Prevention, control, and elimination of PRRSv in breeding herds
Porcine reproductive and respiratory syndrome virus (PRRSv) is one of the most costly and difficult to manage diseases within the swine industry. In breeding herds, it causes reproductive losses such as substantially increased incidence of abortions and mummies, and infertility. Downstream, in pigs of all ages, it causes pneumonia with clinical signs such as thumping and coughing, and impacts production by reducing growth and increased mortality. The control, elimination and prevention of PRRS is an integral part of modern swine management, especially in breeding herds.
- To review sow management practices to control PRRSv
- To review gilt management practices to control PRRSv
- To discuss and review PRRSv vaccination
- To discuss PRRSv elimination strategies from the herd
- To discuss biosecurity risks and prevention of PRRSv entering the farm
The main objective to control porcine reproductive and respiratory syndrome virus (PRRSv) is to wean a PRRSv negative (not infected or virus free) pig while protecting the herd from new PRRSv introductions. Several management strategies have been reported to assist in achieving this goal in swine breeding herds.
PRRSv can be transmitted via semen, therefore it is imperative to avoid introducing contaminated semen into the sow herd. Boar studs should be PRRSv negative and boar serum testing by polymerase chain reaction (PCR) for PRRSv must be performed to detect and eliminate PRRSv from the stud should an infection occur. Even though PRRSv may not be detected in serum until 24 and 96 hours post-infection respectively (Reicks et al., 2006) suggesting there is a window of time when the virus will not be detected, serum continues to be the most sensitive sample for boar stud monitoring.
Suckling Pig Management
Once gestating sows and gilts become infected with PRRSv, an intra-uterine infection can occur leading to the birth of viremic, weak and non-viable newborn piglets (Ladinig et al., 2015). McCaw (2000) introduced a new concept to control the spread of pathogens in suckling pigs. The concept acronym McRebel stands for “Management Changes to Reduce Exposure to Bacteria to Eliminate Losses”. Measures such as minimizing cross-fostering, eliminating poor doing non-responsive pigs, changing needles between litters or pens and taking extra care of small pigs are included. Since then, additional management practices have been included, among them is limiting nurse sow movements at weaning (Garrido et al., 2020), and decreasing piglet handling by farm personnel. Breeding farms that undergo a PRRSv elimination process and did not include thorough and continued McRebel management practices have had recurrent recirculation of the virus in the sow population (Polson et al, 2010) highlighting the importance of these management practices.
Once a breeding herd has become infected with PRRSv, gilt introduction becomes one of the most important factors for PRRSv control. Gilts are susceptible to PRRSv infection if they have not developed protective immunity prior to their introduction into the herd. Additionally, gilt acclimation plays an important role on the production of PRRSv negative pigs at weaning. If gilts are infected during gestation their progeny will be born infected, therefore becoming a source of infection to the herd. Gilts that are infected with PRRSv in their growing phase will develop immunity against the PRRSv strain to which they were exposed.
The goal of a PRRSv acclimation program is to produce immune non-viremic gilts before they are introduced into the breeding herd. This is achieved by exposing them at a young age to the homologous virus circulating in the herd. Early in an acclimation program, gilts can be exposed to PRRSv using different methods. One way is to expose gilts to viremic nursery pigs. A second method is to intentionally expose incoming gilts to the resident PRRSv by live-virus injection (Batista et al., 2002).
Potential risks with these methods are the spread of other pathogens, potential generation of PRRSv aerosols posing a risk for neighboring herds in addition to increased mortality, and poor reproductive performance (Williams et al., 2015), as well as promoting virus mutation. However, one important advantage is that the exposure date will be known and theoretically all gilts will develop immunity against the resident PRRSv around the same time, thereby providing the basis for the virus elimination at the population level. A third acclimation option is to vaccinate gilts during the acclimation period. With this option, gilts receive one or two immunizations with a modified-live vaccine prior to entering the breeding herd.
