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Quest for a vaccine to counter CLA.

Quest for a Vaccine To Counter CLA

The sheep disease known as caseous lymphadenitis (CLA shortens lifespans, reduces wool production, and makes carcasses unfit for consumption. The U.S. sheep industry loses about $250,000 annually from carcasses being condemned because of CLA lesions, says Kim A. Brogden, ARS microbiologist at the National Animal Disease Center in Ames, Iowa.

Caused by the bacterium Coryne-bacterium pseudotuberculosis, the disease produces severely enlarged lymph nodes and ugly lesions filled with pus. The bacterium is a small, club-shaped rod that gets its long name from the tuberculoid lesions. The organism may spread to the lungs and other internal organs, inducing large fibrous abscesses.

Milk production in dairy goats infected with CLA is severely reduced. In sheep, the bacterium has been associated with thin ewe syndrome. In addition, infected animals experience much discomfort.

All U.S. sheep and goats are susceptible. Once an animal is infected, the disease persists the entire life of the animal and can slowly infect the entire flock. "And it never gets better," says Brogden.

In Australia, CLA is the leading cause of economic loss to the sheep industry. Here, CLA is the third leading cause for condemnation of sheep carcasses at slaughter, following emaciation and pneumonia.

"Veterinarians the world over would welcome an improved vaccine for this insidious disease. A company selling this type of product could hope to sell 100,000 doses in the first year," says Don E. Bailey, executive secretary of the American Association of Small Ruminant Practitioners. Bailey lives in Roseburg, Oregon, where he keeps a 300-head flock of sheep. He says about 5 percent of them are infected with CLA.

CLA is spread by contact among sheep, and the bacterium can live for months in dust in barns, shearing sheds, and corrals.

"Because the bacterium becomes walled off in the enlarged lymph nodes, treatment with antibiotics is useless, which makes it all the more important to protect animals in the first place via vaccination," says Brogden.

Research to develop an effective low-cost vaccine began 6 years ago at the National Animal Disease Center.

Brogden made the first vaccine for CLA from killed whole cells of the bacterium and tested it on naturally infected sheep and goats in field trials done in cooperation with researchers at the University of Guelph, Ontario, Canada.

Veterinarians Paula Menzies and Anne Muckle found the vaccine effective in reducing the number of animals that would develop the disease naturally. Fewer than 6 percent of the vaccinated goats developed abscesses, compared to almost 12 percent of the nonvaccinated goats. In sheep, 6 percent of the vaccinated sheep developed lesions, compared to 34 percent of sheep that were not vaccinated.

Results of this study will be reported in the Canadian Journal of Veterinary Medicine.

The Canadian field trial revealed a serious side effect of giving the first-generation vaccine in a 5-mg dose. Animals receiving it reacted with redness, swelling, and lameness at the vaccination site. Almost 30 percent of the goats and 34 percent of the sheep developed lumps from the vaccination.

To overcome this problem, Brogden lowered the dosage of immunizing material. And to keep the same level of protection, he added an immune booster called muramyl dipeptide.

Muramyl dipeptide was developed by Louis Chedid, M.D., who is with VACSYN in Tampa, Florida. Chedid provided the MDP to Brogden under the provisions of a cooperative research and development agreement between VACSYN and USDA.

So far, the current field studies of the second-generation vaccine are being done in cooperation with researchers at the University of California at Davis. Veterinarians John S. Glenn and Nancy East and sheep specialist Rob Rutherford report on side effects from this vaccination after the first year. The results of this trial will not be ready for another year or two. Animals exposed to the bacterium may not show signs of the disease for almost 2 years.

Brogden says of the lower dose vaccine: "It's economical, nontoxic, and fully potent - important concerns for animal protection products." - By Linda Cooke, ARS.

Kim A. Brogden is at the USDA-ARS National Animal Disease Center, P.O. Box 70, Ames, IA 50010. Phone (515) 239-8287.

PHOTO : Microbiologists Howard Lehmkuhl and Kim Brogden collect blood from a sheep to test for antibodies against CLA. (K-4030-1)

PHOTO : Electron micrograph of Corynebacterium pseudotuberculosis, a club-shaped bacterium, the causative agent of CLA.
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Title Annotation:caseous lymphadenitis
Author:Cooke, Linda
Publication:Agricultural Research
Date:Apr 1, 1991
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