Frequently Asked Questions
Initially prepared for Smoke Signals (April issue of 2000) by Donal O’Toole (University of Wyoming), Hong Li (USDA Agricultural Research Service), and Tim Crawford (Washington State University).
Malignant catarrhal fever (MCF) is a generally fatal disease of cattle,
bison, true buffalo species, and deer. It is caused by viruses belonging to the
Herpesvirus family. MCF occurs worldwide and is a serious problem, particularly
for bison in the United States and Canada. MCF in bison is caused by a virus
called ovine herpesvirus-2 (OvHV-2). A closely related virus called Alcelaphine
herpesvirus-l (AlHV-1) is responsible for most cases of MCF in Africa.
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The virus responsible for the sheep-associated form has never been isolated.
Until recently, this made it difficult to develop reliable laboratory tests. The
sporadic nature of the disease and relatively low losses attributed to MCF in
cattle made it a low priority for research funding agencies.
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As far as is known, the disease has been present for hundreds of years.
However, it has been recognized as a defined disease only within the past 80
years or so in the U.S. Reports of an MCF-Iike disease in cattle in North
America appeared as early as the 1920s. The first well-documented case of MCF in
bison occurred in South Dakota in 1973 and was reported in the scientific
literature in 1977.
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Yes. There are reliable scientific reports of MCF in both European and
American bison (Bison bonasus and B.bison, respectively). MCF occurs in the
Plains subspecies (B. bison bison), which is the basis of the commercial bison
industry in North America. We are not aware of confirmed cases of MCF in
Woodland bison (B. bison athabascae) but we recently found that they are
subclinically infected at about the same rate as are the Plains bison.
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Epidemiological studies of field outbreaks strongly suggest that sheep
infected with OvHV-2 are the principal source of MCF outbreaks in bison and
cattle. A strong association between outbreaks in cattle and recent exposure to
sheep has been documented repeatedly since 1929. Recent data indicate that a
similar pattern occurs in bison. In some outbreaks, however, no sheep were in
the vicinity immediately prior to the first case being identified.
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Lambs are infected after birth. Infection occurs between birth and 4 months
of age, with most Iambs infected by the age of 4 months or earlier. Placental
transmission is possible but rare. Horizontal transmission through physical
contact with flock mates is the major mode of transmission among sheep. High
levels of OvHV-2 viral DNA are consistently found in nasal secretions of sheep,
suggesting that the nose is an important portal for OvHV-2 shedding. The virus
does not cause illness in sheep.
Cattle, bison, and deer are also infected by horizontal transmission from
sheep, apparently through transfer of virus via nasal secretions.
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There is no evidence that transmission occurs horizontally from one bison to
another. Currently we are doing a study supported in part by the National Bison
Association to establish whether bison-to-bison transmission is a factor in
natural outbreaks.
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Natural infections occur in many species of deer such as elk, reindeer and
moose, as well as a variety of exotic ruminants. The disease can be transmitted
experimentally to laboratory species, but there is nothing to indicate these
species are involved in natural transmission. MCF was recently reported in pigs
for the first time.
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Most infections are characterized by depression, separation from the rest
of the herd, loss of appetite, and in many bloody diarrhea. Unlike MCF in
cattle, discharge from the eyes and nasal passages of affected bison is minimal.
Animals develop a fever and may pass bloody urine. The clinical course is
generally 1 -7 days. Most animals die within 3 days of developing clinical
signs.
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Yes. There are generally no lesions in the fetus or placenta. It is assumed
that abortion is a result of illness in the dam, rather than infection of the
fetus. Dams die shortly after they abort.
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No. There is no effective treatment for MCF in bison or cattle. There are
scattered reports in the scientific literature of animals recovering from MCF
following medical treatment. In our experience treatment merely prolongs the
clinical course without effecting a cure
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No. Attempts to develop a protective vaccine to MCF in cattle using the
wildebeest virus, alcelaphine herpesvirus-1 (AlHV-1), were unsuccessful. No
comparable studies have been done on bison to date.
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Yes. Bovine viral diarrhea, salmonellosis and pneumonia complex, among
others, can be confused with MCF: Testing sick animals is the key to
establishing an accurate diagnosis.
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Confirm the suspicion of MCF by laboratory tests on sick or dead animals.
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Three tests are available: histopathology, serological testing for MCF and an
OvHV-2 specific test using the polymerase chain reaction (PCR).
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The best one to use for diagnosing active disease is the PCR test. Blood
collected into an EDTA-anticoagulant tube (purple-top tube) is the best sample
to obtain from live animals. Both the PCR test and a newly developed serological
test can be run on one purple-top blood sample.
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The carcass of an animal that dies of MCF-like signs should be examined
post-mortem by your veterinarian or better still a pathologist at a veterinary
diagnostic laboratory. A specific set of tissues should be collected and held at
4° C (or frozen if there will be a delay getting samples to a laboratory) to
confirm the suspicion. A second set of tissues should be collected into a 10%
solution of formalin for microscopic examination. The best tissues to collect
for microscopic examination are kidney, liver, bladder, lymph nodes in thorax
and abdomen, pampiniform plexus (blood vessels to testes), lung, spleen, cecum
and colon, plus ANY tissue that looks grossly abnormal.