Both inactivated and modified-live virus (MLV) vaccines have been used in gilts, sows, and growing pigs for PRRSv control. MLV vaccines, including, both homologous or heterologous PRRSv have shown some efficacy in improving reproductive performance, reducing mortality, impact on growth rate, and reducing wild-type virus shedding (Linhares et al., 2012) even when a heterologous strain was used as the challenge virus (Roof, 2008). A recent study showed that pigs vaccinated twice with MLV outperformed pigs that had been vaccinated only once (Moura et al., 2019). MLV vaccine has also been administered repeatedly within infected populations and the number of persistently infected and shedding pigs was apparently reduced (Cano et al., 2007). MLV vaccines have also been used as an element in the control and elimination of field virus from infected breeding herds.
Elimination methods at the herd level
Different methods have been described to eliminate PRRSv from sow herds, among them whole herd depopulation/repopulation and herd closure have been successfully used. These methods must be implemented in an integral approach including other tools described below
Whole Herd Depopulation and Repopulation
This method, although not routinely practiced, comprises the elimination of all breeding and / or growing swine from the farm, disinfecting the facilities and re-stocking the farm with PRRSv negative pigs. Whole herd repopulation is costly as all the breeding females need to be replaced. An off-site breeding program can be implemented to mitigate the disruption in weaned pig production. One important advantage is that using this strategy, if properly planned, other pathogens can be eliminated and the herd genetic level improved.
Herd Closure and Rollover
Herd closure and rollover has become the most widely used method for eliminating PRRSv from sow herds. This method was first described by Torremorell et al. (2003) and consists of interrupting the introduction of replacement females (whether they were born on-site or off-site) into the breeding herd for at least 6 months plus the elimination of seropositive animals over time.
The objective of interrupting the introduction of new animals into the herd is decreasing the number of susceptible animals in which the pathogen can replicate, thereby favoring the elimination of the virus. To avoid productivity reduction, an adequate number of immune gilts should be introduced into the herd before its closure.
Planned exposure of the breeding herd with homologous virus or MLV vaccine is the last step of this program. The objective of exposing all breeding animals to PRRSv at once is to ensure solid immunity of the entire breeding herd. Although persistently infected animals remain since there are no susceptible animals in the herd, virus circulation within the herd will be significantly reduced or eliminated.
Following intensive testing of processing fluids, family oral fluids or sera of neonatal and due-to-wean piglets by RT-PCR testing, the herd can be declared as stable if consistent negative results are obtained. The number of samples, sample type and sampling frequency to classify a herd as PRRSv stable have been summarized in the AASV PRRS sow herd classification guidelines (Holtkamp et al., 2021). The time between herd closure and weaning consistently negative piglets is variable and depends on several factors such as virus strain, exposure method, presence or absence of previous immunity, number of previous outbreaks, management factors and season when the virus was introduced (Sanhueza et al., 2018; Linhares et al., 2013).
Rollover begins when PRRSv negative gilts are introduced into the herd, and it concludes when the last exposed sow is culled. Herd closure has been reported to be the least expensive method to eliminate PRRSv. Furthermore, breeding herd performance may improve after the elimination plan has been completed.
A biosecurity program for each breeding herd is required to avoid pathogen introduction. Biosecurity programs are the result of the orchestrated implementation of different biosecurity procedures related to daily production practices that represent a risk of pathogen introduction.
As already mentioned, in the United States, the tendency is to have PRRSv naïve boar studs located in areas of low pig density and under strict biosecurity measures and intense monitoring protocols. These protocols are intended to detect PRRSv as early as possible to halt any semen doses from leaving the stud or being used at the sow farm.
All transport vehicles used to move all categories of pigs, feed and any farm related material constitute an important risk as these vehicles can be contaminated with PRRSv that can then be transmitted into the barn. Therefore, vehicles that will come in contact with the herd must go through thorough cleaning and disinfection procedures (Dee et al., 2004). The use of a thermo-assisted drying and decontamination (TADD) systems to complement the cleaning and disinfection procedure has become a popular tool. After washing and disinfecting, the trailer is heated to the point that drying time is significantly reduced but it also contributes to hindering viral survivability (Dee et al., 2007).
