Update on Zika Virus March 2016
2016 has seen an unexpected explosive pandemic of the Zika virus in South America, Central America and the Caribbean. It has been characterised by rapid spread of the virus through the Americas and reports of excess cases of microcephaly (a fetal malformation resulting in small head and brain) and Guillain Barre Syndrome (GBS, a neurological disorder causing severe weakness and paralysis).
Zika virus was discovered in Uganda in 1947 as part of mosquito and primate surveillance. It is a flavivirus similar to dengue. The virus circulated predominantly in wild primates and forest mosquitoes and was mainly confined to the equatorial belt of Africa and Asia where it caused sporadic cases. Zika infection is usually associated with mild dengue like illness displaying symptoms such as fever, rash, joint pain or conjunctivitis. It was in 2007 when it first displayed its epidemic potential causing an outbreak in Micronesia. Zika virus caused an outbreak in French Polynesia in 2013-2014 and it was estimated to have infected 66% of the general population. During this outbreak, Zika virus was associated with an epidemic of GBS. Brazil first reported an outbreak of the Zika virus in the middle of 2015 and the current estimates are that between 400,000 to 1 million persons have been infected. Since then, many other surrounding countries have reported Zika infection and excess cases of microcephaly and GBS.
As of March 22nd, 2016; the Zika virus is known to be circulating in 38 countries and territories. However, the full extent of the pandemic is not yet clear. It is spread by the Aedes aegypti mosquito and everyone is concerned that this pandemic will spread in significant numbers to other countries including Singapore. More than half of the world population lives in areas where the Aedes mosquito is circulating. Countries around Singapore including Malaysia, Indonesia and Thailand have all reported sporadic cases of Zika virus infection. We can expect that cases of this viral infection will emerge in Singapore but it is important that we ensure that the infection does not result in ongoing transmission here. Our population does not have immunity to this virus and if there is ongoing transmission here, there will be serious concern of it spreading quickly and in large numbers. The pattern that has emerged when countries have ongoing Zika transmission is as follows: several weeks after initial detection of viral circulation, an unusual increase in cases of GBS ensue. Detection of microcephaly and other fetal malformations comes later when pregnancies come to term.
In the past 3 months, our knowledge of this viral infection has steadily increased. In addition to the virus causing neurological disease and fetal malformations, we also know that sexual transmission occurs and this information has an impact of recommendations for the control measures for Zika virus. At this time, these control measures are focused on the following
Control Of The Aedes Mosquito (Vector Control)
In Singapore, we have well established and well implemented mosquito control programmes. Unfortunately, mosquito control efforts alone have failed to curtail the spread of dengue and Chikungunya infections in Singapore and many other countries. WHO has convened several expert meetings on vector control and they have focused on deployment of 2 pilot projects. One is on microbial control using the Wolbachia bacteria in adult mosquitos and the second on the use of genetic manipulation to reduce mosquito populations. These are pilot programmes and none were judged to be ready for full scale implementation.
Prevention Of Infection In Vulnerable Groups
Women of child bearing age living in affected countries are encouraged to avoid pregnancy until the current outbreak is over. Women who are pregnant are advised to take meticulous precautions to avoid mosquito bites. Authorities have recommended that women living in non affected countries should avoid non-essential travel to endemic areas. As sexual transmission is known to occur, patients recovering from Zika virus are advised to continue with use of contraception possibly for as long as 6 months. In a recent publication on the Zika outbreak in French Polynesia, it has been estimated that pregnant women infected by Zika virus in the first trimester had a 1% risk of their infants born with microcephaly.
Diagnosis Of Zika Virus Infection
In the acute infection when the virus is circulating in the body (viremia), nucleic acid amplification tests may be done to detect the virus. Reference laboratories in Singapore will be able to carry these tests. However, it is important to emphasize that most patients with acute Zika virus infection have mild symptoms and may not come to medical attention. Zika has often been labelled as a weak cousin of dengue because of the mild symptoms that it causes. Viremia is expected to be short lasting approximately 4-5 days.
The challenge is making a reliable “point of care” test available where results are not only accurate but fast (within 1-2 hours). This will usually require a blood test to detect specific antibodies to the Zika virus. This is not easy especially in Singapore because of our high burden of dengue virus infection. There may be a cross reaction of the tests as both dengue and Zika come from the same family of viruses.
Our knowledge on Zika virus is steadily increasing and the specific recommendations on the diagnosis, treatment and prevention of this infection is expected to evolve in the coming months.