Bronchopneumonia caused by Pasteurella multocida or Mannheimia haemolytica has a cranioventral lung distribution and affects sheep and goats of all ages worldwide. It can be particularly devastating in young animals around weaning. It is a common cause of morbidity and mortality in lambs and kids, especially in those that have not received adequate colostrum or in which passive colostral immunity is waning. The disease appears to occur most often in animals that have undergone recent stress such as transportation, weaning, change of diet, or commingling with animals from unrelated farms. Bibersteinia trehalosi (formerly Pasteurella trehalosi) causes septicemia in lambs 4–9 mo old (systemic pasteurellosis). (Also see Pasteurellosis of Sheep and Goats.)
P multocida and M haemolytica are commensals of the upper respiratory tract that can cause pneumonia either alone or in conjunction with other organisms. Primary infections with respiratory pathogens such as parainfluenza type 3, adenovirus, respiratory syncytial virus, Bordetella parapertussis, or in particular Mycoplasma ovipneumoniae appear to predispose to secondary infection with Pasteurella and Mannheimia.
Stress appears to be an important factor that allows Pasteurella, Mannheimia, Mycoplasma ovipneumoniae, other bacteria, and viruses to multiply and impair the normal physical defense mechanisms, facilitating invasion of lung tissue and development of pneumonia. In calves, alveolar macrophage function is impaired after viral pneumonia. This results in decreased clearance of inhaled bacterial pathogens, allowing them to become established. Pathogen-host interactions result in tissue damage, especially because of massive influx of neutrophils. As these neutrophils are lysed, enzymes are released that cause more lung tissue damage. This mechanism may be similar to that of Pasteurella and Mannheimia pneumonias in sheep and goats.
Acute respiratory disease caused by M haemolytica is uncommon in adult sheep, unless there is a predisposing problem such as ovine pulmonary adenocarcinoma or other viral infection. Clinical signs include acute onset depression, lethargy, and inappetance and are consistent with profound endotoxemia. Sudden death may occur without clinical signs having been observed. Affected sheep are typically separated from the remainder of the flock and are easily caught and restrained. On approach, they may show an increased respiratory rate with an abdominal component.
Affected sheep are typically febrile (>40.5°C [104.9°F]). The mucous membranes are congested, and there may be evidence of dehydration with sunken eyes and extended skin tent duration. Auscultation often does not reveal significant changes other than an increased respiratory rate. Rumen contractions are reduced or absent. There may be evidence of diarrhea. Frothy fluid may be noted around the mouth during the terminal stages.
At necropsy, testing whether lung tissue sinks (pneumonia) or floats (normal) remains a useful screening test. In acute cases, cultures obtained from tracheal swabs or washes or from lung tissue or associated lymph nodes are diagnostic. Histopathologic examination is useful, especially if other types of pneumonia (eg, retrovirus interstitial pneumonia in adult sheep and goats) are also suspected. In chronic cases, bacterial cultures may be less rewarding; Pasteurella or Mannheimia may have been the initial problem, but results of cultures taken later may reveal Trueperella pyogenes, a common causative agent of lung abscesses.
Whenever possible, treatment should be based on bacterial culture and sensitivity, especially in herd or flock outbreaks, when valuable animals are involved, or in acute or chronic cases when initial therapeutic attempts have failed. Commonly recommended antibiotics include oxytetracycline (10 mg/kg/day of non-long-acting product, or 20 mg/kg once of the long-acting product), florfenicol (20 mg/kg, every 48 hr), and tylosin (10–20 mg/kg, once to twice daily). Therapy should continue for at least 24–48 hr after body temperature has returned to normal. Duration of treatment usually is 4–5 days. Tilmicosin and other macrolide antibiotics can also be used but are considerably more expensive than oxytetracycline. Acute cases may also benefit from the use of NSAIDs (eg, flunixin meglumine or ketoprofen) in conjunction with antibiotic therapy for control of endotoxemia and inflammation.
Inadequate ventilation, crowding, commingling of animals from various farms (feedlot or sale barn situations), poor nutrition, failure of passive transfer of antibodies, transportation, and other stresses have all been associated with pneumonia outbreaks. Control and prevention lies with correction of the predisposing factors whenever practical.
Prevention of pasteurellosis is best attempted using vaccines incorporating iron-regulated proteins; because these proteins are antigenically similar, they confer cross-protection against other serotypes. Breeding ewes require a primary course of two injections 4–6 wk apart followed by an annual booster 4–6 wk before lambing. However, this vaccination regimen provides only passive immunity to the lambs for up to 5 wk. Lambs can be protected by two doses of vaccine administered from 10 days of age, because colostral antibody does not interfere with development of active immunity.