What happens to West Nile Virus in the winter?

Where, oh where, does the virus go (in the winter)?

Dr. Rosmarie Kelly, Public Health Entomologist, Georgia Department of Public Health

It’s that time of year when those in mosquito surveillance and control think fondly of consistently cooler temperatures and eagerly await that first hard frost. Of course, this is already happening in some places up north. We may have to wait a while longer in here in Georgia. But that does bring to mind the question: Where do all the mosquitoes go once the colder weather arrives?

Mosquitoes, like all insects, are cold-blooded creatures. As a result, they are incapable of regulating body heat and their temperature is essentially the same as their surroundings. Mosquitoes function best at 80 F, become lethargic at 60 F, and cannot function below 50 F. In tropical areas, mosquitoes are active year round. In temperate climates, mosquitoes become inactive with the onset of cool weather and enter diapause (hibernation) to live through the winter. Diapause induction also requires a day length shorter than 12 hours light (more than 12 hours dark). All mosquitoes pass through four developmental stages: egg, larva, pupa and adult, and diapause can occur in any of these stages depending on the species.

  • The Aedes and Ochlerotatus species, and some Culiseta species, lay eggs in dry or damp, low-lying areas or containers that are subject to flooding from accumulations of precipitation. Winter is passed in the egg stage, with hatching dependent on the presence of water, water temperature, and amounts of dissolved oxygen.
  • Coquillettidia and Mansonia species, and some Culiseta species, have larval stages that overwinter, apparently without total loss of activity, restricting development to very permanent water bodies. These species renew development towards the adult stage once water temperatures begin to rise.
  • Overwintering in Anopheles, Culex and some Culiseta species takes place in the adult stage by fertilized, non-blood-fed females. In general, these mosquitoes hide in cool, dark places waiting for temperatures to rise and days to lengthen before they seek out a blood meal and resume their lives.

What, if anything, does this mean for West Nile virus (WNV)? If the mosquitoes are infected with WNV when they enter diapause, it should overwinter with them to be transmitted to birds when the mosquitoes emerge the following spring. Temperature is the crucial factor in the amplification of the virus. Studies in various states have shown that WNV does indeed overwinter in mosquitoes. The virus does not replicate within the mosquito at lower temperatures, but is available to begin replication when temperatures increase. This corresponds with the beginning of the nesting period of birds and the presence of young birds. Circulation of virus in the bird populations allows the virus to amplify until sufficient virus is present in the mosquito populations (and vector mosquito populations are high enough) that horse and human infections begin to be detected.

In the Northeastern U.S., Culex pipiens, the northern house mosquito, is the most important vector species. This species overwinters as an adult, and has been found harboring WNV during the winter months. This mosquito goes into physiological diapauses (akin to hibernation) during the winter months, and while it may be active when temperatures get above 50°, it will not take a blood meal.

Culex quinquefasciatus, the southern house mosquito and the major vector for WNV in Georgia, also overwinters as an adult, and also goes into diapause when winter comes, and it is likely that this mosquito also harbors WNV throughout the winter months.  However, the southern house mosquitoes go into more of a behavioral diapause when temperatures are below 50°, and are quite capable of taking a blood meal (and maybe transmitting WNV) when things warm up during the winter, which is not an unlikely occurrence here in Georgia especially as one goes further south. So, although the risk for WNV transmission in the south in the winter months is very low, it is certainly possible.

Regulatory Restrictions Protect Human and Animal Health

Nancy C. Hinkle, Ph.D.

Veterinary Entomologist, Dept. of Entomology, University of Georgia

One of the foundations of Integrated Pest Management is prevention, and one of the essential underpinnings of prevention can be regulatory restrictions. If we prevent the introduction of a pest or disease into an area where it does not occur, we avoid the risks associated with the pest or pathogen.

WNV cycleUp until fifteen years ago we had never had a case of West Nile Virus in the U.S. So how did West Nile Virus come to North America? Probably someone smuggled in an infected bird that was carrying the virus. The smuggler didn’t think he was doing anything bad; after all, he had paid good money for the bird and wanted to bring it home with him to New York City. What was wrong with tucking the bird into his pants and not declaring it when the agent asked if he was bringing any living animals as he passed through Customs? Once home, the bird was placed in a cage near the apartment window, a local mosquito flew in and sucked a little of its blood, then flew out and fed on a local sparrow. The sparrow became infected with West Nile Virus, more mosquitoes fed on it and picked up the virus, and a few weeks later dozens of birds at the Bronx Zoo dropped dead of West Nile Virus after being fed on by these infected mosquitoes.

