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Public Health England has confirmed that three Britons have contracted Zika virus following travel to Central and South America, where the virus continues to spread.
Prof. Laura Rodrigues, Professor of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, said:
Where in the world do people contract Zika, and how?
“Zika is a mosquito transmitted virus; it has been isolated in countries in Africa, South Asia, Southeast Asia, Micronesia, Polynesia and Latin America. There have been outbreaks in Micronesia and French Polynesia and Latin America. Transmission could happen anywhere there is dengue (as Zika is transmitted by the dengue vector, Aedes Egypti) if the virus is introduced, for example by someone returning from a country where the virus is circulating.
What does Zika virus do to a healthy (and non-pregnant) adult and what would their symptoms be?
“Zika is normally very mild; symptoms are similar to mild dengue: malaise, rash, low fever, itching and red eyes. Extremely rarely there maybe neurological complications (Guillain-Barré syndrome) but the concern is transmission to the fetus in pregnant women.
What are the potential complications if Zika is contracted during pregnancy, and how strong is the evidence for an association / causative relationship with microcephaly?
“The evidence for a link is relatively strong, and considered strong enough to warrant public health measures. Evidence includes temporal association between Zika outbreaks and microcephaly outbreaks both in Brazil and French Polynesia. The virus is neurotropic – it grows in the brain of the fetus and destroys brain structures so the developing brain is malformed – small – that’s the cause of the microcephaly.
Is Zika contagious between people?
“We know for sure that most transmission is by Mosquitos. There is no evidence of person to person transmission yet but the virus has been isolated in semen so this is a possibility.
Do we know why cases of Zika are on the rise in South America and the Caribbean?
“Once a virus with a competent vector (the mosquito) is introduced in a totally susceptible population it is to be expected that transmission there would be epidemic. There were earlier outbreaks in Micronesia and Polynesia. The mystery is why there were no outbreaks in the other countries were Zika was isolated. Potential factors enhancing transmission are the high population density (large cities) with many breeding sites for the mosquito – dengue is a domestic mosquito and is rarely found more than 100 metres from human habitations.
What does it mean for the UK that three returning travellers have tested positive for Zika – could they pass it onto others? Does the mosquito that transmits the virus live in the UK?
“Because Aedes does not live in the UK (it does not survive cold winters) there is almost no risk of transmission unless there are transmission mechanisms we do not know about yet. Infected cases coming to the UK are therefore not going to transmit infection. All the risk is for pregnant women travelling to areas in the world were there is Zika and being infected there.
“This is a very new situation. Until a few months ago we did not know that Zika could cause congenital infections and microcephaly. It caught us all by surprise. There was very little research as Zika was seen as a virus of no public health importance. There is no vaccine, we do not know if a case becomes immune, there is no good diagnostic test and no treatment. Much research needs to be done – will be done – with a major international effort we hope to have better knowledge and maybe a vaccine in a couple of years.”
Prof. Andrew Easton, Professor of Virology, University of Warwick, said:
“Zika virus was first detected in Uganda in 1947 in a monkey with the first human cases identified in Nigeria in the late 1960s. The virus was then found in humans in small sporadic irregular outbreaks in many parts of Africa, particularly West Africa and subsequently South East Asia. In the cases reported at that time the disease was seen in approximately 20% of infected individuals with the remainder showing no obvious disease. The disease was mild with fever, rash and headaches and some additional symptoms and that remains the most common type of disease. The full range of symptoms is not always seen.
“The virus continued its spread reaching the islands of Micronesia and ultimately French Polynesia in recent years.
“The virus is transmitted by mosquitoes belonging to the Aedes species. Aedes aegypti is a very common mosquitos of this family found in urban areas in tropical and sub-tropical regions and is likely to be the primary species responsible for transmitting Zika virus, and many other viruses. Zika virus is transmitted when the infected mosquito bites a person to take a blood meal and if the person is already infected the mosquito may pick up the virus at this time and transfer it to the next person they bite. The virus must be introduced into the bloodstream to be able to establish an infection and is not transmitted by other routes such as aerosols. There has been a report suggesting that Zika virus may very rarely be transmitted sexually but more work on this is required and even if possible it is very unlikely to be coming and is certainly not a major route of infection.
“The virus was introduced into South America in probably in late 2014. This was most likely due to an infected person (who may have had no symptoms of disease) being bitten by a mosquito which then transferred the virus to other people. Mosquitoes are a major concern in that part of the world as they transmit a number of other diseases and they are extremely numerous.
