal membrane fusion, a conformational rearrangement that depends on cholesterol in the host mosquito midgut membrane. Substituting valine for alanine at residue 226 (E1-A226V)—positioned in the IJ loop adjacent to the hydrophobic fusion peptide—relieves cholesterol dependence during membrane fusion, reducing the oral 50% infectious dose for Aedes albopictus by roughly 100-fold without compromising replication in Aedes aegypti.

Why couldn't the Asian lineage of chikungunya virus evolve the E1-A226V mutation in the Americas?

Reverse-genetics studies revealed that all Asian lineage strains carry a threonine residue at position 98 of the E1 glycoprotein (E1-98T), whereas East, Central, and South African (ECSA) strains carry an ancestral alanine (E1-98A). Through negative sign epistasis, E1-98T structurally blocks the fitness benefit of E1-A226V, meaning the Asian lineage would require two simultaneous point mutations to adapt to Aedes albopictus.

Why does joint pain persist for months or years after acute chikungunya infection?

Chikungunya virus uses the cell-surface receptor MXRA8 to infect synovial fibroblasts, macrophages, chondrocytes, and osteoblasts. Even after neutralizing antibodies clear infectious virions from the bloodstream within 7 to 10 days, viral RNA and antigen persist inside synovial macrophages and fibroblasts, driving chronic Toll-like receptor activation, IL-6 and CCL2 secretion, monocyte infiltration, and an elevated RANKL-to-osteoprotegerin ratio that activates osteoclast-mediated periarticular bone erosion.