YC.com: Respiratory Disease

It takes a lot of work for a calf to get sick, according to Dr. Robert Sprowls, Ph.D., DVM, with the Texas A&M Medical Diagnostic Laboratory in Amarillo, Tex., but then the cattle industry gives disease a lot of help.

The presence of bacteria or a virus alone in an animal will not necessarily make him sick, but a combination of the two, especially coupled with the stress of handling and shipping, can cause severe respiratory disease in feedyard cattle.

Viruses have to have a cell, Sprowls said. They go inside a cell and take over. Instead of the cell function, they produce more viruses. They, in turn, compromise the respiratory defense and allow bacteria access to the respiratory track.

As air enters the nostrils, it’s filtered. It’s heated, and moisture is added so when it gets to the lungs, it’s non-irritating. If the air is too cold, it can’t be heated to body temperature. If the dust particles are too great, there are dust particles in the lungs.

The respiratory tract is constantly trying to move irritants back up the track and out of the body, but a calf under stress will prove susceptible to disease. The stress lowers the immune system.

Sprowls cites studies where as many as 66 percent of cattle will have pasteurella haemolytica bacteria, but these organisms are on the surface of the respiratory coverings unable to penetrate and cause infections. If a virus is introduced, it can give the bacteria the opening it needs. Or if the animal simply becomes dehydrated and can’t produce the moisture needed in the respiratory tract to keep it working properly, the bacteria won’t be moved out. It will penetrate the animal’s defenses.

“Bacteria is secondary to virus,” Sprowls said.

If stress is added from a lack of water, weaning, shipping, dehorning or castration, there will be enough viruses that can penetrate the epithelial cells of the respiratory tract.

“They’ll multiply in the lymph system basically,” Sprowls said. “Viruses will also replicate in the epithelial cells. A few of them will eventually penetrate. If you take a healthy calf, it generally won’t hurt them. If you stress them, you’ve got a sick calf.”

The way the industry handles cattle is the best way to help them get respiratory infections.
“If you want to buy fresh calves from the auction, you really need to take them and set them three or four days with hay and a little water,” Sprowls suggested. “We put them on a truck, then unload them, castrate them and dehorn them. All this puts more stress on them. That lowers the body’s defense mechanism.”

If he wanted to design a way to produce shipping fever, the way the industry handles cattle would be ideal.

“We yank them off mom, we poke them and yell at them and stress them all we can in the process, not intentionally, but we do,” Sprowls said. “Then there are the dehorning and castrations and implants and all the vaccinations. Our marketing scheme is designed to cause shipping fever.”

But with a proper vaccination program and a little thought in handling, the danger of severe bovine respiratory disease can be lessened.

Bacteria is almost omnipresent in cattle, he said. Viruses aren’t necessarily there.

“Our bodies build antibodies, and antibodies are not built just in general,” Sprowls said. “They are built to fight specific infections. If we inject a calf with an IBR virus, he produces a specific set of antibodies that will only attack IBR virus. They won’t attack BVD viruses or Chlamydia or any other agent. It will only protect that animal against IBR viruses. If you vaccinate against BVD viruses, it’s the same thing. It will only protect against BVD viruses.
There are different strains of BVD, he said.

“Sometimes there is enough strain variation that we’ll give a calf a BVD vaccination, and it may still get sick,” Sprowls said. Although this is improving with new strains of BVD vaccines being developed and marketed.

Most commercial vaccines have a good, broad antigen but every once in a while viruses will not be identical to or close enough to the strain in the vaccine. They can still become infected with a different BVD.

“It’s a little more complicated but basically as far as immunology is concerned, a specific cell does a specific job,” Sprowls said. “A cell has to recognize that IBR virus. When the cell recognizes the virus, it turns it over to another type of cell to make an antibody. The first thing it does is make a template. Then off of that template specific antibodies are produced. All of that takes time. It may take two or three or four days to recognize the virus, disassemble it and turn it over to another cell to make the template and start producing antibodies.”

With BVD, it takes 10 days to two weeks minimum before the animal can start producing antibodies.

“It takes time to produce antibodies and all antibodies are different,” Sprowls said.