Featured Research

from universities, journals, and other organizations

Wild sheep show benefits of putting up with parasites

Date:
August 7, 2014
Source:
Princeton University
Summary:
In the first evidence that natural selection favors an individual's infection tolerance, researchers have found that an animal's ability to endure an internal parasite strongly influences its reproductive success. The finding could provide the groundwork for boosting the resilience of humans and livestock to infection.

The researchers examined the relationship between each sheep's body weight and its level of infection by nematodes, tiny parasitic worms that thrive in the gastrointestinal tract of sheep. This scanning electron micrograph shows nematodes on the surface of a sheep's gut with a field of view of approximately one centimeter. An economic detriment to sheep farmers, nematodes infect both wild and domesticated sheep, resulting in weight loss, reduced wool growth and death.
Credit: Photo by David Smith/Moredun Research Institute

In the first evidence that natural selection favors an individual's infection tolerance, researchers from Princeton University and the University of Edinburgh have found that an animal's ability to endure an internal parasite strongly influences its reproductive success. Reported in the journal PLoS Biology, the finding could provide the groundwork for boosting the resilience of humans and livestock to infection.

Related Articles


The researchers used 25 years of data on a population of wild sheep living on an island in northwest Scotland to assess the evolutionary importance of infection tolerance. They first examined the relationship between each sheep's body weight and its level of infection with nematodes, tiny parasitic worms that thrive in the gastrointestinal tract of sheep. The level of infection was determined by the number of nematode eggs per gram of the animal's feces.

While all of the animals lost weight as a result of nematode infection, the degree of weight loss varied widely: an adult female sheep with the maximum egg count of 2,000 eggs per gram of feces might lose as little as 2 percent or as much as 20 percent of her body weight. The researchers then tracked the number of offspring produced by each of nearly 2,500 sheep and found that sheep with the highest tolerance to nematode infection produced the most offspring, while sheep with lower parasite tolerance left fewer descendants.

To measure individual differences in parasite tolerance, the researchers used statistical methods that could be extended to studies of disease epidemiology in humans, said senior author Andrea Graham, an assistant professor of ecology and evolutionary biology at Princeton. Medical researchers have long understood that people with similar levels of parasite infection can experience very different symptoms. But biologists are just beginning to appreciate the evolutionary importance of this individual variation.

"For a long time, people assumed that if you knew an individual's parasite burden, you could perfectly predict its health and survival prospects," Graham said. "More recently, evolutionary biologists have come to realize that's not the case, and so have developed statistical tools to measure variation among hosts in the fitness consequences of infection."

Graham and her colleagues used the wealth of information collected over many years on the Soay sheep living on the island of Hirta, about 100 miles west of the Scottish mainland. These sheep provide a unique opportunity to study the effects of parasites, weather, vegetation changes and other factors on a population of wild animals. Brought to the island by people about 4,000 years ago, the sheep have run wild since the last permanent human inhabitants left Hirta in 1930. By keeping a detailed pedigree, the researchers of the St Kilda Soay Sheep Project can trace any individual's ancestry back to the beginning of the project in 1985, and, conversely, can count the number of descendants left by each individual.

Expending energy to fight infection

Nematodes puncture an animal's gut and can impede the absorption of nutrients. Therefore, tolerance to nematode infection could result from an ability to make up for the lost nutrition, or from the ability to repair damage the parasites cause to the gut, Graham said. "This island is way out in the North Atlantic, where the sun doesn't shine much," she said. "So tolerant individuals might be the ones who are better able to compete for food or better able to assimilate protein and other useful nutrients from the limited forage."

Tolerant animals might invest energy in gut repair, but would then be expected to incur costs. Graham and her colleagues identified a similar evolutionary tradeoff in a 2010 study that compared immune-response levels and reproductive success in female Soay sheep. They found that animals with strong antibody responses produced fewer offspring each year, but also lived longer. The team has not yet been able to detect costs of parasite tolerance in the sheep, but such costs could help explain variation in tolerance if the most tolerant animals were at a disadvantage under particular conditions.

