Thursday, May 1, 2003

BRD Complex Viral Conditions, ETC

BVD Infection of Feedyard Cattle
   If you ever want to start an argument in a feedyard about vaccination programs, mention BVD (bovine virus diarrhea). The folklore, myths and half-truths concerning this virus are many. BVD does play a key role in feedlot respiratory disease, but we are unsure
of its exact role and even more unsure of possible control methods.
   BVD was first described in 1946 as a severe and often fatal disease of dairy cattle. Early researchers named this disorder mucosal diseaseand many people still call the severe digestive form of BVD by this name. We now know that BVD can affect other organs, including the respiratory and immune systems. Because of the complexity of this disease, we do not completely understand all the effects of BVD virus on cattle.
   BVD in the feedyard can mimic many other diseases or show no obvious signs at all. Common clinical signs include depression, off-feed, discharge from the eyes and nose, coughing and rapid respiration. These signs may or may not progress to a severe
watery diarrhea with ulcers in the mouth and entire digestive system. This set of signs could account for any of a number of diseases seen in feedlot cattle and explains why it is important to avoid a diagnosis of BVD from clinical signs alone.
   In order to put this viral disease in perspective, lets look at the effects of BVD on the respiratory system and the Bovine Respiratory Disease complex.
   BVD has two prime effects on the respiratory system. First is a direct negative effect on the local disease fighting mechanisms of the lungs and upper respiratory tract. Second, BVD virus can cause a severe disruption in the bodys immune system.
   The end result is that the BVD infected animal is more susceptible to any infection, but most importantly bacterial pneumonia such as Pasteurella haemolytica(Mannheimia haemolytica), Pasteurella multocida and Hemophilus somnus. Mycoplasma bovis may also play a role.
   In order to have a problem with BVD in a feedyard, you must have two factors present: (1) a susceptible animal and (2) a virus capable of causing disease.
   Of course, salebarn cattle and freshly weaned calves are more susceptible to any disease challenge. Also, some strains of virus are capable of causing disease to greater extent than other strains.
   If you take a highly susceptible calf that has been freshly weaned and expose it to an extremely hot virus in the feedyard, the outcome usually is death. Conversely, if
the cattle are protected against BVD or non-stressed, the same virus may have little to no effect on them.

Control of BVD
   There are three schools of thought concerning BVD vaccination:
   1. First, all cattle must be vaccinated with a modified live virus (MLV). This procedure has one serious drawback. Any MLV vaccination on stressed calves is potentially harmful, but the effects of MLV BVD vaccines can be even more harmful than most by causing immunosuppression of an already compromised animal. There are good MLV BVD vaccines that confer adequate protection to feedlot cattle with minimal side effects, but the potential for this adverse reaction is present with many MLV BVD vaccines.
   2. The second school of thought says vaccinate for BVD with a killed product. Killed products have been much improved over the last few years and offer good protection without causing adverse reactions. Opponents of these vaccines agree that protection is adequate, but rather slow in appearing.
   3. The third school of thought is that BVD vaccination has no effect on respiratory disease in a feedlot.
   In our practice, recommendations on BVD vaccination are based upon the type of cattle fed, past health history and confirmed presence of BVD problems through diagnostic techniques.
   In summary, BVD has the potential to cause serious problems in feedlot cattle. The key to control lies in proper diagnosis and sound preventative measures.

Respiratory Syncytial Virus Vaccine
   Currently, two modified live vaccines and one killed respiratory syncytial virus vaccines are available for use in beef cattle. In order to understand the potential use of these products, let us examine this virus, its clinical manifestations and application of vaccination to a feedlot health program.
   Bovine Respiratory Syncytial Virus (BRSV) is named because of its ability to form syncytia in infected cells. Syncytia are simply an aggregation of cells that clump together in a large mass with an indistinct internal structure.
   This virus was first seen in Europe and Japan. Reports of outbreaks in the U.S. were first seen in Nebraska and Colorado. A human counterpart has been recognized that has many similarities to the virus seen in cattle.
   When BRSV is exposed to susceptible cattle, an incubation period of 2 to 10 days can be expected.
   Clinical signs during an outbreak are of rapid onset and include increased respiratory rate, depression and discharges from the eyes and nose. Salivation or slobbering is common and may be blood-tinged as can the discharge seen coming from the nose. Coughing is usually marked and seems to persist throughout the course of infection. Swelling may occur around the eyes but is usually pronounced in the throat region. Body temperatures are increased and range from 104° to 108° F. Abortions usually are not seen but some have been reported.
   The disease outbreak can affect all ages of cattle, but is usually most pronounced in freshly weaned calves. Infections can range from mild to marked and affects a large number of cattle, but usually with a low death loss. As with any virus infection, bacterial pneumonia can be a secondary invader.
   The clinical picture of BRSV can resemble other viral infections such as IBR, PI3 or BVD, except that the disease is usually mild and the marked swelling in the throat latch is rather unique.
   Other viral infections such as those mentioned can cause much higher death losses; but early in the outbreak there has been confusion as to the actual agent involved. Many times BRSV has not been identified until other common viral agents have been eliminated
as the causative agent.
   As yet, in our feedyard experiences, the problems with BRSV have been minimal. That is, we have not recognized this condition in the pure form. This is not to say that BRSV has not been involved in certain respiratory problems. As with other viral agents, over-
growth of Mannheimia haemolytica or Pasteurella multocida may actually mask the true initiator of the problem. BRSV can cause severe damage to the trachea
and lung, which would then allow for bacterial pneumonia to occur.
   In order to diagnose BRSV as a problem, you must attempt to isolate the virus from the lung, trachea or nasal secretions. The affected lung must also be examined under the microscope for the syncytial formation so characteristic of this disease. These two procedures may prove very unfruitful in the feedlot environment.
   Blood samples and serology taken at the time of respiratory outbreak and repeated 2 to 4 weeks postoutbreak are the most beneficial method of positive diagnosis currently. Continued development of more accurate and effective diagnostic techniques is     under way.    
   In summary, Bovine Respiratory Syncytial Virus is usually a mild viral infection which can be relatively severe under some circumstances. This disease is usually a problem in freshly weaned calves, but can be a problem in any age of cattle.

