Countries at risk for Dengue Transmission
For years the conventional wisdom has been that for a human infected by one of the (4) Dengue viruses to pass the virus on to another mosquito, he/she must be very viremic (have a high concentration of virus in their blood), otherwise they are a dead-end in the chain of transmission.
The CDC describes the chain of transmission on their Dengue Epidemiology page:
Symptoms of infection usually begin 4 - 7 days after the mosquito bite and typically last 3 - 10 days.
In order for transmission to occur the mosquito must feed on a person during a 5- day period when large amounts of virus are in the blood; this period usually begins a little before the person become symptomatic. Some people never have significant symptoms but can still infect mosquitoes.
After entering the mosquito in the blood meal, the virus will require an additional 8-12 days incubation before it can then be transmitted to another human
A new study, conducted in Cambodia by the Institut Pasteur, finds that asymptomatic (or mildly symptomatic) carriers of the Dengue virus – which may account for 3/4ths of all cases – are not necessarily the dead ends previously assumed, and can pass on the virus to mosquito vectors despite having lower levels of virus in their blood.
Veasna Duonga,1, Louis Lambrechtsb,c,1, Richard E. Paulc,d, Sowath Lye, Rath Srey Laya, Kanya C. Longf, Rekol Huyg, Arnaud Tarantolae, Thomas W. Scottf,h, Anavaj Sakuntabhaic,d, and Philippe Buchya,i,2
Our work provides evidence that people who are infected with dengue virus without developing detectable clinical symptoms or prior to the onset of symptoms are infectious to mosquitoes. At a given level of viremia, symptom-free people were markedly more infectious to mosquitoes than clinically symptomatic patients. Our results fundamentally change the current paradigm for dengue epidemiology and control, based on detection of dengue virus-infected cases with apparent illness.
Three-quarters of the estimated 390 million dengue virus (DENV) infections each year are clinically inapparent. People with inapparent dengue virus infections are generally considered dead-end hosts for transmission because they do not reach sufficiently high viremia levels to infect mosquitoes.
Here, we show that, despite their lower average level of viremia, asymptomatic people can be infectious to mosquitoes. Moreover, at a given level of viremia, DENV-infected people with no detectable symptoms or before the onset of symptoms are significantly more infectious to mosquitoes than people with symptomatic infections.
Because DENV viremic people without clinical symptoms may be exposed to more mosquitoes through their undisrupted daily routines than sick people and represent the bulk of DENV infections, our data indicate that they have the potential to contribute significantly more to virus transmission to mosquitoes than previously recognized.
The Institut Pasteur published a press release yesterday with more details on how this study was conducted, and the significance of these findings (excerpts below).
Public Release: 10-Nov-2015
Scientists at the Institut Pasteur in Cambodia, the Institut Pasteur in Paris and the CNRS provided proof that people infected by dengue virus but showing no clinical symptoms can actually infect mosquitoes that bite them. It appears that these asymptomatic people - who, together with mildly symptomatic patients, represent three-quarters of all dengue infections - could be involved in the transmission chain of the virus. These findings, published in the journal PNAS, on the 9 of November, question established theories concerning the epidemiology of dengue.
The scientists performed blood tests on people living in close proximity - in the same household or in the immediate neighborhood - to patients with confirmed symptomatic dengue. People who tested positive for dengue virus in their blood tests but without clinical symptoms were then put into contact with healthy laboratory-bred mosquitoes. Subsequent analysis of the mosquitoes confirmed they had been infected and would be capable of transmitting the virus the next time they bit a human. The research data also confirmed that the viremia level is a critical factor in transmission of dengue virus from a human to a mosquito.
"This finding raises the possibility that people with few or no symptoms - in other words the majority of those infected by dengue - may actually be contributing to the spread of the virus without realizing it," explained Louis Lambrechts, a CNRS scientist in charge of the Insect-Virus Interactions Group at the Institut Pasteur in Paris. Moreover, people who are virtually or completely unaffected by the virus are potentially exposed to more mosquitoes during their daily routines than those who are severely ill, bed-ridden or hospitalized.
While still only a minor problem in North America, Dengue, Chikungunya and the newest threat – the Zika Virus (see WHO WER: Zika Virus Outbreaks In the Americas) – can all be transmitted by the Aedes mosquito which can be found across wide swaths of the United States.
We see hundreds of imported cases of dengue in the United States each year – each with at least the potential to seed local mosquito populations with the virus – but so far locally acquired cases have remained rare. The lack of an abundant non-human animal reservoir for the virus (a Sylvatic cycle) is likely partly responsible.
In 2003, a CDC EID study also found that economics and lifestyle may have a lot to do to with our lack of locally transmitted Dengue (see Texas Lifestyle Limits Transmission of Dengue Virus).
But given the availability of two competent mosquito vectors (Aedes Aegypti & Aedes Albopictus), and repeated introductions of the virus from travelers coming from regions where the virus is endemic, our luck in this matter may not last forever.
Which makes understanding the epidemiology of transmission of Dengue – and the other mosquito borne threats – a high priority.