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Vector-Borne Diseases (Lyme disease, Japanese Encephalitis, Yellow Fever)

Centers for Disease Control, Division of Vector-Borne Infectious Diseases, Fort Collins, CO

Publication date: 08/01/1991

Table of Contents

General facts about Lyme disease and its transmission
Symptoms of acute Lyme disease
Symptoms of chronic Lyme disease
Treatment of Lyme disease
Prevention of Lyme disease
Lyme disease in pregnancy and in nursing mothers
STATE health departments

Japanese Encephalitis Description
Japanese Encephalitis vaccine

Yellow Fever Description
Yellow Fever Vaccine Requirements

St. Louis Encephalitis


General facts about Lyme disease and its transmission

Lyme disease is named after the town of Lyme, Connecticut, where it was discovered in 1977. The disease is a bacterial infection caused by the bites of certain, very small, infected ticks. The two most important ticks are: the deer tick in the northeast and in the north central states; and the black legged tick in the West. These ticks are much smaller than the common dog or cattle tick. The disease most commonly occurs during the summer months when ticks are most active; however, the symptoms of chronic, untreated Lyme disease can occur at any time of the year. Although the deer ticks which carry the disease are slowly expanding into new areas, there are three areas where risk of getting the disease is much greater than elsewhere:
  • the northeast states of Massachusetts, Connecticut, Rhode Island, New York, New Jersey, and Pennsylvania;
  • the north central states of Wisconsin and Minnesota; and finally
  • the Pacific Coast states of California and Oregon.
Lyme disease and the ticks that carry it are rare or non-existent in the Rocky Mountain States, Hawaii, and Alaska.

In nature, the Lyme disease bacteria exist in a cycle involving ticks and small animals, most specifically the wild white-footed mouse. Once this mouse becomes infected, it remains infected for long periods of time without any apparent ill effects and can spread infection to the many hundreds of immature deer ticks that feed upon it. These infected ticks spread infection to other mice and animals as well as to humans. The adult stage of the tick prefers to feed on larger animals -- especially deer. Deer, however, are resistant to Lyme infection and do not directly participate in the life cycle of the Lyme bacteria, except to provide blood meals for adult ticks and to carry ticks into areas where they did not exist before. Birds may also transport these ticks for great distances and be a factor in the spread of the disease.

Lyme disease is not transmitted from person to person. Only ticks have been shown to be of any importance in Lyme disease transmission to humans.

Ticks which transmit Lyme disease do bite and can infect both dogs and cats. Dogs may develop fever, lameness, swollen joints, or other symptoms similar to those seen in man. Symptoms of illness in cats may be less obvious. Although pets do not directly transmit Lyme disease to man, the presence of infected ticks on the pet may pose a hazard to both the pet and owner. Chemical repellents or insecticides may be useful. A veterinarian should be consulted regarding the most effective and safe product for your pet since some combinations of ingredients may be toxic. Pets should be checked daily for ticks and attached ticks should be promptly removed using tweezers.

Symptoms of acute Lyme disease

Acute Lyme disease, except for the peculiar skin rash it produces in 60 to 80% of the patients in which it occurs, is a summer "flu-like" illness without a cough. About half of the persons who are ill will have a slight fever for a few days. Many people complain of general body pain, of pain in a few joints, and headaches. A few people will have swollen lymph glands. Most persons treated with the appropriate antibiotics at this stage will have a quick recovery and will be completely cured.

The most characteristic symptom of early Lyme disease is the skin rash which occurs at the site of the tick bite from 5 to 40 or more days after the bite. A rash which occurs immediately after a bite is due to an allergic reaction and is not Lyme disease. The Lyme disease rash is flat, circular and is, or will become, at least 2 inches in diameter. The central portion of the rash is usually pale while the outer margins are reddened, giving it what has been described as a bull's eye appearance. This single rash in untreated patients can become quite large and/or may be accompanied by one or more scattered rashes on other areas of the skin. These rashes clear quickly with antibiotic therapy.

Patients who do not receive treatment can become completely well, with a possibility of recurrent problems later, or they can develop further problems involving the heart, joints or nervous system as the disease progresses. The most frequent heart problem is a relatively mild disturbance of heart rhythm. In many patients, this rhythm disturbance clears even without treatment; with treatment it almost always clears very quickly with no apparent long-term damage. In very rare cases, there are serious heart muscle infections which may lead to heart failure.

