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Combatting cattle disease

There is an old adage that says an ounce of prevention is worth a pound of cure. When it comes to using vaccines to help control and prevent the spread of disease in animals, that saying rings true.

Vaccinating animals not only improves the overall health of livestock but also reduces the high costs involved in treating animals that become ill. However, some vaccines don't work very well or lose efficacy over time, and new vaccines are needed.

No effective solution

Scientists at the Agricultural Research Service (ARS) are addressing these issues by developing more effective vaccines to combat troublesome diseases like anaplasmosis in cattle.

Also known as yellow bag or yellow fever, this disease can be difficult to detect, especially in the early stages. Clinical symptoms are very seldom seen in young cattle because they are replacing red blood cells at a faster rate than older cattle.

Mainly spread by ticks, the pathogen invades and destroys red blood cells of cattle and other ruminant hosts. Severe infections cause anemia, weight loss, abortions, and sometimes death – the latter being the fate of 50,000 to 100,000 U.S. cattle annually. Those surviving the disease become lifelong carriers, endangering other herd members and impeding U.S. cattle trade.

While ticks and blood-sucking insects are the primary culprits for spreading the disease, proper hygiene can significantly reduce the risk of contamination from equipment especially during castration, implanting, vaccinating, and dehorning.

Though antibiotics can kill A. marginale, a vaccine for cattle would keep the pathogen from infecting the animals to begin with. But besides posing safety issues, vaccination has been hampered by uneven performance, and there is currently no widely accepted vaccine. A contributing factor is A. marginale's ability to reconfigure its surface proteins and to evade detection by an animal's immune system.

Research shows promise

An answer may be on the way. ARS molecular biologist Susan Noh, who is based at the Animal Disease Research Unit in Pullman, Washington, collaborated with scientists at Washington State University to identify significant proteins to include in a potential vaccine that is being tested on animals. They found that small groups of the outer surface proteins of A. marginale induce an immune response that reduces symptoms and also prevents A. marginale infection in some animals.

Among the vaccines being tested, some of those with the most potential have protected 80% to 90% of the animals from clinical disease and have prevented infection in up to 40% of the animals, Noh says.

“This is significant because infected animals may have no clinical evidence of infection, yet they serve as sources of infection for others,” she says. “No vaccine has ever prevented infection from A. marginale in cattle.”

In other countries, a weakened strain (A. centrale) has been used as a vaccine that protects against clinical disease, but not infection, Noh says. This type of vaccine is prepared from live microorganisms or viruses that are cultured in the lab in such a way that they lose their intensity but still give disease immunity.

Thus far, scientists have only tested the vaccine against one strain of Anaplasma, but there are many strains that coexist in the field. Their next step will be to determine whether this particular group of proteins will protect cattle against various strains of Anaplasma. ●

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