The explosive global spread of Zika virus follows the predictable pattern of other mosquito-borne viruses in the past. Global research funding and policy must change if we are to stop such viruses becoming epidemics in the future, Duncan R Smith writes.
In the space of a few short years the mosquito-transmitted Zika virus has changed from being a virus known to only a few specialised researchers, to being a Public Health Emergency of International Concern with millions of people infected in more than 50 countries and territories around the world.
The good news is that about 80 per cent of human infections with Zika virus do not result in any symptoms, and in the majority of cases where symptoms do occur, the disease is relatively mild and self limiting. The bad news is that in a small percentage of cases, more severe consequences of infection can result, including neurological damage. However, of greatest concern is the association with microcephaly (abnormal smallness of the head) in babies whose mothers were infected with Zika virus while they were pregnant, particularly when the infection occurred in the first trimester.
Zika virus was first isolated in Uganda in 1947, but only a handful of cases of human infection were reported in Africa and Asia over the next 60 years. While an outbreak of Zika fever occurred in the Federated States of Micronesia in 2007, it was the large outbreaks in French Polynesia and the islands of the Pacific Ocean starting from 2013, and the introduction of Zika virus into Brazil in 2015 and in particular the association with microcephaly, that started to focus world attention on this virus.
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From Brazil the virus spread rapidly to other countries in South America, as well as to those in Central and North America. Surprisingly, evidence has shown that the virus that has spread around the globe had its origins in Southeast Asia. In Southeast Asia it is known that the virus has been present and causing disease for at least six years, but there has been no large-scale outbreak of Zika virus-associated disease to date. However, at the time of writing, more than 100 cases have been reported in Thailand, and Singapore has reported more than 250 cases of local transmission and it is possible that countries in Southeast Asia will experience an outbreak on the scale of that seen in the Americas.
The explosive spread of Zika virus seems unprecedented, but to those who work with mosquito-transmitted disease (and as famously ascribed to Yogi Berra), “It’s déjà vu all over again”. Starting from 2005 the mosquito-transmitted Chikungunya virus spread rapidly from Africa to countries around the Indian Ocean (including India, Sri Lanka, Malaysia, Thailand and Singapore) causing millions of infections. A second strain of the virus from Southeast Asia subsequently caused large numbers of infections in South America and the Caribbean. Chikungunya had long been known as a pathogen of concern. Vaccine development was underway in the late sixties and early seventies, but languished as there was little evidence of chikungunya circulating in major population centers.
In the case of both chikungunya and Zika there was ample historic evidence that the viruses caused disease in humans, and that they had the potential to spread. And in both cases at the time of emergence there was no sensitive, simple and specific diagnostic test kit available, no vaccine and no antiviral to treat or prevent infection. In both cases science had to play catch up and, more than a decade after chikungunya started to re-emerge, there is still no protective vaccine commercially available.
The bad news is that there are many more mosquito-transmitted viruses out there. Close relatives of Zika virus include Usutu virus, Wesselsbron virus, Ilheus virus and another dozen or so viruses whose ability to cause human disease has been documented, over and above the big brother viruses such as Dengue virus, Japanese encephalitis virus and West Nile virus.
The lesson from Zika virus is that we cannot, and should not, wait for a virus to cause millions of infections before we take notice. There is enough evidence already available to identify which mosquito-transmitted viruses are likely to emerge and we should be working on them now. Governments and funding agencies need to be proactive in supporting research on lesser-known mosquito-transmitted viruses and not turn a blind eye to viruses that are yet to cause epidemics.
While it’s clear that funding for research is not a bottomless resource, it needs to be appreciated that today’s obscure mosquito-transmitted virus is likely to be tomorrow’s epidemic, and it would be nice to be ahead of the curve for once.