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U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3017

March 2010

U.S. Geological Survey National Wildlife Health Center

Climate Change and Wildlife Health: Direct and Indirect Effects

Printed on recycled paper

Climate change effects must be distinguished from other 

human activities that threaten human and animal health, such as 

habitat destruction and urbanization, the introduction of exotic 

and invasive species, and pollution. Clearly, these human 

activities directly affect ecosystem health and thereby indirectly 

affect human and animal health as well.

Climate change must also be viewed within the context of 

other physical and climate cycles, such as the El Niño Southern 

Oscillation (El Niño) (Rasmussen and Carpenter, 1982) and 

cycles in solar radiation (Carslaw and others, 2002) that have 

profound effects on the Earth’s climate and human, domestic 

animal, and wildlife health. 

Still, convincing evidence exists for climate change effects 

on wildlife disease in several areas, four of which are addressed 

here: geographic range and distribution of wildlife diseases, plant 

and animal phenology (Walther and others, 2002), wildlife hostpathogen interactions, and disease patterns in wildlife. Other 

factors of concern, such as ecosystem composition and pathogen 

virulence, are addressed in climate change literature.

Geographic Range and Distribution of Wildlife Diseases

In the Northern Hemisphere global warming has likely 

played a role in northern geographic shifts of disease vectors and 

parasitic diseases that have complex life cycles. For example, 

the geographic range of the lung parasite, Parelaphostrongylus 

odocoilei, of caribou has shifted northward since 1995 from the 

Pacific Coastal Range of the United States and British Columbia 

to include Alaska and the Yukon and Northwest Territories of 

Canada (fig. 2). 

Climate change will have significant effects on wildlife, 

domestic animal, and human diseases, according to scientists. 

The Intergovernmental Panel on Climate Change predicts that 

unprecedented rates of climate change will result in increased 

average global temperatures; a rise in sea levels; changes in 

global precipitation patterns, including increased amounts and 

variability; and increased midcontinental drying during summer 

(Intergovernmental Panel on Climate Change, 2007). Scientists suspect that increasing temperatures, in combination with 

changes in rainfall and humidity, may have significant impacts 

on wildlife, domestic animal, and human diseases. Because of 

expanding human populations, these changes could aggravate 

already limited water resources and increase habitat destruction, 

providing yet more opportunities for infectious diseases to cross 

from one species to another. 

Awareness has been growing in recent years about zoonotic 

diseases—that is, diseases that are transmissible between animals 

and humans. The rise of such diseases is the result of an increasingly close relationship among wildlife, domestic animals, and 

people, allowing more contact with diseased animals, or infected 

vectors and people (Daszak and others, 2000). Thus, it is impossible to separate the effects of global warming on wildlife from 

its effects on the health of domestic animals or people (fig. 1). 

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Fig. 2.  Geographic ranges for protostrongylid parasites in northern

ungulates including caribou, wild thinhorn sheep, mountain goat,

woodland caribou, black-tailed and mule deer (Hoberg and others, 2008).

DISEASE

Fig. 1.  Ecosystem health reflects environmental quality, an

important factor in the well-being of humans, domestic animals,

and wildlife. The interface between these components requires a

holistic approach of “one health” for the benefit of all (Friend, 2006).

Known species diversity since 1995

Known species diversity before 1995

P. andersoni

P. odocoilei

U. pallikuukensis

Protostongylidae n. sp

P. andersoni

P. odocoilei

Drawing by John Evans
In addition, the tick vector, Ixodes scapularis, of Lyme 

disease and several other tick-borne zoonotic diseases in North 

America has been expanding north into southern Ontario and, 

more recently, into western Ontario and Manitoba (Ogden and 

others, 2009).

