Vector-Borne Diseases

Anyone can be infected by West Nile Virus through the bite of an infected mosquito and become ill. However, certain groups of people have a higher risk of developing severe illness and should take extra precautions to avoid mosquito bites. These groups include people:

• Over the age of 60 years
• With weakened immune systems
• With diabetes, kidney disease, or some other medical condition

See your medical provider if you think you may have West Nile Virus.

There are no effective medical treatments or human immunizations for West Nile Virus. Preventing mosquito bites and reducing mosquito populations is the best approach to preventing infections.

• Prevent mosquito bites by using an EPA-registered insect repellent with DEET, picaridin, IR3535, or oil of lemon eucalyptus before you go outside.
• Avoid spending time outdoors around dawn and dusk or take extra precautions to avoid bites during this time. Culex mosquitoes, the vector for West Nile Virus, are most active at dawn and dusk.
• Install window and door screens that fit tightly into their frames. Check for and repair any holes or tears you find.
• Dump standing water around your yard at least once a week. Request free mosquitofish or anonymously report neglected swimming pools here.

Visit our Prevention page to learn more about source reduction and bite prevention.

Other animals can be infected with West Nile Virus if they are bitten by an infected mosquito. However, most animals will not get sick. West Nile Virus can cause severe illness in birds, tree squirrels, and horses.

While many bird species can carry West Nile Virus, some species like crows, ravens, jays and magpies can become very ill and die. Bird die offs can be an indicator that West Nile Virus is in the area. Tree squirrels infected with WNV can develop symptoms including shaking, circling, or paralysis and may die. You can report dead birds or tree squirrels directly to the District at 559-732-8606 during regular business hours or through the California West Nile Virus Hotline online at www.westnile.ca.gov or by calling 1-877-968-2473. If the dead bird or squirrel meets the criteria for disease testing, someone will contact you to arrange a pick up.

Horses infected with West Nile Virus may develop symptoms such as fever, stumbling, muscle twitching, and reduced appetite. The fatality rate for horses showing symptoms is roughly 33%. Horses that recover may have long term side effects including behavioral issues and problems with their gait. A West Nile Virus vaccine is available for horses and is strongly recommended. Cats and dogs can be infected by West Nile Virus but they rarely get sick. 

Contact your veterinarian for more information.

Anyone can be infected by St. Louis Encephalitis Virus through the bite of an infected mosquito and become ill. However, the elderly and those with weakened immune systems have a higher risk of developing severe illness and should take extra precautions to avoid mosquito bites. 

There are no effective medical treatments or human immunizations for St. Louis Encephalitis Virus. Preventing mosquito bites and reducing mosquito populations is the best approach to preventing infections.

• Prevent mosquito bites by using an EPA-registered insect repellent with DEET, picaridin, IR3535, or oil of lemon eucalyptus before you go outside.
• Avoid spending time outdoors around dawn and dusk or take extra precautions to avoid bites during this time. Culex mosquitoes, the vector for St. Louis Encephalitis Virus, are most active at dawn and dusk.
• Install window and door screens that fit tightly into their frames. Check for and repair any holes or tears you find.
• Dump standing water around your yard at least once a week. Request free mosquitofish or anonymously report neglected swimming pools here.

Visit our Prevention page to learn more about source reduction and bite prevention.

Many wild bird species can carry St. Louis Encephalitis Virus without getting sick. You can report dead birds directly to the District at 559-732-8606 during regular business hours or through the California West Nile Virus Hotline online at www.westnile.ca.gov or by calling 1-877-968-2473. If the dead bird meets the criteria for disease testing, someone will contact you to arrange a pick up.

Most other animals infected with St. Louis Encephalitis Virus do not become sick. Contact your veterinarian for more information if you are concerned about your pet.

Anyone can be infected by Western Equine Encephalomyelitis Virus through the bite of an infected mosquito and become ill. However, very young children and the elderly have a higher risk of developing severe illness and should take extra precautions to avoid mosquito bites. 

There are no effective medical treatments or human immunizations for Western Equine Encephalomyelitis Virus. Preventing mosquito bites and reducing mosquito populations is the best approach to preventing infections.

• Prevent mosquito bites by using an EPA-registered insect repellent with DEET, picaridin, IR3535, or oil of lemon eucalyptus before you go outside.
• Avoid spending time outdoors around dawn and dusk or take extra precautions to avoid bites during this time. Culex mosquitoes, the vector for Western Equine Encephalomyelitis Virus, are most active at dawn and dusk.
• Install window and door screens that fit tightly into their frames. Check for and repair any holes or tears you find.
• Dump standing water around your yard at least once a week. Request free mosquitofish or anonymously report neglected swimming pools here.

Visit our Prevention page to learn more about source reduction and bite prevention.

