Mosquito-Borne Encephalitis Viruses

Mosquito-borne viruses that cause illness in humans and domestic animals are known as "arboviruses". There is a potential risk to humans and domestic animals in Washoe County from arboviruses, which can cause mild to serious illness. An arboviral-caused encephalitis (brain inflammation) can develop which may result in long-term disability including partial paralysis and memory loss or death.

The Vector-Borne Disease Prevention Program (VBDPP) staff conducts tests to detect the presence of the arboviruses that cause West Nile Virus, St. Louis Encephalitis, and Western Equine Encephalitis. Doing this type of surveillance is an important initial step in the prevention of these diseases since the virus usually shows up in wild and domestic animals before the first human cases appear.

The VBDPP staff conduct testing of adult mosquitoes to identify the presence of all three viruses. If any of these tests turn up positive, the VBDPP staff will respond rapidly to isolate and eliminate the virus by increasing mosquito control in the area where it has been identified.

West Nile Virus brochure.
CDC - Arboviral Encephalitis


Rabies is a viral disease that affects the central nervous system of humans and other domestic animals. Rabies is usually spread from one animal to another (and to humans) through a bite. The virus is transmitted through the infected animal's saliva. It is always fatal in humans, but can be prevented if the individual receives a series of shots after being bitten by the rabid animal. Immediate washing of a bite wound with soap and water can also play an important role in prevention of this dread disease.

Rabies vaccinations programs for dogs and cats and public education about rabies has been crucial in making this disease a rare occurrence in humans in the United States. Usually there are fewer than five human cases of rabies in the United States each year.

The Vector-Borne Disease Prevention Program will transport rabies susceptible wild mammals that are involved with a possible human exposure to the Nevada State Department of Agriculture Animal Diseases Laboratory for rabies testing. If the animal is still alive, the staff will undertake capturing it. It is important to report any animal bite or other possible exposure from wild animals that are considered high-risk rabies species to the Health Department so that precautionary steps can be initiated to prevent illness to humans and domestic animals.

Rabies Brochure
Rabies information from the CDC

Rabies and Bats Fact Sheet


Hantavirus pulmonary syndrome (HPS) is an infectious disease caused by a virus that can be fatal to humans. Rodents such as the white-footed deer mouse found in Washoe County spread it to people. The droppings and urine of infected mice contain the virus particles which cause infection in humans after being inhaled.

Historical mouse surveys for hantavirus have been completed in Cold Springs, Silver Knolls, Caughlin Ranch Parkway, Spanish Springs, and Galena Creek Park. These surveys allowed our Program staff to determine levels of hantavirus in rodents from different areas and to make appropriate recommendations to residents to prevent illness. The average rate of infection for hantavirus in deer mouse populations in the area is about 15%, although this figure fluctuates greatly and may vary up or down in any given area at any given time. Test results have shown as low as 5% or as high as 50% of the rodents trapped testing positive for hantavirus.

VBDPP no longer tests mice for hantavirus but staff will discuss rodent removal and proper methods of cleaning to prevent human illness with County residents. Please see the brochures listed below outlining rodent prevention and proper cleaning procedures.

CDC page on Hantavirus Pulmonary Syndrome (HPS)

Got Mice? - Seal Up! Trap Up! Clean Up!

Lyme Disease

Lyme disease is a bacterial infection that causes skin, joint, heart, and nervous system problems. It can cause chronic, long-term illness in humans if not identified and treated early.

The Vector-Borne Disease Prevention Program staff does not test ticks for disease but can provide identification for assisting primary care physicians and veterinarians.  

A tick can be easily removed by gripping it at the base of the imbedded mouthparts with a tweezers. By gently pulling, the tick will release within a few seconds. Do not use Vaseline, gasoline, alcohol, or burn the tick with a match. Do not turn the tick clockwise two times. These popular methods of removal can cause the bacteria to be expelled into the host and can increase the chance of infection. These and similar methods should never be used.

Once removed the tick can be put in a small Ziploc plastic bag with a piece of damp paper towel. You may deliver the tick to the Environmental Health Services Office at the District Health Department at the corner of Ninth Street and Wells Avenue for identification.

CDC - Learn about Lyme Disease


Plague is caused by a bacterium called Yersinia pestis. The bacteria are passed between rodents through the bite of an infected flea. It can be passed to humans by a flea bite or by handling animals that have died from the disease. The disease can be fatal if it is not treated with antibiotics or if treated too late in the course of the illness. Domestic cats are particularly vulnerable to plague and often spread the disease to their owner.

There have been no human cases of plague in Washoe County for more than twenty years. However, plague does maintain a presence in wild rodent populations in the area. California ground squirrels, golden mantle ground squirrels, chipmunks, and rabbits are known to harbor plague-carrying fleas in Washoe County.

The Vector-Borne Disease Prevention Program staff will pickup deceased ground squirrels that are in close proximity to human habitation and transport them to the Nevada State Department of Agriculture Animal Diseases Laboratory (ADL) for plague testing. By determining if a rodent has died from plague, our staff can take precautions to prevent the spread of the disease to humans and domestic cats. Public warnings to alert residents against having contact with dead or sick rodents may be posted.

You may contact the Vector-Borne Disease Prevention Program staff at 328-2434 regarding pickup of a dead squirrel.

More information on plague from the CDC

Racoon Roundworm

Raccoon roundworm, known scientifically as Baylisascaris procyonis, is a parasitic infection that has gained some attention in the U.S. as a source of human disease. Although this parasite is relatively harmless to the raccoon, serious illness can occur in humans when infective eggs are accidentally ingested. Ingested eggs hatch as larvae in the small intestine, penetrate the intestinal wall and migrate to other organs such as the liver, lungs, and brain through the circulatory system. If the larvae migrate to the eye, brain, or spinal cord, there can be severe and irreversible damage including blindness, paralysis, and death.

Fortunately, the incidence of human illness in the U.S. is low (less than 30 cases have been reported). Young children are the most likely individuals to become infected as they commonly put dirt and other objects into their mouths. The CDC receives over 4,000 requests annually for laboratory testing for this parasite.

The Vector-Borne Disease Prevention Program (VBDPP) staff have determined that the raccoon roundworm parasite is present in raccoons living in the Truckee Meadows (see surveillance activities). The staff is available for consultation and education regarding raccoon problems at 328-2434.

Racoon Roundworm Brochure

Zika virus

Zika virus is an emerging vector-borne disease spread primarily through the bite of infected Aedes aegypti mosquitoes.  Although Aedes aegypti is not known to occur in Northern Nevada, Northern Nevada Public Health remains committed to active surveillance for this and other closely-related mosquito species in order to prevent their establishment in our region.  Residents are encouraged to contact the Health District if experiencing mosquito problems at any time. 

The greatest risk for Washoe County residents for contracting Zika virus is through travel to countries with active Zika transmission.  It is highly recommended that travelers contact their local travel clinic or primary health care provider before and after traveling to regions reporting active Zika transmission. 

Symptoms of Zika Virus may be similar to those of Dengue and Chikungunya including fever, rash, joint and muscle pain, conjunctivitis (red eyes) and headache.  Because Zika virus has been linked to certain complications with pregnancy and birth defects and the potential for transmission through body fluids, it is recommended that pregnant women and their partners strictly follow the advice of their healthcare providers and to seek additional information before and after travel by either person.   

Additional information may be attained by calling Northern Nevada Public Health at 775-328-2434.

Links:   Zika Virus CDC

Last modified on 08/15/2023