What diagnostic test is best to identify a respiratory parasite in cattle showing signs of respiratory disease unresponsive to antibiotics?

The Baermann exam for larvae in feces is the most appropriate diagnostic test for identifying respiratory parasites in cattle exhibiting signs of respiratory disease that do not respond to antibiotic treatment. This method is particularly effective because it specifically targets the detection of larvae of certain parasitic infections, such as those caused by lungworms, which are known to affect the respiratory system of cattle.

Given that traditional testing methods might focus on adult parasites or systemic infections, the Baermann exam allows for the identification of parasite larvae that can often be more indicative of ongoing respiratory issues. Additionally, respiratory parasites like Dichelobacter nodosus or species of Metastrongylus may not generate significant antibody responses detectable by serological methods; hence serology testing would not provide the clearest insight.

While fecal flotation can be useful for certain gastrointestinal parasites, it is not effective for respiratory parasites, and a tracheal wash, while useful in diagnosing respiratory infections, would not specifically target parasitic infections as effectively as the Baermann exam. Consequently, the Baermann method stands out as the best choice for diagnosing respiratory parasites in this context.