Effects of M. haemolytica Following Exposure to PI Calves on Performance and Health
ACVIM 2008
D.L. Step, DVM, DACVIM; Clinton Krehbiel, PhD; Luis Burciaga-Robles, MVZ, MC; Robert Fulton, DVM, PhD, DACVM; Anthony Confer, DVM, PhD, DACVP
Stillwater, OK, USA

Bovine Respiratory Disease (BRD) was first described in the late 1800's. In dairy calves the disease is commonly referred to as Enzootic Calf Pneumonia (ECP) accounting for 24.5% mortality in pre-weaning heifers and 44.8% mortality in weaned heifers. In beef cattle the disease is more commonly referred to as Shipping Fever Pneumonia and is considered as the major cause of death, clinical disease and results in significant economic losses to the beef cattle industry. More obvious losses associated with morbid cattle include medicine costs for treatment, labor required to handle the cattle, decrease in facility efficiency, and potential decline in morale of employees when animals do not respond to treatments or die. Less obvious and frequently unrealized economic losses can be attributed to decreases in carcass weights, ribeye areas, marbling (lower quality grades), and tenderness (less satisfaction than anticipated eating experienced for the consumer). In the last couple of decades, newer antimicrobial drugs have been approved for the treatment and/or prevention of BRD in cattle. Despite advances in antimicrobial medications, the number of deaths in feeder cattle per 1000 head reported by NAHMS has increased from 10.3/1000hd in 1994 to 14.2/1000hd in 1999 to 17.5/1000hd in 2003.

BRD is multifactorial in nature. The generally accepted pathogenesis of BRD in beef cattle involves an initial infection with a respiratory virus, which changes the normal host clearance mechanism of the mucociliary tree. Additionally, some of the respiratory viruses can result in immunosuppression. Bacteria such as Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni are common inhabitants of the nasopharynx and are thus common opportunistic microorganisms associated with BRD. Stress is the triggering agent. Cattle stressors include weaning, transport (shipping), commingling, dehydration, changes in weather, changes in diet, and viral infections including suppression of the immune system. Replication of these opportunistic bacteria occurs with inhalation into the lungs following a stressful period in cattle.

Bovine Viral Diarrhea Virus (BVDV) was first described in the US in 1946. This virus has received much attention recently and is known to cause immunosuppression by interfering with the normal function of lymphocytes and macrophages. Different strains or genotypes of BVDV have preference for different tissues and organs. The various strains of the virus have been associated with enteritis, lameness, abortions, congenital anomalies, and respiratory disease. If a non-cytopathic biotype infects a susceptible female, crosses the placenta and infects the developing fetus between 45-125 days of gestation, the calf will be born persistently infected (PI) with the virus. These PI animals are a source of acute infections for other cattle, especially during the commingling process that commonly occurs in cattle production in North America.

BVDV has been documented serologically in several studies involving BRD. Investigators in Quebec investigated 11 different outbreaks of BRD in calves and adults by analyzing serum samples. The results indicated that 92% of the calves seroconverted to BVDV and 65% of the calves seroconverted to 2 or 3 respiratory viruses while 38% of the cows seroconverted to 2 or 3 of the viral agents. Studies involving commingled weaned beef calves have concluded that infections of both Type 1 and 2 BVDV can be identified in cases of bovine respiratory disease and cattle in the feedlot that were treated for BRD had greater increases in BVDV titers than non-treated cattle with BRD.

Other studies have examined arrival titers of feedlot beef calves and its association with performance and morbidity. A Canadian study was able to demonstrate that the arrival BVDV titer was associated with risk of developing Undifferentiated Fever (UF). Cattle with higher arrival titers to respiratory viruses involving BVDV were associated with increased weight gains during the feeding period while animals that showed an increase in titer post-arrival were associated with decreased weight gains.

Herd immunity to BVDV in cow/calf operations can influence the future health in the calves. In a study evaluating a retained ownership program in which the calves were vaccinated prior to arrival at the feedlot and weaned 45 or more days, low titers on a herd basis to BVDV reflected higher morbidity in calves during the finishing phase of production.

BVDV can be associated with other bovine pathogens. Using immunohistochemistry (IHC) staining techniques, BVDV has been documented with other common pathogens such as Mycoplasma spp and Histophilus somni in cases of pneumonia, septic arthritis, and myocarditis.

The Bovine Respiratory Group at Oklahoma State University has been working on a model that more closely represents field conditions to study the effects of BRD on the immune response, performance effects, and changes in metabolism in feeder cattle. The model includes treatments comparing challenge with Mannheimia haemolytica with or without prior exposure to BVDV. The model includes a non-challenge negative control treatment group. Twenty-four steers were used to evaluate the effects on health and performance following challenges with or without exposure to BVDV. Precautionary steps were taken to prevent cross-contamination between the treatment groups. Antibody titers confirmed that pathogen challenge occurred and that the assigned treatment group seroconverted accordingly. Clinical observations typical of cattle suffering from respiratory disease were different between treatment groups and the non-challenge controls. Challenge with M. haemolytica and/or BVDV affected rectal temperatures, total WBC and differentials, and acute phase protein production. Average daily gains (ADG) were significantly affected within the first 7 days following challenge. Even though feed intakes and feed efficiencies were numerically different between the different treatment groups, no statistical differences were measured. It was concluded that the model was successful in studying the abnormal clinical signs and physiological changes associated with BRD.

The results from this model will allow researchers an option when studying the effects in cattle with BRD.


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Speaker Information
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Douglas Step, DVM, DACVIM
Stillwater, OK

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