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California
Vet Pathologists
Study Wide Range Of Diseases
By David Bowser
DAVIS, Calif.
— As bovine spongiform encephalopathy has grabbed sensational
headlines for the past decade, veterinarians here at the California
Animal Health and Food Safety Lab's pathology department have
learned a great deal about the disease, but they also deal with more
localized problems in an effort to help the state's livestock
industry.
BSE enters an
animal's body through the digestive tract and travels up the spinal
column to the brain, but it takes three years, says Dr. Mark
Anderson, a pathologist with the California Animal Health and food
Safety Laboratory System.
"It takes a
minimum of three years," Anderson says. "There's no point in
examining the brain before 30 months of age."
Where it
first appears in the brain is at a nerve ganglia that supplies the
Vegas nerve, the nerve that supplies the function to the
gastro-intestinal tract.
"What we find
is that after a cow digests the material from meat and bone meal, it
gets into the intestinal tract, stays there for a long time and
eventually migrates up that Vegas nerve and first appears where the
Vegas nerve nuclei comes to the brain, near where the spinal nerves
enter the skull," Anderson says.
Pathologists,
Anderson says, have found that is where they could cut for the
earliest detection of the abnormal prions connected with BSE.
They've
devised a spoon with sharp edges on either side that can be inserted
into the hole at the base of the skull after the nerves on each side
have been cut, and with two swipes cut. That area is then pulled out
of the brain and used to diagnose BSE. That negates the necessity of
having to go in and take the skull apart.
Anderson says
he's seen people in slaughterhouses in Switzerland who can remove
the area quickly and efficiently with little more than a flick of
the wrist.
"There are
several tests that have been developed to try to detect the disease
before you see clinical signs," says Dr. Bradd Barr, another
pathologist in the lab. "Before these special antibody tests were
developed, you had to wait until the animal developed severe signs.
It could take five years sometimes."
An
examination of the diseased brain tissue under a microscope reveals
vacuoles, or holes, in the tissue, says Barr. The holes from which
the disease gets its name spongiform are not apparent to the naked
eye.
The antibody
tests were developed to try to detect the abnormal prions earlier.
"It's a very
unusual disease," Barr says.
Antibodies
attach to the abnormal prions symptomatic of BSE in cattle or
scrapie in sheep.
"There are
abnormal and normal prions," Anderson says. "The antibodies,
unfortunately, recognize both."
The testing
technique involves digesting the brain. Abnormal prions are
extremely resistant to acids and enzymes, so the tissue is treated
with harsh acids and harsh enzymes. That removes all the normal
prions. All that is left are the resistant, abnormal prions.
This
particular test, Anderson says, requires more time and work, so it
is used as the final confirmatory test.
"It not only
shows the prions, but as a backup, it's in the right area," Anderson
says.
When the
Davis lab was testing for BSE, they would test 96 samples of brain
tissue at a time in a small clear plastic plate with 96 indentations
for samples.
The abnormal
prions would stick to antibodies in the bottom of an indentation.
Then they add a second antibody with a color so any indentations
with abnormal prions would turn yellow.
"Wherever
yellow is," Anderson says, holding up a plate, "would be mad cow."
Three
indentations are control areas. If a color shows up, it is read by a
machine and the results are turned into numbers indicating density.
"If they see
a positive once, they run it again," Anderson says. "Sometimes you
can still get false reactions."
If it still
comes up positive, it is sent to the federal government. They run
this test again and another test and report that it is either
positive or negative.
England is
the authority when it comes to BSE, Anderson says.
The second
and third cows diagnosed with BSE in the United States, Anderson
says, had somewhat different varieties that are now being typed as
unusual or atypical.
But BSE isn't
the only disease with which the lab deals.
One of the
most common, and most puzzling, is epizootic bovine abortion, known
locally as Foothills Abortion.
"It usually
affects heifers," Anderson says. "It can be in older cows."
The disease
usually attacks late term.
"It's
transmitted by tick, and the tick lives in open woodland type
pastures," Anderson says.
That's the
type of pastures found in the foothills of the Central California
Valley.
The tick,
known as the pajaroello tick, is a soft-shelled tick that looks
similar to the common ear tick found in cattle.
The disease
is a major source of economic loss for California cow-calf
producers, according to John Maas of the University of California
Davis veterinary school. He estimates that as much as 10 percent of
the state's calf crop may be lost to it, or anywhere from 45,000 to
90,000 calves per year.
