Of Mice, Mosquitoes, and Men: Identifying a New Potential Vector for Zika Virus

Raidah Jahan
Freshman
School of Life Sciences
Independent University, Bangladesh

A major threat to the world surfaced when a dangerous pathogen named Zika virus was recently discovered to spread quickly from one area to another and soon to other countries. The resulting disease, Zika fever, can range from being mild or asymptomatic to causing high fever, joint pain, rashes, and in the case of pregnant women, babies born with deformities. Zika virus was originally found in rhesus monkeys back in 1947. It passed on to humans in the 1950s. The recent Zika virus epidemic began in Brazil in 2015.

The virus is usually transmitted by female mosquitoes since male mosquitoes do not bite us; they feed on flower nectar whereas the females bite us because they need blood to develop their eggs. The way the mosquitoes transmit viruses to other organisms is quite interesting! When they suck blood, they insert two tubes: one that sucks the blood up and the other which releases saliva into our blood. This saliva contains chemicals that stop the blood from clotting, and in case of mosquitoes that have bitten Zika infected individuals, viruses which thus get transmitted to the body of a new individual. But it is important to know which mosquito species actually transmit this virus to lower the extent of its spread.

Although it is widely known through experiments that several Aedes species mosquitoes act as vectors (organisms which spread a pathogen from one host to another) for Zika virus transmission, a recent study conducted at the Institute of Microbiology and Epidemiology in Beijing provides evidence that Culex species mosquitoes can also be potential vectors for Zika virus.

Zika virus transmission in the first year of the epidemic. Statista

Let’s look into how the scientists came to this understanding. Firstly, they infected female Culex pipiens quinquefasciatus mosquitoes with the virus. Then the scientists took some mice and placed them in a cage containing the infected Culex mosquitoes. Since Zika virus is known to infect brain cells, after some time the mice’s brains were examined. The infected mosquitoes were also dissected (each part, such as the midgut and salivary glands, was dissected separately) to know exactly where the virus is present, in order to determine whether the virus was able to persist in the mosquitoes, and how it could spread. Quantitative PCR (Polymerase Chain Reaction) and other techniques were used to determine the amount of virus present in the organisms. In PCR, known short DNA sequences unique to the virus are used to amplify a region of the viral genome, if present in the sample. Zika virus carries an RNA genome, so viral genomic RNA was collected from the midguts, salivary glands, saliva and ovaries of the mosquitoes and the brains of the infant mice, and then converted to DNA for PCR. In quantitative PCR, the amplification curve is then used to determine the initial amount of viral RNA and thus the amount of the virus itself. The researchers were able to show that Culex pipiens quinquefasciatus could become infected and could also pass on Zika virus through oral exposure. Zika virus was found in the salivary glands and saliva of the mosquitoes, and in the brain cells of the mice, showing that the virus had been transmitted by the mosquitoes. This finding strongly suggests the competency of Culex pipiens quinquefasciatus as a vector of Zika virus.

Back in the day, Zika was not considered a cause for alarm since people thought it was just a mild fever but since the outbreak in 2015, it has become a major threat worldwide, owing to its rapid spread, and the large number of babies being born with microencephaly (characterized by malformed heads) as a result of the virus. It is concerning if different mosquito species can act as vectors for the transmission Zika virus because different vector control strategies are needed for different mosquito species. Culex pipiens quinquefasciatus mosquitoes are known to transmit several other viruses, such as West Nile virus and St. Louis encephalitis virus; they also act as secondary vectors of  Japanese Encephalitis virus and now there is evidence to suggest they could transmit Zika virus.

An important caveat here is that other studies have yielded different results when looking at Zika virus transmission by Culex mosquitoes. This is most likely explained by the fact that the competence of Culex mosquitoes for transmission may vary among species found in different geographical locations, and also by different viral strains. Even so, this finding is worrying, at least in the context of China, where this study was conducted, and where the mosquito strain used in these experiments is present. Culex pipiens quinquefasciatus is very common in urban areas, and it feeds on humans since fewer animals live in urban areas. Even though it is not the primary vector of Zika virus, we cannot ignore its potential of becoming a major vector of Zika virus transmission, especially in Asia.


Raidah is a first year student at IUB who aspires to be a microbiologist. In her free time, she loves being active and partaking in sports such as volleyball and basketball.

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