Arthropod-borne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes, ticks, sand flies, or midges. Other modes of transmission for some arboviruses include blood transfusion, organ transplantation, perinatal transmission, breast feeding, and laboratory exposures.

More than 130 arboviruses are known to cause human disease. Most arboviruses of public health importance belong to one of three virus genera: Flavivirus, Alphavirus, and Orthobunyavirus.

Several types of insect-borne viruses can cause encephalitis. Collectively, these viruses are referred to as arboviruses (because they are arthropod-borne), and the diseases they cause are described as arboviral encephalitis. Most arboviruses are endemic to specific geographical regions. Arborviral encephalitis diseases found in the United States include eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis, and West Nile encephalitis (WNE). Expansion of arboviruses beyond their endemic regions sometimes occurs, generally as a result of environmental changes that are favorable to the virus or its vector. Increased travel of infected humans, animals, or vectors has also allowed arboviruses to spread into new regions.

In most cases, arboviral infections are asymptomatic or lead to a mild disease. However, when symptoms do occur, they include high fever, chills, headaches, vomiting, diarrhea, and restlessness. In elderly patients, severe arboviral encephalitis can rapidly lead to convulsions, coma, and death.

Mosquitoes are the most common biological vectors for arboviruses, which tend to be enveloped ssRNA viruses. Thus, prevention of arboviral infections is best achieved by avoiding mosquitoes—using insect repellent, wearing long pants and sleeves, sleeping in well-screened rooms, using bed nets, etc.

Diagnosis of arboviral encephalitis is based on clinical symptoms and serologic testing of serum or CSF. There are no antiviral drugs to treat any of these arboviral diseases, so treatment consists of supportive care and management of symptoms.

Eastern equine encephalitis (EEE) is caused by eastern equine encephalitis virus (EEEV), which can cause severe disease in horses and humans. Birds are reservoirs for EEEV with accidental transmission to horses and humans by Aedes, Coquillettidia, and Culex species of mosquitoes. Neither horses nor humans serve as reservoirs. EEE is most common in US Gulf Coast and Atlantic states. EEE is one of the more severe mosquito-transmitted diseases in the United States, but fortunately, it is a very rare disease in the United States.

Western equine encephalitis (WEE) is caused by western equine encephalitis virus (WEEV). WEEV is usually transmitted to horses and humans by the Culex tarsalis mosquitoes and, in the past decade, has caused very few cases of encephalitis in humans in the United States. In humans, WEE symptoms are less severe than EEE and include fever, chills, and vomiting, with a mortality rate of 3–4%. Like EEEV, birds are the natural reservoir for WEEV. Periodically, for indeterminate reasons, epidemics in human cases have occurred in North America in the past. The largest on record was in 1941, with more than 3400 cases.

St. Louis encephalitis (SLE), caused by St. Louis encephalitis virus (SLEV), is a rare form of encephalitis with symptoms occurring in fewer than 1% of infected patients. The natural reservoirs for SLEV are birds. SLEV is most often found in the Ohio-Mississippi River basin of the central United States and was named after a severe outbreak in Missouri in 1934. The worst outbreak of St. Louis encephalitis occurred in 1975, with over 2000 cases reported. Humans become infected when bitten by C. tarsalis, C. quinquefasciatus, or C. pipiens mosquitoes carrying SLEV. Most patients are asymptomatic, but in a small number of individuals, symptoms range from mild flu-like syndromes to fatal encephalitis. The overall mortality rate for symptomatic patients is 5–15%.

Japanese encephalitis, caused by Japanese encephalitis virus (JEV), is the leading cause of vaccine-preventable encephalitis in humans and is endemic to some of the most populous countries in the world, including China, India, Japan, and all of Southeast Asia. JEV is transmitted to humans by Culex mosquitoes, usually the species C. tritaeniorhynchus. The biological reservoirs for JEV include pigs and wading birds. Most patients with JEV infections are asymptomatic, with symptoms occurring in fewer than 1% of infected individuals. However, about 25% of those who do develop encephalitis die, and among those who recover, 30–50% have psychiatric, neurologic, or cognitive impairment. Fortunately, there is an effective vaccine that can prevent infection with JEV. The CDC recommends this vaccine for travelers who expect to spend more than one month in endemic areas.

