Terminology for classifying swine herds by porcine reproductive and respiratory syndrome virus status. DJ Holtkamp, DD Polson, M Torremorell, and committee members B Morrison, DM Classen, L Becton, S Henry, MT Rodibaugh, RR Rowland, H Snelson, B Straw, P Yeske, J Zimmerman. Journal of Swine Health and Production, Volume 19, Number 1, 2011
What are they studying?
The purpose of this paper is to provide a herd classification system for describing the PRRSV status of herds, based upon a set of definitions reflecting the biology and ecology of PRRSV.
An accepted set of terms and herd classifications will facilitate regional and national efforts to eliminate PRRSV. Furthermore, the availability of standardized nomenclature will allow communications among researchers, veterinarians, producers, genetic companies, and other industry participants.
How is it done?
This herd classification system was developed by a definitions committee formed jointly by the American Association of Swine Veterinarians (AASV) and the United States Department of Agriculture PRRS-Coordinated Agricultural Project, and was approved by the AASV Board of Directors on March 9, 2010. The committee included veterinarians from private practice and industry, researchers, and representatives from AASV and the National Pork Board.
What are the results?
Breeding herds, with or without growing pigs on the same premises, were categorized as Positive Unstable (Category I), Positive Stable (Category II), Provisional Negative (Category III), or Negative (Category IV) on the basis of herd shedding and exposure status. Growing-pig herds are categorized as Positive or Negative.
|Herd category||Shedding status||Exposure status|
|Positive Unstable (I)||Positive||Positive|
|Positive Stable (II-A)||Uncertain||Positive|
|Positive Stable (II-B)
|Uncertain – undergoing elimination||Positive|
|Provisional Negative (III)||Negative||Positive|
Figure 1. Breeding-herd classification for porcine reproductive and respiratory
syndrome virus (PRRSV) according to shedding and exposure status.
What implications does this paper have?
Standardized nomenclature will facilitate better contractual and business arrangements, especially those between genetic and commercial production companies and agreements between producers that offer premiums for weaned pigs from breeding-herd sources with a specific PRRSV status. Researchers writing proposals and papers and making presentations may save time otherwise spent defining terms related to PRRSV, and standardized definitions will facilitate comparison of field-based research results by clarifying the conditions under which research trials are conducted.
However, anecdotal evidence from the field supports the notion that herds classified as Positive Stable (II) have better reproductive and growing-pig performance than herds that are Positive Unstable (I). Being Positive Stable (II) is the goal for breeding herds that are trying to control the virus. In the context of area or national elimination efforts, subdivision of category II into II-A and II-B is important to convey the likely differences in the risk of current or future shedding of virus by animals in these herds.
The view from the field by Enric Marco
When talking about PRRS virus positive farms it is common among technicians to speak of stable or unstable farms. This terminology is generally used to describe what we see on the farm, i.e, when clinical signs —either reproductive or respiratory— are observed in growing animals, the farm is classified as unstable to PRRS; and when these symptoms are not observed, the farm is commonly called stable to PRRS. In some cases, this classification is based on some laboratory investigations, generally serological tests on breeding pigs to detect antibodies (ELISA). We rarely analyze in detail what such classification is based on. This article explains the criteria that must be applied: sampling population, number of samples, serological tests, repetitions, etc. This standardization allows us to establish a common language when referring to this infection, and it becomes a key element in the development of regional control plans.
It is inconceivable for the fight against PRRS to be conducted in isolation in high density areas. It is not the first time we mention it. Neighbouring farms will influence the success or failure of the measures applied on a particular farm. As with other airborne infections, measures must be implemented jointly in all holdings in an area: hence the existence of regional control plans. When joint actions are initiated, it is essential to establish agreed criteria in order to measure their progress, allowing us to celebrate successes and establish corrective measures when we don't get the expected results. The development of the Aujeszky's disease control and eradication plan, for example, was measured based on prevalences. In this case, and since the primary objective was disease control, measuring prevalences doesn't contribute much, hence the importance of establishing a standardized classification of farms and being able to monitor progress.
The first step of any PRRS regional control plan, which would be a great breakthrough, should be to achieve a positive stable status (i.e, the animals have had contact with the virus but they are not excreting it) for all the farms in the area (or most of them), because that would result in a reduction of the risk of new infection, and most farms would be producing without the economic impact posed by clinical signs. This alone is enough to justify the application of any joint action plan.