New adjunct tests
Several new diagnostic techniques allow for more accurate diagnosis of specific variants of mycobacteria infecting cats with
AMP. These techniques can be performed on biopsy samples and can be used as adjuncts to the routine diagnostic procedures
previously recommended since many mycobacterial species are not easily differentiated by existing morphologic and microbiologic
methods. Genetic tests, such as sequencing procedures in which the genetic code of a targeted mycobacterial gene (i.e. 16S rDNA) is identified and compared with a known database, allow identification of the specific mycobacterial species known
to cause AMP.3,14 In addition, since the mycobacterial species causing AMP are difficult to identify by histologic examination, procedures
such as polymerase chain reaction can demonstrate and amplify mycobacterial nucleic acid in tissue samples, which can then
be used for further diagnostic testing, including sequence analysis.3,14 In one study, immunostaining with polyclonal anti-Mycobacterium bovis antibodies in skin biopsy samples from animals suspected of having AMP facilitated identification of mycobacteria when compared
with conventional histochemical stains alone.15 Practitioners should inquire about the availability and cost of these new ancillary procedures from their pathology service.
Cats with chronic nodular dermatitis and panniculitis pose unique diagnostic problems depending on the ultimate diagnosis,
but the following guidelines should prove useful for identifying most cases of AMP.
1. Determine that the cat's illness is consistent with one or more of the historical, clinical, or clinicopathologic syndromes
characteristic of atypical mycobacterial infections.
2. Exclude more common causes of dermatologic disease by using routine diagnostic methods.
3. Obtain multiple deep tissue biopsy samples early in the clinical course of disease for cytologic, histologic, and microbiologic
4. Alert the diagnostic laboratory receiving aspirate and biopsy samples to the possibility of an atypical mycobacterial infection,
so the laboratory can apply techniques to facilitate mycobacterial species identification.
5. Request antimicrobial susceptibility testing on all successfully cultured and suspected pathogenic mycobacteria.
6. Perform serial biopsies when necessary to confirm a diagnosis of AMP.
John H. Rossmeisl Jr., DVM, MS, DACVIM (internal medicine and neurology)
Thomas O. Manning, DVM, MS, DACVD
Department of Small Animal Clinical Sciences
Virginia-Maryland Regional College of Veterinary Medicine
Blacksburg, VA 24061
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