Zika and chikungunya
Zika and chikungunya are viral diseases carried by Aedes mosquitoes. Each spreads in its own way and does its own harm to the people it infects. Zika is fought mainly by prevention, and chikungunya, though not endemic in Europe, has sporadically been passed on there.
The Zika virus and how it spreads
Zika belongs to the flaviviruses, which are most often transmitted by vectors, most commonly mosquitoes and ticks. Some members of the family, Zika among them, have been associated with sexual and bloodborne transmission. Their antibodies cross-react, which has made it hard to diagnose which flavivirus is present, but the cross-reaction has not proven protective from one species of virus to another. Zika appears to be unique in the family in both crossing the placenta during pregnancy and causing congenital abnormalities.
The virus was first identified in 1947 in primates in Uganda, where it was carried by a forest mosquito that was active at night and fed on primates, not humans. It has since moved to a human-biting mosquito, Aedes aegypti, which feeds at dawn and dusk. Its first serious outbreak was on the Micronesian island of Yap in 2007, and it spread east over the following years. Climate change had let A. aegypti live farther north, which aided the virus's spread farther into the United States.
Both Florida and Texas had hot spot zones monitored by the Centers for Disease Control and Prevention, though Texas had since been removed as a hot spot. Of the more than 80 mosquito species in the two states, and the roughly 80 in Florida, only A. aegypti had been confirmed to transmit Zika. The mosquito has distinctive black and white markings, and it can transmit Zika, dengue and chikungunya singly or infect one person with several of them. Co-infection of Zika with dengue or chikungunya had been suspected of worsening the outcome, but recent studies had not shown a link.
Zika infection and congenital Zika syndrome
About one in five people infected with Zika develop symptoms, which are generally mild and last two to seven days: headache, fever, conjunctivitis, joint pain, a diffuse itchy blanching rash and muscle pain. These symptoms are common to a myriad of diagnoses, so a specific diagnosis is difficult without specific testing. Practitioners had no standard-of-care guidelines, which made the virus harder still to track. As of October 2017 there had been 254 confirmed symptomatic cases in the United States and 554 confirmed cases in US territories. Of the 254 cases, 3 were transmitted sexually and the rest were presumed to have come from mosquitoes while the person was travelling in an endemic area.
Congenital Zika syndrome comprises microcephaly, encephalitis, loss of vision and hearing, arthrogryposis, fetal demise and possible long-term changes in behaviour. Its risk is about 4%, which is higher than the 3% occurrence of congenital abnormalities in the United States, and its treatment costs an estimated $4,000,000 per child. Zika can have dire consequences for infected people and their babies, and it can bring high medical costs.
A single mutation, S139N, which arose in 2013, has been linked to the virus's capacity to cause microcephaly in utero and Guillain–Barré syndrome. The mutation makes the virus deadlier to neuron precursor cells, so it disrupts fetal development and causes congenital Zika syndrome. It has also been linked to Guillain–Barré complications in adult infections, particularly in the elderly.
Preventing Zika
Prevention was the best approach to Zika in the United States because understanding of the disease process and its treatment was still in its infancy. Condoms and other deterrents were recommended for three weeks against sexual transmission. The recommendation was controversial because how long the virus remains transmissible is unknown: it had been detected up to a year after infection, but the risk of transmission was uncertain. People can protect themselves with easy measures such as insect repellent and removing standing water around their homes.
Florida controlled A. aegypti with a biological larvicide containing Bacillus thuringiensis and a chemical adulticide, naled, and both drew environmental protests from the public. Two further, very promising methods of reducing Zika transmission by A. aegypti had been approved for release: male mosquitoes infected with Wolbachia, and Oxitec's genetically modified sterile males.
Wolbachia is a Gram-negative parasitic bacterium that can infect only insects. The World Mosquito Program had found that A. aegypti infected with Wolbachia transmit less Zika to the people they bite. When infected males breed with infected females the eggs remain unhatched, which somewhat lowers the population. Wolbachia then becomes established in the population, and females carrying it pass it to their offspring and live but cannot transmit viruses. So as more of a population is infected, transmission of dengue and Zika falls.
Oxitec's males pass a defective gene to their offspring that stops the larvae developing into adults, and some had already been released. Both methods reduce the A. aegypti population, and with it Zika's prevalence, without harming other ecosystems. Both release only males, while only females bite.
Chikungunya and its symptoms
Chikungunya, also called CHIK fever, is a viral disease caused by chikungunya virus (CHIKV), which infected mosquitoes, primarily Aedes aegypti and Aedes albopictus, transmit to humans. Most infected people develop symptoms within 3 to 7 days, most commonly fever and joint pain. Other symptoms can include headache, muscle pain, joint swelling or rash. Most patients recover within a week, but some have joint pain for several months or longer, which can be disabling. A small proportion may develop severe acute disease, which can lead to multiorgan failure. Severe disease is most often observed in newborns exposed to the virus during childbirth and in adults over 65. As of 2024, there was no licensed treatment for chikungunya.
Where chikungunya occurs
CHIKV infections affect people mostly in the tropics and subtropics. Most countries reporting a high disease burden are in Central and South America. Brazil had outbreaks in a number of regions in 2024, with more than 160,000 cases reported in the first quarter of that year. Chikungunya is not endemic in Europe, and most cases in the EU concern travellers who were infected outside mainland Europe. Infected travellers have, however, sporadically passed the virus on after their return, mainly in southern Europe, where Aedes albopictus is established. The mosquito's spread with climate change could lead to cases in regions so far spared.