Zika fever
Zika fever, also known as Zika virus disease or simply Zika, is an infectious disease caused by the Zika virus. Most cases have no symptoms, but when present they are usually mild and can resemble dengue fever. Symptoms may include fever, red eyes, joint pain, headache, and a maculopapular rash. Symptoms generally last less than seven days. It has not caused any reported deaths during the initial infection. Mother-to-child transmission during pregnancy can cause microcephaly and other brain malformations in some babies. Infections in adults have been linked to Guillain–Barré syndrome.
Zika fever is mainly spread via the bite of mosquitoes of the Aedes type. It can also be sexually transmitted and potentially spread by blood transfusions. Infections in pregnant women can spread to the baby. Diagnosis is by testing the blood, urine, or saliva for the presence of the virus's RNA when the person is sick, or the blood for antibodies after symptoms are present more than a week.
Prevention involves decreasing mosquito bites in areas where the disease occurs and proper condom use. Efforts to prevent bites include the use of insect repellent, covering much of the body with clothing, mosquito nets, and getting rid of standing water where mosquitoes reproduce. There is no effective vaccine. Health officials recommended that women in areas affected by the 2015–16 Zika outbreak consider putting off pregnancy and that pregnant women not travel to these areas. While there is no specific treatment, paracetamol may help with the symptoms. Hospital admission is rarely necessary.
The virus that causes the disease was first isolated in Africa in 1947. The first documented outbreak among people occurred in 2007 in the Federated States of Micronesia. An outbreak started in Brazil in 2015, and spread to the Americas, Pacific, Asia, and Africa. This led the World Health Organization to declare it a Public Health Emergency of International Concern in February 2016. The emergency was lifted in November 2016, but 84 countries still reported cases as of March 2017. The last proven case of Zika spread in the Continental United States was in 2017.
Signs and symptoms
Most people who are infected have no or few symptoms. Otherwise the most common signs and symptoms of Zika fever are fever, rash, conjunctivitis, muscle and joint pain, and headache, which are similar to signs and symptoms of dengue and chikungunya fever. The time from a mosquito bite to developing symptoms is not yet known, but is probably a few days to a week. The disease lasts for several days to a week. It is usually mild enough for people not to go to a hospital.Due to being in the same family as dengue, there has been concern that it could cause similar bleeding disorders. However that has only been documented in one case, with blood seen in semen, also known as hematospermia.
Guillain–Barré syndrome
Zika virus infections have been strongly associated with Guillain–Barré syndrome, which is a rapid onset of muscle weakness caused by the immune system damaging the peripheral nervous system, and which can progress to paralysis. While both GBS and Zika infection can simultaneously occur in the same individual, it is difficult to definitively identify Zika virus as the cause of GBS. Though Zika virus has been shown to infect human Schwann cells. Several countries affected by Zika outbreaks have reported increases in the rate of new cases of GBS. During the 2013–2014 outbreak in French Polynesia, there were 42 reported cases of GBS over three months, compared to between 3 and 10 annually before the outbreak.Pregnancy
The disease spreads from mother to child in the womb and can cause multiple problems, most notably microcephaly, in the baby. The full range of birth defects caused by infection during pregnancy is not known, but they appear to be common, with large-scale abnormalities seen in up to 42% of live births. The most common observed associations have been abnormalities with brain and eye development such as microcephaly and chorioretinal scarring. Less commonly there have been systemic abnormalities such as hydrops fetalis, where there is abnormal accumulation of fluid in the fetus. These abnormalities can lead to intellectual problems, seizures, vision problems, hearing problems, problems feeding and slow development.Whether the stage of pregnancy at which the mother becomes infected affects the risk to the fetus is not well understood, nor is whether other risk factors affect outcomes. One group has estimated the risk of a baby developing microcephaly at about 1% when the mother is infected during the first trimester, with the risk of developing microcephaly becoming uncertain beyond the first trimester. Affected babies might appear normal but actually have brain abnormalities; infection in newborns could also lead to brain damage.
