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Does Cold Weather Contribute to Pneumonia in Calves?
By: Russ Daly, SDSU Extension & State Public Health Veterinarian - 03/14/2019

Prolonged winter cold has some fairly obvious effects on cattle of all ages. Cattle producers calving early on the northern plains are familiar with frostbite that nips ear margins and tail tips in baby calves, but all cattle are affected by the increased nutritional demands inflicted by prolonged cold temperatures. When the outside temperature drops below an animal's thermoneutral temperature, increased energy in the diet is necessary.

Is an increased susceptibility to disease such as Bovine Respiratory Disease Complex (BRDC, or pneumonia) one of these effects of cold weather also? The answer is, it could be.

In order to explore the effects of cold weather on pneumonia susceptibility, it's instructive to review how BRDC develops in a calf. In the vast majority of clinical BRDC cases, bacterial agents such as Mannheimia hemolytica, Histophilus somni, or Pasteurella multocida are the main culprits. Once these bacteria invade the lung tissue, a cascade of inflammation causes the fever, lack of interest in feed, heavy breathing, and cough all too familiar to cattle producers who have pulled and treated such cases.

Unlike some cattle illnesses, simple exposure to these germs isn't enough to cause disease, however. When we sample the nasal passages of healthy cattle, we're quite likely to find all these germs. When bacteria multiply in the nose and upper respiratory tract, though, the chance of some of them finding their way to the lungs increases greatly.

Herein lies the first possible effect of cold weather. Research shows that bacterial populations (particularly Mannheimia hemolytica, the most significant BRDC germ) in calves' noses are as much as 7 times greater when they're exposed to colder weather (34 F vs. 74 F). So, the effect of cold weather here is indirect: more germs in the nose means more of a chance they'll hit the lungs.

Interestingly, this phenomenon has also been demonstrated when outside temperatures change dramatically. In a project that examined an abrupt change from 41 F to 55 F, the numbers of Mannheimia in calves' nasal passages increased, even though the temperature in this case was moving the right direction!

The reason that the mere presence of bacteria in the nose doesn't always result in pneumonia is the incredible capacity of what's called "mucociliary clearance". This refers to the constant "escalator" provided by cilia on the surface of the respiratory tract lining. These tiny hair-like projections beat dozens of times per second in a coordinated fashion to move mucus from the lower respiratory tract back up to the throat, where it -- and any foreign debris and bacteria it has trapped -- can be coughed up or swallowed. When this works, it works really well. Studies have demonstrated that 90% of Mannheimia hemolytica squirted down the windpipe is cleared from the lower respiratory tract in 4 hours or less.

It turns out that mucociliary clearance is affected by cold weather, too. In colder air temperatures, research shows that the mucus moves more slowly, the cilia beat less frequently, and the transport of bacteria and other debris up and out of the respiratory tract slows down.

If cold weather creates these conditions favorable for BRDC development, why doesn't every one of our calves get sick during cold snaps? The answer is adaptation. Over time, our animals' physiology can adjust to cold conditions and the function of these clearance mechanisms improves. We can directly aid this adaptation by providing reasonable shelter from extreme temperatures -- evening things out a bit.

Of course, many other factors go into preventing BRDC. Ensuring good colostrum intake for young calves is essential, and vaccination strategies for all ages of cattle are also important tools to discuss with a veterinarian. Managing temperature acclimation -- to the extent we can -- along with those tools, can help minimize the effect of BRDC in cattle.

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