Cattle Health: To Deworm Or Not To Deworm?

COLLEGE STATION – Recent rainfall across the state has greened up rangeland, but with tender vegetation also comes the threat of wormy beef cattle, according to a Texas AgriLife Extension Service veterinarian. The key for producers is not to evaluate the cattle, but the ground they are grazing. “They must ask themselves the question, ‘Any wet grass over a period of weeks?,” said Dr. Floron Faries, AgriLife Extension program leader for veterinary medicine. To be ingested by cattle, the larvae of gastro-intestinal hairworms swim up the grass blades while the grass is wet from rain or dew and the soil temperatures are above 55 degrees. “As the grass dries, the larvae go back down with the evaporating moisture. When cattle graze dry grass, they do not ingest larvae because larvae cannot crawl up dry grass,” Faries said. While the grass is dry, the cattle do not get worms. After weeks of dry weather larvae die and, without grass mats for cover, the pastures become free of contamination. Pastures during drought conditions and under good management practices are not contaminated. “The key is to time deworming when larvae are developing in the lining of the stomach and intestines in the spring or fall,” he said. “That may also occur during the summer before the larvae emerge in early fall.” While pastures are contaminated with larvae, it may be cost-effective to worm if the cattle have become exposed to enough larvae during three to six weeks of continuous rains when the larvae are active in soil temperatures of 55 degrees to 85 degrees. “If these conditions occur in the spring, summer or fall on contaminated pastures, the time to deworm cattle to kill the developing and inhibited larvae is following three to six weeks of continuous rain,” Faries said. “When the timing is right for deworming, the cost-effectiveness is expected in nursing calves and young cattle since they are more susceptible, but is not always expected in adults since they are more resistant.” If rains do not come, the cost-effectiveness of deworming cattle is questionable, Faries said. “When it is dry in the spring, do not deworm; wait until the cattle get the worms. Spotty rains this year may be an indication that it is coming for some, but that has not been for many places.” The farms that didn’t receive rainfall during last fall “did not receive pasture contamination going into the winter,” Faries said. Contamination on any farm was lost during the winter due to pasture larvae killed by desiccation if the farm has been under drought conditions. “Cattle on drought pastures since last fall do not have worms and do not need deworming at this time,” he said. “One must wait for the right time on their farm. It’s understood by science that deworming drought-pastured cattle would not result in economic benefits.” Meanwhile, recent rainfall could lead to blackleg in calves, which can occur after they swallow blackleg spores. “This means the ground is contaminated with the spores that never die,” he said. “During rains, these spores are normally concentrated by surface water in various spots in the ground, and drought or rains will cause them to surface from the soil. When ingested by a calf, the spores go to the muscles and remain dormant.” Spores can break out of dormancy months or years later, Faries said. The bacteria multiply rapidly and produce toxins in the muscles, killing the muscles (black dead muscles), causing blood poisoning and sudden death. The most common trigger is fast growth. “Another trigger is muscle exertion, such as that caused during working, weaning and hauling,” he said. “Affected calves may be infected at an early age and die of blackleg at a later age. When blackleg occurs, the transmission was not necessarily recent, but possibly months ago.” Sudden death and rapid, gaseous decomposition are the most common signs of blackleg. “The death is so rapid that treatment is normally ineffective,” Faries said. He advises all dead calves be burned with untreated wood products to keep from contaminating the ground. “Because other calves can have the bacteria in dormancy, guard against triggers such as stress and rapid growth. Vaccinate the remaining calves,” he said. “If these calves die, they were already infected with the dormancy of blackleg bacteria before vaccination. Vaccination after exposure will not prevent the dormancy from breaking out.” A proper vaccination program includes annual vaccination of the entire herd (calves, cows, heifers, bulls), “not just calves,” Faries said. “Grown cattle die from four of the seven different blackleg-type bacteria. Cows should be vaccinated during last three months of pregnancy or twice a year.” Editor’s note: The following can be used as a breakout box. Quick tips on deworming and blackleg in beef cattle: - Deworming cattle: Consider the question, ‘Any wet grass over a period of weeks?’ To be ingested by cattle, larvae swim up the grass blades while the grass is wet from rain or dew and the soil temperatures are above 55 degrees. - Blackleg vaccine: The seven-way blackleg vaccine should be used because other strains in addition to blackleg that also cause sudden death can be present. The seven strains can be diagnosed only in a dead calf by necropsy and laboratory tests. In addition to blackleg, the other six clostridial diseases that cause sudden death are black neck, black liver, malignant edema, and B, C, D enterotoxemia. Source: Dr. Floron Faries, Texas AgriLife Extension Service.