As the name suggests, West Nile virus (WNV) and its associated disease, West Nile encephalitis (WNE), did not originate in North America. Until 1999, it was endemic in the Middle East, Africa, and Asia; however, the first US cases were identified in New York in 1999, and by 2004, the virus had spread across the entire continental United States. Over 35,000 cases, including 1400 deaths, were confirmed in the five-year period between 1999 and 2004. WNV infection remains reportable to the CDC.

WNV is transmitted to humans by Culex mosquitoes from its natural reservoir, infected birds, with 70–80% of infected patients experiencing no symptoms. Most symptomatic cases involve only mild, flu-like symptoms, but fewer than 1% of infected people develop severe and sometimes fatal encephalitis or meningitis. The mortality rate in WNV patients who develop neurological disease is about 10%.

Most arboviral infections are asymptomatic. Clinical disease ranges from mild febrile illness to severe encephalitis. For the purpose of surveillance and reporting, based on their clinical presentation, arboviral disease cases are often categorized into two primary groups: neuroinvasive disease and nonneuroinvasive disease.

Neuroinvasive disease
Many arboviruses cause neuroinvasive disease such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP). These illnesses are usually characterized by the acute onset of fever with headache, myalgia, stiff neck, altered mental status, seizures, limb weakness, or cerebrospinal fluid (CSF) pleocytosis. AFP may result from anterior ("polio") myelitis, peripheral neuritis, or post-infectious peripheral demyelinating neuropathy (i.e., Guillain-Barre’ syndrome). Less common neurological manifestations, such as cranial nerve palsies, also occur.

Non-neuroinvasive disease
Most arboviruses are capable of causing an acute systemic febrile illness (e.g., West Nile fever) that may include headache, myalgias, arthralgia, rash, or gastrointestinal symptoms. Some viruses also can cause more characteristic clinical manifestations, such as severe polyarthralgia or arthritis due to Chikungunya virus or other alphaviruses (e.g., Mayaro, Ross River, O’nyong-nyong).

The Basics

Spending time together outdoors is good for the whole family. Don’t let mosquito and tick bites ruin your fun.

Most bug bites are harmless, but some mosquitoes and ticks spread germs that can make you sick. Some of these germs are very serious – and may even be deadly.

• Mosquitoes spread viruses like Zika, West Nile, chikungunya, Eastern equine encephalitis (EEE), and dengue.
• Lyme disease and Rocky Mountain spotted fever are just 2 of the serious diseases you can get from tick bites.

Protect yourself from mosquito and tick bites.

When you spend time outside, take these steps to protect yourself:

• Use insect repellent (also called bug spray) on your skin and clothing.
• Wear long-sleeved shirts, long pants, and socks to cover your skin.
• Check everyone for ticks after spending time outside.
• Take a shower within 2 hours after being outside to help wash away ticks.
• Put dry clothes in the dryer on high heat for 10 minutes to kill ticks.

Protect your pets, too.

Ticks also carry germs that can make pets sick. And when pets bring ticks into the house, those ticks can bite you, too.

• Check your pets for ticks every day, especially after they've been outdoors. If you find a tick, remove it right away.
• Talk with your veterinarian about the best way to protect your pets from ticks. Ask your veterinarian before using any tick prevention products on your pets.

Take Action!

Keep mosquitoes out of and away from your home.

Use screens on windows and doors to keep mosquitoes outside. Repair any holes in the screens, and don’t leave doors propped open.

