Achieving Day 0 for Mycoplasma hyopneumoniae: What is it? Why should it be done?

Day 0 is a common term in swine disease control and elimination. However, a standard definition of Day 0 may not be available for all diseases or pathogens. In the following article, a definition and explanations for its application are offered using Mycoplasma hyopneumoniae (M. hyopneumoniae) as an example.

In general, it could be assumed that Day 0 is a self-explanatory term needing no further clarification, but obviously, being so simple, it can be applied in various scenarios and therefore can become confusing.

Day 0 marks the official beginning of a disease control strategy, being applied as a regular farm control measure or for disease elimination. From this day on, the clock starts ticking and all logistics regarding the control program are counted and organized around this schedule. Usually, Day 0 marks the beginning of the herd closure, which becomes the most important aspect affecting sow farm management during elimination. Herd closure implies that no more gilts are allowed to enter the farm until scheduled (usually several months later), which will inevitably lead to aging of the herd, accompanied with the lack of flexibility for necessary culling.

Herd closure is understood as a method which allows the modification of disease status in the herd by stopping the introduction of potentially susceptible individuals to a population, where pathogen circulation exists. Herd closure programs vary depending on the pathogen in question, but are guided by the epidemiological characteristics of a given infectious agent. Of significant importance is to know that the persistence of the pathogen generally defines the length of the herd closure. Ideally, herd closure will allow ample time for the population to become infected, develop disease, recover from it, and stop shedding.

Using the example of M. hyopneumoniae elimination, Day 0 refers to the date when gilts (Figure 1) in a herd have been confirmed exposed to the live form of the pathogen. This statement is loaded with points were differences and lack of definition can be introduced. Thus, the following aspects are discussed:

• Gilts: In endemically infected farms, it is assumed that females already housed at the farm may have been exposed to M. hyopneumoniae and are therefore not considered part of the newly susceptible population. This assumption maybe stretched by the fact that subpopulations of negative females can be detected over time in naturally infected farms. A potential explanation for this situation is the fact that transmission of M. hyopneumoniae occurs rather slowly, and not all contact between females are effective contacts, resulting in infection. Thus, sows that have already farrowed in the herd should be at least considered when designing control and elimination protocols, even though Day 0 activities will be based on gilts.
• Confirmed exposed: Gilt exposure to M. hyopneumoniae does not simply equal effective exposure. It would be erroneous to assume that all types of exposure will result in the initial objective of making sure that females have encountered and responded to the M hyopneumoniae. Therefore, rapid confirmation of exposure based on diagnostics is highly recommended. The timing and diagnostic protocol for this confirmation of exposure varies, mainly based on the diagnostic and control methods, including, but not limited to route and dose of infection.
• Live: Although gilt acclimation for M. hyopneumoniae relies largely on gilt immunization using commercial vaccines, it is important to remember these products will not render gilts infected with the bacterium, as they are bacterins.

Gilt infection with M. hyopneumoniae can be achieved in the population by several means (which are not the topic of this document) but will definitely have a significant impact on disease transmission and dynamics. Other factors, such as M. hyopneumoniae strain virulence, for example may influence the transmission of the pathogen in the herd and, should therefore be taken into consideration when estimating Day 0, and consequently, the end of the herd closure period.

Although it may result difficult to establish a single definition for Day 0, as explained above, it is usually assumed that it is achieved when the majority of the susceptible animals in the population have been included in the exposure program. Establishing Day 0 is a crucial step in disease control and elimination programs and should be carefully performed by swine health professionals.