Pig production systems have changed dramatically and new diseases have emerged since the first implementation of isolated weaning (Isowean) and multi-site production in the 1980s.

The introduction of these production technologies brought rapid acceptance and application throughout the world.

However, not surprisingly, implementation was often done incorrectly as concepts and procedures were misunderstood.

This overview will review the scientific bases for multi-site production (Figures 1a-c), maximizing disease control and comparing multi-site production to one- and two-site production (Figures 2a-b) for disease elimination.

Concepts, Procedures

In multi-site systems (three-site; multiple-site; isolated, wean-to-finish;), the breeding, gestation and farrowing (B, G, F) stages of production are isolated from the other stages of production (Figures 1a-c).

It was anticipated this concept of production would cause reduced herd immunity. And, in fact, disease agents such as Haemophilus parasuis, which caused few problems in one-site herds, reemerged as a significant pathogen in multi-site systems.

The recent development of parity segregation compensates for this lack of herd immunity, improving productivity in multi-site systems (See April 15, 2004 Blueprint on Parity-Based Management).

In the late '80s, the major disease threat in the U.S. was pseudorabies (PRV). Three-site production proved to be a valuable tool in eradicating the virus from herds without depopulation.

Based on these early studies, it was believed that Isowean pigs from multiple sources could be mixed. However, multi-site systems that mixed weaned pigs infected with porcine reproductive and respiratory syndrome (PRRS) virus from more than one site (B, G, F stages) experienced a profound drop in productivity.

Unfortunately, it took a few years to learn how to produce negative PRRS pigs by Isowean.

Origin of Microbes in Newborns

Usually, piglets are sterile or microbe-free in the uterus of the dam. Some infectious agents such as PRRS and hog cholera viruses may infect piglets in the uterus.

Normally, the piglet's first exposure to microbes occurs as it passes through the cervix into the vagina of its mother. As pigs pass through the vagina, they become infected with microbes. At birth, more microbial exposure occurs by contact with feces, skin of the dam and facilities.

Some microbes grow on the skin, and others are swallowed by the piglet and begin to establish themselves in the mouth, stomach and intestines.

Infection depends on the health and immune status of the sow, overall sanitation of the farrowing facility, colostrum and milk intake, and comfort level of the pig-rearing environment (stress, temperature and dampness).

Medicated early weaning (MEW) and Isowean are two methods to alter the establishment of the microbial flora in the newborn piglet and decrease disease in multi-site rearing systems (Figures 3a-b).

How MEW Prevents Disease

MEW was developed as an alternative to surgical derivation and has the distinct advantage of avoiding surgery on the donor sow.

In MEW, small groups of pregnant sows (near term) from one or more farms are placed in strict isolation and farrowed (Figure 3a). Ideally, each isolated group of sows is induced to farrow within 2-4 days of one another. The sows are heavily medicated both prior to and during their stay in the farrowing unit.

To ensure elimination of some infectious microbes, and achieve higher levels of colostral and milk immunity, the sows may only be from second or higher parities. Vaccines may be administered to the sows 4-6 weeks prior to farrowing to further increase the levels of colostral and milk immunity.

Soon after birth, the piglets are administered heavy doses of antimicrobials to lessen the chance of sow-to-piglet transfer of microbes. The piglets are weaned at 5 days of age or less into isolated nurseries. Often, piglets continue to be medicated for several weeks after weaning.

For best results, each weaning group should have only a 1-2-day age difference when placed into an all-in, all-out (AIAO) nursery.

The move to grow-finish should be well isolated from farrowing and nurseries.

MEW is far more costly than Isowean, which is far more practical and can be readily applied to any production system. However, for elimination of pathogens like Streptococcus suis type 2, MEW is the method of choice.

How Isowean Prevents Disease

Isowean has many synonyms: modified-medicated early weaning (MMEW), segregated early weaning (SEW), age-segregated weaning and segregated disease control (SDC). I prefer Isowean because it is derived from the phrase isolated weaning, which more clearly describes the technique and differentiates it from MEW.

Early weaning is not a feature of Isowean in that pigs are weaned at an older age away and in strict isolation from the other age groups in a multi-site rearing system.

Isowean is different from MEW because in Isowean, the sows are not removed from the farm and placed in isolation to farrow (Figure 3b). The weaning age of an Isowean pig depends on the pathogens for elimination and varies from 8 days to 4 weeks (Table 1).