Besides north-south shifts, scientists also predict disease 

distribution changes in altitude. For example, climate warming 

may lead to year-round transmission of avian malaria at higher 

elevations in the Hawaiian Islands, further threatening endangered native Hawaiian birds that have little or no resistance to 

the introduced disease. Currently, on the island of Hawai’i, avian 

malaria, caused by the parasite Plasmodium relictum, is limited 

to warmer elevations below 1,500 meters (fig. 3) (Van Riper III 

and others, 1986). If the higher elevations become warmer as 

predicted, mosquito activity and parasite development in these 

areas will increase. Conservationists are concerned that 

climate change may lead to increased avian malaria transmission 

throughout the year at increasingly higher elevations (fig. 3).

MONTH

MONTHLY MEAN LAND SURFACE

TEMPERATURE IN DEGREES CENTIGRADE

1 2 3 4 5 6 7 8 9 10 11 12

10

30

20

0

warmer spring

temperatures

MONTH

1 2 3 4 5 6 7 8 9 10 11 12

300

150

0

TICK NUMBERS

nymphs

overlapping

larvae

non-overlapping

larvae

Phenology: Effects on Wildlife Disease

The timing of recurring seasonal biologic cycles of a 

number of plant and animal species has already been affected by 

climate change (Walther and others, 2002). The study of these 

seasonal cycles is called phenology. The timing of biological 

cycles, such as the arrival of a bird species in the spring and the 

availability of its preferred food source, is critical for successful 

breeding and survival. Variability in the timing of these biological cycles also can lead to an increase or decrease in the risk for 

infectious disease, particularly vector-transmitted disease. In 

Europe, transmission of tick-borne encephalitis (TBE) to humans 

is often increased when warmer temperatures in the early spring 

result in the overlap of feeding activity of nymphal (virus 

infected) and larval (uninfected) Ixodes ricinus ticks. Under 

these weather conditions, infection is more readily passed from 

infected ticks to uninfected ticks through small rodents. Because 

the viral infection is brief in tick-infested rodents, feeding of 

both stages of tick at the same time results in more infected 

larval ticks and greater risk for TBE infection in humans (Randolph, 2009). Cooler spring temperatures result in less overlap 

of feeding activity of nymphal and larval ticks. Therefore, under 

these conditions the virus-infected rodents have time to recover 

from infection and are less likely to pass the virus to feeding 

larval ticks (fig. 4). 

Fig. 3.  Abundance of avian malaria parasites in Hawaiian birds,

mosquito vectors, and native Hawaiian birds, in relation to elevation

on the island of Hawai’i (Van Riper and others, 1986)

Fig. 4.  Human risk for tick-borne encephalitis in Europe is

heightened in some years by slightly warmer temperatures in early

spring (red line and arrow, top figure) that are associated with

overlapping feeding of infected nymphal ticks and uninfected

larval ticks (grey and red line, bottom figure). A slower rise in

spring temperatures (black line, top figure) is associated with

non-overlapping feeding (graphs provided by S. Randolph, 2010).

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ELEVATION IN METERS

ABUNDANCE

V

Native Birds

ectors

Malaria Parasites

At a number of sites in North America, the same seasonal 

temperature effect has been observed in the transmission of the 

bacterium Borrelia burgdorferi, the cause of Lyme disease, from 

infected nymphal Ixodes scapularis ticks to uninfected larval 

ticks. When feeding of larval ticks occurs at or nearly at the same 

time, it not only contributes to the successful transmission of the 

pathogen to larvae, but it also results in greater genetic diversity 

in this zoonotic pathogen (Gatewood and others, 2009). Climate 

change has the potential to affect these natural cycles by altering 

seasonal weather patterns. 

Wildlife Host–Pathogen Interactions

In nature, pathogens can be transmitted directly from one 

diseased animal to another, or they may be transmitted indirectly 

though intermediate hosts, such as infected prey, or through 

vectors such as biting insects. Indirect transmission cycles are 
often affected by environmental conditions such as temperature 

and rainfall. Higher temperatures associated with climate change 

may contribute to an increase in disease-producing agents within 

intermediate hosts and vectors, or increased survival of animals 

that harbor disease. For example, warmer summer temperatures 

in the Arctic now allow the lung nematode (Umingmakstrongylus 

pallikuukensis) larvae often found in muskoxen to develop to 

the infectious stage within the intermediate host, the marsh slug 

(Deroceras laeve), at a rate that has reduced the parasite’s life 

cycle from 2 years to 1 year (Kutz and others, 2005). 

