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EWG's Guide to Bug Repellents in the Age of Zika
April 18, 2016

EWG's Guide to Bug Repellents in the Age of Zika: What diseases do bugs transmit?

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What diseases do bugs transmit?


The primary mosquito-borne disease threat in the U.S. is West Nile virus. Travelers to tropical regions and some other places could encounter malaria, dengue fever, yellow fever and a few other diseases.

The most common diseases carried by mosquitoes are:

Mosquito genus



Dengue fever, Rift Valley fever, yellow fever, chikungunya


Malaria, lymphatic filariasis


West Nile fever, Japanese encephalitis, lymphatic filariasis

Source:  World Health Organization, 2012

West Nile virus struck 5,674 Americans last year.  Half those cases were complicated with neuroinvasive disease such as meningitis, encephalitis or acute flaccid paralysis, according to the CDC.  Symptoms can be high fever, headache, stupor, tremors, convulsions and neurological damage. 

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Between 1999 and 2008, 64 different mosquito species tested positive for West Nile virus.

According to the CDC, infections from mosquitoes carrying West Nile virus occur primarily in the summer months, peaking in August. West Nile cases have been found in almost every state in the nation, with the highest numbers reported in Texas, California, Illinois, Louisiana, Michigan, Mississippi and Oklahoma. 

Malaria can cause fever, chills, headache, vomiting, diarrhea and in severe cases, organ failure, coma and death. According to the World Health Organization, more than 125 million travelers annually visit the 100 countries or areas where malaria has been reported. 

Some 30 to 40 of the 3,500 species within the Anopheles genus transmit malaria. These species are widespread throughout the world.

The CDC recommends DEET, Picaridin, IR3535 and Oil of Lemon Eucalyptus/PMD to repel mosquitoes that transmit malaria. The WHO recommends DEET, IR3535 and Picaridin.

What tests tell us – and what they don’t

Repellents are not tested against all mosquito species. The most common mosquito used in tests is the Aedes aegypti mosquito, because it is very sensitive to repellents and easy to cultivate in laboratories.  Other Aedes mosquito species require higher doses and more frequent reapplication of repellents (Badolo 2004).

One repellent may not always protect a person from all the different mosquito species, nor from other pests. 


Diseases carried by ticks are rare but can be severe.  EWG recommends prudent tick prevention methods to anyone who spends time in tick-infested areas.

Seven tick species in the U.S. may carry disease:




Blacklegged tick, also known as deer tick

Lyme disease

Anaplasmosis: fever, headache, chills and muscle aches
Babesiosis: varies from none to flu-like and life-threatening
Lyme disease: bull’s eye rash, fatigue, chills, fever, headache, aches, swollen lymph nodes

Western blacklegged tick

Lyme disease

American dog tick

Rocky Mountain spotted fever

Rocky Mountain spotted fever: fever, headache, vomiting, muscle pain and in some cases death

Brown dog tick

Rocky Mountain spotted fever

Gulf Coast tick


Rickettsiosis: fever, scabs, rash

Lone star tick


Ehrlichiosis: fever, headache, chills, malaise, muscle pain, nausea/vomiting/diarrhea, confusion, red eye, rash

Rocky Mountain wood tick

Rocky Mountain spotted fever Tularemia

Rocky Mountain spotted fever: fever, headache, vomiting, muscle pain and in some cases death
Tulameria: Skin ulcer with regional lymph node swelling

Source: CDC 2010

Lyme disease, carried by the blacklegged tick and Western blacklegged tick, is the most prevalent tick-borne disease in the U.S., with 24,364 known and 8,733 probable cases reported in 2011, the latest year for which data are available.

Most cases occur from Virginia to Maine and the upper Midwest, mostly Wisconsin and Minnesota (CDC 2013F).

Adult blacklegged ticks are about the size of a sesame seed.  Most Lyme disease is transmitted via bites from immature nymphs that feed during the spring and summer. You are most likely to contract tick-borne Lyme disease between late April and mid-July.

The lone star tick, found in the eastern half of the U.S., carries Southern tick-associated rash illness, an infectious disease similar to Lyme disease.

Rocky Mountain spotted fever can cause fever, headache, vomiting, muscle pain and in some cases death. It can be transmitted to humans by bites of the American dog tick, Rocky Mountain wood tick and brown dog tick.  The disease has been reported throughout the lower 48 states; most cases occur in North Carolina, Oklahoma, Arkansas, Tennessee and Missouri.  The incidence rate has increased over the past 20 years to about 2,000 cases annually.  But the fatality rate has greatly decreased over the past 50 years from 5 to 10 percent to less than 0.5 percent (CDC 2012A).

Do repellents stop ticks?

No single chemical completely repels important American ticks. Do not rely on any product to keep ticks away.  Perform tick checks at the end of the day or when returning indoors.

To help protect from Lyme disease, the CDC recommends that people use a product with at least 20 percent DEET or Pemethrin-coated clothing (CDC 2011). In lab studies DEET stops 3 of every 4 ticks.  Researchers test tick repellents by timing how long it takes for a tick to walk over a volunteer’s repellent-coated skin. The effectiveness of tick repellents can vary according to species and lifecycle stage of the tick.

The EPA has approved Picaridin, IR3535 and Oil of Lemon Eucalyptus/PMD for use as tick repellents with a protection time greater than two hours (EPA 2013).  There are few peer-reviewed scientific studies of the efficacy of these chemicals against deer ticks. Two studies indicate that IR3535 repels deer ticks as well as or better than DEET (Bissinger 2009, Carroll 2005). Direct evidence of Picaridin efficacy is limited, but Consumer Reports gave it top marks (Consumer Reports 2010).

In one study, three repellents – 33 percent DEET, 20 percent Picaridin and 20 percent IR3535 – repelled lone star ticks for some hours, but 10 percent IR3535 did not (Carroll 2005).

Does treated clothing keep ticks away?

ExOfficio BugsAway products, sold at outdoor stores like REI, L.L. Bean, and Cabela’s, are pretreated with permethrin, which is not a repellent but an insecticide meant to kill ticks on contact. 

This chemical is neurotoxic and has been classified by EPA as a likely human carcinogen.  It is highly toxic to the environment, especially to fish and other aquatic life.  But a 2009 EPA review said that “permethrin factory-treated clothing is unlikely to pose any significant acute or chronic hazard to people,” including toddlers, pregnant women and nursing mothers (EPA 2009, EPA 2012).

Use permethrin products with caution, read labels, and wash all treated clothing separate from other clothing.

ExOfficio claims that the bug-killing effect of its clothing lasts for 70 washings (ExOfficio 2013). Efficacy testing indicates a higher level of protection than that offered by repellents applied to the skin (Miller et al. 2011).  In a pilot study, 16 North Carolina state employees in outdoor occupations chose between treated clothing or repellents.  The group that used repellents received 62 tick bites. Among the group who wore treated clothing, one person had one tick bite (Vaughn & Meshnick 2011).