However, age variation of farrowing groups should be very narrow, as in MEW. Weaning age should only vary 2-5 days.

In Isowean, sows are usually vaccinated in late gestation, as in MEW, but often are not medicated. Sows should be placed in an AIAO farrowing room and all sows should farrow within a 4-7-day period.

If bacterial or respiratory pathogens are to be eliminated, the piglets should be medicated in nursing and postweaning periods. At weaning, the piglets are moved to an isolated nursery (and subsequently to an isolated finisher) or AIAO wean-to-finish building.

Creating a Disease-Free Isowean Pig

There are many factors involved in successfully creating an Isowean pig free of infectious disease agents present in the dam or the environment. The factors include:

  • Level of immunity in the dam to the infectious agent(s);

  • Level of immunity passively acquired by the piglet from colostrum or milk;

  • Age of piglet at time of infection;

  • Medications given to the dam and/or the piglet; and

  • Overall throughput, sanitation and biosecurity practices on the farm.

The age of the piglet at weaning (and lactation length) may be critical to successful elimination of some infectious agents (Table 1). In general, the younger the weaning age, the more likely the piglet will be weaned disease-free. If a very high level of immunity can be created and maintained in the sows, then this may increase the weaning age required to eliminate a pathogen.

Multi-Site Production Approaches

Multi-site rearing systems rely on Isowean for the production of disease-free weaned pigs residing in the adult population in the breeding, gestation and farrowing site(s). One-site and traditional two-site farms can utilize Isowean by incorporating an isolated nursery and finisher unit for all or part of weaned pig production.

In fact, prior to construction of the first three-site system in 1988, all Isowean experimental trials involving elimination of PRV, toxigenic Pasteurella multocida and Mycoplasmal pneumonia were conducted by removing a portion of the weaned pigs from one site and traditional two-site farms infected with those agents.

Successfully using Isowean to exclude pathogens depends upon many factors. Most importantly, weaning and finishing facilities must not be operated on a continuous-flow basis, since not every weaning group can be expected to be healthy. AIAO nurseries and finishers also have distinct advantages over systems that are AIAO by room or pen within a barn.

There are eight steps to eliminate infectious agents by multi-site systems:

  1. Establish procedures for isolation and acclimation of incoming replacement breeding stock. If the infectious agent has been recently introduced into the breeding herd, it may be necessary to wait until a level of immunity has been achieved in the adults. Furthermore, it may be important to add replacement stock (negative to the infectious agent in question) to the herd immediately following a disease outbreak so that the replacements also become immune to the agent. The length of acclimation is determined by the infectious agent to be eliminated. For the control of PRRS virus, it is imperative that PRRS-negative replacements be utilized.

  2. Vaccinate sows prefarrowing to prevent diseases caused by infectious agents in Isowean pigs. Vaccines are available for microbes such as toxigenic Pasteurella multocida, Mycoplasmal pneumonia, Haemophilus parasuis, Streptococcus suis, Actinobacillus pleuropneumonia, Actinobacillus suis, swine influenza virus and PRV. The use of live-virus vaccines for Transmissible gastroenteritis (TGE) and PRRS virus are not recommended for elimination of these agents by Isowean.

  3. Administer medications to sows prior to farrowing based on the infectious agents to be eliminated.

  4. Establish management steps for AIAO flow for each farrowing room.

  5. Set the weaning age based on the infectious agents to be eliminated (Table 1).

  6. Administer medications to piglets prior to and after weaning based on the infectious agents to be eliminated, using a consulting veterinarian for advice and prescriptions, as required.

  7. Establish management procedures for AIAO throughput for each weaning group in the nursery and finisher.

  8. Maintain strict biosecurity protocols.

Multi-Site's Role in Disease Emergence

The development of multi-site systems may be contributing to the emergence and reemergence of pathogens. Depending upon the infectious agent to be eliminated, the weaning age for Isowean pigs may vary considerably, but is nearly always less than 21 days of age.

The evolution toward multi-site systems has further supported the concept of AIAO production. Weaning at less than 21 days, AIAO throughput and improved hygiene certainly reduce microbial exposure of the piglet prior to weaning in the presence of maternal antibodies.

The result is a requirement in multi-site Isowean systems for high standards of biosecurity to minimize the introduction of infectious agents into grow-finish facilities.

