Thursday, March 03, 2016

Fiocruz: Zika Virus Experimentally Introduced Into Culex Mosquitoes

# 11,088

It is widely assumed that the Aedes Aegypti mosquito (and very possibly the Aedes Albopictus) are the primary mosquito vectors for Zika, Dengue and Chikungunya. Both are well distributed in Central and South America, and both make serious inroads into North America as well.

But there are other mosquito species known to carry diseases that can  infect humans, and few have been examined to see if they can transmit Zika. 

During yesterday's PAHO press conference the speaker from the FIOCRUZ (Fundação Oswaldo Cruz) scientific research institute announced their researchers had experimentally infected Culex mosquitoes with the Zika virus, and that the virus was was subsequently detected in their salivary glands. 

Culex is a large genus of mosquitoes that contains hundreds of species, which are well distributed around the globe, and range much farther than do Aedes mosquitoes.

Several breeds of Culex are known to vector diseases including West Nile virus, Japanese encephalitis, and St. Louis encephalitis.  Whether they can vector the Zika virus is still not known, and may not be for several more months.

Last January, in FIOCRUZ Researchers Investigate Other Possible Zika Mosquito Vectors, we looked at ongoing research into non-Aedes Zika vectors by researcher Constância Ayres, followed again in February by a Lancet commentary (see Identification Of Zika Virus Vectors).

While some headlines overnight are prematurely declaring Culex a vector of Zika, for now it is only a possibility, as explained in the following (translated) report from Pernambuco nordeste.

02/03/2016 17h36 - Updated 03/02/2016 17h36
Still can not confirm that the common mosquito transmits the disease. The  end result of the research will be known within eight months.

The ease of spread of zika virus in Culex mosquitoes infected in the laboratory was confirmed on Wednesday (2) by the researcher Constance Ayres, of vector design of the Oswaldo Cruz Foundation institution ( Fiocruz ) in Pernambuco . "This means that, in the laboratory, the virus managed to escape some barriers in the mosquito and reached the salivary gland," the researcher explained. The Culex is the common mosquito, popularly known as muriçoca or stilt.

During the second day of the workshop A, B, C, D, E Zika virus, held in Recife , the biologist presented the preliminary results of research showing the spread of the virus to the mosquito 's salivary gland, where happen to transmission disease to humans. After performing three infections in 200 Culex mosquitoes (the first two in December last year and the third in February), research shows the vector competence of mosquito in the laboratory.

The survey results are still partial, it is still not possible to say whether the mosquito is capable of transmitting the virus to zika people. "To complete this, lack identify in the field the species of mosquito infected with the virus zika," says the biologist. 

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Culex mosquitoes, while more plentiful and better distributed than the Aedes mosquito, are more likely to get a blood meal from birds than from humans.  Which is not to say they don't bite humans, only that humans are not their primary target.

Some Culex mosquitoes feed exclusively on birds, while others feed on birds and mammals, providing a `bridge' for vectoring diseases like West Nile Virus from birds to humans. From the EID Journal, we get:

To evaluate the role of Culex mosquitoes as enzootic and epidemic vectors for WNV, we identified the source of vertebrate blood by polymerase chain reaction amplification and sequencing portions of the cytochrome b gene of mitochondrial DNA.
All Cx. restuans and 93% of Cx. pipiens acquired blood from avian hosts; Cx. salinarius fed frequently on both mammals (53%) and birds (36%). Mixed-blood meals were detected in 11% and 4% of Cx. salinarius and Cx. pipiens, respectively. American robin was the most common source of vertebrate blood for Cx. pipiens (38%) and Cx. restuans (37%). American crow represented less than 1% of the blood meals in Cx. pipiens and none in Cx. restuans.
Human-derived blood meals were identified from 2 Cx. salinarius and 1 Cx. pipiens. Results suggest that Cx. salinarius is an important bridge vector to humans, while Cx. pipiens and Cx. restuans are more efficient enzootic vectors in the northeastern United States.  
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While it has been estimated that humans provide less than 5% of blood meals to Culex mosquitoes, given their number - particularly in densely populated urban areas - their ability to spread diseases remains substantial.

But whether they will prove to be a significant factor in the spread of Zika is something we'll simply have to wait to see.