"With this
disease," Anderson says, "these are usually almost ready to be born
kind of fetuses. They're fresh. The striking thing about it is
they've been infected interuteral anywhere from three to four months
by this infection."
Cattle abort
because of that chronic infection.
"As we open
those fetuses up," Anderson says, "there will be an area underneath
their tongue with little hemorrhages. There may be a lot of fluid in
the abdominal cavity, maybe with some fiber. About half of them have
a thick, swollen liver."
The spleen
can also be enlarged.
The lymph
nodes, especially in front of the shoulder, will be enlarged.
Anderson says
one of the reasons they set up the veterinary school next door was
to figure out the cause and eradicate EBA or Foothills Abortion.
"We're still
working on it," Anderson says.
He says many
things have been incriminated and then ruled out over the years.
"With the
advent of molecular techniques where you're basically looking for
DNA with various agents, that has really pushed this in search for
the agent of EBA much more forward," Anderson says. "One of the
things that's been done is that they have mechanisms where they can
basically amplify bacteria DNA. It's a very generic amplification."
They take the
DNA that they get and sequence it, he says, and they figure out what
kind of bacteria it might be.
"That has
been done," Anderson says, "both in our lab and at the veterinary
school."
The results
show a rather novel bacteria, he says. It's apparently a bacteria
that cannot be cultured.
"We've tried
culturing it on many different kinds of culture media," Anderson
says.
But with the
information they have on the bacterial DNA, they have made progress
and eventually plan to develop a vaccine using basic techniques.
"It's a
pretty simple process," Anderson says.
They take
antibodies from calves that had the disease.
"We know that
when calves are infected — they're infected three to four months
before they abort — they really respond to this disease," Anderson
says. "They really put out a lot of antibodies. We know that because
we can test the blood and see what the antibody levels are. In some
cases, they can be at adult animal levels. There's a very strong
antibody response."
They've also
used some chemical tests that have proven to be quite effective.
"That's where
we're at today with this disease," Anderson says.
He says they
still need to figure out how to grow it in other tissue to make a
vaccine for the disease.
At least one
researcher has taken infected tissue and injected it into a
non-pregnant cow. He then challenged the cow and she was protected.
While this
helps manage the disease, Anderson acknowledges that it hasn't
helped eradicate it.
"For years,"
Anderson says, "this is the way we managed around this disease. You
take your open heifers and you put them in tick-infested areas.
Hopefully, they all get bit, and you're protecting most of them."
Right now, he
says, they have identified a bacteria that they are confident is the
cause.
"We've also
found the bacteria DNA in the ticks," Anderson says, "but we don't
know where the ticks are getting it. We know the ticks aren't born
with it. The earliest stage of ticks do not have the infection."
As they get
older, particularly the female ticks, they tend to acquire the
infection someplace in the environment.
“We don't know if they're
getting it from deer or mice or from the environment. The organism
seems to persist for a very long time in the tick, but we don't know
where it comes from in nature."
There is some
conjecture that because the ticks are found in bedding areas for
both deer and cattle, deer might be the primary host for the tick.
The larval
forms of the tick attach themselves to the host animal and feed for
10 to 14 days. If deer are host to the tick, this would allow
migrating deer to carry the tick to a variety of areas.
The mature
tick, however, feeds only 10 to 20 minutes, Maas says, making it
difficult to find the ticks on cattle even though the mature ticks
are visible to the naked eye.
Because the
ticks spend such a short time on cattle, external parasite controls
are generally not effective against the pajaroello tick.
The ticks
also feed on the hindquarters of cattle, so eartag insecticides do
not seem to be an effective control measure, Maas notes.
Research
indicates that the ticks need a blood meal every two or three
months, but ticks have lived in a laboratory environment for up to
three years between blood meals.
The ticks are
most active during the hottest, driest months of the year. Their
activity declines in the winter months when soil temperatures drop
below 45 degrees.
The ticks are
not found in wet areas.
Their peak
feeding time in northeastern California at elevations above 4000
feet is in June, July and August. At lower elevations and in warmer
parts of the state, the peak feeding time is between May and
October.
Heavy rains
appear to decrease the tick's activity, which may explain the
differences in occurrence of EBA from one year to another.
Maas says
it's thought that only a fraction of the ticks are capable of
transmitting EBA, but that is still being studied.
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