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West Nile Virus and Mosquito Prevention
What is West Nile Virus?
West Nile Virus (WNV) is a mosquito-borne disease that was originally found in Africa, and as of 1999, has moved to the United States. Approximately 80% of people do not develop any symptoms. However, about 1 in 5 will get flu-like symptoms that last for a few days, with fatigue and weakness that can last for weeks or months. In a very small percentage of people, <1%, severe neurological illness, such as encephalitis or meningitis (inflammation of the brain or surrounding tissue) may occur. They may last several weeks or become permanent; however, this happens in roughly 1 in every 150 people. Animals are also susceptible to WNV, including dogs and cats, but especially horses. To learn more about WNV and horses click here What is West Nile Virus?.
The main way that people contract WNV is a bite from infected mosquitoes. WNV cannot spread through touching.
Mosquito Prevention
There are three main ways to reduce the risk of being infected by WNV:
- Use insect repellent: DEET, picaridin, or oil of lemon eucalyptus is recommended.
- Protect yourself most at DAWN and DUSK: mosquitoes are more likely to bite during the early morning and evening, so make sure you wear proper clothing and repellent if outside during these times.
- Drain standing water: mosquitoes lay eggs on standing water; so, it is important to eliminate or drain all standing water sources around homes and properties.
- Learn more about how to prevent and protect yourself from West Nile Virus.
Currently, there is no vaccine for WNV; the best method of not contracting the disease is to prevent contact with mosquitoes using the above methods.
2025 Mosquito Fish Giveaway Information
Mosquito Fish Giveaway Dates, Location and Time
All Mosquito Fish Giveaways are located at the Eric Rood Center (950 Maidu Ave) in Nevada City and are held from 10:00am-12:00pm.
- May 7th and 21st
- June 4th and 18th
- July 9th
Request Mosquito Fish:
Please call the Vector Control Program at 530-265-1500.
About Mosquito Fish & the Best Environments for Them to Thrive
Mosquito Fish can consume large quantities of insect larvae in your pond or other standing bodies of water. Reducing the mosquito population can reduce your risk of contracting mosquito-borne diseases, including the West Nile Virus.
These fish are not native; they are invasive species. These fish are only meant for closed/stagnant ponds and should not be placed into ponds that can drain into the local creeks and streams. This can have negative impacts on the native ecosystem. If your pond has outlets, please do not acquire these fish.
If your pond already has larger fish, you don’t need mosquito fish. The larger fish will eat the smaller mosquito fish. Also, the larger fish will eat mosquito larvae, and certainly, their smaller offspring will eat larvae too. Nevada County Environmental Health can inspect your water source and offer other options.
In situations where a property has temporary, seasonal ponds, drainages, or ditches, and they cannot be manually drained, store-bought bacterial larvicide products such as BTI can be used. Most home-use larvicide products are harmless to the environment and will not hurt people, pets, or wildlife when used properly and according to label directions. These products are often found at various hardware, garden, or pond supply stores.
Also, the Nevada County Environmental Health Department provides on-site service visits for treatment requests.
It is very rare to catch the virus, and most infected people will not even get sick or will only experience mild flu-like symptoms. However, West Nile Virus can be fatal, especially for people over 50 years of age. Remember to protect yourself from mosquito bites before doing any outdoor maintenance and while enjoying outdoor activities. Also, you can refer to the Home and Garden Checklist for best practices to fight the bite.
A West Nile Virus Glossary
The A to Z of fighting mosquitoes:
- Animals: Dogs and cats can become infected, but rarely become ill and do not spread the virus. Keep animal areas dry. Keep food and water bowls clean. A vaccine is available for horses.
- Birdbaths: Clean or hose out birdbaths weekly.
- BTI: A natural bacterium that kills mosquito and fly larvae yet is non-toxic to animals when used properly. Kills larvae developing in water (see dunks below.)
- Building Material: Unused pipe should be kept inside or turned over so that no water accumulates.
- Chain Link Fence: Cover chain link fence posts with metal or plastic caps since they are hollow pipes.
- Clothing: Wear long-sleeved shirts and long pants when working around mosquito-infested areas.
- Containers: Cover containers or turn them upside down so that they do not hold water, including ashtrays, boxes, buckets, cans, cups, jars, and pots.
- Dawn/Dusk: The times of day when biting mosquitoes are most active, since they avoid the heat of the day.
- Dead Birds: Pick up dead birds with a shovel or gloves. Double-bag in plastic and dispose of in the trash. Dead birds should not be handled directly in order to avoid exposure to the virus. Wash your hands thoroughly afterward.
