Animal health matters: Why is stress in calves associated with pneumonia?

Russ Daly
Special to the Farm Forum
Columnist

Bovine respiratory disease (pneumonia) isn’t a new condition by any means, but we’ve learned a lot about it and its underlying causes as the years have passed. Despite that, it still costs the cattle industry between 500 and 900 million dollars every year. 

The emergence of bovine respiratory disease complex (BRDC) really coincided with the phenomenon of moving large numbers of calves via trains and trucks. The term “shipping fever” was coined when the association between sickness and long hauls and co-mingling at auction markets became apparent. Even as long ago as 1942, this connection was made.

In that year’s USDA Yearbook of Agriculture, Dr. C. D. Stein wrote, “The vitality of an animal is lowered by the hardships of transit, and its resistance to infection is decreased.” 

Despite the link becoming well established, the mechanisms behind it weren’t yet well known. We have, however, learned a lot in the past 79 years.

The term “shipping fever” has dropped from our vocabulary, mostly because we now understand that it wasn’t necessarily the shipping itself that caused the problem. The problem was long-term stress, which can come from sources besides transportation: weaning or a prolonged blizzard, for example. 

Short-term stresses can actually be temporarily beneficial for the animal’s system: consider the “flight or fight” response to an immediate threat. But once a stress becomes chronic, it becomes detrimental. The brain is connected to the adrenal glands through a complex network of nerves and hormones. When the animal’s psychological capacity to cope with a stress becomes overwhelming, the adrenal glands produce a hormone called cortisol. In fact, when researchers measure pain or stress in cattle, it’s this hormone they measure. 

Cortisol does some good things. It regulates an animal’s energy metabolism and how it stores and uses fat and sugars. If not for cortisol, a calf’s system would crash when stressful events occur. 

But unfortunately, the good effects of cortisol are usually overshadowed by its bad effects during stress. Cortisol is hard on the immune system. It decreases the ability for white blood cells to multiply and mobilize in response to an infection.  These include lymphocytes and neutrophils, which are crucial to mounting an immune response sufficient to nip an infection in the bud.

Cortisol interferes with the production of immune system chemicals that also help the immune system. Dr. Stein’s book chapter long preceded the discovery of these chemicals and hormones, yet he was right on with his understanding that the effects of long-term stress ruined an animal’s resistance to disease. 

Knowing about the effects of long-term stress is one thing; doing something about it is another. Some of these contributors to stress (“…overcrowding…hard driving, lack of rest and proper shelter…” as Dr. Stein put it) were already well-known 79 years ago.  Today’s beef production systems build in stressful events like weaning, trucking and processing. Performing these tasks in a manner that takes stress reduction into account will improve those animals’ ability to keep infectious diseases in check. 

A key concept to remember is that stresses are additive. A calf that is weaned, transported, mixed with unfamiliar cattle and processed all in a manner of hours experiences a much higher intensity and duration of increased cortisol and immune suppression than had those processes been spread out.

In particular, working animals immediately after transportation can be problematic. For every hour of transportation, calves should get at least an hour to rest and acclimate themselves to their new environment before they’re run through a chute.

We are also learning more about the interaction of respiratory vaccines and stress. The same suppression of the body’s defenses against infectious disease agents applies to its response to vaccines as well. Calves undergoing or recovering from long-term stresses do not respond well to vaccines. Consequently, an increasing body of evidence supports delaying vaccinations for 10-14 days after an animal has been weaned or transported to a new lot.

Even though we now know about important immune system components such as cytokines, interleukins and cortisol – substances not even conceived of back in 1942, our goals for stress reduction in cattle still remain. Incorporating those concepts into how we manage our calves means a much better chance of keeping them healthy from start to finish. 

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at (605)688-5171.