‘Zika’s weak point is the mosquito’

The mosquito Aedes Aegypti is known to transmit Zika Keystone

What does the decision to declare the Zika virus a global health crisis mean for Switzerland? asked Marcel Tanner, epidemiologist and former head of the Swiss Tropical and Public Health Institute in Basel.

This content was published on February 6, 2016

In past decades the world has worried about the Ebola virus, polio and swine fever. Now the latest “Public Health Emergency of International Concern” declared by the World Health Organisation is called Zika. WHO is concerned about the presumed link – strongly suspected, though there is still no scientific proof – between the virus and birth defects (microcephaly) in newborns. What does the WHO alert mean for Switzerland?

Marcel Tanner: It means that Switzerland too has to be concerned about the problem. This means monitoring at national level, but also getting involved in international efforts to contain the epidemic. It is important to understand that this is not a problem that concerns just Brazil.

Travellers need to be appropriately informed. They need to know that, for the time being, there are no vaccines or medicines available. The virus is transmitted by mosquitoes and the only solution is to keep them away with repellents and nets. Swiss hospitals dealing with fevers, in particular with people just back from Latin America, need to consider the Zika virus as one of the possibilities. But let’s be careful: we must avoid any hysteria. In 80% of cases, infection with Zika is asymptomatic. The tiger mosquito, one of the potential carriers of the virus, is now present in the southern Alps, in canton Ticino. Do we need to worry about that?

M. T.: The Swiss Tropical and Public Health Institute is responsible for monitoring populations of tiger mosquitoes in Switzerland. If they should expand, or if any changes should be observed, the government will be informed. The Federal Office of Public Health will then go into action with measures to control mosquitoes. They may target sites favourable for these insects breeding. This is our contribution to the worldwide effort.

In the tiger mosquitoes we have studied, we have found no Zika virus. But if the mosquitoes were in contact with an infected patient from Brazil, they could in theory transmit the virus to other humans. I think that an unlikely scenario, particularly now in the winter. What questions do we need to find an answer to as fast as possible?

M. T.: What has alarmed the international community is the comorbidity aspect [more than one disease occurring in the same patient], with the appearance of microcephaly in newborns of infected women. It had never happened before in this form. There are reported to be about 4,000 cases.

The correlation between Zika and these birth defects is not 100% sure. It might be due to other factors, such as genetic ones. But it is imperative that we investigate any causal relation with focused studies. This will take several months, probably. What with the Olympics in Brazil, we will have to develop specific tests to diagnose the virus quickly. I am sure that in the past many cases of dengue fever, a virus belonging to the same family, were in fact attributable to Zika. Have there been other situations where relatively harmless viruses have triggered serious disease outbreaks?

M. T.: Certainly – just think of some of the influenza viruses. We know that some can get particularly dangerous. There are also the coronaviruses: some of them are harmless, others can cause serious illnesses like SARS [severe acute respiratory syndrome]. The appearance of major pathologies can be linked to mutations of the virus, but also just to the genetic features of the host. The general health of people infected – I am thinking of nutrition, say – may also be a factor. Some public health experts believe that the Zika epidemic is worse than the Ebola. This is a silent infection which strikes a highly vulnerable group of individuals, namely pregnant women. Do you concur with this assessment?

M. T.: No. With Ebola, all the body fluids are infectious. We saw that in West Africa: the capacity of Ebola to spread was enormous, unless we reacted. With the Zika virus, however, one infected patient cannot infect others*. There has to be a carrier, the mosquito. This is precisely its weak point, and we know how to fight it. How?

M. T.: By focussing on controlling mosquitoes. Brazil already had success that way with dengue fever. For the Zika virus we need to see the same kind of effort, like the 220,000 soldiers mobilised by the Brazilian government. Good systems of monitoring are needed. But we need not just to observe, but to intervene. Once we have identified a case, there is need to get to the location where the infection seems to have happened and fight the mosquitoes there. Everywhere in the world that mosquitoes live or can survive, we need to be vigilant. What other viruses are there that could yet become an international threat?

M. T.: Although these epidemics have now been contained, we should not forget SARS and MERS [Middle East respiratory syndrome coronavirus]. We know of about 1,300 infectious diseases, and 800 of them are carried by other species. We can’t afford to let down our guard. That means national public health programmes, ongoing monitoring, getting the right information out to the people, and also exchanging information and close collaboration between human and veterinary medicine.

I think that, in general, researchers should be asked to focus more on what are classified as rare, emerging and re-emerging diseases. The microcephaly cases in Brazil are terrible. But they are not so significant if we think of some of the other health problems around. Malaria, tuberculosis and HIV, just to mention a few, cause much more in the way of deaths and suffering.

Zika in 28 countries

Isolated for the first time in 1947 in the tropical forest of that name in Uganda, Zika is carried by mosquitoes of the genus Aedes, the same group of species that carries the dengue and chikungunya viruses. Infected patients present mild symptoms not much different from a common flu. There are no vaccines or specific treatments.

After an initial epidemic on Yap, an island in Micronesia (2007) and another one in French Polynesia (2013), a new outbreak was detected in May 2015 in northeast Brazil. Cases of infection have been recorded in 28 countries and territories, and according to WHO estimates, between 500,000 and 1.5 million people have the disease. That number could jump to 3-4 million by the end of the year.

On February 2, WHO set up a global response unit to coordinate action against the Zika epidemic.

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Link to microcephaly?

The outbreak of Zika in Brazil coincided with an increase in newborns with birth defects. Whereas  147 cases of congenital microcephaly were diagnosed in 2014, about 4,200 have been noted since last October.

WHO “strongly suspects” a direct link with the virus, without having scientific proof as yet. Unlike HIV or Ebola, Zika is a “relatively mild illness”, said Dr Anthony Costello, a microcephaly expert with the Geneva-based organisation, but it could have “potentially devastating effects for families” that have children with birth defects. 

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*This interview was held before a case of sexually transmitted Zika virus became known in the US.

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