Many wild bird species can carry Western Equine Encephalomyelitis Virus without getting sick. You can report dead birds directly to the District at 559-732-8606 during regular business hours or through the California West Nile Virus Hotline online at www.westnile.ca.gov or by calling 1-877-968-2473. If the dead bird meets the criteria for disease testing, someone will contact you to arrange a pick up.

Horses that are infected with Western Equine Encephalomyelitis Virus may develop symptoms such as fever, stumbling, muscle twitching, and reduced appetite. The fatality rate for horses showing symptoms is between 15-30%. Horses that recover may have long term side effects including behavioral issues and problems walking. A Western Equine Encephalomyelitis Virus vaccine is available for horses and is strongly recommended.

Contact your veterinarian for more information if you are concerned about your horse.

Dengue is a mosquito-borne disease that is caused by dengue virus 1, 2, 3, or 4. It is spread through the bite of an infected Aedes aegypti or Aedes albopictus mosquito. These mosquitoes become infected when they bite a person who has dengue virus in their blood. Pregnant women can transmit dengue to their child during the pregnancy or around the time of birth.

About 25% of people with dengue will develop a fever, headache, pain behind the eyes, rash, joint pain, bone pain, or muscle pains. Of those who get sick, about 1 in 20 will develop severe dengue. Symptoms of severe dengue can develop within a few hours and include stomach pain or tenderness, vomiting, respiratory distress, internal bleeding, shock, and even death. Severe dengue is a medical emergency.

There is no effective medical treatment for dengue. A dengue vaccine is available for people living in certain areas of the world who have had a confirmed prior dengue infection. Preventing mosquito bites and reducing mosquito populations is the best approach to preventing infections. Continue to avoid mosquito bites for 3 weeks after traveling to an area with dengue.

Contact your healthcare provider if you believe you may have dengue.

Chikungunya is a mosquito-borne disease that is spread through the bite of an infected Aedes aegypti or Aedes albopictus mosquito. These mosquitoes become infected when they bite a person who has chikungunya virus in their blood.

Most people infected with chikungunya virus will develop symptoms within a week of being bitten by an infected mosquito. Symptoms may include fever, joint pain, joint swelling, headache, and rash. These symptoms may be severe and disabling with joint pain lasting months. Newborns, people over the age of 65 years, and people with medical conditions have a greater risk for more severe disease.

There is no vaccine or medical treatment available for chikungunya. Preventing mosquito bites and reducing mosquito populations is the best approach to preventing infections. Continue to avoid mosquito bites for 3 weeks after traveling to an area with chikungunya. 

Contact your healthcare provider if you believe you may have chikungunya.

Yellow fever is a mosquito-borne disease that is spread through the bite of an infected Aedes aegypti or Aedes albopictus mosquito. These mosquitoes become infected when they bite a person who has yellow fever virus in their blood.

Most people who are infected with yellow fever virus will not develop symptoms. For those that do, symptoms will typically appear within 3 to 6 days and include fever, chills, headache, back pain, body aches, nausea, vomiting, and fatigue. About 1 in 7 people will develop a more severe illness that includes jaundice (yellow skin), bleeding, organ failure, shock, and death. Between 30% to 60% of people who develop severe illness will die.

There is no medicine to treat or cure a yellow fever infection.  A vaccine is available that prevents yellow fever. If you are traveling to an area where yellow fever is endemic, being vaccinated is highly recommended. Preventing mosquito bites can help prevent infections. Continue to avoid mosquito bites for 3 weeks after traveling to an area with yellow fever.

Contact your healthcare provider if you believe you may have yellow fever or before traveling to an area where yellow fever is endemic.

Tick-borne relapsing fever is a bacterial disease that can be spread to people through the bite of an infected soft tick. Soft ticks are mostly found in forested mountain and foothill areas where they can easily feed on rodents, their preferred meal choice. These ticks typically live in rodent nests, wood piles, under floorboards, and cracks and crevices.

People infected with tick-borne relapsing fever typically develop symptoms about a week after being bitten by an infected tick. Symptoms may include a sudden high fever (104-105 F), chills, headache, muscle aches, nausea, vomiting, abdominal pain, or a rash. The symptoms occur in cycles, lasting between 3-5 days before disappearing for a few days, then occurring again.

If you develop any symptoms within 30 days of being bitten by a tick, contact your healthcare provider. Antibiotics can successfully treat tick-borne relapsing fever and people rarely develop chronic health problems or die. For more information about tick-borne relapsing fever, visit the California Department of Public Health website.

Tularemia is a bacterial disease that can be transmitted to people through the bite of an infected Pacific coast tick (Dermacentor occidentalis), American dog tick (Dermacentor variabilis), or deerfly (Chrysops discalis). People are most commonly infected through direct contact with an infected wild animal. Wild rabbits are the most common source of infection. People can also become infected if they eat meat from an infected animal.

