An alpaca presents with crusting dermatitis affecting multiple animals in its herd. What should be the next step in diagnosis?

When an alpaca presents with crusting dermatitis affecting multiple animals within a herd, a microscopic examination of a skin scrape is an appropriate next step in diagnosis. This method allows for the identification of parasitic infestations, such as mites, which can cause similar skin lesions. By scraping the skin and examining it microscopically, veterinarians can look for the presence of ectoparasites like Sarcoptes or Demodex mites, which are common culprits in cases of dermatitis among livestock.

This diagnostic approach is particularly useful in a herd situation, as it helps to pinpoint the underlying cause of the dermatitis that could be affecting multiple animals. Other methods, such as bacterial culture or fecal egg counts, would not provide the specific information needed to diagnose a skin condition likely caused by ectoparasites. An immunohistochemical exam of a biopsy may be more invasive and typically reserved for cases where a neoplastic process or deeper tissue infection is suspected, rather than initial dermatological evaluations. Thus, the microscopic examination of a skin scrape is not only less invasive but also directly pertinent to diagnosing conditions manifesting as dermatitis in the herd.