Pneumonia in adult dairy cattle (Proceedings)
Respiratory disease continues to present challenges in the dairy industry in the 21st century. Traditionally, veterinary curricula have emphasized an etiologic approach to the subject and veterinary practitioners are very familiar with the well documented and heavily researched infectious causes such as bovine respiratory syncytial virus (BRSV), infectious bovine rhinotracheitis (IBR), bovine viral diarrhea virus (BVDV), Mannheimia haemolytica, Pasteurella multocida and Mycoplasma spp. It is no coincidence that these well documented forms of respiratory disease are the ones for which commercial vaccines are widely available and heavily utilized. Despite a good understanding of these primary causes of bovine respiratory disease and widespread compliance with established herd vaccination protocols, it is still common for veterinarians to be asked to trouble shoot outbreaks of respiratory disease in adult dairy cattle.
Veterinarians are comfortable with the clinical signs that classically accompany infectious lower airway disease, namely tachypnea, pyrexia, nasal discharge and coughing as well as the auscultation of adventitious lung sounds. Unfortunately, some of these clinical signs, particularly pyrexia, tachypnea and nasal discharge are not specific for pneumonia. An elevated body temperature in an adult dairy cow is not always a true fever but is almost always accompanied by a compensatory increase in respiratory rate that, taken together, frequently result in a diagnosis of pneumonia, especially amongst non-veterinarians. A rectal temperature greater than 103 °F is usually ascribed to fever in adult dairy cattle, the root of which may be infection, inflammation or an immune response. Increased environmental temperature, elevated temperature/humidity index, exercise, and/or heat exposure (limited access to shade, shelter or fans) are other reasons for hyperthermia in cattle. If one adds some degree of nasal discharge to a cow with hyperthermia, perhaps attributable to nothing more than a reluctance to clean off her nose as is common in cattle with any number of conditions, it may be surprising that pneumonia isn't diagnosed even more frequently than it is. We find that "pneumonia" becomes an escape diagnosis for all cattle with fever and tachypnea, signs that may accompany other diseases. Individuals in the fresh cow pen that demonstrate a fever in the absence of uterine discharge/odor, abnormal mammary gland evaluation or diarrhea are all too frequently assigned this diagnosis. However their signs may be nothing more than the normal physiologic response to fever with an increased respiratory rate or effort. To complicate a pneumonia diagnosis further, finding a normal temperature in cattle does not necessarily rule out an infectious or inflammatory response and many cows with chronic pneumonia have no fever.
Thoracic auscultation is both an art and a science and, as such, is subject to a variety of interpretations, even amongst trained veterinarians. Pneumonia in adult dairy cattle may not be associated with auscultation of classical crackles and wheezes when there is pulmonary consolidation, abscessation, pneumothorax, or pleural fluid accumulation. Increased intensity of thoracic breath sounds, when not based on a comparison to tracheal breath sound intensity can simply be the result of rapid breathing, proximity to the hilus or thin body condition of the patient. Stethoscope quality, the listening environment, cow temperament, thoracic wall thickness or body condition of the cow can also impair the potential value of lung auscultation. While thoracic auscultation can be a valuable tool in the hands of a trained veterinarian examining a cow suspected to have pneumonia, we find no substantive value in making thoracic auscultation a routine part of the examination of adult cattle by lay persons working the fresh or hospital cow pen on the dairy.