Meanwhile people in Queens were developing high fevers, severe headaches, and nerve problems like paralysis. Even though New York mobilized and started treating for mosquitoes, the virus was already established in birds and mosquitoes. West Nile did not exist in the U.S. prior to 1999; since that year mosquitoes have spread West Nile westward through the continental U.S., resulting in over 1,700 human deaths and ten times that many paralysis cases.

WNV incidence in the US
Source – http://tinyurl.com/qdm4u65

There is a reason the Customs Declaration Form that people entering the U.S. fill out contains the question, “Are you bringing with you meats, animals, or animal/wildlife products?”  While we don’t often think about it, animals in other countries can contain pathogens that we don’t have here in North America and that can be lethal to humans or animal life on our continent. If these hosts get moved into our country, the pathogens can rapidly spread to local wildlife and then to humans.

Before traveling outside the U.S., travelers should visit the Centers for Disease Control website to determine which vaccinations and medications are needed for the areas to which they’ll be traveling. It’s important to follow appropriate precautions to avoid insect bites. And people reentering the U.S. should not bring back with them any living animal or plant, meat, or other animal products. The fellow who smuggled in the West Nile-infected bird had no idea that his action would result in the death of over 1,700 Americans, thousands of horses, and countless wild birds.

Demise of Small Mosquito Control Programs (and the Effect on West Nile Virus Transmission)

State public health officials in Georgia support an integrated approach for mosquito control. Local officials can contact the Department of Public Health for more information about how to conduct an integrated program in their counties.

Learn more at the Georgia Department of Public Health Web Page.

Also check out the Georgia Mosquito Control Association website.

Summary of an article by Dr. Rosmarie Kelly, Georgia Department of Public Health Read the entire article here

Asian tiger mosquito, Ary Farajollahi, Bugwood.org
Asian tiger mosquito, Ary Farajollahi, Bugwood.org

A number of published reports suggest that mosquito control programs, and especially those using Integrated Mosquito Management techniques, are needed to reduce the risk of arboviral (West Nile Virus and some other mosquito vectored diseases) transmission at the local level. A study from Michigan indicated that people in communities with no mosquito control program had a tenfold greater risk of West Nile fever/encephalitis than those in areas where mosquitoes were controlled http://www.cdc.gov/ncidod/dvbid/westnile/conf/pdf/Walker_6_04.pdf

A Chicago area study suggested that mosquito control programs made a difference in WNV infection rates. The Des Plaines Valley District, with an intensive program to kill mosquito larvae, had four West Nile fever/encephalitis cases per 100,000 people, while the North Shore District, with a less ambitious program, had 51 cases per 100,000. This study showed that the program with the most mosquito surveillance and best documented larviciding and adulticiding operations had the fewest number of West Nile fever/encephalitis cases (Tedesco, Ruizand and McLafferty 2010).

This is not new information. The efficacy of aerial insecticide applications to reduce the transmission of Saint Louis Encephalitis (SLE) virus was shown during an epidemic in Dallas, TX in 1966. This study presented evidence that infection rate is reduced as a consequence of anti-mosquito measures. Before aerial spraying there was an SLE virus infection rate of 1 in 167 mosquitoes tested. After aerial control operations the SLE virus infection rate was 1 in 28,639 mosquitoes (Hopkins et al. 1975)

So, are small programs important? There was a documented increase in vector populations after the temporary closure of Clayton County, Georgia’s mosquito control program. There was an apparent increase in the risk of West Nile fever/encephalitis based on the presence of increased numbers of vector species and the detection of an early human case of West Nile fever/encephalitis in 2010. There was also a suspected increase in nuisance species and mosquito complaints, although these data were not collected. The Clayton County program has since been re-instated and is administered by Public Works.

Since the size of mosquito populations is crucial to disease transmission, it is important to reduce these populations below transmission thresholds. Even small programs can provide a reduction in vector populations and reduce the risk of vector-borne disease transmission.

Read the entire article here


Hopkins, CC et al. 1975. The epidemiology of St Louis encephalitis in Dallas, Texas in 1966. Am J Epidemiol 102: 1-15.

Tedesco C, Ruiz M, and McLafferty S. 2010. Mosquito politics: Local vector control policies and the spread of West Nile Virus in the Chicago region. Health & Place, 16 (6): 1188-1195.