Infection of the nervous system can progress to meningitis, paralysis of facial nerves, encephalitis, or painful disorders and weakness in the limbs caused by nerve inflammation. When these conditions occur relatively soon after the initial infection, they generally respond well to intravenous antibiotics. Patients with untreated arthritis may develop more severe involvement of one or more joints, but most often, a single knee joint. Fluid often collects in the affected joints and causes swelling. These early stages of arthritis respond well to intravenous antibiotics in most instances.

Lyme disease can be diagnosed in the laboratory by isolating the bacteria or by detecting antibodies against the bacteria in blood. The first method, isolation of the bacteria, is difficult, and is frequently only done in research situations. Detecting antibodies in blood is the most common test performed. The detection of antibodies in blood is reliable in chronic cases of Lyme disease, but is frequently not positive in the early stages of illness. Research to remedy this problem is underway.

Symptoms of chronic Lyme disease

Chronic Lyme disease involves abnormalities in either the skin, the joints, or the nervous system. Abnormalities in the skin are rare, but include localized swelling especially in the ear lobe, arm pit, and nipple areas, and thinning of the skin on hands and feet.

Chronic arthritis is the most widely recognized result of untreated Lyme disease. It most often affects one or a few large joints and usually the knee. It is considered chronic because it recurs in episodes lasting for as long as 6 months. Arthritis persisting beyond 6 months is seldom due to Lyme disease. Unlike most other forms of arthritis, chronic Lyme arthritis does not usually attack the same joint on both sides of the body at once, and does not affect many joints at once. When Lyme disease does affect a joint, it usually causes marked swelling with redness and accumulation of fluid in the joint. Lyme arthritis usually responds to antibiotic treatment; however, if severe joint damage has occurred, complete recovery may not occur or may take a long period of time.

Chronic Lyme disease infection of the nervous system most often produces pain in arms or legs, along with weakness and/or numbness in the affected limbs. These problems are caused by Lyme disease infection of the spinal cord. With infection of the brain, a number of other problems can occur. These include headaches, severe fatigue, impaired vision, double vision, hearing impairment, facial paralysis, and difficulties with memory and thinking.

It is important for you to know that all these symptoms including those of the skin, joints and nervous system can be caused by many things other than Lyme disease. A number of special but readily available tests are required to determine the cause of these symptoms in any individual. If you are concerned about this, you should consult your physician for advice and treatment.

Lyme disease can be diagnosed in the laboratory by isolating the bacteria or by detecting antibodies against the bacteria in blood. The first method, isolation of the bacteria, is difficult, and is frequently only done in research situations. Detecting antibodies in blood is the most common test performed. The detection of antibodies in blood is reliable in chronic cases of Lyme disease, but is frequently not positive in the early stages of illness. Research to remedy this problem is underway.

If you and your physician are concerned about the accuracy of your Lyme disease test results, your state health department laboratory can act as a reference laboratory to assist you.

Treatment of Lyme disease

If your symptoms are diagnosed as due to Lyme disease, your physician will prescribe an antibiotic for treatment. The antibiotic he chooses for your treatment will usually be in one of four general groups; a penicillin, a cephalosporin, a tetracycline or an erthyromycin. The antibiotic chosen will depend on a number of factors such as allergic history, age, pregnancy, and stage of disease. Antibiotics may be given orally, intravenously or possibly intramuscularly as determined by your physician.

The response of patients to treatment for chronic Lyme disease is variable.

If a person had untreated Lyme disease for many years, damage to the nervous system or joints may require a prolonged period for repair after the infection has been eradicated. In some instances, treatment seems to have little or no beneficial effect.

Prevention of Lyme disease

The only certain way to prevent Lyme disease is to avoid all situations of exposure to infected ticks. For many people, this is not possible or is not acceptable. In these situations, there are a number of steps you can take which will greatly reduce your chances of acquiring Lyme disease.
  1. Wear long pants. Tuck pants legs into long socks or seal pants legs with masking tape or rubber bands.
  2. Spray a permethrin-containing tick repellent on clothes if it is available in your state.
  3. Use a repellent containing the compound DEET on your skin areas that are exposed except for the face area. Follow label directions carefully, and be especially cautious when using DEET on children.
  4. Check your entire body carefully for ticks twice a day, including inspection of the neck and scalp. If you are alone, the use of a fine tooth comb will help locate adult ticks in your scalp.
  5. Remove attached ticks from your skin immediately with tweezers by grasping the tick's head parts as close to your skin as possible. Do not attempt to get ticks out of your skin by burning them or coating them with anything such as nail polish remover or petroleum jelly. If you remove a tick before it has been attached for more than 24 hours, you greatly reduce your risk of infection.
  6. There is presently no human vaccine available for protection against Lyme disease.

