The duration of herd closure is based on the expected time required for all animals on the farm to mount an immune response, clear the pathogen, and no longer be infectious.

Herd closure of at least 240 days (eight months) is based on research indicating that pigs can shed 
M. hyopneumoniae up to 200 days post infection. However, persistence may not always follow the book. Factors that drive persistence are unknown, but management practices, exposure method, coinfections, etc. should be assessed.

Studies aimed at shortening the persistence of M. hyopneumoniae by various vaccination or medication strategies have yet to succeed. The natural course of the disease currently dictates the length of closure, although constant reevaluation is necessary.