Thursday, September 08, 2016

Nature: Culex Pipiens Quinquefasciatus - A Potential Vector To Transmit Zika Virus



Yesterday, in PLoS NTD: Culex quinquefasciatus Not Competent To Transmit Zika Virus - the latest in the `do Culex mosquitoes transmit Zika' debate - I remarked that `it may be a bit naive to assume we've heard the last on this . . . '.

Well, thanks to a head's up from we didn't have very long to wait.   

No sooner had the electrons settled down from yesterday's  blog, Nature's  Emerging Microbes & Infections has come along with a new study on Culex carriage of Zika -  one that comes to  a decidedly different conclusion.

First the abstract, then I'll return with a bit more. 

Culex pipiens quinquefasciatus: a potential vector to transmit Zika virus
Xiao-xia Guo1,*, Chun-xiao Li1,*, Yong-qiang Deng2,*, Dan Xing1, Qin-mei Liu1, Qun Wu1, Ai-juan Sun1, Yan-de Dong1, Wu-chun Cao3, Cheng-feng Qin2 and Tong-yan Zhao1


Zika virus (ZIKV) has become a threat to global health since the outbreak in Brazil in 2015. Although ZIKV is generally considered an Aedes-transmitted pathogen, new evidence has shown that parts of the virus closely resemble Culex-transmitted viruses. Therefore, it is important to evaluate the competence of Culex species for ZIKV to understand their potential as vectors. 

In this study, female Culex pipiens quinquefasciatus were orally exposed to ZIKV. Mosquito midguts, salivary glands and ovaries were tested for ZIKV to measure infection and dissemination at 2, 4, 6, 8, 12, 16 and 18 days post exposure (pe). In addition, saliva was collected from mosquitoes after infection and infant mice were bitten by infected mosquitoes to measure the transmission ability of Cx. p. quinquefasciatus

The results showed that the peak time of virus appearance in the salivary glands was day 8 pe, with 90% infection rate and an estimated virus titer of 3.92±0.49 lg RNA copies/mL. Eight of the nine infant mice had positive brains after being bitten by infected mosquitoes, which meant that Cx. p. quinquefasciatus could be infected with and transmit ZIKV following oral infection.

These laboratory results clearly demonstrate the potential role of Cx. p. quinquefasciatus as a vector of ZIKV in China. Because there are quite different vector management strategies required to control Aedes (Stegomyia) species and Cx. p. quinquefasciatus, an integrated approach may be required should a Zika epidemic occur.


In this study, we demonstrated that Cx. p. quinquefasciatus was able to be infected with and transmit ZIKV after oral exposure. The IR was as high as 90% in the salivary glands and 80% in the saliva on day 8 pe. The decrease in mosquito midgut infections observed over time in our study is consistent with other published studies31 and was probably due to virus clearance by the mosquito’s immune system.32, 33
In the other transmission experiment, infant mice were infected after being bitten by virus-positive mosquitoes and disseminated to and replicated in the mouse brains. These findings verified the potential role of Cx. p. quinquefasciatus as a vector of ZIKV.
(Continue . . . )

This isn't the first time we've seen conflicting results from similar scientific studies conducted by reputable research teams (see When Flu Vaccine Studies Collide), but I don't ever recall seeing so many emerge in such a short span of time.

Six weeks ago, in Fiocruz: A Culex Mosquito With The Potential To Transmit Zika and followed up just last week in BioRxIv: Zika Virus Replication In Culex Quinquefasciatus In Brazil we looked at research by FIOCRUZ researcher Constance Ayres that strongly suggested Culex could carry and spread the Zika virus. 

But since mid-July we've seen no fewer than four studies pretty much dismissing the idea.
PLoS NTD: Culex quinquefasciatus Not Competent To Transmit Zika Virus 
Eurosurveillance: Experimentally Infected Culex Mosquitoes Unable To Spread Zika
EID Journal: Culex pipiens and Aedes triseriatus Mosquito Susceptibility to Zika Virus

To say the jury is still out on all of this is an understatement.  So if you're confused, you aren't alone.

Of course, being physically able to transmit the virus in the laboratory setting, and actually contributing to the community spread of the Zika virus in a substantial and meaningful way, are two different things. 

That said, none of these journals are lightweights, and these studies were all conducted by reputable researchers, so I'm not inclined to try to pick a side. 

The best I can do is continue to update this blog whenever new information is published and hope a consensus is reached soon.

But with nearly daily releases on Culex's competence in transmitting Zika, this one topic is beginning to feel like a full time job.