Mosquito saliva enhances virus replication and disease

Biting mosquitoMosquito saliva, which is injected into the host as a mosquito probes for a blood vessel, contains a collection of chemicals which include anticoagulants to prevent blood clotting, vasodilators to keep blood vessels wide, and anesthetics to prevent us from sensing the mosquito. Saliva also contains components that enhance viral replication, dissemination, and pathogenesis by inducing an inflammatory response that inadvertently promotes infection by providing new cell targets for infection (paper link).

To separate the bite from virus inoculation, mice were first exposed to Aedes aegyptii mosquitoes, and then infected at the bite site with two different mosquito transmitted viruses, Semliki Forest virus or Bunyamwera virus. Mosquito bites caused more virus replication at the inoculation site, greater dissemination of virus, and more lethality compared with control mice that received only virus.

How does mosquito saliva enhance virus replication and dissemination? Part of the story is that as the mosquito probes for a blood vessel, it causes damage that leads to vascular leakage and accumulation of fluid (edema) which inhibits movement of virus to draining lymph nodes.

But delaying dissemination of virus alone does not promote infection and disease. Mosquito bites cause an infiltration of neutrophils (a type of white blood cell) into the bite site. The edema at the bite site is enhanced by neutrophils, because depleting these cells from mice greatly reduced edema. This depletion also returned viremia to levels observed in unbitten control mice, and restored dissemination of virus to draining lymph nodes. Neutrophils are not susceptible to infection with Semliki Forest virus, and therefore cannot explain the increase in virus replication at the bite site.

Enhanced virus replication in the skin occurs because the neutrophils elaborate chemokines that attract macrophages, which can be infected by Semliki Forest virus and Bunyamwera virus. One of the chemokines produced by neutrophils that is a macrophage attractant – CCL2 – binds a receptor on macrophages. Mice lacking the gene encoding the CCL2 receptor are protected from bite enhancement of Semliki Forest virus enhancement.

When a mosquito bites a host, it delivers saliva along with a virus. The saliva induces an inflammatory response and attracts neutrophils into the bite site. The resulting edema holds virus at the bite site until chemokines produced by neutrophils attract macrophages, which are then infected. The virus produced disseminates widely, reaching secondary tissues and causing disease.

It seems likely that the ability to replicate in macrophages that are recruited to the bite site is a property that was selected during evolution of mosquito-transmitted viruses. By replicating in macrophages, the amount of virus in the blood is increased, as well as the likelihood that the virus will be picked up by another mosquito and transmitted to a new host – a powerful selection mechanism. The down side – increased disease in the mammalian host – is an accidental side effect.

Think about that the next time you are scratching that raised bump on your skin caused by a mosquito bite.

Virus Watch: How mosquitoes spread viruses

In this episode of Virus Watch, I explain how mosquitoes spread viruses. We’ll look at how a mosquito finds a host, how it finds a blood vessel, and how it delivers viruses to a new host. Don’t blame mosquitoes for viral diseases: it’s not their fault!

TWiV 390: Building a better mosquito trap

TWiVProject Premonition, a Microsoft Research project that uses drones to capture mosquitoes and analyze them for pathogens, preprint servers, and three mouse models for Zika virus induced birth defects are the topics of episode #390 of the science show This Week in Virology.

You can find TWiV #390 at, or listen below.

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TWiV 344: Glasgwegians go viral

Episode #344 of the science show This Week in Virology was recorded at the Glasgow Science Festival microTALKS, where Vincent spoke with Ruth, Glen, and Esther about their research on viruses and Hodgkin lymphoma, adenovirus structure and entry into cells, and interactions between arthropod borne viruses and their hosts.

You can find TWiV #344 at

Sushi protects mosquitoes from lethal virus infections

mosquito brainAs far as I know, mosquitoes do not eat sushi. But mosquito cells have proteins with sushi repeat domains, and these proteins protect the brain from lethal virus infections.

Mosquitoes are vectors for the transmission of many human viral diseases, including yellow fever, West Nile disease, Japanese encephalitis, and dengue hemorrhagic fever. Many mosquito-borne viruses enter the human central nervous system and cause neurological disease. In contrast, these viruses replicate in many tissues of the mosquito, including the central nervous system, with little pathological effect and no alteration of behavior or lifespan. The defenses that allow such persistent infection of mosquitoes are slowly being unraveled.

A protein called Hikaru genki, or Hig, is crucial for controlling viral infections of the mosquito brain. Originally discovered in the fruit fly Drosophila, Hig is produced mainly in the brain of Aedes aegyptii, the natural vector for dengue and yellow fever viruses. Experimental reduction of Hig mRNA or protein in the mosquito leads to increased replication of dengue virus and Japanese encephalitis virus. This increase in viral replication is accompanied by more cell death in the mosquito brain, and decreased survival.

How does Hig protein impair virus replication? The Hig protein of A. aegyptii binds dengue virus particles via the E membrane glycoprotein. As Hig protein is located on the cell surface, binding to virus particles prevents virus entry into cells. Impairment of endocytosis is limited to insect cells – introduction of Hig into mammalian cells had no effect on virus replication. Clearly other components of insect cells must participate in the Hig-mediated antiviral mechanism.