Lyme disease in pregnancy and in nursing mothers

There is currently very limited information about Lyme disease infection during pregnancy. Presently there is no conclusive evidence that Lyme disease produces an increase in spontaneous abortions, stillbirths, or fetal abnormalities. However, there have been several reports of the Lyme bacteria being found in stillborns, and in infants born with severe abnormalities. Therefore, pregnant women should be promptly treated if suspected of having been infected.

There is also no direct evidence to date that nursing mothers infected with Lyme disease transmit infection through their milk. However, if a nursing mother is suspected of being infected, she should stop nursing her infant until she has had a complete course of antibiotic therapy. The infant should be observed for signs of infection and its blood tested for evidence of infection if illness develops.

STATE health departments

For additional detailed information, please contact your state health department. State health departments are familiar with diseases in their area and can give more specific advice and recommendations.

Alabama             205-261-5052
Alaska              907-465-3090
Arizona             602-255-1024
Arkansas            501-661-2111
California          916-445-1248
Colorado            303-520-8333
Connecticut         203-566-2279
D.C.                202-673-7700
Delaware            302-736-4701
Florida             904-487-2705
Georgia             404-894-7505
Hawaii              808-548-6505
Idaho               208-334-5930
Illinois            217-782-2016
Indiana             317-633-8400
Iowa                515-281-5605
Kansas              913-296-1500
Kentucky            502-564-3970
Louisiana           504-568-5052
Maine               207-289-3201
Maryland            301-225-6500
Massachusetts       617-727-2700
Michigan            517-335-8024
Minnesota           612-623-5100
Mississippi         601-960-7634
Missouri            314-751-6400
Montana             406-444-2544
Nebraska            402-471-2133
Nevada              702-885-4740
New Hampshire       603-271-4477
New Jersey          609-292-7837
New Mexico          505-827-2615
New York            518-474-2011
North Carolina      919-733-3446
North Dakota        701-224-2372
Ohio                614-466-2253
Oklahoma            405-271-4200
Oregon              503-229-5032
Pennsylvania        717-787-6436
Rhode Island        401-277-2231
South Carolina      803-734-1880
South Dakota        605-773-3361
Tennessee           615-741-3111
Texas               512-458-7375
Utah                801-538-6111
Vermont             802-863-7200
Virginia            804-786-3561
Washington          206-753-5871
West Virginia       304-348-2971
Wisconsin           608-266-1511
Wyoming             307-777-7121

Japanese Encephalitis Description

Japanese encephalitis is a mosquito-borne viral disease that occurs chiefly in three areas: (1) China and Korea, (2) the Indian sub-continent consisting of India, parts of Bangladesh, southern Nepal, and Sri Lanka, and then (3) the southeast Asian countries of Burma, Thailand, Cambodia, Laos, Vietnam, Malaysia, Indonesia and the Philippines. Japanese encephalitis also may occur with a lower frequency in Japan, Taiwan, Singapore, Hong Kong, and eastern Russia. In all areas, Japanese encephalitis is primarily a rural disease.

Transmission is usually seasonal, following the prevalence of mosquitos. In China, Korea and other temperate areas, the transmission season extends through the summer and fall. In other subtropical and tropical regions, risk is associated with the rainy season, which varies in each country. For instance recent epidemics have occurred in northern India, Nepal, and Sri Lanka, from October to December. However, in tropical areas sporadic cases may occur at any time of the year.

The chance that a traveller to Asia will develop Japanese encephalitis is probably very small. Only 5 cases among Americans travelling or working in Asia are known to have occurred since 1981. Only certain mosquito species, are capable of transmitting Japanese encephalitis. In areas infested with mosquitoes, usually only a small portion of the mosquitoes are actually infected with Japanese encephalitis virus. Among persons who are bitten by an infected mosquito, only 1 in 50 persons will develop an illness. The majority of infected persons develop mild symptoms or no symptoms at all. However, among persons who develop encephalitis, the consequences of the illness may be grave.