- DEET: The active ingredient in insect repellent. A 10% concentration is recommended for kids.
- Drains: Keep outdoor drains flowing freely.
- Dripping Water: Fix leaky faucets, air conditioners, and hoses.
- Drought: Drought conditions may help spread the virus since birds and mosquitoes share the same few water sources, even in yards.
- Dumpsters: Keep dumpsters covered and remove any water inside and under the dumpster.
- Dunks: Dunks are donut-sized pellets that kill mosquito larvae but are non-toxic to animals. They dissolve slowly in water. Available at hardware and garden stores. (See BTI above.)
- Flat Roofs: Inspect weekly to remove any puddles. Fix leaks from air conditioners and pipes. Keep rain gutters clear of debris and flowing freely.
- Irrigation: Do not overwater. Eliminate any areas of excess standing water weekly.
- Landscaping: Remove plastic sheeting under bark or rock and replace it with landscape fabric that prevents weeds yet allows water through.
- Lawn Ornaments: Should be checked for areas that hold water and drained or flushed weekly.
- Lighting: Check garden lights and eliminate water from the tops of fixtures and from inside floodlights.
- Playgrounds: Drill drainage holes in tire swings and playground equipment that holds water.
- Ponds: Stock with mosquito-eating fish. Use mosquito dunks or BTI in ponds and larger bodies of water. Follow instructions for safe use.
- Pools/Spas: Maintained pools and whirlpool spas are not a hazard since pool chemicals and filters kill any larvae. Use dunks in deserted pools and spas.
- Potted Plants: Do not overwater plants. Empty saucers weekly or flush with a garden hose. Drill small drainage holes in outdoor saucers.
- Rain Barrels: Cover tightly with screening.
- Rain Gutters: Keep gutters clear of debris. They can become breeding areas with standing water.
- Recycling Bins: Cut top and bottom from tin cans and flatten. Invert glass jars. Crush soft drink bottles and cans. Store newspapers on end, not flat. Drill drainage holes in the bottom of the recycling bin.
- Screens: Install and maintain tight-fitting window and door screens.
- Shrubbery: Trim and thin shrubs and bushy plants since they can be mosquito hiding areas.
- Tires: Properly dispose of old tires used in retaining walls and in landscaping.
- Tool Sheds: Keep shed roofs maintained and eliminate water around the foundation.
- Trash: Remove anything that can hold water, including cans, cups, tin foil, plastic, and paper, since mosquito larvae can grow in them.
- Trash Cans: Keep trash cans covered. Remove buckets and containers from around trash areas.
- Trees: Eliminate water from dead tree stumps and hollow areas of live trees. Fill cavities with sand and flush weekly with a hose.
- Umbrellas: Table umbrellas and basketball bases are often filled with water. Cap tightly, seal with duct tape, or fill with sand and cover tightly.
- Weeds: Remove weeds, especially around areas close to water. This information is available thanks to Fight the Bite Colorado.
This information is available courtesy of Fight the Bite Colorado.
Insect repellents help people reduce their exposure to mosquito bites that may carry potentially serious viruses such as West Nile Virus. Please read the following to learn more about commonly used active ingredients in insect repellents.
DEET
Insect repellents containing DEET are the most effective mosquito repellents available. Products vary in their concentration of DEET from below 10% to above 30%. Products with concentrations around 10% are effective for periods of approximately two hours. As the concentrations of DEET increase, the duration of effectiveness during activity increases. For example, 24% has been shown to provide an average of 5 hours of protection.
For safety, select the lowest concentration effective for the amount of time spent outdoors. It is generally believed that DEET should not be applied more than once a day.
The active ingredient in insect repellents containing DEET is N, N-Diethyl-meta-toluamide, or diethyltoluamide.
For more information regarding DEET, please visit the National Pesticide Information Center's informational page or the Environmental Protection Agency's DEET Page.
Use Insect Repellents (DEET) According to the Label Directions
DEET repels insects, it does not kill them. If you are aware of mosquitoes around you, and they are not biting, the repellent is working or still working, and you do not need to reapply the repellent.
- DEET should not be used in a product in combination with sunscreen, since typically, sunscreen should be applied multiple times a day and it is advised that DEET should only be used once a day. Be sure to follow label directions for the product you are using.
- Apply DEET sparingly on exposed skin and do not use under clothing.
- Do not use DEET on the hands of young children. Avoid applying to areas around the eyes and mouth.
- Do not use DEET over cuts, wounds, or irritated skin. Wash treated skin with soap and warm water after returning indoors. Wash-treated clothing.
