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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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