Carlos Pijoan, DVM, and his colleagues at the University of Minnesota have extensively studied the respiratory tract colonization patterns of the bacterial flora of the pig. They list five risk factors for the development of disease due to Streptococcus suis type 2 and Haemophilus parasuis:

  1. Early weaning;

  2. Isowean;

  3. Degree of virulence of the infectious agents in the pig population;

  4. Proportion of sows of lower parity (high numbers of gilt introductions); and

  5. Immunosuppressive infectious agents present in the population (such as PRRS).

Thus, there appears to be an emergence of some infectious agents in multi-site systems, which have not been significant problems in one-site or traditional, two-site farms. Emerging pathogens include Actinobacillus suis, Haemophilus parasuis, circovirus and Strep suis type 2.

Haemophilus parasuis, for instance, was not a significant pathogen in one-site and traditional two-site herds, except when first introduced into a herd. After the initial outbreak of Haemophilus parasuis or Glasser's Disease, adequate herd immunity would develop in most herds and the piglets would be exposed prior to weaning.

Alternately, it has been suggested by Iowa State University researchers Pat Halbur, DVM, and Eileen Thacker, DVM, that herds infected with either PRRS virus, mycoplasma or both may be immunologically suppressed and thus more susceptible to these emerging pathogens.

Due to a lack of epidemiologic knowledge regarding PRRS, the initial introduction of the virus around 1990 into multi-site systems was devastating, and similar (if not worse) than the occurrence of the virus in single-site farms. Subsequently, it was shown that PRRS virus could be eliminated by Isowean in most batches of pigs at a similar rate as via surgical derivation.

In general, PRRS continues to be a serious threat to hog operations. But multi-site farms appear to be able to either eliminate or control the agent better than large, one-site farms.

It is interesting to note that small, traditional, one-site farms appear to eliminate the PRRS virus when effective acclimation of replacement gilts and biosecurity are implemented.

Porcine circovirus type 2 (PCV 2) can be eliminated from pigs by Isowean (Table 1). However, it is unknown if Postweaning Multisystemic Wasting Syndrome (PMWS) can be eliminated or controlled effectively in multi-site systems of production.

Eradication of Infectious Agents

Modern-day multi-site production systems apply Isowean to exclude or minimize infectious agents at weaning to decrease their effect on the performance of the growing pig. The adult population in one-site systems may or may not remain infected with the pathogen(s) eliminated using Isowean.

Prior to the development of Isowean-rearing techniques in the late '80s, eradication methods focused on elimination of pathogens from the entire herd. These methods will be addressed in the following articles in this Blueprint Series. They include: depopulation, cleaning, disinfecting of facilities and repopulation with high-health status pigs, testing and removal of infected animals, and increasing herd immunity or medication.

Although the original intent of multi-site production was to eliminate infectious agents by isolated weaning, leaving Site 1 infected, there have been approaches to also remove pathogens from the entire system. Around 1998, Jorgan Plomgaard, a veterinary practitioner in Denmark, developed a method for eradicating certain pathogens from entire three-site herds. The so-called Plomgaard Method is based on the work of Von E. Zimmermann in Switzerland regarding eradication of mycoplasma from small, traditional herds without total-herd depopulation.

The Plomgaard Method

This technique has been used to eradicate PRRS virus, mycoplasma and Actinobacillus pleuropneumonia (APP) from an entire three-site farm by these four steps:

  1. No replacement of breeding stock for a period of three months;

  2. Whole-herd medication directed against mycoplasma and APP;

  3. Removal of all breeding animals less than 10 months of age and those with no serologic titers to PRRS virus and APP; and

  4. 4.New replacement stock free of PRRS virus, mycoplasma and APP.

These steps resulted in the elimination of the above three swine disease agents from Site 1 of a three-site system. The nursery and finisher buildings at Sites 2 and 3 were depopulated prior to receiving Isowean pigs free of the three agents.

It is likely that PRV and TGE viruses could be eradicated by this method, but it has not been reported as yet.

The efficacy for eradication of specific infectious agents from entire herds is presented in Figure 4.

Depop/ repop is the surest way to eradicate infectious agents, assuming a supply of negative animals is available and the location of the herd is such that reintroduction is unlikely.

However, as compared to the other methods, depop/repop is far more expensive due to the interruption in income when no pigs are being produced or sold. Test and removal, increased herd immunity, whole-herd medication and the Plomgaard Method are only applicable to specific infectious agents, but are not as costly as depop/repop.

