Resources

This page contains resources to help you manage a PRRSv outbreak in the breeding herd. There is more PRRSv-related content on PRRS.com.

Swiss Cheese Model

The ‘Swiss cheese model’ acknowledges that no single intervention alone can control PRRSv. Each layer of the Swiss cheese model gradually helps you to achieve robust internal biosecurity and improve PRRSv control.

While performing a whole herd PRRS control program, remember the...

Ten Golden Rules

Rule #1: When cross-fostering, move pigs only when necessary

At the time of farrowing, it is not unusual for the sows within a farrowing group to have a high number of piglets per litter, and after considering factors such as the number of functional teats and teat conformation, farrowing technicians commonly carry out a process called “cross-fostering”. This movement of piglets to a foster sow of the same group is also referred as “litter equalisation” it may actually negatively impact health stability so it should always be minimized, especially for PRRS positive farms!

• Since all piglets are not born with the same immune status (PRRS), and have not received the same colostrum, any piglet movement between litters has the potential to spread diseases (PRRS) within the lactation room.⁶
• Any piglet movement between litters has the potential to disrupt the milk production of the sow causing weight loss and affecting the well-being of the moved piglets and the entire litter, so avoid unnecessary movements.⁷
• It has been shown that piglets that were cross-fostered once were 11.69 times more likely to die and were at higher risk of pericarditis and heart condemnation compared with pigs that were not cross-fostered (p < 0.05).⁸

Rule #2: No cross-fostering later than 48 hours

After farrowing, providing adequate colostrum intake while minimizing cross-fostering at the same time is an industry challenge. Colostrum intake is one of the main determinants of piglet survival since it provides the essential energy and immunity that every piglet needs in early life. Colostrum from the birth sow also helps to maximize the quality of immunity. But the pressure to cross-foster early can be high due to reasons such as increased sow prolificacy, inadequate functional teat numbers, and disease instability. Since early cross-fostering can result in the variability of the quality and volume of colostrum intake of piglets within the litter it is important to delay the process as long as possible. At the same time, the cross fostering that is absolutely necessary must not occur too late in lactation in order to avoid litter disruption and the associated negative impacts.

• Piglets are born immunologically naïve as the sow is unable to transfer antibodies in utero to the piglets via placenta, so antibody transfer from colostrum is crucial for adequate immune function in early life. Immunoglobulin (Ig) G is the most predominant in colostrum and its concentration decreases dramatically during the first 24-30 hours of life.⁹
• Piglet ability to absorb antibodies from colostrum decrease rapidly after 6 hours from the first feeding due to a permeability decrease of large proteins through gut membranes.

Rule #3: Keep piglets in the farrowing pen and avoid handling to minimise the spread of disease

After the farrowing event, there are several management practices that require the handling of piglets. Examples include piglet processing (e.g. clipping teeth, tail docking, umbilical cord management, iron administration, and castration), split suckling using warm boxes, and cross-fostering. To perform all of these processes, it is not uncommon for the farrowing technicians to step into the farrowing pens and share common tools to hold and treat piglets from each litter in order to ensure the most efficient work processes. However, all of these management practices can facilitate the spread of diseases such as PRRS between litters. Keeping the piglets in their own pen and minimising the number of handling processes that share tools and spaces is critical for disease management.

• Possible indirect routes of PRRS virus transmission include urine, blood, saliva and faeces from infected animals to susceptible ones. Warming boxes and processing carts when shared within the same room can serve as fomites to transmit the virus between litters.¹⁰
• Avoid stepping into the farrowing crates of each sow. It has been demonstrated that boots and coveralls can serve as fomites for the PRRS virus from infected to susceptible animals.¹¹

Rule #4: Change needles between litters

Syringes and needles are used to administer injectable antibiotics and analgesics (i.e. pain killers) as well as other therapeutic products such as iron and vitamins. In the swine industry, it is common to use the same needle to inject the same product into different animals. The practice of sharing needles between piglets during the lactation period can play a significant role in the transmission of infectious diseases, such as PRRS.

• At the peak of viremia, infected animals have a viral load of at least 10³-10⁴ TCID₅₀/L.
• Assuming a minimum infectious dose of 10¹-10² TCID₅₀/L through a percutaneous (i.e. injectable) exposure, a simple drop of blood (i.e. 1-10 µL) could transmit a sufficient amount of virus between animals.¹² Even if a needle change is performed between litters, an additional measure to minimize within litter spread would be to inject small and rough-haired animals last.
• Bloodborne transmission of the PRRS virus has been demonstrated in controlled field studies using both the same needle and needle-free injection devices. ^13,14

Rule #5: Do not move sick piglets

During the lactation period, management practices and factors impacting stress levels and disease status have the potential to influence milk production and, with that, piglets and litter performance. Therefore, litters and piglets within litters, do not always show the same growth performance. This reality is common for PRRS positive farms. To fix this issue, producers tend to practice cross-fostering of piglets that are falling behind, when compared to their pen mates without considering that the disease transmission risk could be higher than the potential improvement in growth performance.

