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Mosquito-borne Illness

Yellow FeverJapanese Encephalitis | Malaria | Dengue

Yellow Fever

Yellow fever is a mosquito-borne viral disease that occurs in parts of Africa and South America. Illness varies in severity from a flu-like illness to severe hepatitis and hemorrhagic fever. The yellow fever immunization has almost total efficacy, while the mortality of yellow fever is more than 60% in nonimmune adults. International regulations require proof of vaccination for travel to and from certain countries. Some countries require an individual, even if only in transit, have a valid International Certificate of Vaccination (ICV) with proof of yellow fever vaccination if they have been in countries either known or thought to harbor the yellow fever virus.

The yellow fever vaccine is manufactured using a live-attenuated virus produced in chick embryos by manufacturers approved by the WHO and administered at an approved yellow fever Vaccination Center. Vaccines should receive an ICV that is completed, signed, and validated with the stamp of the center where the vaccine is administered. The vaccine is well tolerated, with more severe side effects occurring in persons with a history of egg allergy, the very young and the very old. Vaccine-associated encephalitis (brain infection) occurs rarely, more often in persons < 4 months old. This vaccine should be avoided in travelers less than 9 months old, pregnant women, and immunocompromised people. Exemption from the requirements may be made on the basis of age or medical contraindication: some countries exempt infants younger than 6 months or 1 year of age. If immunization is medically contraindicated, it should be noted in the space provided on the ICV, and the traveler should carry a letter of immunization exemption.

There have been recent case reports of severe side effects, including (rarely) death, in recipients of the yellow fever vaccine. A recent (2001) review examining this issue determined that the risk of severe reaction appears to increase with the elderly, although remains an unusual event. With this in mind, our Travel Center doctors study the travelers' individual itinerary, with special attention to Yellow Fever risk areas, together with WHO guidelines and recommend this vaccine only for those in whom it is clearly required or indicated. It is important to remember that the tiny risk of the vaccine may be outweighed by the risk of acquiring a disease that may be fatal.

Japanese Encephalitis

Encephalitis caused by the Japanese encephalitis (JE) virus is the most common form of epidemic viral encephalitis in the world and is the leading cause of viral encephalitis in Asia. In areas where JE is endemic, annual incidence ranges from 1 to 10 per 10,000. It is transmitted by the Culex genus mosquitoes, which breeds in rice fields. JE is largely transmitted in rural areas, with domestic pigs the most important amplifying hosts. Although most infections are subclinical, one of approximately 200 infections results in neuroinvassive disease. After an incubation period of 7 to 14 days, the clinical illness begins with a rapid onset of fever, chills, malaise, headache, nausea and vomiting followed by signs and symptoms of generalized central nervous system infection including a stiff neck, photophobia, delirium, convulsions, tremors and localized paralysis.

From 1981 to 1993 only 11 U.S. residents were infected with JE virus; eight of these were military personnel or their dependents. Risks for acquiring JE include travel during the transmission season and exposure in rural areas, especially for extended periods of time, and are related to the extent and nature of outdoor activity, and use of insect protective measures. The overall risk of acquiring JE among U.S. travelers is extremely low, although travelers to rural areas where JE is endemic, during the transmission season, have a risk of being infected of 1 per 5,000 per week.

The approved JE vaccine in the U.S. (BIKEN) contains an inactivated virus derived from infected mouse brain. This is generally well tolerated and provides immunity in ³ 88% of persons who receive the primary immunization series of 3 doses over a 30 day period. Because of the possibility of vaccine side effects that may include generalized urticaria (rash) and angioedema (head/neck swelling) that could be life-threatening, persons receiving this immunization should be observed in the medical office for 30 minutes. An important feature of the vaccine reactions has been the interval between vaccination and onset of symptoms. Reactions have occurred hours to 3 days following the first vaccination, and up to 14 days after the second vaccination. Vaccines should defer international travel and remain in areas with ready access to medical care for 10 days after receiving the last dose of this, and be counseled to seek immediate medical care at the onset of any reaction. There is an increased risk of side effects in people with a history of urticaria.

JE vaccination should be considered for travelers who plan to reside in or travel to areas where JE is endemic or epidemic during transmission season, in persons spending a month or longer in epidemic areas during the transmission season, especially if travel will include rural areas. JE vaccine is NOT recommended for all persons traveling to or residing in Asia. In all instances travelers to endemic areas should be advised to take personal precautions to avoid exposure to mosquito bites.


Malaria is a mosquito-transmitted parasitic disease caused by four Plasmodia species that infects approximately 270 million people worldwide and is responsible for 1-2.5 million deaths per year. Approximately 1000 cases of malaria are imported into the U.S. annually by American travelers, which include 5-7 fatalities. At our institution we see an average of five imported malaria cases per year. Effective malaria chemoprophylaxis (preventative therapy) is recommended with either chloroquine, mefloquine, atovaquone/proguanil (Malarone), or doxycycline depending upon the local resistance patterns. Travelers to East Africa have roughly a 1% chance per month of developing severe malaria if they do not take chemoprophylaxis, which is fatal 2% of the time. All of the antimalarial prophylactic medications have potential side effects.


Dengue fever (DF) (dengue hemorrhagic fever,DHF, in the severe form) is a viral infection caused by the dengue fever virus. This illness is transmitted by the bite of mosquitoes of the Aedes genus, and is becoming a serious problem in many tropical areas, especially Central and South America. In 1994 there were 37 confirmed imported cases in the U.S., which was twice the normal rate; in some Travel Centers it has become the second most frequent cause of hospitalization of returning travelers. More recently (2001) there have been numerous cases of DF acquired in Maui, Hawaii. There is no treatment or preventative medication for this disease. The incubation period is brief (10 days) and the disease is usually self-limited (DF). A more severe form of illness, DHF, is less common, and may be related to past episodes of DF.

The risk of acquiring insect-borne diseases can be minimized by avoiding insects with the use of an insect repellent such as DEET (N,N-diethyl-m-toluamide), protective clothing, and insecticide (permethrin) impregnated clothing / bed nets. DEET is primarily effective against ticks and mosquitoes and has been approved for use in the U.S. since 1954. In the U.S. there are numerous brand name repellents that are available, with DEET concentrations ranging from 6.5% to 100%. All DEET-containing repellents should be labeled with their DEET content. When DEET as a 33% cream is used alone it is 89% effective for up to 14 hours in keeping away flying insects. DEET has been associated with neurotoxicity, especially in children, so minimizing prolonged exposures, and higher concentration formulations are advised. Permethrin is a non-toxic insecticide that is applied only to fabric, such as clothing or mosquito nets. It is poorly absorbed through the skin and kills insects that come into direct contact with it. Studies using the combination of 33% DEET cream plus permethrin-impregnated clothing demonstrated > 99% protection against mosquito bites.