Stochastic model of porcine reproductive and respiratory syndrome virus control strategies on a swine farm in the United States. Jaewoon Jeong; Sharif S. Aly; Jean Paul Cano; Dale Polson; Philip H. Kass; Andres M. Perez. 2014. Am J Vet Res 2014;75:260–267
What are they studying?
The authors studied the effectiveness of different combinations of control strategies for porcine reproductive and respiratory syndrome (PRRS) virus at the individual herd level. The goal is to determine the most effective strategy for PRRS virus control.
How is it done?
Stochastic models simulated PRRS outbreaks on a hypothetical small, medium, or large farrow-to-weaning swine farm in the Midwestern United States. Different control strategies were compared in the models: from none to 4 several combinations of immunization, herd closure, and gilt acclimatization. In addition, by combining 3 virulence status of PRRSv and 3 herd sizes, nine different models were simulated.
The goal in each simulation was to have negative PCR results on 3-week old piglets (PRRS stability).
What are the results?
Increasing PRRS virus virulence and herd size were negatively associated with the probability of achieving a stabilized status. Virulence level appeared to have a greater influence on the probability of the farm achieving a stabilized status than did herd size.
For the baseline scenario (no PRRS control strategies implemented), a stabilized status was achieved only in the model for small herd size and low PRRS virus virulence.
Repeated mass immunization with herd closure or gilt acclimatization was a better alternative than was single mass immunization for disease control within a farm.
What implications does this paper have?
For a farrow-to-weaning swine farm, repeated mass immunization with a PRRS modified-live virus vaccine with herd clousure or gilt acclimitization was the scenario most likely to achieve a stabilized status. Also, herd closure appeared to be more beneficial than only gilt acclimatization.
The view from the field by Enric Marco
Those of us who, during these last years, have worked as veterinary surgeons in the pig sector and have had to "deal" with the PRRS are not surprised to see that the approaches to achieve PRRSV stabilization differ in different parts of the world (USA, EU), or even within Europe. All of them have some elements in common but, at the same time, they all have differences. What are the common elements?
All the different control plans always combine different strategies in order to achieve their goal: they all try to prevent the destabilization the arrival of replacements generates, either with the most drastic methods such as its temporary herd closure, or less drastic methods such as different adaptation plans of varying lengths. They all avoid elements that pose a risk of introducing new strains, such as the arrival of positive replacements or the use of semen doses from positive artificial insemination centres. They all try to avoid the presence of hot spots (areas where the virus circulates continuously: post-weaning) using different techniques such as partial depopulations, batch handling or the implementation of strict internal biosecurity measures. And they all implement a vaccination plan: Post-outbreak emergency vaccination and vaccination of gilts, blanket vaccination several times a year, "6-60" vaccination protocols, combination vaccines with live attenuated and inactivated virus, sows-only vaccination or vaccination of the entire herd, etc. The truth is that, after 24 years of experience, we still do not have a control plan agreed and accepted by all our technical community. Why?
Although a theoretical exercise (based on certain assumptions), this article helps us to understand what happens in real life. The different combinations of circumstances we find in the field, such as the type of farm (farrow-to-finish or piglet producing farm), size, handling of replacements and pathogenesis of a viral strain in particular, largely explain why, in real life, the result obtained with the implementation of these measures is not always the same. In short, the worst starting combination will always require more aggressive measures if we want to achieve the same results.