By the time animals show clinical signs of illness, the disease may have spread to herd mates. The body responds by sending in more white blood cells to combat the intruder, and eventually the intestinal walls thicken. Finally, the walls become so thick, the animal can't absorb nutrients, and it develops chronic diarrhea. Despite a healthy appetite, the animal finally wastes away. Other less obvious signs in the herd can include decreased fertility, lower weaning weights, animals that must be culled sooner, and increased feed costs without related weight gain.
Infected animals can shed the bacteria onto feed or grass, where it can be ingested.
Once outside the body, the bacteria contaminates the ground and water, and although it doesn't multiply, it can live up to a year in the environment. Young animals, from birth to a year of age, are most susceptible to the disease, said Dr. Schwartz. The vaccine's use must have state approval, and a veterinarian is to administer the injection only to animals less than 35 days old. Johne's Widespread Dr. Schwartz said the general thought for years was that Johne's affected herds primarily in the North. Tests for Johne's fall into three categories, explained Dr. Schwartz.