Q&A: Zika and dengue a potential double whammy?

SINGAPORE — With the traditional dengue peak season looming ahead, Singapore could see a potential double whammy hit its shores. The onset of locally-transmitted Zika cases comes as the National Environment Agency (NEA) noted in its latest weekly dengue advisory that there has been a “steady increase” in the Aedes mosquito population since April. Earlier in February, it was reported that the number of dengue cases this year may exceed 30,000 — higher than the 2013 record when 22,170 cases were reported. This is due to factors like warmer temperatures, increased mosquito population and changes in the circulating virus.

TODAY spoke to several infectious disease experts to find out the implications for Singapore in having to grapple with two mosquito-borne viruses simultaneously.

Q: Is it possible for the same mosquito to carry both dengue and Zika viruses? What about someone being infected with both viruses at the same time?

Associate Professor Hsu Li Yang, programme leader of the antimicrobial resistance programme at the Saw Swee Hock School of Public Health: The major urban mosquito in Singapore, Aedes aegypti, is able to transmit both Zika and dengue viruses. However, I am not aware of any research that has attempted to infect a single mosquito with both viruses concurrently. In theory, it should be possible. There is a report documenting concurrent infection with dengue and Zika in two patients from New Caledonia. Both survived and the concurrent infection did not appear to result in a worse illness. There is also a report this year of a rare triple infection with dengue, Zika and chikungunya in a pregnant woman from Colombia. She also apparently did well, as did the unborn baby.

Prof Ooi Eng Eong, deputy director of Emerging Infectious Diseases Programme, Duke-NUS Medical School: The chance of getting two viruses are extremely small, so it’s more of a hypothetical question… Once the mosquito is infected, it also reacts to the infection, and that would inhibit a second infection… So even if mosquito infected with dengue and bites someone with Zika, the chance of it acquiring the Zika virus is very small. And immediately after a person recovers from a viral infection, for a limited period we are quite resistant to infections from other viruses because (the) immune system is already up and that’s what’s going to kill the viruses. So when new virus come in, it doesn’t survive. So to get two infections at the same time — it’s very uncommon. So each has their own concerns, I don’t think (you) can add the two together to become an even bigger problem.

Prof Ary Hoffmann, Entomologist at the Faculty of Science, University of Melbourne, Australia: This might be possible but it is expected to be extremely rare. A female would need to feed on a person with both viruses. Or take blood meals from one person with Zika and another with dengue (and then transmit this to a third person). But very, very few female mosquitoes would live long enough to do this

Dr Low Chian Yong, infectious disease expert in private practice: If you ask if it’s possible to happen concurrently, the answer is yes. But whether we have seen it, the answer is no. We’ve had chikungunya and dengue for quite a while, but I have not seen a co-current infection in a patient at the same time…. We’ve not have enough experience with patients with co-current illnesses to know the complications. But for adults, it does not cause bleeding like dengue, or severe myalgia like chikungunya, it does not cause encephalitis… For (most) adults it’s largely benign. We don’t have enough experience with patients to know the complications (that) might arise.

Q: Would there be a scenario where a person who has already contracted dengue might suffer from an even worse case of Zika?

Assoc Prof Hsu: This has been difficult to prove or disprove in a real world setting. However, in a study published this year by investigators from Imperial College London, it was shown that under laboratory conditions, the dengue antibodies in blood collected from patients that had recovered from dengue cross-reacted to the Zika virus (both are related viruses belonging to the flavivirus family), but were unable to neutralise it. Instead, they resulted in the enhancement of Zika infection in cell lines. These results seem to suggest that prior infection with dengue may result in greater vulnerability to Zika infection.

Dr Lam Mun San, Infectious Disease Physician, Mount Elizabeth Hospital: Possible as well. No one knows what happens in concomitant simultaneous infection but there are concerns that circulating antibodies to dengue can potentiate Zika infections.

Q: What are some of the issues that you foresee cropping up if dengue and Zika both hit?

Assoc Prof Hsu: The two diseases mimic each other (although dengue generally results in a more severe illness) to a considerable extent and therefore one may be mistaken for another. But the primary issue is that of a strain on the hospitals and healthcare system if the number of cases of both diseases escalates rapidly. Thankfully, this does not appear to be the case – the number of weekly dengue cases, while high, is actually similar or lower compared to last year’s figures. The hospitals have also been able to discharge Zika-infected patients relatively quickly, as the duration of viremia (when the patients are able to transmit the virus to mosquitoes, and hence potentially to other persons) is short.

Dr Leong Hoe Nam, infectious disease specialist  at Mount Elizabeth Novena Hospital: Dengue and Zika come from the same family of viruses, and the test kit might have false positive results, as they have similarities, which means a person may have Zika, but is wrongly diagnosed as having dengue. Is there any danger with this? No, because the management is still the same, (just need) adequate rest and water, as the risk of death in Zika is extremely low.

Dr Low: However, in cases of sexual transmission, there could be a prolonged transmission period. In the literature, if you have a male patient infected with Zika, his semen has been reported to secrete Zika virus for up to 93 days. For women, the recommendation for them to stay abstinent is about eight weeks, so potentially sexual transmission might be a concern but we have to learn more about it.

Q: Why is there a situation of local transmissions going on in Singapore when elsewhere in the region it doesn’t seem as pervasive? What could be the reasons behind this occurrence?

Prof Hoffmann: Peak dengue season is associated with greater mosquito breeding activity, and the same species that transmit dengue also transmit Zika, leading to an increased risk if Zika is circulating in the human population. Might just be bad luck of course (e.g. chance arrival of a person carrying Zika into Singapore). And while Singapore has Aedes mosquitoes that can transmit the virus, mosquito populations in Singapore are likely to be lower than in many other areas in the region, so again this is unlikely to be a factor.

Dr Lam: We are in the midst of a dengue outbreak and we have the vector mosquitoes that can transmit Zika. There is also active surveillance and active testing which may account for the ‘high’ pickup rate.

Dr Leong: I think Singapore got hit because it’s a major port of call for many airline companies, and we have a ready pool of Aedes mosquitoes ready to transmit… I believe that Zika would probably exist in other neighbouring countries now. The reason is because they may not have detected it yet.

Dr Vernon Lee, Adjunct Assoc Prof at Saw Swee Hock School of Public Health: Our contact case tracing is robust, so we looked back and tried to actively look for cases. We have an efficient and rapid laboratory testing, with accurate results, which takes about four hours. Advisories are also sent to healthcare professionals in a timely manner, so doctors are then able to assist in this surveillance and detection of cases. If you add all these together, we are very much able to detect cases and report them.

Mr Laurent Renia, executive director of Singapore Immunology Network (SIgN): (Singapore cases) are not a lot if you compare it to Brazil … The key is the vector. All you need is one person that’s infected, and that’s enough. One is enough to start a mini-outbreak. There is nothing particular to Singapore in this matter, in fact there’s an even lower (Aedes mosquito) population compared to other places. As long as we have mosquitoes in Singapore, we will be prone to any mosquito-borne infection.

Ms Lisa Ng, Principal investigator at SIgN: At this point, it’s too early to comment. We really need more effort to understand it further, to do further research on serological tests (which look for antibodies in the blood).