While the PLoS Biology findings provide strong evidence that natural selection favors infection tolerance, they do raise questions, such as how the tolerance is generated, and why variation might persist from one generation to the next despite the reproductive advantage of tolerance, Graham said. The data in this study did not permit the researchers to detect a genetic component to tolerance. If genetics do play a role, she suspects multiple genes may interact with environmental factors to determine tolerance; ongoing research will help to tease apart these possibilities.

Understanding the genetic underpinnings of nematode tolerance could someday guide efforts to boost tolerance in livestock by identifying and selectively breeding those animals that exhibit a heightened parasite tolerance, said David Schneider, an associate professor of microbiology and immunology at Stanford University.

"This study shows that parasite tolerance can have a profound effect on animal health and breeding success," said Schneider, who is familiar with the work but was not involved in it. "In the long term, this suggests that it could be profitable to invest in breeding tolerant livestock."

In humans and domesticated animals, intestinal parasites are becoming increasingly resistant to the drugs used to treat infections, Graham said. If the availability of nutrients, even just during the first few months of life, impacts lifelong parasite tolerance, simple nutritional supplements could be an effective way to promote tolerance in people. About 2 billion people are persistently infected with intestinal nematode parasites worldwide, mostly in developing nations. Children are especially vulnerable to the worms' effects, which include anemia, stunted growth and cognitive difficulties.

"Ideally, we would clear the worms from the bellies of the kids who have those heavy burdens," Graham said. "But if we could also understand how to ameliorate the health consequences and thus promote tolerance of nematodes, that could be a very powerful tool."


Story Source:

The above story is based on materials provided by Princeton University. The original article was written by Molly Sharlach. Note: Materials may be edited for content and length.


Journal Reference:

  1. Adam D. Hayward, Daniel H. Nussey, Alastair J. Wilson, Camillo Berenos, Jill G. Pilkington, Kathryn A. Watt, Josephine M. Pemberton, Andrea L. Graham. Natural Selection on Individual Variation in Tolerance of Gastrointestinal Nematode Infection. PLoS Biology, 2014; 12 (7): e1001917 DOI: 10.1371/journal.pbio.1001917

Cite This Page:

Princeton University. "Wild sheep show benefits of putting up with parasites." ScienceDaily. ScienceDaily, 7 August 2014. <www.sciencedaily.com/releases/2014/08/140807145938.htm>.
Princeton University. (2014, August 7). Wild sheep show benefits of putting up with parasites. ScienceDaily. Retrieved December 22, 2014 from www.sciencedaily.com/releases/2014/08/140807145938.htm
Princeton University. "Wild sheep show benefits of putting up with parasites." ScienceDaily. www.sciencedaily.com/releases/2014/08/140807145938.htm (accessed December 22, 2014).

Share This


More From ScienceDaily



More Plants & Animals News

Monday, December 22, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Earthworms Provide Cancer-Fighting Bacteria

Earthworms Provide Cancer-Fighting Bacteria

Reuters - Innovations Video Online (Dec. 21, 2014) Polish scientists isolate bacteria from earthworm intestines which they say may be used in antibiotics and cancer treatments. Suzannah Butcher reports. Video provided by Reuters
Powered by NewsLook.com
Existing Chemical Compounds Could Revive Failing Antibiotics, Says Danish Scientist

Existing Chemical Compounds Could Revive Failing Antibiotics, Says Danish Scientist

Reuters - Innovations Video Online (Dec. 21, 2014) A team of scientists led by Danish chemist Jorn Christensen says they have isolated two chemical compounds within an existing antipsychotic medication that could be used to help a range of failing antibiotics work against killer bacterial infections, such as Tuberculosis. Jim Drury went to meet him. Video provided by Reuters
Powered by NewsLook.com
Hugging It Out Could Help You Ward Off A Cold

Hugging It Out Could Help You Ward Off A Cold

Newsy (Dec. 21, 2014) Carnegie Mellon researchers found frequent hugs can help people avoid stress-related illnesses. Video provided by Newsy
Powered by NewsLook.com
Researchers Test Colombian Village With High Alzheimer's Rates

Researchers Test Colombian Village With High Alzheimer's Rates

AFP (Dec. 19, 2014) In Yarumal, a village in N. Colombia, Alzheimer's has ravaged a disproportionately large number of families. A genetic "curse" that may pave the way for research on how to treat the disease that claims a new victim every four seconds. Duration: 02:42 Video provided by AFP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins