Bob Glock, DVM, PhD, Dipl. ACVP, Arizona Veterinary Diagnostic Laboratory, says upper respiratory disease in cattle can be just as problematic as lower respiratory disease or pneumonia. The real worrisome upper respiratory disease is infectious bovine rhinotracheitis (IBR) which can precipitate major losses, he said at the 2010 Western Veterinary Conference.
Glock outlined four types of pneumonia:
- Bronchopneumonia: Bronchopneumonia is an exudative lesion originating at the bronchoalveolar junction. It is characteristically cranio-ventral, presumably because of distribution via the respiratory tree and gravity.
- Interstitial Pneumonia: Interstitial pneumonia is inflammation of the alveolar walls and interlobular septa or interstitium of the lung. It tends to be more diffuse in distribution. Etiology is not typically bacterial and may involve viruses, toxins, irritants and metabolic products including 3-methlyindole.
- Broncho-interstitial Pneumonia: Broncho-interstitial pneumonia is a combination of the previous two types and is an indication that the pathologist is more or less undecided based on gross and histologic evidence of lesions that fit definitions of both bronchopneumonia and interstitial pneumonia. Viruses (the list is pretty short) are generally thought of.
- Metastatic Pneumonia: Metastatic pneumonias must also be mentioned but are generally sporadic resulting from septic material entering the circulation. The classical presentation is micro-abscesses scattered throughout the lung following erosion of a liver abscess.
Glock discussed the identification and relevance of these specific respiratory pathogens:
- Infectious Bovine Rhinotracheitis Virus: The significant presence of IBR is pretty much a gross diagnosis based on clinical information and necropsy with typical fibrinonecrotic tracheitis. Bronchopneumonia tags along because of destruction of the mucociliary apparatus. The best means of detection is PCR or immunohistochemistry (IHC) on lung. Finding virus without typical lesions is meaningless.
- BVD: There are no obvious specific lesions resulting from BVD virus. Significance is almost entirely due to immunosuppression which is bad with acute transient infection and worse with persistent infection. The best detection method is PCR but other tests work. Again, mere presence of a positive test for BVD doesn’t help us much.
- BRSV: This virus is frequently present in cattle populations and probably most often is simply a contributor to the bronchopneumonia complex. It can also be a primary pathogen resulting in individual cases or multiple animals with severe interstitial pneumonia. Some clinicians are adept at identification but necropsy with histopathology and virus identification are definitive.
- Mannheimia haemolytica: Typical cranioventral fibrinous bronchopneumonia is most often caused by M. haemolytica. It is the most important respiratory pathogen and is best identified by culture of the affected lung. The value is confirmation of what the necropsy has already suggested. Isolates have secondary value for sensitivity testing. Nasal swab comments will follow but isolation of M. haemolytica from nasal cultures requires typing or other focused effort to be considered significant. A variant that resembles M. haemolytica but which ferments trehalose has been described as perhaps a separate pathogen called P. trehalose.
- Pasteurella multocida:This organism can cause acute bronchopneumonia similar to M. haemolytica but more often becomes a pest in animals that fail to respond well to therapy. These pneumonias are often observed at more than 30 days on feed and may become chronic. Cultures used for sensitivity testing often indicate sensitivity to multiple antibiotics but clinical response is often disappointing.
- Histophilus somni:This is often hard to differentiate from M. haemolytica infections unless additional lesions such as cardiomyopathy, fibrinous arthritis or thromboembolic meningoencephalitis are detected. The cautions regarding detection by culture are that the organism seems to not grow well every time or from specimens that aren’t fresh. Special culture requirements must be recognized by laboratory personnel.
- Arcanobacterium pyogenes: This opportunist usually shows up late and is easily found after lesions become suppurative or abscessed. Finding it doesn’t help us much other than to tell us we’ve missed the opportunity for an early response.
- Mycoplasma bovis:This pathogen is gaining in significance, particularly in cattle on feed where it seems to be more commonly recognized as a significant cause of pneumonia and lameness.