Survival of another nematode, Parelaphostrongylus tenuis, 

the brain worm of white-tailed deer, may also be increased by 

recently warmer temperatures and milder winters in the North 

Central United States and southern Canada. The parasite, which 

overwinters as larvae in snails, causes neurological disease in 

moose (Alces alces) (fig. 5). Moose are already heat stressed 

by climate change (Lenarz and others, 2009), and may be more 

susceptible to parasitic and infectious diseases (Murray, 2009), 

including the brain worm of white-tailed deer.

Questions to Ponder about Climate Change

1. Long-term interdisciplinary projects can help 

determine climate’s impacts on biological 

factors associated with disease emergence (that 

is, species abundance, interactions and movements, vector populations). How might various 

physical, social, and economic factors contribute 

to disease emergence, persistence, and spread?

2. How are threatened and endangered free-ranging 

wildlife populations currently threatened by 

disease? How might climate change affect the 

current situation? 

3. How will climate change play a role in the threat 

of wildlife-associated water- and vector-borne 

diseases for free-ranging wildlife, other animals, 

and humans? 

4. How will climate change play a role in the lives of 

native peoples who are dependent upon wildlife 

as a major source for food? Will wildlife population 

declines or wildlife-associated food-borne disease 

threaten native peoples?

because of their symbiotic association with photosynthetic algae. 

Corals that have lost these algae due to increased water temperature, changes in salinity, or pollution are unable to attain enough 

energy from their environment and may be susceptible to disease 

leaving white coral skeletons, referred to as coral bleaching 

(fig. 6). Elevated temperatures will likely increase coral bleaching, which can lead to coral die-offs (Baker and others, 2008). 

Corals that fail to recover sufficiently may lead to loss of coral 

reefs and associated tropical marine life that depend on these 

animals for food and shelter. As a result, local economies that 

depend on coral reefs for sustenance or tourism could be significantly affected by climate change (Pandolfi and others, 2005).

Fig. 6.  Extensive coral bleaching on a reef, St. John, U.S. Virgin

Islands (Caroline S. Rogers, USGS).

Fig. 5.  Healthy North American bull moose (USFWS).

Diseased North American cow moose in the final stages of a brain

worm infection in St. Louis County, Minnesota (Mike Schrage,

Wildlife Biologist, Fond du Lac Band).

Disease Patterns in Wildlife

Predicting the effects of climate change on disease patterns 

across a geographic region is difficult because the effects are 

likely to be highly variable. This may be especially true among 

marine ecosystems. Since the 1980s, coral reefs in the western 

Atlantic Ocean have suffered massive declines due to disease 

(Porter and others, 2001). It is likely that coral mortalities were 

initially due to widespread mortality of sea urchins that allowed 

algal overgrowth of reefs followed by environmental degradation 

and increased susceptibility to disease (Lessios, 1988). Since the 

early 1980s, mass coral bleaching has been observed worldwide, 

especially following the major 1998 El Niño event, and has been 

linked to elevated sea-surface temperatures (Hoegh-Guldberg, 

1999). Corals are able to survive in nutrient-deficient waters 
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By Erik Hofmeister, Gail Moede Rogall, Kathy Wesenberg,

Rachel Abbott, Thierry Work, Krysten Schuler, 

and Jonathan Sleeman.

For additional information contact:

Erik Hofmeister, (608) 270-2476, ehofmeister@usgs.gov

Gail Moede Rogall, (608) 270-2438, gmrogall@usgs.gov

USGS National Wildlife Health Center

6006 Schroeder Rd

Madison, WI 53711

http://www.nwhc.usgs.gov/

Banner photo credits (from left to right): Caroline S. Rogers (USGS),

Carter Atkinson (USGS), USGS Pacific Island Ecosystems Research

Center, Lawrence Igl (USGS), U.S. Fish and Wildlife Service,

and Craig Ely (USGS).