Cause
Reservoir
is a mosquito-borne flavivirus closely related to the dengue and yellow fever viruses. While mosquitoes are the vector, the main reservoir species remains unknown, though serological evidence has been found in both West African monkeys and rodents.Transmission
Transmission is via the bite of mosquitoes from the genus Aedes, primarily Aedes aegypti in tropical regions. It has also been isolated from Ae. africanus, Ae. apicoargenteus, Ae. luteocephalus, Ae. albopictus, ''Ae. vittatus and Ae. furcifer. During the 2007 outbreak on Yap Island in the South Pacific, Aedes hensilli was the vector, while Aedes polynesiensis'' spread the virus in French Polynesia in 2013.Zika virus can also spread by sexual transmission from infected men to their partners. Zika virus has been isolated from semen samples, with one person having 100,000 times more virus in semen than blood or urine, two weeks after being infected. It is unclear why levels in semen can be higher than other body fluids, and it is also unclear how long infectious virus can remain in semen. There have also been cases of men with no symptoms of Zika virus infection transmitting the disease. The CDC has recommended that all men who have travelled to affected areas should wait at least 6 months before trying to attempt conception, regardless of whether they were ill. To date, there have been no reported sexual transmissions from women to their sexual partners. Oral, anal, or vaginal sex can spread the disease.
Cases of vertical perinatal transmission have been reported. The CDC recommends that women with Zika fever wait at least 8 weeks after they start having symptoms of the disease before attempting to conceive. There have been no reported cases of transmission from breastfeeding, but infectious virus has been found in breast milk.
Like other flaviviruses, it could potentially be transmitted by blood transfusion, and several affected countries have developed strategies to screen blood donors. The U.S. FDA has recommended universal screening of blood products for Zika. The virus is detected in 3% of asymptomatic blood donors in French Polynesia.
It is predicted that climate change will expose more than 1.3 billion new people to Zika by 2050, largely due to the expansion of habitats conducive to mosquito vector growth and life cycles, particularly areas with temperatures ranging from 23.9 °C to 34 °C.
Pathophysiology
In fruit flies, microcephaly appears to be caused by the flavivirid virus protein NS4A, which can disrupt brain growth by hijacking a pathway that regulates the growth of new neurons.Diagnosis
It is difficult to diagnose Zika virus infection based on clinical signs and symptoms alone due to overlaps with other arboviruses that are endemic to similar areas. The US Centers for Disease Control and Prevention advises that "based on the typical clinical features, the differential diagnosis for Zika virus infection is broad. In addition to dengue, other considerations include leptospirosis, malaria, rickettsia, group A streptococcus, rubella, measles, and parvovirus, enterovirus, adenovirus, and alphavirus infections."In small case series, routine chemistry and complete blood counts have been normal in most patients. A few have been reported to have mild leukopenia, thrombocytopenia, and elevated liver transaminases.
Zika virus can be identified by reverse transcriptase PCR in acutely ill patients. However, the period of viremia can be short and the World Health Organization recommends RT-PCR testing be done on serum collected within 1 to 3 days of symptom onset or on saliva samples collected during the first 3 to 5 days. When evaluating paired samples, Zika virus was detected more frequently in saliva than serum. Urine samples can be collected and tested up to 14 days after the onset of symptoms, as the virus has been seen to survive longer in the urine than either saliva or serum. The longest period of having a detectable level of the virus has been 11 days, and the Zika virus does not appear to establish latency.
Later on, serology for the detection of specific IgM and IgG antibodies to the Zika virus can be used. IgM antibodies can be detectable within 3 days of the onset of illness. Serological cross-reactions with closely related flaviviruses such as dengue and West Nile virus as well as vaccines to flaviviruses are possible. As of 2019, the FDA has authorized two tests to detect Zika virus antibodies.