Mosquitoes lay eggs in or near standing water. To help keep them away from your home:

• Once a week, check items in or near your home that can hold water ­– like planters, wading pools, or trash cans. Empty and scrub, turn over, or cover the items.
• Get rid of things you’re not using that can collect water, like tires or old toys.
• Tightly cover water storage containers, like rain barrels.
• Add chemicals that kill mosquito eggs (called larvicides) to water that won’t be used for drinking and can’t be covered or dumped out, like a pond or fountain.

Keep ticks away from your home.

Many types of ticks live in areas with woods, bushes, or high grass – so if you have a yard, it’s important to keep it clear. Animals like mice, dogs, and deer may also carry ticks in their fur.

To help keep ticks away from your home:

• Clear bushes, tall grasses, and fallen leaves from around your home.
• Mow the lawn often.
• Use wood chips or gravel to separate your patio or play equipment from wooded or brushy areas.
• Remove plants that attract deer and put up a fence to keep deer out of your yard.
• Consider applying tick control products to your yard. You can do this yourself or hire a pest control company.

Use insect repellent.

Insect repellent makes it harder for mosquitoes and ticks to find you.

What type of repellent do I need?

• Use an insect repellent that’s registered by the Environmental Protection Agency (EPA).
• To prevent mosquito and tick bites, use an EPA-registered insect repellent with one of these ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone.
• Use a spray with .5% permethrin on your clothes, shoes, and camping gear to repel and kill ticks and mosquitoes. Never use permethrin directly on your skin.

How do I use insect repellent?

• Always follow the instructions on the label.
• Apply it to your exposed skin before you go outside.
• Apply sunscreen first, then put on insect repellent.
• Don’t use insect repellent that has sunscreen already mixed in.
• Don’t spray insect repellent directly on your face. Instead, spray it in your hands and carefully rub it on your face.
• Don't put insect repellent on your eyes or mouth, or on skin that's cut or irritated. Don't put repellent on children's hands, so they won't rub it in their eyes or mouth.
• Don’t use insect repellent on babies under 2 months old. Instead, cover their stroller or car seat with mosquito netting.
• Don’t use OLE or PMD on children under 3 years old.

When you go back inside, be sure to wash insect repellent off skin with soap and water.

Wear long-sleeved shirts, long pants, and socks.

Cover up your skin to prevent mosquito bites.

Take a shower after being outside in an area that might have ticks.

A shower can help get ticks off you and lower your risk of Lyme and other diseases ticks can spread. Try to shower within 2 hours of going back inside.

Check for ticks after spending time outside – even in your yard.

Check everybody in the family, including pets. Check the entire body, especially:

• Under the arms
• In and around the ears
• Behind the knees and in the groin (crotch)
• Around the waist and inside the belly button
• In and around hair

Use tweezers to remove a tick as soon as you see it.

• Grab the tick near its head or mouth (the part closest to your skin).
• Gently pull the whole tick straight out. Be careful not to crush or twist the tick.
• Clean your hands and the tick bite with rubbing alcohol or soap and water.
• Never try to use other things, like nail polish or a hot match, to kill and remove a tick.

Get rid of the tick safely.

Get rid of the tick by putting it in rubbing alcohol or flushing it down the toilet. You can also put a tick in the trash, but only if it’s in a sealed container or wrapped tightly in tape.

Remember, never crush a tick with your fingers.

Tell the doctor if you get sick after a tick bite.

If you or your child gets a rash or fever after getting bitten by a tick, call your doctor. Tell the doctor about the tick bite, when it happened, and where you think you were when you got the bite.

Protect yourself from mosquito bites when you travel.

• Before your trip, find out if the country you are visiting has any travel health notices.
• Stay in places that have air conditioning or window and door screens to keep mosquitoes outside.
• If you are pregnant and traveling to an area with risk of Zika, take steps to protect yourself from the Zika virus.

Because vector control programs vary in coverage and effectiveness, travel health practitioners should advise travelers to use repellents and other general protective measures against biting arthropods. Although prophylactic drugs are available to protect against malaria, the effectiveness is variable depending on patterns of drug resistance, bioavailability, and compliance with medication. And while vaccines are available for diseases such as yellow fever and Japanese encephalitis, there are no available vaccines or chemoprophylaxis for other mosquitoborne diseases such as dengue, chikungunya, Zika, filariasis, and West Nile encephalitis; for tickborne diseases such as Lyme borreliosis, tickborne encephalitis, and relapsing fever; for sand fly–borne diseases such as visceral and cutaneous leishmaniasis; and for black fly–borne diseases such as onchocerciasis (river blindness).

The Environmental Protection Agency (EPA) regulates repellent products in the United States. CDC recommends that consumers use only those repellent products registered by the EPA. Registration indicates that the EPA has determined that the product is both efficacious and safe for human use, when applied according to the instructions on the label.

GENERAL PROTECTIVE MEASURES

Avoid outbreaks. As much as possible, travelers should avoid known foci of epidemic arthropodborne disease transmission. The CDC Travelers’ Health website provides updates on regional disease transmission patterns and outbreaks (www.cdc.gov/travel).

Wear appropriate clothing. Travelers can minimize areas of exposed skin by wearing long-sleeved shirts, long pants, boots, and hats. Tucking in shirts, tucking pants into socks, and wearing closed shoes instead of sandals may help reduce risk. Application of repellents or insecticides, such as permethrin, to clothing and gear can provide an added layer of protection. Remind travelers to always follow instructions on the label when applying repellents to clothing. (Additional information on clothing repellents and their proper use is included below.)

Check for ticks. Travelers should inspect themselves and their clothing for ticks during outdoor activity and at the end of the day. Prompt removal of attached ticks can prevent some infections. Showering soon after leaving a tick-infested area may prevent ticks from attaching or facilitate detection of recently attached ticks.

Bed nets. Bed nets provide essential protection to travelers staying in accommodations lacking adequate window screens and air conditioning. Bed nets that do not reach the floor should be tucked under mattresses. Treating bed nets with a pyrethroid insecticide helps maximize their efficacy. Travelers may opt to buy a pretreated net or, as an alternative, apply the insecticide after purchase. Nets treated with a pyrethroid insecticide will be effective for several months if not washed. Long-lasting pretreated nets may be effective for much longer.

Insecticides and spatial repellents. Active ingredients in these products, metofluthrin and allethrin, provide protection from mosquito bites over a wide area. Spray aerosols can clear rooms or areas of mosquitoes; coils, vaporizing mats, and spatial repellents repel mosquitoes from a circumscribed area. Use insecticides and repellent products with caution, avoiding direct inhalation of spray or smoke.

Although many of these products demonstrate insecticidal or repellent activity under particular conditions, their efficacy in preventing vectorborne disease has not been evaluated adequately in peer-reviewed studies. For this reason, reliance on these products alone may afford insufficient protection from bites. Encourage travelers to apply an EPA-registered repellent to their skin and/or clothing and to use bed nets wherever vectorborne diseases are a risk.

Optimum protection can be provided by applying the repellents described in the following sections to clothing and to exposed skin.

REPELLENTS FOR USE ON SKIN AND CLOTHING

CDC has evaluated information published in peer-reviewed scientific literature and data available from EPA to identify several types of EPA-registered products that provide repellent activity sufficient to help people reduce the bites of disease-carrying insects. Products containing the following active ingredients typically provide reasonably long-lasting protection:

