Recommendations for 

Protecting Outdoor Workers 

from West Nile Virus Exposure 

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
Cover photograph courtesy of William Brogdon,
 

Centers for Disease Control and Prevention (CDC)
 
Recommendations for 

Protecting Outdoor Workers 


from West Nile Virus Exposure
 

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention
 

National Institute for Occupational Safety and Health
 
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copied or reprinted. 

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DHHS (NIOSH) Publication No. 2005–155 

September 2005 

Safer • Healthier • PeopleTM 
Introduction 

The West Nile virus (WNV) is most often spread to humans from the bite of an 

infected mosquito. The virus may also be 

transmitted in other ways—through organ 

transplants, blood transfusions, and breast 

milk, and from mother to fetus. But the risk 

of such transmission is very low. WNV was 

first reported in the United States in 1999, 

and occupational exposures have been documented. By 2004, the virus was reported 

throughout the continental United States. 

Most human infections with WNV (about 80%) cause no symptoms, 

and about 20% cause flu-like symptoms, including fever, fatigue, headache, and muscle or joint pain. Fewer than 1% of humans infected with 

WNV become severely ill. Severe symptoms include high fever, stiff 

neck, disorientation, tremors, muscle weakness, and paralysis. Severely 

affected persons may develop encephalitis (inflammation of the brain) 

or meningitis (inflammation of the membranes of the brain or spinal 

cord). Severe cases may be fatal. People of all ages and conditions may 

be affected. However, those who are above age 50 or who have had an 

organ transplant are at increased risk of severe illness. 

Frequently Asked Questions 

Which workers may be exposed to WNV? 

Outdoor workers may be exposed to WNV whenever mosquitoes are 

biting. Outdoor workers at risk include farmers, foresters, landscapers, 

WNV is spread to humans from the bite of 

an infected mosquito. Photograph courtesy of 

Jim Gathany, Centers for Disease Control and 

Prevention (CDC). 

1 
Outdoor workers are at risk of 

WNV infection. 

Anyone who works where there 

are infected mosquitoes is at 

risk of WNV infection. Photograph 

courtesy of Ken Hammond, U.S. 

Department of Agriculture (USDA). 

groundskeepers and gardeners, painters, roofers, 

pavers, construction workers, laborers, mechanics, and other outdoor workers. Entomologists and 

other field workers are also at risk while conducting 

surveillance and other research outdoors. All outdoor workers should follow the recommendations 

in this brochure to reduce their potential for WNV 

exposure. 

In addition to outdoor workers, laboratory workers 

are at risk of WNV infection if their skin is penetrated or cut while performing necropsies or handling 

WNV-infected tissues or fluids. 

Who is at risk of WNV infection? 

Anyone who lives or works in an area where there 

are WNV-infected mosquitoes is at risk of WNV 

infection. People of all ages are at risk of the flu-like 

symptoms of this infection. Although people usually 

recover, the symptoms may be severe enough to result in lost workdays for extended periods. 

People above age 50 and those who have had organ transplants are known to have the highest risk 

of severe illness from WNV infection. Others with 

compromised immune systems are also likely to be 

at high risk. However, people of all ages and conditions can become severely ill. Therefore, ALL workers should be careful to follow the recommendations 

listed in this brochure to reduce their potential for 

exposure to WNV. 

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Have any outdoor workers been infected 

with WNV on the job? 

Workers were infected with WNV at a turkey 

breeder farm in 2002 and an alligator farm in 2004. 

Although it is uncertain how these workers were 

infected, it may have been through mosquito bites, 

contact of cut or scraped skin with infected feces, 

swallowing or being splashed in the eyes or mouth 

with contaminated material, or inhaling airborne 

virus from infected feces. 

It is likely that more outdoor workers have been 

infected with WNV when bitten by infected mosquitoes, but information about workers’ occupations 

may not be collected when cases are reported. Many 

cases have occurred in rural areas where people 

work in farming and other outdoor occupations. 

When are outdoor workers at greatest 

risk of WNV exposure? 

Outdoor workers are at risk of WNV exposure any 

time infected mosquitoes are biting. Most WNV infections occur from July through September. Many 

mosquitoes are most active from dusk to dawn. 

However, some are active during the day. If possible, 

avoid working outdoors during peak activity times 

for mosquitoes. When you must work at such times, 

pay special attention to the use of personal protection such as protective clothing and insect repellent 

to reduce the potential for exposure. 

