Detecting Stress in Animals
Left: Normal liver cells.
Middle: The reddish-brown color
indicates presence of nitrated
proteins that developed in response
to stress from bacterial toxin.
Right: Pretreatment with vitamin E
appears to limit development of
|When people get stressed, they seek
relief in many ways. Some exercise, some practice specific breathing
techniques, and some eat chocolate. Farm animals become stressed, too. In
domesticated food animals, stress can affect meat quality, milk production, and
general health. But animal stress must often be intuited from clues such as
lower-than-anticipated weights and increased illness.
Certain stressful events can be anticipated, for example, birth and weaning.
But how do you know if an animal is stressed for some other reason? Ted H.
Elsasser of ARS' Growth Biology
Laboratory in Beltsville, Maryland, is investigating nitrated protein as a
biomarker of stress. Nitrated proteins may serve as an early-warning system to
point out an animal that may yield unsafe products or flag a need for
therapeutic or intervention strategies to promote recovery from illness.
The biggest problem for animal producers is disease management, so an
early warning system would certainly be helpful, says Elsasser.
Animal scientist Ted Elsasser
prepares an injection of vitamin E
to test its ability to relieve
some effects of bacterial toxins.
| Other researchers have found that
nitrated proteins are formed when certain components of
proteinstyrosinesbecome nitrated along critical parts of the
regulatory pathway, altering the function of the proteins. This occurs after
high levels of infectious stress, injury, or the presence of immune system
hormones called inflammatory cytokines.
These changes in proteins occur in humans and other animals. But all nitrated
proteins are not the same. Specific protein nitrations are associated with
specific disorders, depending on the protein altered and the position of the
alteration on the protein. Proteins are like jigsaw puzzle pieces that are
designed to fit together. When proteins are altered, they no longer fit with
other proteins, and they become ineffective. Different stresses are reflected
in different biochemical systems of the bodyresulting in different
patterns and profiles of nitrated proteins. To complicate matters more, not all
animals' biochemical pathways respond to the same stresses in the same way.
The protein nitrification process begins with arginine, an amino acid that is
present in the diet and circulates in the bloodstream. If significant stress is
present, more arginine is transported into cells by a specific transporter.
This is one of the body's initial attempts to fight harmful consequences of
Various biochemical pathways cause nitric oxide to be generated from the
arginine. Depending on how cells have responded to stresses, their internal
oxidative environment can change. Under some circumstances, the nitric oxide
reacts with oxygen, creating a compound called peroxynitrite. It is this
peroxynitrite that reacts with amino acids in proteins, tyrosines in
particular, to form a nitrated protein.
Antioxidant compounds can control some of the processes through which
peroxynitrite is formed. Just as we get vaccinations before foreign travel to
ward off illness, Elsasser is investigating the possibility of preconditioning
animals with antioxidants before predicted stresses to head off subsequent
illness. Just as they mop up free radicals in the body, antioxidants combat
some of the chemical reactions in cells that result in nitrated protein
formation. It is important to note that nitrated proteins are not the same as
Stress causes several undesirable effects, including slow animal growth and
lowered immune response.
Farmers often use small amounts of antibiotics to avert these effects. But
sustained dosing can accelerate development of antibiotic-resistant bacteria,
which may go on to infect humans. So Elsasser is investigating another method.
An Alternative Approach
Enter vitamin E, a standard antioxidant supplement in many people's nutritional
regimen. Our studies using vitamin E as a weapon against animal stress
may offer some relief, says Elsasser. He gave six calves vitamin E as a
pretreatment before challenging them with a toxin taken from bacterial cell
walls. This toxin causes the immune system to react as if an infection were
present. Elsasser also gave six calves only the toxin, and four received
neither the toxin nor the vitamin E.
Compared to the four untreated calves, all the calves challenged with the toxin
had significantly lower concentrations of IGF-I, a metabolic growth factor, in
the liver and blood. However, those calves that were injected with toxin and
pretreated with vitamin E had higher IGF-I concentrations than the calves that
received only the toxin. The cattle given vitamin E also recovered more quickly
from the systemic effects of the toxin. Pretreating with vitamin E could have
Using vitamin E as a preconditioner allows us to maintain growth rates
better and perhaps prevent secondary infections, which crop up when stress
creates an opportunity for a pathogen to gain a competitive foothold,
says Elsasser. Being able to prevent infections due to stress could lead to
lower disease-management costs, less antibiotic use, and healthier animals. And
healthier animals mean that fewer disease-causing microbes make their way to
Durham, Agricultural Research Service Information Staff.
This research is part of Food Animal Production, an ARS National Program
(#101) described on the World Wide Web at http://www.nps.ars.usda.gov.
Ted H. Elsasser is with the
Growth Biology Laboratory,
Bldg. 200, Room 210, 10300 Baltimore Ave., Beltsville, MD 20705; phone (301)
504-8281, fax (301) 504-8623.
"Detecting Stress in Animals" was published in the
issue of Agricultural Research magazine.