Japanese encephalitis begins clinically as a flu-like illness with headache, fever, and often gastrointestinal symptoms. Confusion and disturbances in behavior also may occur at an early stage. The illness may progress to a serious infection of the brain, i.e., encephalitis, and in one third of cases, the illness may be fatal. Another one third of cases survive with serious neurologic aftereffects such as paralysis or other forms of brain damage, and the remaining one third of cases recover without further problems. After the onset of the infection, and until the illness has run its course, only supportive treatment is available.

Japanese Encephalitis vaccine

The vaccine for Japanese encephalitis is no longer available in the United States except to military personnel on official duty. The vaccine is available in many Asian countries including Japan, Korea, Taiwan, Hong Kong, Thailand, Nepal, India, Sri Lanka, and Singapore, and in Canada, Australia, and many European countries. Travellers are advised to contact the local U.S. Embassy or consulate for a list of reputable clinics that may have the vaccine. In China, the vaccine may be obtained at the U.S. Embassy and consulates in major cities, but only by citizens who will live in China for an extended period.

Travellers to Asia may seek the vaccine abroad, but CDC recommends the vaccine only to those who will work or have extensive visits during the transmission season to rural areas of the previously mentioned countries. Risk of acquiring Japanese encephalitis is proportional to exposure to the mosquitos that breed chiefly in rural rice-growing and pig farming regions. Therefore, risk is low among persons whose itineraries are limited to cities or who will travel to the countryside for short periods. Arbitrarily, many authorities have recommended the vaccine for persons who will travel in rural areas for three weeks or more.

The vaccine is given in 3 doses on days, 0, 7 and 14 and protection can be expected 10 days following the last dose. No serious permanent side effects are known to be associated with the vaccine. Fever and local reactions such as redness, swelling and pain are reported in fewer than 10% of those vaccinated.

Because of the potential for other mosquito-borne diseases in Asia, all travellers, but particularly those who are unable to obtain Japanese encephalitis vaccine, are advised to use precautions to avoid mosquito bites. The mosquitoes which transmit Japanese encephalitis feed chiefly outdoors from dusk to dawn. Travellers are advised to minimize outdoor exposure at these times, to wear mosquito repellents containing DEET as an active ingredient, and to stay in air-conditioned or well-screened rooms. Travellers should bring a bednet, which can be obtained at army-navy surplus stores, and aerosol room insecticides to kill indoor mosquitoes. Repellents containing DEET should be used with care on children, because of the potential for neurological side effects.


There are some contraindications to the vaccine. The manufacturer recommends that the vaccine not be administered to the following persons:

  • Those acutely ill or with active infections.
  • Persons with heart, kidney, or liver disorders.
  • Persons with generalized cancerous malignancies such as leukemia, lymphoma.
  • Persons with a history of hypersensitivities.
  • Pregnant women.

Yellow Fever Description

Yellow fever occurs only in Africa and South America. In South America sporadic infections occur almost exclusively in forestry and agricultural workers who are exposed occupationally in or near forests. In Africa the virus is transmitted in three geographic regions:
  • principally and foremost in the moist savanna zones of West and Central Africa during the rainy season,
  • secondly, at intervals, massive outbreaks resulting in thousands of cases may develop in urban locations and villages in the dry savanna of Africa.
  • and finally, to a lesser extent, in jungle regions.
The CDC maintains a 24-hour-a-day International Travelers Hotline where doctors or travelers can receive vaccine requirements based on the countries to be visited.

Yellow Fever Vaccine Requirements

Yellow fever vaccine is one of two vaccines that may be required for entry into certain countries in Africa and South America. (The other vaccine is cholera vaccine.) After immunization an International Certificate of Vaccination is issued and will meet entry requirements for all persons traveling to or arriving from countries where there is active or a potential for yellow fever transmission. The Certificate is good for 10 years. Most countries will accept a medical waiver for persons with a medical contraindication to vaccination (e.g., infants less than 4 months old, pregnant women, persons hypersensitive to eggs, or those with an immunosuppressed condition. CDC recommends obtaining written waivers from consular or embassy officials before departure.

The CDC maintains a 24-hour-a-day International Travelers Hotline where doctors or travelers can receive vaccine requirements based on their travel itineraries. Alternately, persons can contact state or local health departments for the most recent recommendations.

Vaccine is obtained from Yellow Fever Vaccine Centers designated by your state health department.

Yellow fever vaccine is a live attenuated viral vaccine. A single dose confers long-lived immunity lasting 10 years or more. Administration of immune globulin does not interfere with the antibody response to yellow fever vaccine. For some individuals, there are precautions and contraindications associated with this vaccine, and you are encouraged to listen to that section of these messages.


The vaccine generally is associated with few side effects: fewer than 5% of vaccinees develop mild headache, muscle pain, or other minor symptoms 5 to 10 days after vaccination.

However, three groups of individuals should not receive the vaccine, and a fourth group should be closely evaluated. The three groups contraindicated for the vaccine are:

  1. Yellow fever vaccine should never be given to infants under 4 months of age due to a risk of developing viral encephalitis. In most cases, vaccination should be deferred until 9 to 12 months of age.
  2. Pregnant women should not be vaccinated because of a theoretical risk that the developing fetus may become infected from the vaccine.
  3. Persons hypersensitive to eggs should not receive the vaccine because it is prepared in embryonated eggs. If vaccination of a traveler with a questionable history of egg hypersensitivity is considered essential, an intradermal test dose may be administered under close medical supervision.

A fourth group should be closely evaluated before administering the vaccine:

  1. Persons with an immunosuppressed condition associated with AIDS or HIV infection, or those with their immune system altered by either diseases such as leukemia and lymphoma or through drugs and radiation should not receive the vaccine. People with asymptomatic HIV infection may be vaccinated if exposure to yellow fever cannot be avoided.

In all cases, the decision to immunize an infant between 5 and 9 months of age, a pregnant woman, or an immunocompromised patient should be made on an individual basis. The physician should weigh the risks of exposure and contracting the disease, against the risks of immunization, and possibly consider alternative means of protection.

Most countries will accept a medical waiver for persons with a medical contraindication to vaccination. CDC recommends obtaining written waivers from consular or embassy officials before departure. Travelers should contact the embassy or consulate for specific advice. Typically, a physician's letter stating the contraindication for vaccination and written on letterhead stationery is required by the embassy or consulate.

St. Louis Encephalitis

The following message gives information on St. Louis encephalitis and its recent outbreak in Central Florida, and Harris County, Texas, the Houston area.

St. Louis encephalitis is caused by a virus spread by certain mosquitoes. Of all mosquitoes, only a small group called vector mosquitoes can transmit St. Louis encephalitis, and even during an outbreak fewer than 1% of these vector mosquitoes are infected with the virus. Most people who are infected by the virus develop no more than a flu-like illness and a headache. About 1 in 100 infected people develop a more severe illness with an aseptic meningitis or encephalitis, which is an inflammation of the brain. Of those severe cases about 7% are fatal. The disease is much more severe in the elderly, who are more likely to develop encephalitis, and to have a fatal case.

There are no drugs and no vaccines that can prevent or treat the disease. The best form of prevention is to avoid outdoor activity at night, especially at dusk and dawn when the mosquitoes are most active, and to repair screens on houses to prevent entry of mosquitoes indoors. While outdoors, wear clothing that completely covers the skin, and use mosquito repellents on clothing and any exposed skin. Repellents containing DEET are the most effective; however, they should always be used according to label directions. Use DEET sparingly on children, because in rare cases, DEET has caused seizures in children.

In central Florida, cases of St. Louis encephalitis have declined but continue to be reported; however, the risk of exposure has greatly diminished for two reasons. (1) The continued monitoring of the mosquito population has shown a significant decline in the number of infected mosquitoes and (2) the cooler and dryer weather has decreased mosquito activity.

Since July, more than 190 St. Louis encephalitis cases have been documented in central Florida. Twenty-seven counties have reported cases with Indian River, Orange, and Palm Beach counties accounting for over 20 cases each. The rest of the cases have been spread throughout the central Florida peninsula. These statistics change from week to week and for current exact numbers, please call the Florida State Health Department at 904-488-4854.

The Houston area outbreak of St. Louis encephalitis appears to have concluded, with the last reported case occurring in mid-October. For additional information on the occurrences of St. Louis encephalitis in the Houston area, please call the Houston City Health Department at 713-794-9181. Travelers and residents in central Florida should wear clothing that completely covers the skin, and use insect repellents as described earlier.


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This page last reviewed: Wednesday, January 27, 2016
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