Cow Tales: Grazing alfalfa & sudden death
August 20, 2012
Widespread drought has the nation's ag economy treading water in an ocean of angst. Many producers have plunged into a high-stakes poker game, but as always we are still waiting for Mother Nature to deal the hand. Feed resources are at an extreme premium and livestock producers are trying to become real resourceful. Gossip floods the local coffee shops with different strategies to survive. Poor hay production has some ranchers turning cows onto hay ground to salvage what little hay did grow. This can lead to a gale storm of negative consequences for the herd.
Interstitial pneumonia refers to any disease process affecting the tissue in-between the alveoli of the lungs. Inspired air travels through the trachea, bronchi, bronchioles, and into the terminal alveoli. This the functional part of the lung where oxygen exchange takes place. The alveoli are microscopic blind sacs separated by extremely thin layers of tissue providing structural support and a location for the vast network of blood vessels and nerves traveling through the lungs. Consequently, most of the lung is air space surrounding blood vessels and the lungs appear pink. Disease processes that disrupt the normal tissue between the small air sacs result in interstitial pneumonia.
Every rancher and feedlot operator has encountered interstitial pneumonia. Summer pneumonia on a ranch is typically caused by a virus called BRSV that leads to interstitial pneumonia. Feedlots are plagued by a poorly understood condition called atypical interstitial pneumonia, a separate disease from shipping pneumonia not caused by a specific infectious pathogen. Other causes of interstitial pneumonia include migrating parasite larvae, digestive disturbances, and some toxic plants.
Acute interstitial pneumonia resulting from digestive disturbances occurs when pastured cattle are rapidly moved from poor-quality pasture to lush forages.
In our region the culprit is usually late season regrowth from an irrigated alfalfa field as the herd returns from summer grass. However, interstitial pneumonia can occur on any lush forage.
L-tryptophan, an amino acid in the forage, is metabolized by the bacteria in the rumen to 3-methylindole and absorbed into the blood stream. Once in the blood the metabolite travels to the lungs where it is further transformed into other toxic substances that begin to destroy the normal lung tissue. Fluid and air leak into the spaces between the air sacs. The cells lining the air sacs become damaged. Caged in the clamshell of the ribs, the lungs continue to swell with fluid decreasing the space available in the air sacs. The animal begins to suffocate from the inside.
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Affected cattle show signs of distress, rapid shallow breathing with head and neck held straight out, open-mouth breathing, and may have an expiratory grunt. Clinical signs usually begin to show up about one to two weeks after introduction into the lush field but may occur as early as 3 days or as late as 3 weeks. Large numbers of the group may be affected and many cattle may die suddenly before a problem is detected.
The severity of the disease can make handling affected animals counterproductive and treatment attempts are often unrewarding. Less severely affected animals can recover with time but the lungs will contain varying levels of scar tissue. These animals are somewhat more susceptible to additional lung disease as a result.
Managing the transition from dry forage to lush forage is the key to prevention and a challenge for producers. Recommendations usually include a stair-step approach to exposure by limiting access to lush forage to just a few hours with hay supplementation. Over the course of weeks the exposure is increased to full access. However, driving cows off lush pasture can resemble herding cats. The herd needs to be monitored closely for the next 2-3 weeks for immediate intervention should a problem arise. Lush forages are usually safe after a hard freeze, something hard to come by in July or August.
Acute interstitial pneumonia is a disease process hounding all producers at some point. Producers can limit their exposure to some potential causes with awareness and management considerations.
Questions? Feel free to contact us or a local veterinarian. Producers will need to be inventive to manage the torrent of feed-related decisions in the coming months. As Red Green often says. "I'm pulling for ya. We're all in this together."
Nerd Word: Emphysema – the abnormal accumulation of air within a tissue such as under the skin or within lung tissue, subcutaneous emphysema and pulmonary emphysema; respectively.
Kenny Barrett Jr. is a veterinarian at the Belle Fourche Veterinary Clinic in Belle Fourche, SD and pens "Cow Tales" monthly. Learn more about the clinic at http://www.bfvetclinic.com, or drop them an e-mail at: email@example.com to suggest a topic for the next installment of "Cow Tales."