• The movement of sick piglets and runts, often referred to as ‘fall-back piglets’, increases the probability of disease transmission between litters due to animal-to-animal contact with different immune status for PRRS and other pathogens.¹⁵
• Management practices at the farrowing, such the use of nurse sows, has been demonstrated to facilitate PRRS virus transmission to piglets from the sow and to sow from sick piglets.¹⁶

Rule #6: Wean all piglets from the same farrowing group at the same time, and do not allow any weaned piglets to remain in farrowing rooms

Correct growing pig flow starts in the farrowing room when piglets, that were born into the same farrowing group, are weaned away from the farrowing rooms at the same time to create a batch of growing animals with a similar age. Pig flow errors at the time of weaning can compromise overall health stability. Examples of these errors include cross-fostering underweight wean-aged piglets onto younger litters (i.e. ‘hold back piglets’) for quality/weight improvement and allowing piglets that have been weaned from their sow to stay in the farrowing crate without a sow. Despite the potential benefits, these practices put the health status of the farm at risk.

• When the milk supply is terminated at weaning, maternal antibody levels decline which leaves the piglet vulnerable to infections that can drive changes in the dynamics of disease transmission within the population.^17.18 Therefore, if weaned piglets remain in the farrowing rooms, they become a potential source of pathogens for disease transmission to other litters and sows that are on their way back to the breeding area.
• The movement of older hold-back piglets from one batch to the next increases the probability of disease transmission between animals of different ages via direct and indirect contacts between piglets with different immune statuses for PRRS and other pathogens.¹⁹

Rule #7: Strict batch production (all in/all out)

All-in/all-out systems keep pigs together within batches as they move through the different phases of production. Each group is considered to be an intact unit and, once the group has moved forward, the facility or room is completely emptied and cleaned for the next group. Although the principle of all in/all out pig flow seems simple, it is one of the most difficult rules to implement due to variation in production parameters such as: pigs per batch, weight range, and growth rates. When the causes of variation are not addressed, managers are forced to compensate by breaking the all in/all out rule.

• Cleaning and disinfection strategies between batches is probably the most important internal biosecurity measure to break the infectious cycle of pathogens from one production batch to the next.²⁰
• Mixing or sorting pigs is a source of stress to the animals and it increases the probability of disease transmission due to differences in the immune status for PRRS virus and other pathogens.²¹
• Do not share needles, equipment, personnel and protective equipment between batches (unless cleaned and disinfected) since it could increase indirect transmission of the PRRS virus.²²

Rule #8: No contact between different age groups

In well designed production systems, correctly dimensioned facilities allow producers to house each production batch in separate buildings/rooms (i.e. independent air spaces) at the correct pig density to maintain proper All In/All Out (AI/AO) pig flow management. This strategy ensures no contact between pigs of different ages. However, good AI/AO flow can be challenged due to factors such as significant variation in batch size or the presence of disease. These examples can result in differences in pig growth and quality which can force the need to mix animals from different batches (i.e. different ages) in the same air space.

• Mixing pigs of different age groups at any stage of the production is a risky practice, as it can bring pathogens to a susceptible population due to differences in the immune status for the PRRS virus.^23,24
• Nathues et al., estimated that contact between fattening pigs of different ages during restocking of compartments increased by 13 times the risk for respiratory disease occurrence in enzootic pneumonia positive herds²⁵

Rule #9: No contact between pigs less than six months of age and sows

After farrowing, piglets receive powerful immunity from the sow via colostrum and milk that makes them immune to most of the pathogens to which the sow has been exposed. However, this immune status starts waning in the piglets right after weaning with the removal of the milk supply. After weaning, piglets must use their own active immunity to protect against the infectious challenges they meet. Several factors can inhibit or challenge the growing pig’s immune response such as changes in nutrition, stress due to poor management practices, and commingling with other pig populations that are a source of infectious challenges. Ensuring the separation of growing pig batches from the sow herd protects the sow herd from potential disease challenges.

• Several studies have demonstrated that younger animals have significantly longer viremias, and higher viral loads in lymph nodes, and lungs.^26,27
• Sow farms with a growing pig population (e.g. wean to finish gilt development units attached to a sow unit) that is not properly isolated from the sows are more likely to show a longer persistence of PRRS infection following an outbreak.²⁸

Rule #10: Always introduce incoming and home-produced gilts via quarantine with administration of PRRS MLV vaccination upon entry to the quarantine area

Maintaining a flow of gilt replacements into a herd is a pillar of stable production since a sow herd with stable health performs the best. As a result, one of the most important strategies of gilt development is a good gilt health acclimation process. This becomes especially important with high annual replacement rates in large production herds where gilts make up a relatively large proportion of the productive herd, and in herds with endemic diseases, such as PRRS. Ensuring a well controlled PRRS immunization and exposure of gilts during their quarantine/adaptation period is key to protect them against field viruses and to prepare them for the natural infection challenges they are likely to experience in endemic farms.

• Natural immunization of gilts should be avoided since it offers a poorly controlled immunization process and allows for the re-introduction of the wild-type PRRS virus to the herd.²⁹
• Modified live vaccines can replicate in the host and induce an immune response similar to that induced by mildly virulent PRRSV isolates.³⁰ Therefore, all gilts should be quarantined and immunized two times (3 - 4 weeks apart) with an MLV vaccination.
• The virological and clinical protection afforded by MLV vaccination is considered partial against a heterologous PRRSV strains; however, in general, vaccinated pigs experience fewer clinical signs and a viraemia of shorter duration compared to naïve piglets when infected with field isolates.³¹