“In parts of the world where mosquitoes do not survive well or are relatively uncommon transmission of the virus by a returning infected traveller is unlikely to represent a problem.
“It has only been in the last few months that there have been suggestions of Zika virus infection leading to problems with newborns. In French Polynesia there was a significant increase in the incidence of microcephaly and, as we have seen recently in Brazil, this appears also to be the case there with up to 4000 cases in a very short period. Some studies have detected Zika virus in the fluid that surrounds the embryo during pregnancy and this suggests that infection of foetus in the womb can lead to these problems, though this is still an area of active investigation. These investigations will become more urgent in light of the huge increase in incidence of complications.
“The link between Zika virus infection of pregnant women and microcephaly has not yet been definitively proven but the increase in spread of the virus incidence alongside the increase in microcephaly is a concern. Until the situation is clear pregnant women should seek advice about travel plans to parts of the world where Zika virus is found. There is no treatment or vaccine available for Zika virus and the only options available are to reduce the risk of acquiring the infection. All travellers to these regions should take precautions to reduce the risk of bites from mosquitoes by using repellents and covering their skin as far as possible with e.g. long sleeves.”
Dr Alain Kohl, MRC Programme Leader, MRC Centre for Virus Research, University of Glasgow, said:
Where in the world do people contract Zika, and how?
“The virus is presents in parts of sub-saharan Africa but also SE Asia and more recently Pacific Ocean Islands. The virus has now spread to the Americas and local transmission has occurred in several countries.
Does the mosquito that carries Zika live in the UK?
“Aedes aegypti and Aedes albopictus – which are likely to be involved in transmission in Brazil – are not present in the UK.
What does Zika virus do to a healthy (and non-pregnant) adult and what would their symptoms be?
“Most people are asymptotic or develop fever, rash etc.
What are the potential complications if Zika is contracted during pregnancy, and how strong is the evidence for an association / causative relationship with microcephaly?
“Data is increasing that there may a link between microcephaly and Zika but this isn’t certain yet. The virus has been found in foetal tissues. There will certainly be strong research efforts to investigate this possibility.
Is Zika contagious between people?
“There is no evidence of airborne transmission and the main route of infection is through mosquitoes. There has been a case of sexual transmission, and theoretically transmission by transplantation or transfusion cannot be ruled out.
Do we know why cases of Zika are on the rise in South America?
“There are several possibilities – firstly there is no pre-existing immunity in a very large population exposed to mosquitoes that can transmit the virus, or potentially changes in the virus. There may be other factors that we don’t know about.
Is there advice for what pregnant women should do if they are planning travel to affected areas?
“Protecting themselves from mosquito bites is important and precautions should taken.”
Prof. Trudie Lang, Director of the Global Health Network, University of Oxford, said:
“Zika virus is spread by the same mosquito as Dengue virus, and so can occur in the same countries where aedes mosquitos occurs. It has been know about for many years and was first observed in the Zika forest in Uganda. Typically it causes a mild illness, often with a fever and a rash, and 3 out of 4 people might never have symptoms.
“However there has been a increase in the number of people in Brazil and the wider region who are infected with Zika virus. At the same time there has been a dramatic increase in babies born with microcephaly, which means smaller than normal heads. These seem likely to be related, but it has not been proven yet; hence the advice in the USA from the CDC to pregnant women to avoid these regions.
“The mosquito that carries Zika does not live in the UK.
“This virus is not like Ebola in that there is no evidence for human to human transmission; Zika is not contagious.
“However, this is an important emerging disease outbreak situation and we really must apply the lessons that we learnt from Ebola because Zika could be a major public health issue in these countries. There are many unknowns and so research is urgently needed to understand what is happening and how to prevent further cases.
“There is no treatment and no vaccine and so this would need addressing through clinical trials as quickly as possible. The international research community are pulling together through the ISARIC network to support local health research groups to set up studies as quickly as possible by sharing research methods and protocols.
“During the Ebola outbreak the research response was too slow and lagged behind the immediate medical humanitarian action. Research simply must be embedded in the global response to emerging outbreaks such as this in order that the impact to public health is limited by understanding the disease and evaluating interventions such as prevention strategies, treatments and vaccines.”
* https://www.gov.uk/guidance/zika-virus#zika-in-the-uk
Declared interests
Prof. Laura Rodrigues: No conflicts to declare.
Dr Andrew Easton: No interests to declare.
Dr Alain Kohl: “I work on this virus and have MRC funding on the subject.
Prof. Trudie Lang: “Trudie is Professor of Global Health Research at the University of Oxford, is a member of the ISARIC network and director of the Global Health Network.”