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Skip to 0 minutes and 15 secondsROSANNA PEELING: We cannot really detect Zika infection based on symptoms alone, as most people who are infected do not show any symptoms or the symptoms are very mild. It's estimated that one in five infected individuals will show symptoms. And in those who show symptoms, they are fever, joint pain, a rash, which is really very generally considered a flu-like illness and it's not good enough for us to really say that the cause of the infection

Skip to 0 minutes and 47 secondsis actually Zika: due to Zika. Right now we have two types of tests that could help us with the diagnosis of Zika infection. One is to detect virus, and since the virus is only present in blood in the first five days after the onset of symptoms, they can only be used during that time period. The other is to detect antibodies in blood in response to the infection. And that typically can be used around six days after infection.

Skip to 1 minute and 20 secondsMARTIN HIBBERD: So Zika virus can be diagnosed and detected directly using a polymerase chain reaction, or PCR test. This is a common test that we use for dengue virus as well. But the primers can be very specific to Zika virus. Unfortunately for Zika virus, the presence of the virus itself is difficult to recognise. The symptoms, clinical symptoms, can be pretty mild in the majority of people. And unlike dengue, where you get a very clear fever which often the signals the viremic stage, for Zika virus this can be difficult to detect.

Skip to 2 minutes and 0 secondsAnd with the virus being present in blood perhaps just a few days, actually getting the chance to isolate the virus or detect the virus in those samples can be very difficult. We know that Zika actually can be detected in other samples, such as urine and semen. And this then offers, perhaps, a longer window of time where we can do a diagnostic test. However, pinpointing that time can still be very difficult. And so typically, we would rely on the antibody test, the indirect measurement of the response to the infection as a way of actually diagnosing it. So Zika is actually very similar to dengue virus. It's not only transmitted by the same mosquito, but it's also genetically similar.

Skip to 2 minutes and 52 secondsAnd it actually in particular has a similar envelope protein which is so related to the dengue viruses that it makes it difficult to distinguish them in many ways. This similarity causes problems with the detection, because antibodies that raised against dengue virus can also cross-react then with Zika virus and this is causing the communities a lot of difficulty in recognising what actually was a Zika virus compared to a dengue virus.

Skip to 3 minutes and 26 secondsROSANNA PEELING: Right now the situation is very difficult, because without good test that is assessable to definitely diagnose infection, we cannot make the association between, say, a pregnant woman who's infected and if their baby is born with microcephaly. The other is that we usually use antibody-based tests to try and look at the extent of the outbreak in the country. But since these antibody test are cross-reactive and could be false positive or false negative, we don't have a tool to assess the extent of the outbreak.

Skip to 4 minutes and 6 secondsMARTIN HIBBERD: The similarity of that envelope protein, it makes not only diagnosis difficult, but potentially a vaccine difficult as well. For dengue virus, there are four separate viruses. And people have known for a long time that you need to make a vaccine against all four of those viruses. Because if you fail to make a good antibody immune response - neutralising protective immune response - against all four of the viruses, then you might end up with what's called antibody enhancement. That's where the antibodies from one virus, instead of protecting you, might lead to enhancement with a different virus.

Skip to 4 minutes and 43 secondsWhile we don't know yet whether Zika virus actually does cross-react sufficiently to enable this antibody enhancement procedure to occur for Zika virus, It's a good likelihood that it will based on our diagnostic problems. And if that's the case, then we need to make a vaccine that protects you against all five similar viruses, the dengue viruses and the Zika virus, in order to make sure that you don't suffer from this enhanced disease. And we don't know yet whether Zika can enhance dengue or whether dengue can enhance Zika. And all of these things make making a vaccine quite complicated.

Diagnosis and vaccines

Symptoms alone are not enough to diagnose Zika, and many people experience no symptoms at all. How do we diagnose a case of Zika virus? Are the tests we’re using for diagnosis effective?

In the video Professor Rosanna Peeling and Professor Martin Hibberd discuss some of ways in which we test for Zika, the difficulties we are experiencing with testing, and how these issues impact our ability to develop a vaccine.

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Preventing the Zika Virus: Understanding and Controlling the Aedes Mosquito

London School of Hygiene & Tropical Medicine

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