Gilt Origin and Introduction
Introduction of replacement animals into breeding herds and boar studs is a high-risk event. Replacement animals should be sourced from high health multiplication herds that have a historical record of being PRRSv negative. When leaving the multiplier, gilts should be loaded onto clean, disinfected trucks, driven to an isolation unit, strategically located on a biosecure site, tested and confirmed they are virus free before they are introduced into the breeding herd.
Even though PRRSv does not infect humans, personnel can serve as a vector should they come onto the farm wearing contaminated clothes, boots or personal objects (Otake., 2002). People play a key role in preventing the introduction of the virus into the herd as they are responsible of respecting and implementing the biosecurity protocols established for the herd. Therefore, all personnel should be trained and aware of the potential routes of PRRSv transmission. Current recommendations include that people shower-in and wear farm specific clothes and boots before entering in contact with the animals within the breeding herd. Personal belongings such as watches, phones, wallets, and jewelry should remain outside. If shower-in is not an option, an alternative method is the Danish entry system where personnel change their street clothes for farm-specific clothes and boots together with handwashing. This system is set in a way that a dirty and a clean area is clearly designated.
Breeding herds require delivery of various supplies for their daily operation. Supply entry poses a risk as supplies can be contaminated with pathogens; therefore, it is recommended that there is a process in place for these supplies to be disinfected before going into the farm. This process can be done through different procedures such as using a fogging chamber or a disinfection and drying room. The objective is to ensure that the external surface of these supplies comes in contact with a disinfectant. The disinfectant is allowed to dry and, in some cases, supplies are not entered until they have met a specific downtime. There are other methods used for killing viruses potentially present on surfaces of fomites. For instance, exposing supplies to 37°C for more than 1 day proved to hinder viral survival (Kordas et al., 2020) and exposure of fomites (Dee et al., 2011) and aerosols (Cutler et al., 2012) to ultra-violet (UV) rays has also been shown to be effective at decreasing pathogen load.
One way that PRRSv can be transmitted between farms is through the airborne route. Positive pigs can exhale particles containing live virus that can exit the barn. It is now known that the larger the airborne particles, the more virus these particles will contain (Alonso et al., 2015). Therefore, when PRRSv becomes airborne it can exit the barn and represent a risk of infection to other pigs. Thus, filtering incoming air has become an alternative for breeding herds, especially those located in areas where there is a large concentration of pig farms. Air filtration has proven to be efficient at reducing the break rate of breeding herds (Alonso et al., 2013) and increase overall productivity of filtered farms. A pre-filter and a filter located in the attic or a side filter bank removes particles from the incoming air that may harbor the PRRSv.
At the farm level, disposal of dead animals usually occurs either through rendering or composting in the majority of breeding farms in the United States. The use of rendering has been associated to a PRRSv positive status (Lambert et al., 2012). Composting has been considered a key intervention in the reduction of PRRSv incidence as it avoids the visit of a rendering truck to the site (Donovan, 2019).
PRRSv can survive in feces and represent a risk, especially when temperatures are low (Linhares et al., 2012). When scheduling a manure pumping event, producers should work with their manure hauling company and set biosecurity guidelines to protect the herd. When possible, schedule this event first when the pumping equipment has been cleaned and allowed to dry before visiting your site. Personnel carrying out this process should understand which areas within the farm they are allowed to enter. Manure-hauling crew members should not enter barns.
In summary, protecting the breeding herd against PRRSv requires multifaceted methods implemented at each event and stage of swine production. It is especially necessary to implement strategies in the breeding herd to protect from clinical disease and production losses directly and in the downstream pig supply to reduce overall pig mortality and economic losses due to PRRSv. Exercising the above strategies discussed may help achieve proper control, prevention and effectively eliminate PRRSv within a breeding herd.
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