Animals at this stage appear healthy but are shedding adequate numbers of M. paratuberculosis organisms in their manure to be detected on fecal culture. Blood tests are not reliable to detect animals at this stage. These animals pose a major but often hidden threat of infection to other animals through contamination of the environment. Cattle at this stage have acute or sometimes intermittent watery pea-soup manure. Animals lose weight and drop in milk production but continue to eat.
Some animals appear to recover but often relapse in the next stress period.
Most of these animals are shedding billions of organisms and are positive on culture. Many are positive on serologic tests. Clinical signs often last several weeks before the animals are sent to slaughter in thin emaciated condition. The carcass may not pass meat inspection for human consumption. Thus, in a herd of 100 milking cows, two clinical cases at once suggest 20-30 others are infected, and less than half of the infected cattle are detectable by fecal culture.