A recent study was published addressing the long-term effects of the presence of persistently infected (PI) cattle among groups of calves throughout the feeding period. PI cattle are known to shed Bovine Viral Diarrhea (BVD) virus continuously and in large amounts throughout their lifetimes, spreading the virus to all cattle they contact. BVD virus is important due to its immunosuppressive effect that shuts down a calf’s ability to fight disease in the lungs. Does this negative effect last throughout the feeding period? Perhaps more importantly, how do these findings apply to backgrounded calves in KY?

Trial Details (see diagram): The first trial involved 184 calves (steers and heifers) averaging 500 pounds from two Alabama farms and all were tested and declared BVD PI free (study did not specify if calves were weaned). They were divided equally into two groups of 92 head based on weight, sex, frame score, and farm of origin. Each group of 92 was loaded on a truck but to the second truck was added 2 known PI calves, one with Type 1 and one with Type II BVD infection. The calves were then trucked 1120 miles to Michigan and unloaded into two pens located 600 feet apart. Within 24 hours, all of the calves were processed and each received blackleg vaccine, Mannheimia toxoid (“One Shot”), pour-on dewormer, and an implant. In each of the two pens, half of the calves received a 5-way respiratory vaccine (IBR-PI3- BRSV and 2 types of BVD) and half received a respiratory vaccine with IBR-PI3-BRSV but no BVD vaccine. Pen 2 had 2 known PI calves present throughout the feeding period. All calves were fed for 168 days during the summer and fall then transported to slaughter. Throughout the feeding period, the calves were weighed and samples of blood, serum, and nasal swabs were collected and analyzed for BVD virus. Two calves died during the trial; one from each pen due to pneumonia. “Retreatment” was defined as a calf re-treated within 48 hours of first treatment. “Repull” was defined as treatment again at least 7 days after initial treatment. Drugs used for treatment were not given.

The Quiz:

1. Which pen had more calves treated for pneumonia, Pen 1 or Pen 2? Answer: Pen 2. Calves running with PI cattle during the feeding period (Pen 2) were twice as likely to be treated for pneumonia as were calves not exposed (Pen 1).

2. Of all the treated calves, calves in which pen required retreatment (a second shot of antibiotic within 48 hours of the first dose) most often? Answer: Pen 2. Calves treated for pneumonia and running with PI calves in the same pen were 5.7x more likely to require retreatment than those with no PI exposure.

3. Among all calves treated for pneumonia, which pen had the most repulls (2nd dose of antibiotic at least 7 days after the first treatment) or were the repull rates the same? Answer: The repull rate was the same for both pens.

4. Which calves gained more in the first 30 days, Pen 1 or Pen 2? Answer: Pen 1. The average daily gain (ADG) was greater in the first month for calves not exposed to PI cattle; this coincides with the period when most of the sickness was observed.

5. Which pen of calves gained better by the end of the 168 day feeding period (Pen 1 or Pen 2) or did they gain the same? Answer: The ADG did not differ between treatment groups on day 168. This suggests calves in Pen 2 adjusted to the feedlot, developed immunity against disease, and recovered from illnesses allowing those calves to “catch up” (have compensatory weight gain).

6. Of the calves treated for pneumonia at least once, were most vaccinated against BVD (Groups 1a and 2a) or not (Groups 1b and 2b) or were treatment rates the same? Answer: Treatment at least once for pneumonia was the same for calves that were and were not vaccinated against BVD virus.

7. Of the calves treated for pneumonia at least once, which were less likely to require retreatment within 48 hours- those vaccinated or unvaccinated for BVD virus? Answer: Vaccinated. Calves treated for pneumonia but had been vaccinated against BVD were 57% less likely to require retreatment than those unvaccinated.

8. In Pen 2, was the sickness rate higher, lower, or the same for calves vaccinated against BVD virus (Group 2a) as those not vaccinated with BVD virus (Group 2b)? Answer: Lower. Calves constantly exposed to PI cattle but administered a modified-live (MLV) BVD vaccine at arrival had less sickness than unvaccinated pen mates.

Take Home Messages:

• Pens with PI calves have more sickness and retreatments for pneumonia, at least during the first 30 days, than pens without PI calves.

• Early setbacks in weight gain due to sickness may be overcome (at least in this trial).

• Vaccination with a modified-live BVD virus vaccine decreased the amount of sickness ina pen with PI exposure and decreased the need for retreatment in all cases of pneumonia.

• The magnitude of the effect of constant PI exposure on the health and performance of calves depends on many factors, including the type and age of cattle, genetics, management practices, nutrition, immune status and additional stressors such as transportation, weather and presence of other diseases.

Is it possible to apply these findings to backgrounded calves in KY? The effect of a PI calf in a pen of high risk calves largely depends on how “mean” or virulent the strain of BVD virus that the PI calf is carrying and spreading. Not all BVD viruses are the same; some are relatively mild while others are extremely immunosuppressive and deadly. BVD virus also has the ability to mutate or change into a meaner, deadlier form. In KY, we must also deal with poor calf immunity due to deficiencies of essential trace elements such as selenium and copper, and other management problems such as weaning on the truck and leaving bulls intact forcing late castration that pose additional hazards for purchased calves. Lastly, KY weather is a roller coaster of temperature and humidity throughout the year that can have a profound effect on health not observed in other parts of the US with more consistent weather patterns. Ultimately, vaccination of calves against BVD virus in combination with identification and immediate removal of PI calves will minimize the effect of BVD virus on backgrounded cattle.

Source: Grooms DL, Brock KV, Bolin SR, et al. Effect of constant exposure to cattle persistently infected with bovine viral diarrhea virus on morbidity and mortality rates and performance of feedlot cattle. J Am Vet Med Assoc 2014;244:212-224.