Other Respiratory Diseases
   Many times when we talk about respiratory disease, infectious bronchopneumonia or Bovine Respiratory Disease Complex (BRDC) is the focal point.
   In the feedyard, other forms of respiratory disorders exist. Diphtheria and hardbreathersor honkerscan be problems, although not as common as BRDC.

   Diphtheria is an infectious disorder of the upper airways. A focal point of damage usually occurs to the larynx or voice box and allows for opportunistic invasion of the damaged airway lining by bacteria. The bacteria that invades the airway is the same organism that causes foot rot in cattle.
   Symptoms of the calf affected with diphtheria may include difficulty in breathing, excessive salivation, swelling around the throatlatch and increased body temperature. Often the most obvious clinical sign is the foul smelling breath that occurs. Because of the bacterial invasion of airway tissues and the particular pathogenic organisms involved, dead tissue is
formed at the site of infection causing the fetid odor.
   Diphtheria in a feedyard may often be an uncommon occurrence, but may occur secondary to feeding coarse feedstuffs, in the form of hay or finished feed. Other outbreaks of widespread diphtheria have not been attributed to any known factors.
   Treatment for the condition is generally quite satisfactory, if treated symptomatically with a broad spectrum antibiotic. Treatment should also include therapeutic agents to decrease swelling and increase the overall airway opening. Swelling in the voice box or larynx
area can lead to a greatly decreased airway capacity.

   Hardbreathers or honkers are frequently seen in the feedyard. A hardbreather is an animal that usually has 45 to 60 days or more on feed. It generally is seen in a pen of cattle having difficulty breathing. Depending on the severity, a honkmay be heard as air is expelled.
   Hardbreathers are not caused by a single condition, but by several possible conditions. These conditions may include (1) IBR, (2) acute spread of a low level pneumonia,
(3) allergic tracheitis, (4) atypical interstitial pneumonia, or (5) respiratory syncytial virus.
   It is difficult, if not impossible, to tell which of these conditions the animal has when it is pulled. Laboratory tests and response to treatment are necessary to separate these five causes of a hardbreather. And it is important to know the causes before proper
treatment can be instituted.

Acute IBR
   Acute IBR infection is seen as a single condition in cattle with greater than 45 to 60 days on feed. Cattle often have severe respiratory difficulty and a high body temperature (in excess of 105°F). A blood tinged nasal discharge may also be present.
   Animals with low grade pneumonia which previous therapy failed to completely resolve the condition, often are triggered by the IBR virus into an active, more severe respiratory condition.
   As the animal becomes closer to finish and fatter, the digestive tract is pushed forward into the diaphragm causing pressure on the previously compromised lung thus causing less total function air space and many times death.

Allergic Tracheitis
   Allergic tracheitis and atypical interstitial pneumonia are actually an acute response to chronic exposure to allergy causing moldy feed or other airborne allergenic particles. These animals appear very suddenly with no previous indication that there was a problem.
   Such cases may occur when the animals are moved longer distances, as might occur to reimplant. Airway space was adequate when in the pen, but the exertion of movement caused the animal to become decompensated and thus suffer an acute lack of necessary

Respiratory Syncytial Virus
   Respiratory Syncytial Virus (RSV) has been reported by some to be a cause of allergic tracheitis and atypical interstitial pneumonia. It is unclear at this time as to the significance of this condition in fat cattle pneumonia.
   Therapeutic goals for these conditions are relatively simple: (1) avoid any procedure at the time of treatment that would increase the animals need for additional oxygen; (2) supply therapy to open narrowed airways; (3) reduce swelling and possible allergic reaction; and
(4) treat with a short withdrawal antibiotic.
   Animals treated may respond favorably at first only to relapse soon after treatment is completed. Therefore, you should keep the option of early slaughter open should treatment response be unsuccessful.
   Even though the incidence of fat cattle respiratory conditions is much less than BRDC in newly arrived cattle, the economic losses incurred are much greater per animal affected. For example, an animal lost soon after arrival will realize an economic loss that is roughly two-thirds that of an animal that is near finish. Therefore, it is easy to see that keeping the animal
healthy throughout the feeding period is important for efficient economic returns.

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