As illustrated in Figure 4, many infectious agents cannot be eliminated from the entire herd except by depop/repop. Thus, the clear, main advantage of multi-site production is the capability to eliminate all the infectious agents (listed in Table 1 and Figure 4) by Isowean without the need to depopulate site one (B, G, F stages).

It is up to the producer to design the correct system to take advantage of the power of Isowean to achieve this and thus maximize productivity.

Table 1. Weaning Age Required to Eliminate Infectious Agents by Isowean
Infectious agent Age of Weaning (days)
Actinobacillus pleuropneumoniae 21-28
Bordetella bronchiseptica 10
Brachyspira hyodysenteriae 21
Haemophilus parasuis 10
Lawsonia intracellularis 10
Leptospira spp. 14-16
Mycoplasmal pneumonia 17-21
Pasteurella multocida (toxigenic) 8-10
Porcine circovirus - Type 2 21
PRRS virus 14-16
Pseudorabies 21
Salmonella 14-16
Streptococcus suis type 2* 5
Swine influenza virus 21
Transmissible gastroenteritis virus (TGE) 21
*Medicated early weaning may be required.

Sow and Piglet Immunity

If the dam has been previously infected with a microbial pathogen, it is likely that antibodies will be present in her colostrum and milk, which could aid in protecting the suckling piglet against infection.

However, if the dam has never been infected, and the microbial pathogen is present in the pig-rearing environment, piglets nursing non-immune sows will become infected.

The degree of sow immunity depends on parity, type of rearing system, presence or absence of the infectious agent in the various stages of production, and the type of pathogen.

In general, first-parity sows, and sometimes late-parity sows, may have low or no immunity to a pathogen. One-site and traditional, two-site farms tend to have higher levels of sow immunity because the housing of various age groups of swine in the same air space or in close proximity results in greater transmission and infectious rates of pathogens.

The level of sow immunity can be enhanced by intentional exposure to infectious agents during acclimation of replacement gilts, prior to breeding all sows, and 3-4 weeks prior to farrowing. Some, but not all, vaccines will also enhance sow immunity and aid in the protection of piglets against infection during nursing.

Piglets must receive colostrum within 36 hours of birth in order to absorb protective antibodies. After 36 hours, the piglet's gut closes and these colostral antibodies are no longer absorbed into the blood. The presence of antibodies in the blood is important for protection against certain types of infectious agents like Streptococcus suis, Haemophilus parasuis and erysipelas.

Both colostrum and milk contain antibodies that also help protect the piglet against some enteric infections by coating the mouth, tonsils, stomach and small and large intestine.

Inducing the pig to produce its own antibodies through vaccination or intentional exposure to an infectious agent may enhance piglet immunity.

Ideally, each pig in a farrowing room should be of identical age. In practice, this is difficult to achieve. But in the original medicated early weaning (MEW) experiments, sows were farrowed in isolated farrowing rooms within a 24-hour period.

A variation in the age of piglets in the farrowing room could result in a wide variation in the age at weaning, thus increasing the possibility of lower immunity levels and increasing chance of infection in the oldest weaned pigs.

Management Practices

Administering Medications

Administering medications to both sows and piglets may aid in the prevention of colonization of piglets by most bacterial agents (not viruses).

Sow medications may be given via feed, water or by injection. Piglets must receive medications by oral dosing or injection.

Oral dosing may require 2-3 administrations every 24-hour period. Injectable antibiotics may need to be given every 12 hours or as infrequently as every 3-6 days, depending on antibiotics used and pathogens involved.

Administering medications, particularly to piglets, may increase the weaning age required to eliminate a pathogen.


Overall sanitation of the farrowing room environment will determine the level of infectious agent exposure to the young piglet.

If there is a very high level of pathogens present on floor surfaces, feeding area and in the air, then the piglet may become infected more readily and at a younger age.

All-in, all-out throughput combined with proper cleaning and disinfecting is essential for minimizing the level of infectious agents in the overall pig-rearing environment.


All farms must strive for a biosecure system to lower the introduction of new infectious agents onto the farm or production site. If a new infectious agent affects either farrowing or late-gestating sows, the odds are high that both medicated early weaning and Isowean piglets will be infected with the agent at weaning, primarily due to low or no sow immunity.