...From the pages of Agricultural Research magazine
Battling Bisons' Mysterious MCF Disease
A disease that has buffaloed scientists, veterinarians, and bison ranchers
is yielding some of its secrets. That's because of innovative research
by ARS scientists and their Washington
State University colleagues. They've developed new tests for detecting
and correctly identifying the disease, known as malignant catarrhal
fever, or MCF. These tests are the work of veterinary microbiologist
Hong Li of the ARS Animal Disease Research Unit and veterinary virologist
Tim B. Crawford of Washington State University, both in Pullman, Washington.
"MCF's history in veterinary medicine can be traced back to the late 1700s," Li says. "The disease affects many domestic and wild ruminantsanimals that have multichambered stomachssuch as cattle, bison, and deer. It is caused by a group of herpesviruses with very complex life cycles. Several ruminants serve as carriers of MCF viruses. In the United States, the most prominent carrier is the domestic sheep."
Veterinary microbiologist Hong Li
(left) and veterinary virologist
Tim Crawford select sheep for
an MCF transmission study.
Sheep Don't Succumb
"Sheep MCF virus, or ovine herpesvirus 2, is the cause of most
MCF cases in the United States," says Li. "Sheep carry MCF
virus, but apparently are not susceptible to the disease. It is often
fatal to some other animals but is harmless to humans.
"MCF occurs sporadically," adds Li. "There's much about
it that we still don't understand. Though both cattle and bison can
die from the disease, bison seem more susceptible. In fact, MCF is one
of the leading infectious diseases of bison, so it's a top research
priority of the American bison industry."
Some 300,000 bison are currently being raised in the United States for their unique, low-fat meat. That's according to Donal O'Toole, who collaborates with Li and Crawford. O'Toole is a veterinary pathologist at the University of Wyoming, Laramie.
|Sheep inadvertently spread the
virus, mainly from their nasal secretions. Bison or other animals sharing
the same range, pasture, feed or, perhaps, water, with the sheep may come
into contact with the virus particles shed by the sheep. Notes O'Toole,
"The virus doesn't live very long once it's shed."
An early and telltale sign of the disease is a severe runny nose and often a custardlike discharge that eventually encrusts the afflicted animal's muzzle. Other symptoms that follow may include mouth ulcers; cloudy, whitened eyes; swollen lymph nodes; bloody diarrhea; and a high feveras much as 107°F, as compared to a healthy bison's normal 101° to 102°F.
Tests Identify Antibodies and Virus
Today there is no treatment or cure for MCF and no vaccine. Yet the
tests that Li and Crawford developed may someday help prevent this disease.
One assay is a CI-ELISA, short for competitive inhibition enzyme-linked
immunosorbent assay. This blood test is best used for screening healthy
herds of MCF-susceptible animalsbison or cattle, for example.
It determines whether any of the animals, even if they are not showing
any signs of illness, are carrying an MCF virus.
Li points out, "Besides screening MCF-susceptible animals, the
test is also very useful for screening carrier animals, such as sheep,
for MCF virus." The team's CI-ELISA can detect even very small
amounts of antibodies that the animal makes in response to the invading
virus. Li explains that the CI-ELISA is the first test capable of detecting
antibodies that are formed in response to MCF viruses. It is a significant
improvement over earlier MCF blood tests.
Although the assay can indicate whether an animal has made antibodies
to MCF, it can't distinguish among members of the MCF virus family.
That's the job of tests that are based on what's called a polymerase
chain reaction, or PCR.
"Researchers in Scotland," says Li, "developed a PCR
for sheep MCF virus. We adapted that to develop other PCRs, including
a quantitative one for MCF research. The quantitative PCR tells us not
only which MCF virus is present, but also how much of it there is."
PCR tests are useful for identifying new MCF viruses. For example,
Li's group was the first to identify a new MCF virus in domestic goats.
What's more, they used PCR technology to discover another new MCF virus
that causes the disease in white-tailed deer. Li did the deer work with
veterinary pathologist Neil W. Dyer at North Dakota State University.
Having this array of PCRs may help reveal which viruses in the MCF family are deadly to which species of livestock or wildlife. That information could help livestock producers, wildlife specialists, and zoo managers.
Diagnostic Tests Prove Useful
Today these tests can be performed at regional veterinary diagnostic
laboratories on behalf of researchers and veterinarians who send in
specimens. For instance, the Washington Animal Disease Diagnostic Laboratory
at Washington State University processes between 800 and 1,000 MCF diagnostic
assays a year in addition to the thousands it runs for research purposes.
Zoos are also increasingly relying on the tests, reports Li. "They
want to test their own animals as well as those they are interested
in adding to their collections."
Li and Crawford have used the assays to determine a previously unknown
interval during which newborn lambs are virus-free. "We found that
most lambs are virus-free for about 6 to 8 weeks after birth,"
Li states. "So you can establish a virus-free flock if you take
lambs from their infected mothers before that time is up."
For that work, they collaborated with ARS animal scientist Gary D.
Snowder at the agency's U.S. Sheep Experiment Station, Dubois, Idaho.
Li says several zoos in North America have begun using this regimen
to produce MCF-free sheep.
In addition, the tests have been "very crucial to bison MCF research,"
asserts Li. Says colleague O'Toole, "The tests, for example, have
shown that cattle are significantly less susceptible to ovine herpesvirus
2 than are buffalo."
A study by researchers in Pullman and Laramie revealed that at least
25 to 35 percent of all bison are infected with the MCF virus.
Li received a top regional award from ARS in 2000 for his pioneering
research. In addition, he has served as special expert for the United
Nations in establishing diagnostic assays for MCF in West Africa.
Li and colleagues have reported their findings in the Journal of
Clinical Microbiology and the Journal of General Virology
as well as many veterinary science journals, including the Journal
of Veterinary Diagnostic Investigation.
VMRD, Inc., of Pullman, Washington, sells the reagents for the MCF
This research is part of Animal Health, an ARS National Program
(#103) described on the World Wide Web at http://www.nps.ars.usda.gov.
"Battling Bisons' Mysterious MCF Disease" was published
in the June
2002 issue of Agricultural Research magazine.