• DEET (chemical name: N,N-diethyl-m-toluamide or N,N-diethyl-3-methyl-benzamide). Products containing DEET include, but are not limited to, Off!, Cutter, Sawyer, and Ultrathon.
• Picaridin (KBR 3023 [Bayrepel] and icaridin outside the US; chemical name: 2-(2-hydroxyethyl)-1-piperidinecarboxylic acid 1-methylpropyl ester). Products containing picaridin include, but are not limited to, Cutter Advanced, Skin So Soft Bug Guard Plus, and Autan (outside the United States).
• Oil of lemon eucalyptus (OLE) or PMD (chemical name: para-menthane-3,8-diol), the synthesized version of OLE. Products containing OLE and PMD include, but are not limited to, Repel and Off! Botanicals. This recommendation refers to EPA-registered products containing the active ingredient OLE (or PMD). CDC does not recommend using “pure” oil of lemon eucalyptus (essential oil not formulated) as a repellent. It has not undergone similar, validated testing for safety and efficacy and is not registered with EPA as an insect repellent.
• IR3535 (chemical name: 3-[N-butyl-N-acetyl]-aminopropionic acid, ethyl ester). Products containing IR3535 include, but are not limited to, Skin So Soft Bug Guard Plus Expedition and SkinSmart.
• 2-undecanone (chemical name: methyl nonyl ketone). The product BioUD contains 2-undecanone.

EPA characterizes the active ingredients DEET and picaridin as “conventional” repellents. Biopesticide repellents—OLE, PMD, IR3535, and 2-undecanone—are derived from, or are synthetic versions of, natural materials.

Repellent Efficacy

Published data indicate that repellent efficacy and duration of protection vary considerably among products and among mosquito and tick species. Ambient temperature, level of activity, perspiration, water exposure, abrasive removal, and other factors affect efficacy and duration of protection. In general, higher concentrations of active ingredients provide longer duration of protection, regardless of the active ingredient. Products with <10% active ingredient may offer only limited protection, often 1–2 hours. Products that offer sustained-release or controlled-release (microencapsulated) formulations, even with lower active ingredient concentrations, may provide longer protection times. Studies suggest that concentrations of DEET above approximately 50% do not offer a marked increase in protection time against mosquitoes; DEET efficacy tends to plateau at a concentration of approximately 50%. CDC recommends using products with ≥20% DEET on exposed skin to reduce biting by insects that may spread disease.

Recommendations regarding use of repellents are based on peer-reviewed scientific studies and data submitted to regulatory agencies. People may experience some variation in protection from different products. Regardless of the product used, travelers getting insect bites should reapply the repellent according to the label instructions, try a different product, or, if possible, leave the area.

Ideally, travelers should purchase repellents before traveling. They are sold online as well as in hardware stores, drug stores, supermarkets, camping, sporting goods, and military surplus stores. When purchasing repellents overseas, look for the active ingredients specified above on the product labels.

Repellents and Sunscreen

Combined sunscreen/repellents are not recommended. Advise travelers to use separate products, applying sunscreen first, followed by repellent. Repellents applied according to label instructions may be used with sunscreen with no reduction in repellent activity. Limited data show that DEET-containing insect repellents applied over sunscreen decrease the sun protection factor of the sunscreen by one-third; travelers using both products may therefore need to reapply sunscreen more frequently. In general, travelers typically need to apply sun protection more often and in larger amounts than they do insect repellent.

Repellents and Insecticides for Use on Clothing

Travelers can apply permethrin to clothing, hats, shoes, bed nets, jackets, and camping gear for added protection. Products such as Permanone and Sawyer, Permethrin, Repel, and Ultrathon Permethrin Clothing Treatment are EPA-registered specifically to treat clothing and gear. Alternatively, commercially available clothing pretreated with permethrin is marketed to consumers in the United States as Insect Shield, BugsAway, or Insect Blocker.

Permethrin is a highly effective insecticide, acaricide (pesticide that kills ticks and mites), and repellent. Permethrin-treated clothing repels and kills ticks, chiggers, mosquitoes, and other biting and nuisance arthropods. Clothing and other items must be treated 24–48 hours in advance of travel to allow them to dry. As with all pesticides, follow the label instructions when using permethrin clothing treatments.

Permethrin-treated materials retain their ability to repel or kill insects after repeated launderings. To provide continued protection, travelers should retreat their clothing and gear as described on the product label. Clothing treated before purchase is labeled for efficacy through 70 launderings. Clothing treated with the other repellent products described above (such as DEET) provides protection from biting arthropods but will not last through washing and will require more frequent application.