Outdoor workers should be trained 

about the risks of WNV exposure 

and infection. 

Many mosquitoes are most active 

from dusk to dawn. Photograph 

courtesy of USDA. 

3
 
 

 

 

 

 

 

 

 

 

 

 

Tires provide breeding sites for 

mosquitoes. Photograph courtesy of 

Graham Heid and Harry D. Pratt, CDC. 

When pregnant workers are outside, 

they should avoid mosquitoes, 

wear protective clothing, and use 

an effective repellent. Photograph 

courtesy of CDC. 

Where are mosquitoes most commonly 

found? 

Mosquitoes develop in any standing body of water that 

persists for more than 4 days. Stagnant pools, ponds, 

watering troughs, irrigation ditches, rain barrels, 

manure lagoons, and other stagnant bodies of water 

increase mosquito populations. Weedy, bushy, and 

wooded work environments may also have mosquito 

populations. Emptying containers of stagnant water 

every 4 to 5 days or treating these breeding sites with 

larvicides can help to reduce mosquito populations. 

How does WNV affect a woman’s 

pregnancy? 

The Centers for Disease Control and Prevention 

(CDC) has followed a number of women who became ill with WNV during pregnancy. Most of these 

women delivered apparently healthy babies with 

no evidence of WNV infection. Although some 

of the babies born to WNV-infected mothers did 

have health problems, it is unknown whether the 

WNV infection caused these problems. Because of 

the limited number of cases studied so far, it is not 

yet possible to determine what percentage of WNV 

infections during pregnancy result in infection of 

the fetus or medical problems in newborns. More 

research is needed to understand the possible effects 

of WNV on pregnancy. 

When pregnant workers are outside, they should follow the recommendations at the end of this brochure 

4 
 

 

 

 

to reduce their risk for WNV infection. These recommendations include avoiding mosquitoes; wearing protective clothing; using repellents containing 

DEET (N,N-diethyl-m-toluamide), picaridin, or 

oil of lemon eucalyptus on the skin; and spraying 

clothes with one of these repellents or permethrin. 

Always follow label instructions for repellents. Pregnant women who become ill should see their health 

care providers. 

What should I do if I must handle dead 

animals? 

Avoid handling dead animals when possible. If you 

must handle them, avoid direct contact and wear 

gloves that provide a protective barrier. See additional 

recommendations at the end of this brochure. 

What should I do if I develop symptoms 

of WNV infection? 

Tell your supervisor about any symptoms that might 

be caused by WNV infection. If you develop severe 

symptoms such as high fever, stiff neck, disorientation, tremors, muscle weakness, or paralysis, seek 

medical attention immediately. Be sure to tell your 

health care provider that you work outdoors. The 

period between receiving the infected bite and having symptoms is 3 to 14 days. 

Testing for WNV infection is available. No vaccine 

is currently available to prevent WNV infection in 

humans. 

5 

Wear gloves if you must handle 

dead animals. 

Visit your health care provider if 

you develop WNV symptoms. 
Removing tires from worksites 

helps to reduce mosquito 

populations. Photograph courtesy 

of CDC. 

Recommendations for 

Employers of Outdoor Workers 

Employers should protect their workers from WNV 

exposure by taking the following steps: 

■	 Provide training that describes how WNV is 

transmitted and reinforces knowledge about 

the risks of WNV exposure and infection. 

■	 Stress to workers the importance of reporting all work-related injuries and illnesses in a 

timely manner. 

■	 Provide a medical surveillance system that 

monitors, records, and assesses the symptoms 

and absenteeism associated with WNV infection. 

■	 Provide workers with protective clothing 

(long-sleeved shirts, long pants, and socks) 

and repellents to use on skin and clothing: 

— Use repellents containing DEET (more 

than 20% DEET for longer protection), 

picaridin, or oil of lemon eucalyptus on 

both skin and clothing. 

— Use permethrin on clothing only. 

■	 Reduce worker exposure to mosquitoes by 

taking the following steps: 

— Avoid having workers outdoors when 

mosquitoes are most active and biting 

(most often from dusk to dawn). 

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— Recommend that outdoor workers wear 

long-sleeved shirts, long pants, and socks 

when possible. 

— If worker uniforms are provided, include 

long-sleeved shirts and long pants as options. 

