Disease Information


We would like to acknowledge and thank the Centers for Disease Control for allowing the use of the following information and links concerning mosquito-borne illness.

West Nile virus is a flavivirus (a genus of Japanese Encephalitis and closely related to Louis Encephalitis) commonly found in Africa, West Asia and the Middle East (it was first discovered in Uganda, Africa in 1937). It was discovered in the United States in 1999 in the New York City area and has been moving across the U.S. since.

WNv is primarily a mosquito/bird disease, but incidental infections can occur with humans and many other animals. Mosquitoes become infected when they feed on infected birds, such as ravens and crows. After an incubation period of 5 to 15 days, the mosquito can then transmit the virus to humans and animals by biting them. Following this transmission, the virus multiplies in the bloodstream. In severe cases the virus crosses the blood-brain barrier, reaching the brain and causing inflammation of the brain tissue. This inflammation interferes with the central nervous system.

Questions & Answers

Q. What is the basic transmission cycle?

  • A. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit WNv to humans and animals while biting to take blood. The virus is located in the mosquito’s salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.
  • Q. What Are the Symptoms of WNv?
  • Serious Symptoms in a Few People. About one in 150 people infected with WNv will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, paralysis and even death. These symptoms may last several weeks, and neurological effects may be permanent.
  • Milder Symptoms in Some People. Up to 20% of the people who become infected have symptoms such as fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for a few days or many weeks or months.
  • No Symptoms in Most People. Approximately 80% of people (about 4 out of 5) who are infected with WNv will not show any symptoms at all.
  • Q. What Is the Risk of Getting Sick from WNv?
  • A. Anyone at any age can be infected with WNv and become sick. Though people over 50 are at higher risk to get severe illness, serious illness has occurred if every age group and being young is not a defense against this disease. Being outside means you’re at risk. The more time you’re outdoors, the more time you could be bitten by an infected mosquito. Pay attention to avoiding mosquito bites if you spend a lot of time outside, either working or playing. Risk through medical procedures is very low. All donated blood is checked for WNv before being used. The risk of getting WNv through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. If you have concerns, talk to your doctor.
  • Q. What Should I Do if I Think I Have WNv?

A. People typically develop symptoms between 3 and 14 days after the infected mosquito bites them. Milder WNv illness usually improves by itself and people do not necessarily need to seek medical attention for this infection (though they may choose to do so). If you develop symptoms of severe WNv illness, such as unusually severe headaches or confusion, seek medical attention immediately. Severe WNv illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could indicate WNv infection.

Washington State Department of Health; West Nile virus Home Page:


Centers for Disease Control West Nile Virus Home Page:


Western Equine Encephalitis:

Human WEE cases are usually first seen in June or July. Most WEE infections are asymptomatic or present as mild, nonspecific illness. Patients with clinically apparent illness usually have a sudden onset with fever, headache, nausea, vomiting, anorexia and malaise, followed by altered mental status, weakness and signs of meningeal irritation. Children, especially those under 1 year old, are affected more severely than adults and may be left with permanent sequelae, which is seen in 5 to 30% of young patients. The mortality rate is about 3%.

Clinical Features:

• Symptoms range from mild flu-like illness to frank encephalitis, coma and death.

Etiologic Agent:

• Western equine encephalitis virus, member of the family Togaviridae, genus Alphavirus. Closely related to eastern and Venezuelan equine encephalitis viruses


• 639 confirmed cases in the U.S. since 1964.


• Mild to severe neurologic deficits in survivors.


• Total case costs range from $21,000 for transiently infected individuals to $3 million for severely infected individuals

• Insecticide applications can cost as much as $1.4 million depending on the size of area treated


• Mosquito Borne

Risk Groups:

• Residents of endemic areas and visitors

• Persons with outdoor work and recreational activities


• National Notifiable Diseases Surveillance System


• Epidemic disease that is difficult to predict

• Risk exposure increases as population expands into endemic areas


• No licensed vaccine for human use

• No effective therapeutic drug

• Unknown overwintering cycle

• Control measures expensive

• Limited financial support of surveillance and prevention


SLE is the most common mosquito-transmitted human pathogen in the U.S. While periodic SLE epidemics have occurred only in the Midwest and southeast, SLE virus is distributed throughout the lower 48 states.

Since 1964, there have been 4,437 confirmed cases of SLE with an average of 193 cases per year (range 4 – 1,967). However, less than 1% of SLE viral infections are clinically apparent and the vast majority of infections remain undiagnosed.

Illness ranges in severity from a simple febrile headache to meningoencephalitis, with an overall case-fatality ratio of 5-15 %. The disease is generally milder in children than in adults, but in those children who do have disease, there is a high rate of encephalitis. The elderly are at highest risk for severe disease and death.

Clinical Features:

• Aseptic meningitis or encephalitis.

• The majority of infections are subclinical or result in mild illness

Etiologic Agent:

• St. Louis encephalitis virus – flavivirus related to Japanese encephalitis virus


• Hospitalization for CNS infection – 95% of recognized cases.


