Zika
virus
Zika virus (ZIKV) is
a member of the Flaviviridae virus family and the Flavivirus genus.
In humans, it
initially causes a mild illness known as Zika fever, Zika, or Zika
disease, which since the 1950s has been known to occur within a
narrow equatorial belt from Africa to Asia. In 2014, the virus spread
eastward across the Pacific Ocean to French Polynesia, then to Easter
Island and in 2015 to Central America, the Caribbean, and South
America, where the Zika outbreak has reached pandemic levels. Zika
virus is related to dengue, yellow fever, Japanese encephalitis, and
West Nile viruses. The illness it causes is similar to a mild form of
dengue fever, is treated by rest, and cannot yet be prevented by
drugs or vaccines. There is a possible link between Zika fever and
microcephaly in newborn babies by mother-to-child transmission, as
well as a stronger one with neurologic conditions in infected adults,
including cases of the Guillain–Barré syndrome.
In January 2016, the
U.S. Centers for Disease Control and Prevention (CDC) issued travel
guidance on affected countries, including the use of enhanced
precautions, and guidelines for pregnant women including considering
postponing travel. Other governments or health agencies soon issued
similar travel warnings,while Colombia, the Dominican Republic,
Ecuador, El Salvador, and Jamaica advised women to postpone getting
pregnant until more is known about the risks.
Virology
Along
with other viruses in this family, Zika virus is enveloped and
icosahedral with a nonsegmented, single-stranded, positive-sense RNA
genome. It is most closely related to the Spondweni virus and is one
of the two viruses in the Spondweni virus clade.
The
virus was first isolated in April 1947 from a rhesus macaque monkey
that had been placed in a cage in the Zika Forest of Uganda, near
Lake Victoria, by the scientists of the Yellow Fever Research
Institute. A second isolation from the mosquito A. africanus followed
at the same site in January 1948. When the monkey developed a fever,
researchers isolated from its serum a transmissible agent that was
first described as Zika virus in 1952. In 1968, it was isolated for
the first time from humans in Nigeria.From 1951 through 1981,
evidence of human infection was reported from other African countries
such as the Central African Republic, Egypt, Gabon, Sierra Leone,
Tanzania, and Uganda, as well as in parts of Asia including India,
Indonesia, Malaysia, the Philippines, Thailand, and Vietnam.
The
pathogenesis of the virus is hypothesized to start with an infection
of dendritic cells near the site of inoculation, followed by a spread
to lymph nodes and the bloodstream. Flaviviruses generally replicate
in the cytoplasm, but Zika virus antigens have been found in infected
cell nuclei.
There
are two lineages of Zika virus, the African lineage and the Asian
lineage. Phylogenetic studies indicate that the virus spreading in
the Americas is most closely related to French Polynesian strains.
Complete genome sequences of Zika viruses have been published. Recent
preliminary findings from sequences in the public domain uncovered a
possible change in nonstructural protein 1 codon usage that may
increase the viral replication rate in humans.
Transmission
Zika
virus is transmitted by daytime-active mosquitoes and has been
isolated from a number of species in the genus Aedes, such as A.
aegypti, and arboreal mosquitoes such as A. africanus, A.
apicoargenteus, A. furcifer, A. hensilli, A. luteocephalus, and A.
vitattus. Studies show that the extrinsic incubation period in
mosquitoes is about 10 days.Zika virus can migrate between humans
through sexual contact and it can also cross the placenta, affecting
an unborn fetus. A mother already infected with Zika virus near the
time of delivery can pass on the virus to her newborn around the time
of birth, but this is rare.
The
vertebrate hosts of the virus are primarily monkeys and humans.
Before the current pandemic, which began in 2007, Zika virus "rarely
caused recognized 'spillover' infections in humans, even in highly
enzootic areas".
The
potential societal risk of Zika virus can be delimited by the
distribution of the mosquito species that transmit it (its vectors).
The global distribution of the most cited carrier of Zika virus, A.
aegypti, is expanding due to global trade and travel. A. aegypti
distribution is now the most extensive ever recorded – across all
continents including North America and even the European periphery.
Some mosquitoes have been found to have survived the last four
winters in Washington D. C.
Recent
news reports have drawn attention to the spread of Zika in Latin
America and the Caribbean.The countries and territories that have
been identified by the Pan American Health Organisation (PAHO) as
having experienced "local Zika virus transmission" are
Barbados, Bolivia, Brazil, Colombia, the Dominican Republic, Ecuador,
El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti,
Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint
Martin, Suriname, and Venezuela.
In
2009, Brian Foy, a biologist from the Colorado State University,
sexually transmitted Zika virus to his wife. He visited Senegal to
study mosquitoes and was bitten on a number of occasions. A few days
after returning to the United States, he fell ill with Zika, but not
before having had unprotected intercourse with his wife. She
subsequently showed symptoms of Zika infection with extreme
sensitivity to light. Foy is the first person known to have passed on
an insect-borne virus to another human by sexual contact.
In
2015, Zika virus RNA was detected in the amniotic fluid of two
fetuses, indicating that it had crossed the placenta and could cause
a mother-to-child infection. On 20 January 2016, scientists from the
state of Paraná, Brazil, detected genetic material of Zika virus in
the placenta of a woman who had undergone an abortion due to the
fetus's microcephaly, which confirmed that the virus is able to pass
the placenta.
Vaccine
development
Effective
vaccines exist for several Flaviviruses. Vaccines for yellow fever
virus, Japanese encephalitis, and tick-borne encephalitis were
introduced in the 1930s, while the vaccine for dengue fever has just
recently become available for use.
Work
has begun towards developing a vaccine for Zika virus, according to
Anthony Fauci, director of the National Institute of Allergy and
Infectious Diseases. The researchers at the Vaccine Research Center
have extensive experience from working with vaccines for other
viruses such as West Nile virus, chikungunya virus, and dengue fever.
Nikos Vasilakis of the Center for Biodefense and Emerging Infectious
Diseases has predicted that 10 to 12 years may be needed before an
effective Zika virus vaccine is available.
Clinical
Common
symptoms of infection with the virus include mild headaches,
maculopapular rash, fever, malaise, conjunctivitis, and joint pains.
The first well-documented case of Zika virus was described in 1964;
it began with a mild headache, and progressed to a maculopapular
rash, fever, and back pain. Within two days, the rash started fading,
and within three days, the fever resolved and only the rash remained.
Thus far, Zika fever has been a relatively mild disease of limited
scope, with only one in five persons developing symptoms, with no
fatalities, but its true potential as a viral agent of disease is
unknown.
As
of 2016, no vaccine or preventative drug is available. Symptoms can
be treated with paracetamol (acetaminophen), while aspirin and other
nonsteroidal anti-inflammatory drugs should be used only when dengue
has been ruled out to reduce the risk of bleeding.
Neurological
complications
In
a French Polynesian epidemic, 73 cases of Guillain–Barré syndrome
and other neurologic conditions occurred in a population of 270,000,
which may be complications of Zika virus.
In
December 2015, the European Centre for Disease Prevention and Control
issued a comprehensive update on the possible association of Zika
virus with congenital microcephaly and this syndrome. Data suggests
that newborn babies of mothers who had a Zika virus infection during
the first trimester of pregnancy are at an increased risk of
microcephaly. In December 2015 it was suspected that a transplacental
infection of the fetus may lead to microcephaly and brain damage. The
Brazilian Ministry of Health has since confirmed the relation between
the Zika virus and microcephaly.
