Most who work in the cattle feeding industry have a good feeling for their cattle's major health problems. It's based on experience, however, and there may be more than meets the eye.

This is especially true in light of new diagnostic technologies and emerging pathogens, says Dan Grooms, Michigan State University (MSU) College of Veterinary Medicine.

Historically, most surveys support the conclusion that bovine respiratory disease (BRD) is the major cause of feedlot cattle death in the U.S. In the Texas Ranch-to-Rail mortality summary from 1997-2000, BRD was cited for more than 50% of cattle deaths; bloat claimed another 20%. Similar results have been reported out of western Canada and Ontario.

This past year, as part of a larger project looking at feedlot morbidity and mortality in Michigan, Grooms and MSU veterinarian colleague, Paul Coe, followed 3,303 calves in seven feedlots from entry to harvest.

The calves made up 36 lots, ranging in size from 32-194 head. In-weights ranged from 400-800 lbs. Necropsies were performed on all cattle that died.

A total of 55 calves (1.7%) died. Cause of death is shown in Figure 1.

“The general conclusion is Michigan is no different from the rest of the U.S. in that BRD is the primary cause of feedlot death,” Grooms says.

The next question Grooms and Coe put to the test was, which major pathogens were associated with fatal BRD in Michigan feedlot cattle?

“The most common virus isolated was bovine viral diarrhea (BVD) virus,” Coe says. “Historically, fatal bacterial pneumonia has primarily been associated with Pasteurella haemolytica (now known as Mannheimia haemolytica).”

In their study, the most common bacterial pathogen isolated from cattle succumbing to BRD was Mycoplasma bovis (Figure 2). This supports concerns that M. bovis is becoming more of a problem in fed cattle.

“Speculation is that, with newer-generation vaccines, antibiotics and management schemes, we're doing a good job of reducing incidence of pneumonia and mortality caused by P. haemolytica,” Grooms explains. “But we may have created a niche other pathogens are looking to fill, and M. bovis appears to be doing so.”

Coe says there are few proven tools to prevent and mange M. bovis. “Work is underway to gain a better understanding about this bacteria and ways to prevent and control it,” he adds.

Other predictors

Dan Thomson, Kansas State University DVM in Manhattan, says the average in-weight for a pen of cattle is a strong mortality predictor. There appears to be an increase in feedlot death rates among lighter cattle.

“Lighter cattle generally have higher death loss than the same source of cattle at heavier arrival weights,” Thomson says.

USDA National Health Monitoring System (NAHMS) Sentinel program and VetLife Benchmark data show death loss in cattle with in-weights of 700-899 lbs. didn't increase significantly from 1998 to 2003.

“However,” Thomson notes, “death loss in lighter-weight groups (400-599 lbs.) increased significantly over that period.”

NAHMS data indicate an average increase in overall death loss of 69% (6%/year) from 1994 to 2003, and a 119% increase in death due to respiratory disease (9%/year) in all cattle on feed.

“Newly arrived animals are at greatest risk of BRD,” says Guy Loneragan, West Texas A&M Feedlot Research Group epidemiologist. “Most feedlot animals that die of respiratory tract disorders do so soon after arrival.”

Conversely, he says, most animals dying of digestive tract disorders do so during the later stages of the feeding period.

The NAHMS Sentinel survey indicates monthly mortality ratio for animals that died of respiratory tract disorders was highest in November through January. In December, 17.3 animals died of respiratory tract disorders/1,000 cattle entering the feedlot, compared with only 3.5 animal deaths/1,000 in May.

“Distinct patterns in mortality ratios for animals that died of digestive tract disorders and for animals that died of other disorders were not seen in the NAHMS survey,” Loneragan adds.

Some take-home points

  1. BRD is still the major cause of fed cattle mortality. The feeding industry must develop creative ways to prevent and manage this major cause of cattle sickness and death.

  2. The causes of BRD are changing. Emerging pathogens, such as M. bovis, may require different strategies to control. Strategies that worked for Pasteurella haemolytica may not work as well for other pathogens.

  3. Understanding why and when cattle are sick and dying is necessary to direct proper control and treatment strategies. Necropsies are the best way to learn cause of death.

Because many disease conditions result in similar clinical abnormalities, Loneragan warns categorizing cause of death on the basis of antemortem clinical abnormalities alone can lead to misclassification.

“A thorough postmortem exam is more likely to correctly identify the affected body system,” he says.

“If you have an animal die, don't assume anything,” Grooms notes. “Think about having a necropsy done so disease problems can be addressed properly.”