• National expenditures for mosquito control activities – $150 million

• SLE surveillance and control activities 0-70% of total; (varies by state)


• Mosquito Borne

• Specific mosquito vectors vary regionally

• Gulf Coast, Ohio and Mississippi Valley: (Culex pipiens, Cx. quinquefasciatus)

• Florida: Cx. nigripalpus

• Western States: Cx. tarsalis

Risk Groups:

• Elderly – biological risk factor

• Low SES areas – environmental risk factor

• Outdoor occupation – exposure risk factor


• Mosquito Control Districts

• Active surveillance in collaboration with state and local health departments


• Largest outbreaks in 15 years occurred in 1990

• Urban transmission in west first recognized in 1987

• Deterioration of inner cities global warming may increase vector abundance and transmission

• Unpredictable and intermittent occurrences of outbreaks

• Multiple environmental, biological and social factors contributing to disease occurrence

• Virus maintenance and overwintering cycle

• Develop more effective disease prevention and treatment

Questions & Answers About St. Louis Encephalitis

Q. How do people get St. Louis encephalitis?

A. By the bite of a mosquito (primarily the Culex species) that become infected with St. Louis encephalitis virus (a flavivirus antigenically related to Japanese encephalitis virus).

Q. What is the basic transmission cycle?

A. Mosquitoes become infected by feeding on birds infected with the St. Louis encephalitis virus. Infected mosquitoes then transmit the St. Louis encephalitis virus to humans and animals during the feeding process. The St. Louis Encephalitis virus grows both in the infected mosquito and the infected bird, but does not make either one sick. See Figure.

Q. Could you get the St. Louis encephalitis from another person?

A. No, St. Louis encephalitis virus is NOT transmitted from person-to-person. For example, you cannot get the virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

Q. Could you get St. Louis encephalitis directly from birds or from insects other than mosquitoes?

A. No. Only infected mosquitoes can transmit St. Louis encephalitis virus.

Q. What are the symptoms of St. Louis encephalitis?

A. Mild infections occur without apparent symptoms other than fever with headache. More severe infection is marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic (but rarely flaccid) paralysis.

Q. What is the incubation period for St. Louis encephalitis?

A. Usually 5 to 15 days.

Q. What is the mortality rate of St. Louis encephalitis?

A. Case-fatality rates range from 3% to 30% (higher in the elderly).

Q. How many cases of St. Louis encephalitis occur in the U.S.?

A. Since 1964 there have been 4,478 reported human cases of St. Louis encephalitis, with an average of 128 cases reported annually.

Q. How is St. Louis encephalitis treated?

A. There is no specific therapy. Intensive supportive therapy is indicated.

Q. Is the disease seasonal in its occurrence?

A. In temperate areas of the United States, St. Louis encephalitis cases occur primarily in the late summer or early fall. In the southern United States where the climate is milder St. Louis encephalitis can occur year round.

Q. Who is at risk for getting St. Louis encephalitis?

A. All residents of areas where active cases have been identified are at risk of getting St. Louis encephalitis.

Q. Where does St. Louis encephalitis occur?

A. See map: St. Louis encephalitis outbreaks can occur throughout most of the United States. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974-1977. During that outbreak, over 2,500 cases in 35 states were reported to the CDC. Currently, outbreaks of St. Louis encephalitis have been limited in size (usually <30 cases), although the potential still exists for epidemic St. Louis encephalitis. The most recent outbreak of St. Louis encephalitis occurred in New Orleans, Louisiana in 1999, with 20 reported cases.

Q. Is there a vaccine against St. Louis encephalitis?

A. No.


Our thanks to the American Heartworm Society, who provided the following
information. Please visit www.heartwormsociety.org for more information

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms. The specific roundworm causing heartworm in dogs and cats is known as Dirofilaria immitis. Until recently canine heartworm disease was considered to be a problem only in warm climates, but in the past few years it has been found in almost all areas of the United States and Canada. Since dogs travel widely with their owners, and infected dogs can carry heartworms for several years, heartworm disease may be a problem anywhere in the nation. Heartworm infection is transmitted by mosquitoes. When a mosquito bites an infected dog, it takes up blood which may contain microfilarae. These incubate in the mosquito for about two weeks, during which they become infective larva. Then, when the mosquito bites another dog, the infective larvae are passed into the second dog, infecting it. The infective larvae migrate through the tissues of the body for 2-3 months. They develop into several stages called L1, L2, and L3 stages. The L1 stage only lasts for 1-2 days. The L2 and L3 stages last for approximately two months. They then enter the heart where they reach adult size approximately 3 months after infecting your pet.

The mosquito is the only natural vector of transmission for canine heartworms, and about 70 species are capable of carrying the disease. As you might expect, heartworm infection is more common in areas where mosquitoes are numerous, and outdoor dogs constantly exposed to mosquitoes are the most frequent victims.

How can I find out if my dog has heart disease?

Your veterinarian is your dog’s healthcare expert. Regular veterinary visits are important for early detection of health problems.

Your veterinarian may ask you for specific information about your dog before performing a thorough physical examination. If indicated, blood and urine tests, X-rays, an EKG or other tests may be ordered. Regular testing is important for early detection of heart disease in dogs.

“Too often, dog owners do not take their dogs to visit the veterinarian until they are displaying severe signs of heart failure, and by then it may be too late,” says Dr. Bicknese. “When heart disease is detected in your dog, your veterinarian can recommend a schedule of regular visits and discuss a treatment plan that can help.”

Can dogs with heart disease be treated?

Yes. Although there is no cure for most heart disease in dogs, new treatments are available. Success of treatment depends on various factors, but early detection is always best. By following your veterinarian’s recommendations, you can help your dog live a longer, more comfortable life.


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