■ Eliminate as many sources of standing water from the worksite as possible to decrease 

mosquito populations: 

— Change the water every 4 to 5 days in 

animal drinking troughs, birdbaths, and 

other water containers. 

— Scrub the sides of water containers to 

dislodge eggs. 

— Add an aerator to ponds and water gardens to keep the water circulating, or add 

fish that will eat the mosquito larvae or 

adults. 

— Remove discarded tires or keep them dry 

and under cover. 

— Turn over, cover, store, or remove equipment such as tarps, buckets, barrels, 

wheelbarrows, and containers to prevent 

standing water. 

— Place drain holes in containers that collect water and cannot be discarded. 

— Clean out rain gutters. 

Preventing standing water helps 

to reduce mosquito populations. 

Photograph courtesy of Graham 

Heid and Harry D. Pratt, CDC. 

7 
Using an effective insect repellent 

helps prevent mosquito bites. 

Photograph courtesy of CDC/PHPPO/ 

DPDE/CAB/PhotoServices. 

Long-sleeved shirts, long pants, 

and socks will help prevent 

mosquito bites. Photograph courtesy 

of Joe Larson, USDA. 

— Remove debris (leaves, twigs, trash) from 

ditches. 

— Fill in ruts and other areas that collect 

standing water. 

Recommendations for Workers 

Outdoor workers can reduce their risk of WNV exposure by taking the following steps: 

■	 Use insect repellent if you work outdoors 

when mosquitoes are biting: 

— Apply insect repellent containing DEET 

(more than 20% DEET for longer protection), picaridin, or oil of lemon eucalyptus to exposed skin and to clothing. 

— Use permethrin on clothing only. 

— Carefully follow label directions for repellent use. 

— Do not apply pump or aerosol products 

directly to the face. Instead, spray these 

products onto the hands and carefully 

rub them over the face, avoiding the eyes 

and mouth. 

— Use a repellent that provides protection 

for the amount of time that you will be 

outdoors and reapply it as needed. The 

percentage of active ingredient in the repellent determines the length of protection. 

8 
— Wash skin treated with insect repellent 

with soap and water after returning indoors. 

■	 Use protective clothing if you work outdoors 

when mosquitoes are biting: 

— Wear long-sleeved shirts, long pants, and 

socks. 

— Spray clothing with products containing 

DEET, picaridin, oil of lemon eucalyptus, 

or permethrin, as mosquitoes may penetrate thin clothing. 

— Use permethrin repellents on clothing as 

directed; do not apply them directly to 

skin. 

— Wash clothing treated with insect repellent before wearing it again. 

— Do not apply repellent to skin that is 

covered by clothing. 

■	 Avoid handling dead animals when possible. 

If you must handle them, take the following 

precautions: 

— Use tools such as shovels to avoid direct 

contact with the animals. 

— Wear medical examination gloves that 

provide a protective barrier between 

your skin and blood or other body fluids: 

•	 Wear two pairs of gloves if one pair 

alone might tear. 

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•	 Wear the medical examination gloves 

as the inner pair. 

•	 Make sure that any latex gloves used 

are reduced-protein, powder-free 

gloves to reduce workers’ exposure to 

allergy-causing proteins. 

•	 Wear cotton or leather work gloves as 

the outer pair when heavy work gloves 

are needed. 

•	 Discard both inner and outer gloves 

immediately after use. 

•	 Remember that cotton, leather, and 

other absorbent gloves are not protective when worn alone. 

— If gloves are not available, use a plastic 

bag, which may act as a protective barrier 

between the animal and your skin. 

Additional Resources 

For more comprehensive information about WNV, 

contact the Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention at 

www.cdc.gov/westnile 

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For further information about other occupational 

safety and health topics, contact the National Institute for Occupational Safety and Health, Centers for 

Disease Control and Prevention at 

www.cdc.gov/niosh 

Acknowledgments 

This brochure was written by Kathleen MacMahon, 

DVM, Education and Information Division; and 

Amanda G. Harney, MPH, Office of Compensation 

Analysis and Support. Scientists from the Division 

of Vector-Borne Infectious Diseases, National Center 

for Infectious Diseases, provided critical input. Susan 

Afanuh, Anne Hamilton, and Jane Weber provided 

editorial services. Vanessa Becks provided desktop 

design and production. 

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