Study assesses tissue ISH test for Johne’s disease in goats

A newly published study in the Journal of Veterinary Diagnostic Investigation suggests an RNAscope-based chromogenic in situ hybridization test could strengthen postmortem diagnosis of paratuberculosis, or Johne’s disease, in goats. The paper, published online May 14, 2026, assessed the assay in archived intestinal tissue and found positive hybridization in 19 of 20 goat cases, with no signal in 5 negative controls. (journals.sagepub.com)

That matters because Mycobacterium avium subsp. paratuberculosis remains a stubborn diagnostic problem in small ruminants. USDA APHIS notes that Johne’s disease is harder to recognize clinically in sheep and goats than in cattle, and standard fecal culture can take 12 to 16 weeks on solid media, even though newer liquid systems may shorten that window. Necropsy diagnosis still relies heavily on histopathology, culture, and PCR from intestinal and lymphoid tissues. (aphis.usda.gov)

In the new study, investigators reviewed archived intestinal cases of goat paratuberculosis and applied a pan-Mycobacterium ISH assay to formalin-fixed, paraffin-embedded tissue. The 20 positive cases included 11 multibacillary and 9 paucibacillary infections identified on Ziehl-Neelsen stain, plus 2 postmortem autolysis cases. Among the subset with ancillary testing, 9 of 10 cultured cases were culture-confirmed for MAP, 7 of 7 cases tested by MAP PCR were positive, and pan-Mycobacterium PCR from FFPE tissue was positive in 18 of 20 cases. ISH detected signal in 19 of 20 cases overall, including 8 of 9 paucibacillary cases; the only ISH-negative case had severe autolysis. All 5 negative controls were negative. (acvp.org)

The study builds on a broader shift toward molecular pathology tools that can work on fixed tissue, not just fresh samples. Cornell’s current Johne’s testing guidance already uses caprine fecal PCR as the default fecal detection test for goat herd workups, while noting that ELISA is better suited to herd screening than individual diagnosis. What this paper adds is a tissue-based option for cases that arrive after necropsy, especially when fresh tissue for culture wasn't collected. (vet.cornell.edu)

There wasn't much public expert commentary available at publication, but the findings align with existing pathology experience: Ziehl-Neelsen staining can reveal abundant acid-fast organisms in some tissues, yet may miss them in others, particularly when lesions are subtle or bacillary load is low. The authors’ result in paucibacillary goats is likely the clinically important piece for diagnosticians, because those are the cases where morphology can be suggestive but not definitive. More broadly, prior veterinary pathology work has shown RNAscope performance can remain strong in FFPE tissues, although prolonged fixation and tissue degradation eventually reduce signal. (merckvetmanual.com)

Why it matters: For veterinary professionals, this is less about replacing culture or PCR than about closing a familiar diagnostic gap. Goat Johne’s cases often surface after weight loss, poor production, or chronic wasting, and by then the submitted material may be fixed tissue only. In that setting, an ISH assay that localizes mycobacterial RNA within lesions could improve confidence in diagnosis, help distinguish true infection from look-alike granulomatous enteritis, and support herd-level management decisions sooner than culture alone. That could be especially useful for diagnosticians, pathologists, and mixed-animal veterinarians advising dairy goat operations or seedstock herds. (aphis.usda.gov)

There are still practical limits. This was a small retrospective study, the assay was pan-Mycobacterium rather than MAP-specific, and one severely autolyzed case was missed. Labs will likely want larger validation sets, workflow data, and cost comparisons before treating ISH as a standard add-on. Still, the negative-control performance and the signal in most paucibacillary cases suggest real diagnostic value. (acvp.org)

What to watch: The next step is whether larger studies confirm sensitivity in low-burden and autolyzed tissues, and whether veterinary diagnostic laboratories begin offering RNAscope-based tissue ISH alongside culture, PCR, and conventional histopathology for caprine Johne’s investigations. (acvp.org)