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Animals older than 6 months, particularly if stressed by bad weather,
transportation and handling. Large outbreaks occur in feedlots, where stress due
to crowding is likely.
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Most outbreaks occur in bison in winter months (December-May). Losses due to
MCF can, however, occur at any time of the year.
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Little information has been collected to answer this question. We have seen
outbreaks continue for 16 months in range herds. In feedlots the problem can be
a continuing yearly source of losses.
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With currently available test methods, between 15 and 23%. This is a minimal
figure. No test is perfect. It is entirely possible that the tests currently
being used for identifying infected normal bison miss a significant percentage
of animals. The true rate of subclinical infection could be significantly higher
than this.
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Following transmission, the MCF virus is presumed to infect lymphoid tissues
(spleen and lymph nodes) and cells lining the gut, urogenital tract and nasal
passages. The severe inflammation generally causes gastroenteritis, with
widespread formation of ulcers particularly in the large bowel. This is the
basis for bloody diarrhea, a common and useful diagnostic sign of MCF in bison.
The virus causes encephalitis (inflammation of the brain), panophthalmitis
(inflammation of the eye) and vasculitis (inflammation of bloodvessels).
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We are aware of cases in 11 states (Utah, Wyoming, Colorado, Montana,
California, Oregon, Ohio, Kansas, Nebraska, North and South Dakota) and three
Canadian provinces (Saskatchewan, Ontario and Alberta). The disease is
widespread in the bison industry and is commonly misdiagnosed due to lack of
specific clinical signs. Many veterinarians are not aware that the post-mortem
changes of MCF are somewhat different from those in cattle, and that additional
tests are required to make a firm diagnosis of MCF.
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Our limited knowledge on this subject suggests that bison may be infected for
extended periods, in some instances years, before developing MCF. More work is
needed on this aspect of the disease.
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There is no reason at this time why such animals should be culled. An
important question we are trying to answer is whether healthy test-positive
bison are at a significantly higher risk of developing MCF compared to healthy
test-negative bison.
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In most instances by contact with sheep. The virus is shed from the nasal
passages. It is not clear precisely how the virus is transmitted from sheep to
bison, but it is probably via the nose and/or mouth. There may be other avenues
of transmission that have not yet been recognized.
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MCF virus is transmitted by sheep in most instances. There is no documented
evidence of person-to-bison transmission. Normal veterinary infection control
precautions should be followed when caring for a bison suspected to have this
viral infection.
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Since there is no evidence that infectious virus is shed from affected bison,
disinfection is not considered to be a worthwhile endeavor, as it will
contribute little or nothing to controlling the disease.
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We think it is rare. There are inherent problems in tracking diseases like
MCF in wild bison. The number of studies of MCF in public herds is small.
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Two things:
- Keep sheep and sheep wastes away from your herd.
- Keep stress to a minimum, since excessive handling appears to be a risk
factor.
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The laboratory of Drs. Tim Crawford and Hong Li at Washington State
University has been engaged in basic research on MCF for the past 10 years. The
laboratory can test animals for evidence of antibodies for MCF and for DNA
unique to OvHV-2 as well as other members of MCF virus group. You or your
veterinarian can contact them by phone (509-335-6035 and 509-335-6002) or email
(crawford@vetmed.wsu.edu; hli@vetmed.wsu.edu). The cost per ELISA is 6.00/blood
sample for out-of-state parties, plus a submission fee of 10.00 (one-time).
There is a 20% discount on all samples over 10. Dr. Donal O'Toole is a
veterinary pathologist at the University of Wyoming. You or your veterinarian
can reach him at 307-742-6638, by fax at 307-721-2051, or by email dot@uwyo.edu
at the Wyoming State Veterinary Laboratory. The three of us can make available
additional information such as reprints of scientific articles published on MCF
over the past 10 years.
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Yes. There are several laboratories in Canada offering the test at present.
Those include the Abbotsford laboratory in BC (604-556-3003) and the Prairie
Diagnostic Service in SK (306-966-7316).
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Q and A from a Field Veterinarian
A cattle herd (about 40 calves) has been with sheep (150 ewes) around for a
couple of years. This winter (2002), the owner got some calves from a
dairy and they were going to custom raise them and co-graze them with the sheep.
Around May 1 a heifer acutely developed bilateral corneal edema, fever, and
depression. Later it developed salivation and the animal was euthanized in
5 days. Post mortem revealed that there were oral lesions but little else
except the eyes. It was eventually positively diagnosed with the ovine
strain of MCF by NVSL. Within 3 weeks another calf developed corneal edema but
was fine for 3 weeksand then suddenly got sick and died within 2 days.
Post mortem was not done on this animal due to a short notice. The owner
has another one that has been blind 6 weeks but the animal is still alive
although it is thin (presumably from being blind). Calves and sheep were
separated on initial symptoms although now by a great distance. Separation
is good now and they are on separate pastures
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Couldn't put a number on it, but there is definitely a chance of a few cases
over the next few months, from recrudescence of infections established recently.
They should trail off, however.
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It would only be academic at this point. You can assume all the sheep
are carriers (over 98% are). Trying to determine how many of the cows
became infected recently is not only fraught with the possibility of error due
to lack of a perfect test, but the results have little predictive value of
disease, anyway.
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No, not a problem. The virus comes from sheep by contact, but does not
spread from cow-to-cow by contact.
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