A Veterinarian's Perspective:
Pasteurella pneumonia is considered one of the most important infectious bacterial diseases of sheep. It is usually caused by Pasteurella haemolytica, a bacteria that is a normal inhabitant of the upper respiratory tract of the sheep. Stress to the sheep weakens the immune system, and leads to opportunistic infection of the lungs by P.haemolytica. The bacteria becomes highly pathogenic and infectious.
It is usually a combination of stresses that weaken the sheep’s immune system. Common stresses include:
Once the sheep are stressed and P.haemolytica infects the lungs it becomes highly contagious and outbreaks can occur. The bacteria is spread through inhalation of respiratory droplets or direct contact with respiratory secretions. Typically, fifty percent or more of the flock may be affected.
The common clinical signs of pneumonia are coughing, thick nasal discharge, thick discharge around the eyes, wheezing, and rapid breathing. Other early signs include fever (104-108 F), depression, decreased or no appetite, weight loss, and isolation from the flock. Occasionally the first sign of pneumonia is sudden death with no apparent warning signs. The disease course can last 12 hours to 3 days, and with treatment recovery occurs in 14 to 20 days.
Treatment of Pasteurella pneumonia can be accomplished with appropriate antibiotic therapy. Most cases respond to treatment with long-acting oxytetracycline (LA 200), ceftiofur (Naxcel), or florfenicol (Nuflor.) Tilmicosin (Micotil) may be effective in some sheep, however it is highly specific for pasteurella, and may not be effective if other bacteria are involved in the pneumonia. Medication of the entire flock is necessary to stop an outbreak of Pasteurella pneumonia, and is most easily accomplished by administering a sulfonamide antibiotic in the drinking water. The flock veterinarian should definitely be involved in any treatment decisions and regimens.
As a veterinarian in private practice, diagnosis of pneumonia is fairly straightforward, however the specific diagnosis of Pasteurella pneumonia can be extremely difficult. The only way to definitively diagnose P.haemolyticais through culture of secretions taken from the lungs- this is done through fairly invasive trans-tracheal sampling (a needle and catheter are placed through the trachea and secretions are aspirated.) Unfortunately, definitive diagnosis of P.haemolytia is often made after cultures taken at necropsy.
I do not advocate trans-tracheal sampling for every sheep that coughs or shows symptoms of pneumonia. What I do advocate is paying attention to coughing sheep and involving your flock veterinarian right away. One sheep in the flock that coughs occasionally is probably not a big problem. However, if more than one sheep in the flock is coughing, especially if the cough is accompanied by nasal or ocular discharge or other signs of illness, then an outbreak is a possibility and the whole flock should receive medication. Any sheep that dies suddenly should have a necropsy to determine cause of death. A single sheep that is chronically ill and unthrifty should undergo more intensive and/or invasive diagnostics (or perhaps culling and/or necropsy should be considered in this case, depending on the value of the animal.)
The key to preventing Pasteurella pneumonia is minimizing stress to the sheep. This can be accomplished by practicing good husbandry, ensuring adequate nutrition, parasite prevention programs and adequate facilities and handling. Practicing good isolation and quarantine procedures for new flock additions and sheep returning from shows is always important. Treating with appropriate antibiotics prior to travel may be helpful in preventing Pasteurella pneumonia- consult your flock veterinarian. Observing sheep closely during extreme weather conditions and treating the flock at the first sign of illness is also important- again consult your flock veterinarian!
Another good strategy for preventing Pasteurella pneumonia is by vaccinating sheep for the viruses that may predispose them to P.haemolyticainfection. Clinical studies have shown that commercially available, modified live vaccines for PI3, Bovine Herpesvirus, and Bovine RSV are effective at protecting against these viruses in sheep and lowering the incidence of P.haemolyticapneumonia. Unfortunately, none of the commercially available vaccines for P.haemolyticaitself appear to be clinically effective in sheep at this time. Hopefully, more effective vaccines specifically created for P.haemolyticain sheep will become available in the future.
Again- remember to consult your flock veterinarian regarding prevention and treatment of Pasteurella and other infectious diseases!
©Dr. Heather Ludlam DVM