Bovine respiratory disease complex (BRDC) is usually regarded as a disease of feedlot cattle. It is the leading cause of death in Australian feedlots due to the combination of predisposing stress factors and infectious agents1,2. These two cases of BRDC were seen in grass fed steers on good quality and quantity pastures and water with no crowding or inclement weather. In the first case stress factors could be identified in the calves having been recently weaned transported, co-mingled and introduced to a new location and cohorts. In the second case co-mingling was the only identifiable stress2,3. Identifying the causal agents in both cases helped in understanding the disease progression and response to treatment.
Two properties in the Yass district suffered an outbreak of respiratory disease in weaner cattle recently purchased from the calf sales in the autumn of 2014. The first property had purchased 4 lots of Angus steers, to make a mob of 120. The second property added one lot of 30 Hereford steers to his own during yard weaning to make a mob of 90. On both properties water and feed quality was excellent and the weather conditions were mild. The steers were grazing in large paddocks of fertilised improved pasture.
The District Veterinarian was called to both properties after they had each lost 2 steers in the previous weeks, and now had a third dead with more showing signs of depression. Clinically, on the first property those affected had an elevated temperature between 40° and 41°, rapid respiration and heart rate, a silent rumen and an occasional soft cough. Some were lame and some had a small amount of nasal discharge. On the second property the most obvious clinical sign was depression and dehydration, with steers being found drooling and being reluctant to feed or move. No nasal discharge or coughing was noted, but temperatures were elevated to between 40° and 41°.
Occasionally only one agent is involved in bovine respiratory disease, but more commonly obvious disease results from a bacterial infection, secondary to viral infection.2 On both properties 2 of the cohorts were tested for underlying infectious viral agents.
The results showed that on the first property one of the affected steers was a persistent carrier of pestivirus, presumably infecting its cohorts and serology showed that bovine parainfluenza virus 3 (BPIV-3) was also circulating. A foot swab showed that the lameness was associated with an infection with an unstable isolate of Dichelobacter nodosus. On the day of the post-mortem 12 steers were identified as requiring treatment with oxytetracycline (oxytetracycline 100 mg/ml, Engemycin, MSD Animal Health) for either respiratory disease and/or painful foot rot. This property lost 4 of the 120 purchased steers but losses stopped shortly after the initial treatment.
On the second property bovine respiratory syncytial virus (BRSV) was demonstrated to be circulating with the Histophilus. The combination of these two organisms resulted in a 3 week outbreak of disease on the property requiring intensive management to prevent losses. The steers were observed daily for signs of depression and pulled out of the paddock for antibiotic and anti-inflammatory treatment as required. Clinical signs of illness would progress to death within 24-48 hours in untreated animals. A total of 17 of the 90 steers required antibiotic treatment and 4 steers died from the disease.
Mycoplasmas may be a component of BRDC in Australia5. While contagious bovine pleuropneumonia was ruled out as an exotic differential no other testing was conducted for mycoplasmas.
Bovine respiratory disease complex results from a combination of stress and infectious agents. On the first property the young cattle were presumed to have been weaned straight onto the truck prior to consignment to large weaner sales, before being split from their cohorts, comingled, then trucked again. Disease occurred within 2-3 weeks of arriving on the property. A persistently infected pestivirus animal within the sick steers presumably exacerbated this outbreak of both the respiratory disease and footrot4.
On the second property the Histophilus outbreak did not occur until 6 weeks after the steers had been mixed during yard weaning. Losses occurred in both bought and homebred cattle. Identifying the circulating bovine respiratory syncytial virus (BRSV) did help explain the outbreak but Histophilus can cause disease as the primary agent. It was decided that it was preferable to observe the steers and promptly treat only clinically ill animals rather than to blanket treat the whole mob with antibiotics to stop the outbreak. This recommendation has previously been adopted to stop losses occurring during Histophilus outbreaks3.
On both properties the management was extremely progressive demonstrating the risk of BRDC when purchasing and mixing different cohorts to background steers. While not always practical, one of these well managed properties may have reduced the risk of BRDC by buying weaners, direct from breeders that yard wean and have a pestivirus vaccination program in place.