Wednesday, March 12, 2003

Respiratory Disease of Feeder Cattle

Respiratory disease of cattle is known by various names – pasteurella pneumonia, shipping fever or Bovine Respiratory Disease complex.

No matter what it is called, this disease accounts for huge economic losses every year. So it is important for cattlemen to be familiar with recognition, treatment and prevention of this disease.

There are several types of respiratory disease, but the one of most concern is the infectious form. We all are familiar with the calf that has pneumonia. This sick calf will be depressed, drawn, gaunt and unwilling to move. It will cough, breathe rapidly, have a yellowish
discharge from the eyes and nose, and have a fever of 103° to 106° F.

The first thing we often do when we see these signs is reach for a bottle of antibiotic. Rather than reach for a bottle of antibiotic, let’s look at the factors that interact to cause pneumonia. Once we better understand these factors, we can work toward more effective prevention and treatment programs.

Pneumonia results from a complex series of steps that are only partly understood. Research has shown that three factors must be present for a calf to get sick with pneumonia: (1) stress, (2) viral infection and (3) bacterial infection. Stress can be caused by routine
procedures such as castrating and dehorning. Also, new situations – nutrition, weather, congregation and transportation – can induce stress.

When a calf is stressed, its body reacts by secreting hormones that positively and negatively affect bodily functions. On the negative side, stress causes the entire body to be more prone to infection. This occurs by allowing disease causing organisms easier routes of entry and by disarming some of the body’s important defense mechanisms against disease.

So the first step toward pneumonia occurs following a stressful event.

The second step takes place when the body’s defenses are down and viruses invade the nose and other parts of the respiratory system. Viruses that commonly take advantage of this situation usually include IBR and BVD. Other viruses such as BRSV and PI3 sometimes cause problems.

Virus infection in cattle could be compared to the common cold or flu in humans, which rarely cause death. Although viruses may cause death, the major role they play in pneumonia is to allow the third factor of this disease – bacteria – to gain a foothold and possibly lead to death.

Pasteurella haemolytica (Mannheimia haemolytica) and Pasteurella multocida and Hemophilus somnus account for the majority of bacterial infections of pneumonia.

These bacteria gain entry via infected nasal secretions (much like virus particles may), or they may be stimulated to grow from normally low populations in the throat. Either way, without proper recognition and antibiotic treatment, these organisms can cause death
rather quickly in affected animals.

Since antibiotics kill or inhibit growth of bacteria, these are the organisms that we are trying to eliminate when we use drugs such as an antibiotic.

Respiratory Viral Disease

Respiratory viral disease can be devastating to animal health. It continues to be poorly understood and treated with mediocre results.

Viruses are not actually alive but are particles of genetic material that must infect the animals own living cells before they can reproduce and cause damage. The use of antibiotics will not directly fight a viral infection; but in most instances, antibiotics are indicated to treat the coinciding bacterial disease.

Viruses are spread from one animal to another via (a) direct nose-to-nose contact,
(b) contact with nasal
secretions or other bodily exudates from an infected animal, or
(c) through aerosol particle transfer between animals. Exposure to viruses is unavoidable
in a feedyard, so we must concentrate our efforts on prevention and control-mainly with timely vaccination- preferably before exposure to the disease causing virus.

The four most common respiratory viruses are IBR, PI3, BVD and BRSV.

IBR (infectious bovine rhinotracheitis) has also been called rednose.It causes a reddened, tender nasal surface that crusts over and peels, as well as sore and runny eyes. Internally, the animals trachea or windpipe becomes very inflamed and may fill up with
a thick exudate. IBR has also been implicated in genital infections in cows, as well as in central nervoussystem disorders.

PI3 (parainfluenza 3) has long been recognized as a respiratory pathogen in cattle, especially lightweight calves. Its most harmful characteristics is that it destroys the cilia (hairlike projections) in the trachea, which prevent foreign matter and bacteria from
entering the lungs.

BVD (bovine virus diarrhea) has long been recognized to cause problems in the digestive tract, but in recent years has been implicated as a respiratory agent. BVD is very immunosuppressive; it weakens the animals immune system and thereby lessens its
ability to fight disease. Some cattle have been proven to be chronic carriers of one type of BVD and develop severe digestive and respiratory disease when exposed to another strain. Unfortunately, there is no practical and quick way to identify these animals, and we have
no accurate estimate of the extent of this problem.