- Do not apply aerosols or pump products directly to your face. Spray your hands and then rub them carefully over the face while avoiding the eyes and mouth.
- Avoid spraying in enclosed areas.
- Do not use near food and drink.
- DEET can also be applied to clothing to avoid application to the skin altogether.
- After returning indoors, wash treated skin with soap and warm water.
Insect Repellents and Children
The American Academy of Pediatrics and the Center for Disease Control have suggested a cautious approach is to use products with a low concentration of DEET, 10% or less, on children. Most guidelines cite that it is acceptable to use repellents containing DEET on children 2 years of age or older, but the concentration should be low (less than 10%).
When using a repellent on a child, apply it to your own hands and then rub it on your child. Avoid children's eyes and mouth and use it sparingly around their ears. Do not allow young children to apply insect repellent to themselves, have an adult do it for them. Keep repellents out of the reach of children. Repellent production that does not contain DEET is not likely to offer the same degree of protection from mosquito bites, however, there are several alternatives (see below).
The use of insect repellents may cause skin reactions in rare cases. Cases of serious reactions to products containing DEET have been related to misuse of the product, such as swallowing, using over broken skin, and using it for multiple days without washing skin in between uses. Always follow the instructions on the product label. If you suspect a reaction to these products, discontinue use and wash the treated skin and hands. Seek professional health care if the reaction continues.
Alternatives to DEET:
IR3535
A naturally occurring compound that is synthetically produced. Nearly as effective as DEET and not harmful to the skin. View the EPA's fact sheet here Version OptionsWest Nile Virus and Mosquito PreventionHeadline .
Oil of lemon eucalyptus (OLE) or PMD
This natural alternative is useful in repelling against mosquitoes and other insects; however, the duration of time is much shorter than that of DEET. However, you can reapply multiple times as it is not harmful to the skin in approved products. View the EPA's Fact Sheet here.
Picaridin
Picaridin is an insect repellent that is applied directly to the skin. It is in the piperidine chemical family. Picaridin appears to work by preventing the mosquito from finding or recognizing its host. Picaridin products can be formulated as solids, liquids, sprays, aerosols, or wipes. Insect repellents can help prevent disease, but they must be used correctly; always read and follow the label directions. For more information, please visit the National Pesticide Information Center's page.
Repellent Ingredients
Other main ingredients in repellents can be found at the Environmental Protection Agency's Skin Applied Repellents.
Information on the County's Use of Mosquito Treatment Products
The County's Agricultural and Environmental Health Departments have an active mosquito treatment program and provide information to the public. Nevada County currently does not have a mosquito abatement district.
Where appropriate, the mosquito treatment program does include applying larvicides to stagnant or abandoned water sources. A larvicide program is generally the best and most economical strategy to prevent the emergence of adult mosquitoes. Oils and adulticide fogs are used in very controlled and limited applications.
Our mosquito control personnel will visit your property if you fill out a Citizen Service Request Form online, or call (530) 265-1500 and leave a detailed message.
Mosquito Control Products:
For Altosid Pellets
For VectoBac G Biological Larvicide
Mosquitoes are found all over the world, except in Antarctica. These two-winged insects belong to the order Diptera. Members of the genera Anopheles, Culex, and Aedes are most commonly responsible for the bites in humans. There are approximately 170 species of mosquitoes in North America alone.
Feeding
Only female mosquitoes bite. Male mosquitoes feed primarily on flower nectar, whereas female mosquitoes require a blood meal to produce eggs. They usually feed every 3 to 4 days. In a single feeding, a female mosquito typically consumes more than its own weight in blood. Certain species of mosquitoes prefer to feed at twilight or nighttime, while others bite mostly during the day.
Life Cycle
The life cycle of the mosquito has four stages. The female mosquito lays her eggs, up to several hundred at a time, on the surface of the water or in an area subject to flooding. Unhatched eggs of some species can withstand weeks to months of desiccation, remaining viable until the right conditions for hatching occur. The eggs of most species hatch within 48 hours, and the larvae feed on organic matter in the water for about a week until they change into pupae. The pupae live at the surface of the water for 1 to 4 days before metamorphosing into adult mosquitoes.
Water
Mosquitoes require an environment of standing water. As a group, they have adapted to complete their life cycle in diverse aquatic habitats, including freshwater, saltwater marshes, brackish water, or water found in containers, old tires, or tree holes.
Please refer to our Frequently Asked Questions Opens a New Window. for more information, or contact us via phone at (530) 265-1222 or email at the Environmental Health Department.