Johne’s disease (paratuberculosis) remains one of the toughest infectious challenges in sheep and goat systems worldwide. Caused by Mycobacterium avium subspecies paratuberculosis (MAP), it targets the intestinal tract and advances slowly, often without overt signs for years—yet shedding can be massive in this “silent period,” seeding pens, water, and feed while the herd appears normal.

When clinical signs do emerge—weight loss, diarrhea, poor appetite and sharp declines in milk yield—the economic damage is already underway. In many operations, losses extend beyond clinically affected animals: herd productivity slips, replacement rates rise, breeding value is discounted, and control costs escalate. Enfected animals frequently produce 10–20% less milk and gain weight more slowly, compounding revenue impacts at the flock level. Trade is not immune either: flocks certified free of Johne’s retain a price premium in breeding markets, while infected herds forfeit that advantage.

Transmission is primarily dam-to-offspring at birth or via milk, with additional oral exposure from contaminated environments. MAP’s environmental persistence—up to a year—amplifies within-flock spread. Youngstock are especially vulnerable, and clinical disease often appears 2–5 years after infection, which explains why the first visible cases are often just the tip of a much larger iceberg.

Diagnosis is the chokepoint. Conventional serology and fecal culture tend to detect MAP late, limiting the effectiveness of test-and-manage programs. Newer PCR-based molecular methods can identify MAP DNA earlier in the course of infection, improving decisions on isolation and culling.