People who are infected with tularemia may develop a severe illness or even death. Symptoms typically appear about 3 to 5 days after infection and may include chills, fever, headache, body aches, cough, and pain or tightness in the chest. Lymph nodes in the neck, underarms, or groin may be swollen and painful. The bite or wound site may form an ulcer. If left untreated, tularemia bacteria can cause pneumonia, blood infection, conjunctivitis, or meningitis.

To prevent tularemia, avoid touching or handling animal carcasses. Use gloves when wild game meat and cook thoroughly before eating. Use an EPA registered insect repellent to prevent tick bites and check for ticks frequently.

If you develop any symptoms after handling a wild animal, especially rabbits, or being bitten by a tick, contact your healthcare provider. Antibiotics can successfully treat tularemia. For more information about tularemia, visit the California Department of Public Health or the Centers for Disease Control and Prevention website.

The Spotted Fever Group Rickettsia are a group of bacterial diseases that are associated with a rash and that are transmitted through the bite of an infected tick. They include Rocky Mountain Spotted Fever and Pacific Coast Tick Fever. Pacific Coast Tick Fever is transmitted through the bite of an infected Pacific coast tick (Dermacentor occidentalis), while Rocky Mountain Spotted Fever can be spread through the bite of an infected American dog tick (Dermacentor variabilis), Pacific coast tick (Dermacentor occidentalis), or brown dog tick (Rhipicephalus sanguineus).

Symptoms of Rocky Mountain Spotted Fever typically occur between 2 to 14 days after being being by an infected tick. Symptoms can include fever followed by weakness, muscle pain, severe headache, and a painful abdomen. Some people may develop a rash that covers most of the body including the palms of the hands and soles of the feet. If not treated, the fever can last for two to three weeks and lead to other symptoms, such as weakness, deep muscle pain, severe headache, chills, blood-shot eyes, and a painful abdomen. Kidney failure and even death may result in severe cases. Pacific Coast Tick Fever causes similar but milder symptoms with the addition of a small, open wound that turns into a dark scab.

If you develop any symptoms within 30 days of being bitten by a tick, contact your healthcare provider. Antibiotics can successfully treat infections form the spotted fever group Rickettsia. Left untreated, up to 25% of people infected with Rocky Mountain Spotted Fever may die. For more information, visit the California Department of Public Health website or the Centers for Disease Control and Prevention RMSF page or Other Spotted Fever Group page.

Avoiding tick bites can reduce your risk of developing a tick-borne disease.

• Use an EPA-registered repellent for use against ticks such as those with at least 20% DEET. Visit the EPA Repellent Search Tool to find a repellent that is right for you.
• Before hikes, treat clothing with permethrin to kill ticks.
• Stay in the middle of trails when hiking or walking. Avoid walking through grassy areas, sitting on logs, tree trunks, or other vegetation.
• Keep grass mowed along trails, buildings, camping, and other outdoor gathering areas.
• Check yourself, others, and pets for ticks during and up to 3 days after outdoor activities such as hiking.
• Shower soon after returning from outdoor activities in tick habitat.
• Place clothes worn during outdoor activities in a hot dryer for 10 minutes before washing.

If you are bitten, promptly removing the tick can help prevent disease transmission. 

• Grasp the tick’s mouthparts with tweezers as close to the skin as possible.
• Pull the tick straight out using a gentle, steady motion. Avoid twisting or jerking the tick which can cause the mouthparts to break off in the skin.
• Wash the bite site and your hands with soap and water.

Contact your healthcare provider if you develop any symptoms within 30 days of being bitten by a tick.

Dogs may be at risk for Rocky Mountain spotted fever (RMSF), ehrlichiosis, and Lyme disease. These are bacterial infections that can be transmitted through the bite of an infected tick. Removing ticks promptly can reduce the risk of disease transmission. Fever and lameness or stiffness is a common sign of a tick-borne infection in dogs. Other symptoms may include coughing, lack of energy, loss of appetite, and swelling of the lymph nodes, joints, legs, or face. 

Talk to your veterinarian about treatment options to prevent tick bites. Removing ticks promptly can help reduce the risk of disease transmission. Check your dog for ticks between the legs and toes, under collars, and around the ears and the eyes.

Cats may show a wide variety of symptoms from tick-borne infections. Cats may have ticks in and around the ears, around the eyelids, under collars, under front legs, between the back legs, between the toes, or around the tail. Remove ticks from pets promptly to reduce disease risk.

Cats can be very sensitive to different chemical treatments for ticks. Always talk to your veterinarian before applying a tick treatment to your cat.