Precautions when Using Insect Repellents

Instruct travelers to take the following precau­tions when using repellents:

• Apply only to exposed skin or clothing, as directed on the product label. Do not apply to skin covered by clothing.
• Never use on cuts, wounds, or irritated skin.
• When using sprays, do not spray directly on face—spray on hands first and then apply to face. Do not apply to eyes or mouth, and only sparingly around ears.
• Wash hands after application to avoid accidental exposure to eyes or ingestion.
• Children should not handle repellents. Instead, adults should apply to their own hands first and then gently spread on the child’s exposed skin. Avoid applying directly to children’s hands. After returning indoors, wash children’s treated skin and clothing with soap and water or give the child a bath.
• Use just enough to cover exposed skin or clothing. Heavy application and saturation are generally unnecessary for effectiveness. If biting insects do not respond to a thin film of repellent, apply a bit more.
• After returning indoors, wash repellent-treated skin with soap and water, or bathe. Wash treated clothing before wearing it again. This precaution may vary with different products—be sure to check the label.

If travelers experience a rash or other reaction (itching or swelling) from an insect repellent, they should wash off the product using mild soap and water and discontinue its use. For severe reactions, call a local poison-control center for further guidance. Travelers seeking health care because of a reaction to a repellent should bring the product container with them to show the doctor. Never apply permethrin to skin but only to clothing, bed nets, or other fabrics as directed on the product label.

Children and Pregnant Women

Most repellents can be used on children aged >2 months. Protect infants aged <2 months from mosquitoes by using an infant carrier draped with mosquito netting with an elastic edge for a tight fit. Products containing OLE specify that they should not be used on children aged <3 years. Other than the safety tips listed above, EPA does not recommend any additional precautions for using registered repellents on children or on pregnant or lactating women.

BED BUGS

The recent resurgence in bed bug infestations worldwide, particularly in developed countries, is giving travelers cause for concern. Although bed bugs do not transmit diseases, their bites may be a nuisance. Travelers can take measures to avoid bed bug bites and avoid transporting them in luggage and clothing (Box 3-02).

Box 3-02. Bed bugs and international travel

Bed bugs are small, flat insects that are reddish-brown in color, wingless, and range from 1 to 7 mm in length. Although bed bugs have not been shown to transmit disease, their bites can produce strong allergic reactions and considerable emotional stress.

A recent resurgence in bed bug infestations worldwide, particularly in developed countries, is thought to be related to the increase in international travel, pest control strategy changes in travel lodgings, and insecticide resistance. Bed bug infestations have been reported increasingly in hotels, theaters, and any locations where people congregate, including the workplace, dormitories, and schools. Travelers may transport bed bugs in luggage and on clothing. Personal belongings transported in vehicles infested with bed bugs is another means of spreading these insects.

PROTECTIVE MEASURES AGAINST BED BUGS

Encourage travelers to take the following precautions to avoid or reduce their exposure to bed bugs:

• Inspect the premises of hotels or other sleeping locations for bed bugs on mattresses, box springs, bedding, and furniture, particularly built-in furniture with the bed, desk, and closets as a continuous structural unit. Travelers who observe evidence of bed bug activity—whether it is the bugs themselves or physical signs such as blood-spotting on linens—should seek alternative lodging.
• Keep suitcases closed when they are not in use and keep them off the floor.
• Remove clothing and personal items (such as toiletry bags and shaving kits) from the suitcase only when they are in use.
• Carefully inspect clothing and personal items before returning them to the suitcase.
• Keep in mind that bed bug eggs and nymphs are very small and can be overlooked easily.

Prevention is by far the most effective and inexpensive way to protect oneself from these pests. The cost of ridding a personal residence of these insects is considerable, and efforts at control are often not immediately successful even when conducted by professionals.