Case Study: PI exposure can impair performance

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Bovine viral diarrhea virus (BVDV) can leave animals with symptoms that can reduce production efficiency. Fever, pneumonia, diarrhea and compromised immunity are among the telltale signs of infection with the group of viruses that cause bovine viral diarrhea, an economically significant disease that affects cattle herds throughout the world. Calves exposed to a BVDV in utero may develop persistent infections and shed the virus throughout their lives. Post-birth exposure to BVDV usually leads to acute infections that last 7–10 days.

With lifelong compromised health, persistently infected (PI) cattle are obviously a drain on economic resources, but they may be even more costly than previously assumed. A collaborative study involving scientists from the Agricultural Research Service (ARS) shows that PI cattle can actually decrease the profitability of surrounding cattle — even those that never develop clinical disease. This work was published in the January 2009 issue of the American Journal of Veterinary Research.

PI cattle have higher mortality rates and lower production efficiency than other cattle. But the economic consequences of BVDV don’t end there, according to a study initiated by Bill E. Hessman, DVM, Haskell County Animal Hospital in Sublette, Kan. In collaboration with ARS and university colleagues, Hessman showed that after exposure to PI cattle, non-PI cattle had higher morbidity rates and lower production efficiency than cattle with absolutely no exposure to PI animals.

Microbiologist Julia Ridpath, PhD, ARS National Animal Disease Center, Ames, Iowa, helped design and analyze the study, which was conducted in a newly constructed feedlot. The collaborators tested 21,743 calves as they entered the feedlot. They identified PI animals, characterized the BVDV strains present, and tracked the spread of strains within and between pens.

Some pens held one or more PI cattle. Others had no PI cattle, but were adjacent to infected pens. The remaining pens neither held infected cattle nor adjoined infected pens. The scientists found that the mortality rates were 25.6% for PI cattle and 2.4%, overall, for non-PI cattle. Of the non-PI cattle, those that were exposed to PI cattle had a mortality rate of 3.6%, and those that had no exposure had a mortality rate of 1.7%.

The higher mortality and morbidity rates due to PI exposure have been reported previously. But this study was one of the first to compare performance outcomes, such as production efficiency, of PI-exposed animals and non-PI-exposed animals. Production efficiency, based on the ratio of feed intake to weight gain, for PI-exposed animals was less than half that of non-PI-exposed animals. This is a significant observation for livestock producers because it demonstrates that the economic damage incurred by exposure to PI animals is not limited to increased treatment costs. Even PI-exposed animals that remained clinically healthy gained weight less efficiently than non-PI-exposed animals.

Based on this study, estimated economic losses caused by exposure to PI cattle could be between $40 and $90 per animal, due to increased mortality and morbidity and decreased performance.

This information was excerpted from the Agricultural Research Service’s April 2010 Healthy Animals Newsletter. 



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