Fifteen of a group of calves are lame after arriving at the feedlot, with some developing pneumonia. What organism is likely causing these signs?

Histophilus somni, previously known as Haemophilus somnus, is often associated with respiratory diseases and lameness in cattle, making it a likely candidate for the signs presented in this case. This organism can cause pneumonia in calves, particularly in stressful environments such as feedlots, where newly arrived animals may be more susceptible to respiratory pathogens due to stress and commingling with other animals.

Additionally, Histophilus somni can lead to various clinical conditions, including bovine respiratory disease complex (BRDC) and infectious thromboembolic meningoencephalitis, which can manifest as neurological issues and lameness. The combination of lameness and pneumonia is consistent with infections caused by this organism, especially in a post-transport setting where calves might be experiencing stress-related immunosuppression.

Other organisms listed may also contribute to lameness or pneumonia but do not as consistently produce the specific combination of symptoms indicated. Arcanobacterium pyogenes is typically associated with abscesses and does not primarily cause pneumonia. Mannheimia haemolytica and Pasteurella multocida are more directly involved in respiratory disease but are less typically linked to lameness as a primary symptom. Thus, considering the context and