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Mosquito Borne Diseases

West Nile Virus

West Nile Virus is the number one mosquito transmitted disease in the United States. The disease is caused by a mosquito transmitted virus.

West Nile Virus is a mosquito-borne disease that was introduced to North America in 1999 and detected in California in 2003.  The affects of this mosquito-borne disease have been significant and costly to humans and wildlife throughout the U.S. including California. Researchers have discovered that many native species of mosquitoes, including some that were not known to vector diseases before, can effectively transmit this disease to humans.  In Napa County there are four very common and widespread species of mosquito (the Little House Mosquito, The Foul Water Mosquito, The Tule Mosquito and the Western Encephalitis Mosquito) that are known to be excellent carriers of this disease.  Two other species (the Fish Pond Mosquito and Western Treehole Mosquito) are presumed carriers although research is still pending.  All of the aforementioned species of mosquitoes are specifically targeted by this District.

  • Incubation: period 5 to 15 days.
  • Symptoms: Many people show little or no symptoms.Those that become ill can have fever, headache, nausea, body aches and swollen lymph nodes.1 in 150 infect persons may require hospitalization.
  • Susceptibility: The elderly are the most susceptible to the virus.
  • Diagnosis: By physician using acute phase serum or cerebral spinal fluid.
  • Treatment: There is no specific treatment for infection. Supportive care is important (contacting your physician, fluids, electrolytes, managing fever, etc.)
  • Mosquito vectors: efficient vectors are:

Known Occurrences in Napa County

West Nile Virus was first detected in Napa County birds in the summer of 2004. This virus has been found every year since and is expected to be present in the County for many years to come. Our first human case was diagnosed in 2006 with the second and last case in 2007.

Coordinated surveillance work will continue with neighboring Mosquito Abatement District’s and the California Department of Heath Services, testing birds and mosquito populations for the presence of this virus.

Napa Country West Nile Virus Confirmed Case Data

2003 2004 2005 2006 2007 2008
Human 0 0 0 1 1 0
Horse 0 0 0 0 0 0
Dead Squirrels 0 0 1 0 0 0
Chickens 0 0 2 0 0 0
Dead Birds 0 6 43 8 2 1
Mosquitoes 0 0 0 0 0 0

To download detailed maps of California’s West Nile Virus cases, click the links below:

For more information about this disease go to the The California Department of Public Health West Nile Virus Website.

Malaria

Disease caused by parasites belonging to the genus Plasmodium

  • Incubation:  period varies from 12 to 30 days depending on the species of plasmodium responsible for the infection. Plasmodium vivax is the most prevalent and typically takes about 14 days.
  • Symptoms: Typical symptoms are violent fever lasting six to eight hours that recur every two to three days (chills and fever cycles). Severe chills and shivering lasting approximately an hour, followed by high fever for one to two hours more. Left untreated anemia and enlargement of the spleen develop.
  • Diagnosis: with blood test by physician.
  • Treatment: with anti-malaria drugs.
  • Mosquito vector:Anopheline mosquitoes:

Known Occurrences in Napa County

No locally contracted cases of malaria have occurred in Napa County in more than 65 years. The last imported cases occurred in 2006 (3 individuals).

Encephalitis

Technically, encephalitis means swelling of the brain. Many things including a bite from a mosquito infected with the encephalitis virus can cause this serious disease. Clinical symptoms of mosquito-transmitted encephalitis vary from mild cases showing little or no symptoms to more severe infections, which show fever, headache, convulsions and coma. Most people infected with the virus show little or no disease symptoms. The two most common types of mosquito-borne encephalitis in California are St. Louis Equine Encephalitis and Western Equine Encephalitis.

St. Louis Encephalitis

  • Cause: Disease caused by a mosquito transmitted virus.
  • Incubation: period usually 5-10 days.
  • Symptoms: Fever, severe headache, nausea, and stiff neck. Severe cases may include vomiting, disorientation and paralysis.
  • Diagnosis: By physician using acute phase serum or cerebral spinal fluid.
  • Treatment: There is no specific treatment for infection. Supportive care is important (contacting your physician, fluids, electrolytes, managing fever, etc.).
  • Mosquito vector:

Known Occurrences in Napa County

No confirmed human cases reported in Napa County.

Western Equine Encephalitis

  • Cause: Disease caused by a mosquito transmitted virus.
  • Incubation:period usually 5-10 days.
  • Symptoms: May have flu-like symptoms including fever, headache and lethargy. Severe cases may include vomiting, anorexia, disorientation and paralysis.
  • Diagnosis: By physician using acute phase serum or cerebral spinal fluid.
  • Treatment: There is no specific treatment for infection. Supportive care is important (contacting your physician, fluids, electrolytes, managing fever, etc.).
  • Mosquito vector:

Known Occurrences in Napa County

No confirmed human cases reported in Napa County. The virus for Western Equine Encephalitis was last detected in the Calistoga area in 1996.

Dog Heartworm Disease

Dog Heartworm Disease is a clinical condition in dogs caused by a roundworm, Dirofilaria immitis, which resides within the dog’s heart and lungs.  This disease, a serious and possibly fatal veterinary problem, is associated with dogs, coyotes and foxes.  Canine Heartworm is transmitted by the bite of an infected Western Treehole Mosquito.

The adult worm lives in the right side of the heart and the adjacent large blood vessels and lungs, where it may attain a length of 6-12 inches.

Many other mosquito species feed on dogs, but the Western Treehole Mosquito is the most common carrier of heartworm.

The outward symptoms of the disease are not noticeable in most cases until reduced blood flow caused by adult worms damages the heart, lungs, liver and kidneys.  Advanced symptoms of heartworm may include: rapid tiring, shortness of breath, chronic soft dry cough, listlessness and weight loss.

If you live in or travel to areas where treehole mosquitoes occur, check with your veterinarian regarding treatment and prevention.  Drugs are available to prevent the disease, and it is curable if diagnosed in the early stages.

In the San Francisco Bay area, the time of highest risk for dogs to contract heartworm is April through August; however, unseasonable rainfall may extend this period.

  • Cause: Disease caused by a mosquito transmitted filarial worm that primarily affects dogs.
  • Incubation: period – none. 70 to 90 days for the worms to become mature in dogs.
  • Symptoms: Long term symptoms:  chronic cough that is aggravated by exercise, unusual tiring after exercise, and shortness of breath.
  • Diagnosis: By veterinarian.
  • Treatment: With anti-filarial drugs.
  • Mosquito vector:

Known Occurrences in Napa County

Dog Heartworm is prevalent in Napa County. No current statistics are readily available concerning the number of new cases diagnosed each year in dogs.


For more information about these diseases go to the Centers for Disease Control – vector-borne diseases page.