The antiviral activity of Hig protein depends on the presence of sushi repeat domains, also known as complement control protein (CCP) domains. These consist of 60 amino acid repeats with four conserved cysteines and a tryptophan. The CCP domain is found in many proteins of the complement system, a collection of blood and cell surface proteins that is a major primary defense and a clearance component of innate and adaptive immune responses. The sushi domain mediates protein-protein interactions among complement components. Capturing the dengue and Japanese encephalitis viruses by the A. aegyptii Hig protein is just one example of the virus-binding ability of proteins with CCP domains. An insect scavenger receptor with two CCP domains is a pattern recognition receptor that recognizes dengue virus and recruits mosquito complement to limit viral replication. Some CCP containing proteins are virus receptors (complement receptor 2 binds Epstein-Barr virus, and membrane cofactor protein is a receptor tor measles virus).

Because the Hig antiviral machinery is largely limited to the mosquito brain, it is possible that it prolongs mosquito life to allow virus transmission to other hosts. Transmission of virus to other hosts requires replication in the salivary gland, which cannot take if the mosquito dies of neural infection. I wonder why humans do not have have similar mechanisms to protect their neural tissues from virus infections. Is neuroinvasion a less frequent event in humans, compared with mosquitoes, thereby providing less selective pressure for protective mechanisms to evolve?

TWiV 325: Wildcats go viral

On episode #325 of the science show This Week in Virology, Vincent visits the ‘Little Apple’ and speaks with Rollie and Lorena about their work on mosquito-born viruses and baculoviruses.

You can find TWiV #325 at

The press concludes that arboviruses can be sexually transmitted

zika virus distributionWhat would you conclude if you read the following headlines: Man sexually transmits insect-borne disease to wife (Fox News); Zika virus: First insect borne STD? (HuffPo); Scientist gives insect-borne disease to wife during sex (New York Magazine), and A scientist contracts a mosquito-borne virus and gives it to his wife as std (Time). What would be your impression if you read the journal article on which these headlines are based, which does not conclude that the infection was transmitted sexually?

Zika virus was isolated in 1947 from a monkey in Uganda, and subsequently shown to be transmitted by mosquitoes. Zika is classified as a flavivirus, along with well-known human pathogens such as yellow fever virus, dengue virus, and West Nile virus. Infection of humans with Zika virus leads to headache, fever, malaise, myalgia, and formation of  a maculopapular rash on the face, neck, trunk, and arms. The virus is found mainly in African and parts of Asia (see map).

The case that has precipitated incorrect reporting began with two American scientists working in Senegal in 2008, where they were sampling mosquitoes. Between 6-9 days after returning to their homes in Colorado, they developed a variety of symptoms of infection including fatigue, headache, chills, arthralgia, and a maculopapular rash. The wife of one patient, who had not traveled to Africa, developed similar symptoms three days after her husband. Analysis of paired acute and convalescent sera from all three patients revealed antibodies against Zika virus. The two individuals who had traveled to Africa also had antibodies to yellow fever virus, a consequence of immunization with the vaccine.

Here is what the authors conclude from these data:

Evidence suggests that patients 1 and 2 were infected with ZIKV, probably in southeastern Senegal, by bites from infected mosquitoes…Circumstantial evidence suggests direct person-to-person, possibly sexual, transmission of the virus (italics are mine).

The authors do not conclude that transmission from husband to wife was via sexual activity – they suggest it as a possiblity. The authors know that one cannot prove sexual transmission from such a small study. They go on to write:

If sexual transmission could be verified in subsequent studies, this would have major implications toward the epidemiology of ZIKV and possibly other arthropod-borne flaviviruses.

What other ways might the infection have been transmitted from husband to wife? Virus is likely present in the skin of infected individuals, as a rash is a prominent feature. It is possible that virus was transmitted via cuts or abrasions in the skin. Another possibility is that virus is present in saliva or other body fluids, and is transmitted to others by close contact. The authors don’t believe this to be the case because they write that ‘illness did not develop in the 4 children of patients 1 and 3‘. However physical contact between husband and wife is substantially different from the contact between parents and children, which could have a major role in determining virus transmission.

Here is another way to put this puzzling state of affairs into context. In 2009 a group of scientists published a paper in Science indicating that they had found a retrovirus, XMRV, in the blood of 68 of 101 patients with chronic fatigue syndrome. To this day whether or not XMRV causes chronic fatigue syndrome is still being debated, despite studies in hundreds of individuals. In light of this situation, why does the press conclude from a study of three individuals that Zika virus can be sexually transmitted? Could it be that the journalists didn’t read the journal article (poor excuse – it’s quite short), or if they did, they decided that the conclusions were not sufficiently interesting? Or maybe the sex angle – always a good way to get the reader’s attention – was too good to resist, never mind that it might not be correct. Either way, the public is being misinformed about science – again.

Update: There has been discussion in the comments section that the news articles I cite don’t do such a bad job in presenting the science, and it’s the headlines that are the main problem. I don’t agree with that conclusion about the articles – in my opinion they don’t accurately portray the content of the paper. My journalist friends tell me that the headline writers often take liberty with conclusions; but I don’t see why we should use that as an excuse to forgive inaccurate headlines. How many people never get past the headlines? Both the headline and the article need to be consistent and they need to accurately represent the science.

Foy, B.D., Kobylinski, K.C., Foy, J.L.C., Blitvich, B.J., da Rosa, A.T., Haddow, A.D., Lanciotti, R.S., & Tesh, R.B. (2011). Probable non–vector-borne transmission of Zika virus, Colorado, USA. Emerging Infectious Diseases : 10.3201/eid1705.101939

Hayes, E. (2009). Zika Virus Outside Africa Emerging Infectious Diseases, 1347-1350 DOI: 10.3201/eid1509.090442