Diseases of feedlot cattle

Summary

Six feedlots located in Eastern Australia were selected for participation in a detailed study of the causes of disease and deaths, especially the Bovine Respiratory Disease (BRD) complex. In order to assess possible seasonal variations in disease occurrence, a pen of animals was recruited at each feedlot in each season. These surveillance pens contained a total of 5,306 cattle while during the 18 month study period (October 1991 - March 1993) there were approximately 233,450 cattle on feed in the project feedlots. Blood samples from sentinel animals were examined for antibodies against the following four viruses - pestivirus (Bovine Viral Diarrhoea Virus or BVDV), bovine herpesvirus 1 (Infectious Bovine Rhinotracheitis or IBR virus), Parainfluenza virus Type 3 (PI3) and Bovine Respiratory Syncitial Virus (RSV) - at entry, after 6 weeks on feed and at slaughter.

At entry to the feedlot, there were antibodies present (indicating previous infection) against all four of the viruses surveyed. The levels of past infection with the respective viruses on entry of the cattle to the feedlot were: pestivirus 68%; IBR 13%; PI3 57%; RSV 27%. Of the animals susceptible to infection with pestivirus at entry, it was found that 68% became infected in the first 6 weeks. The comparable rates for the other viruses were: IBR 30%; P13 48%; RSV 57%. At the end of six weeks 9.8% were still susceptible to pestivirus. The comparable rates for the other viruses were: IBR 61 %; PI3 22%; RSV 31 %. By the time of slaughter, there were 6% of animals still seronegative and therefore susceptible to pestivirus. The rates for the other viruses were: IBR 24%; PI3 22%; RSV 29%. Disease was diagnosed in 6.8% of animals.

Fever at the time of entry to the feedlot and BRD are the most important types of sickness and account for 66% of c~ses of sickness. Seventy eight per cent of all cases of disease (including fever at entry) were found to occur in the first 4 weeks after entry to the feedlot. Viral infections were identified in 72% of sick animals and several of viruses were implicated. BVDV and IBR virus were found to be significantly associated with cases of BRD while BVDV, ISR and RSV viruses were associated with fever at entry in some feedlots. Bacteria were only cultured from 19% of sick animals. Pasteurella haemolytica was cultured much more frequently than P. multocida but most isolates were from animals with fever at entry, rather than respiratory disease. Deaths were reported from 0.9% of animals. It was found that 53% of the deaths investigated were due to BRO. Pestivirus and IBR were isolated with similar frequency. Pasteurella multocida was more common than P. haemolytica, in autopsy material. Other bacteria commonly isolated were Salmonellae and A. pyogenes. The IBR (BHV1) viruses isolated were all found to belong to the same subtype, 1.2, as found in Australia previously.

BRD was still found to be the most frequently diagnosed disease condition in most feedlots. Infections with each of the 4 viruses was common in sick cattle, either individually or in various combinations. The small numbers of sick cattle in most groups limited the capacity to apply an intensive statistical analysis of the relative importance of the 4 viruses studied. However, across all feedlots, IBR infection alone or combined with RSV infection was positively associated with illness due to BRD. When results from individual feedlots were examined, there were also significant associations of BVDV infection with respiratory disease and BVDV, IBR and RSV infections with fever at entry. The frequency and complexity of multiple infections and interactions between both viruses and bacteria and the intensity of sampling in this stu_dy limited the ability of this study to identify the role of viruses such as BVDV as predisposing agents to other infections.

Respiratory disease and fever at entry appear to be the most common disease problems in feedlot cattle. As most of the illness in these animals occurs in the first 4-6 weeks from the time of entry, control measures need to be effective at the time of entry to the feedlot, or even from the time of departure from the property of origin, when cattle begin to be exposed to a different range of pathogens and stress. As it usually takes animals at least three weeks to develop effective immunity by natural exposure or after vaccination, vaccination on entry to the feedlot will not provide optimal control of BRO. Vaccines and other control measures will preferably need to be delivered to animals on the property of origin and before mixing with animals of different origins. A sufficient lead-time will be required to provide optimal protection. Measures to assist cattle adapt to the changed environment are also likely to be of benefit. Strategies to provide adequate protection will require changes in both the steer production and feedlot sectors of the beef industry.