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Managing Immunity in Replacement Gilts

Article-Managing Immunity in Replacement Gilts

, the goals of the gilt development unit are not only to ensure that gilts are the proper breeding age and weight and have shown cycling activity, but it also provides a place for gilts to be acclimated (exposed and recovered to herd pathogens) without disrupting herd health stability

A sow herd with stable health performs the best. Health stability is defined as animals having active immunity from prior vaccination or infection with a pathogen without the presence of overt clinical signs or pathogen shedding.

Gilts entering the sow herd are a significant challenge to maintaining this stability. Many herds do not produce their own gilts. Most gilts are sourced from a genetic multiplication unit — either produced internally or purchased from a genetic supplier.

Consequently, the goals of the gilt development unit are not only to ensure that gilts are the proper breeding age and weight and have shown cycling activity, but it also provides a place for gilts to be acclimated (exposed and recovered to herd pathogens) without disrupting herd health stability.

Source-Herd Health Status

Identifying a source of gilts and its respective health status is the first step in getting gilts into a sow herd.

It’s important to start with gilts that are as high health as possible. This eliminates the risks of adding new pathogens (porcine reproductive and respiratory syndrome [PRRS], Actinobacillus pleuropneumonia [APP], swine influenza virus [SIV], E. coli, etc.) to your herd. In some cases, one may choose to match the health of your herd with the health of the gilt source herd. For example, a mycoplasma-positive farm may choose to purchase gilts from a mycoplasma-positive source.

In order to understand the health status of the source herd and ensure that it’s compatible with your herd, a veterinarian-to-veterinarian communication should be completed between your herd veterinarian and the source herd’s veterinarian. This discussion allows the source herd veterinarian to list the pathogens known to be present in the herd and explain the disease monitoring programs that are in place (blood testing, necropsy, slaughter check, etc.), production flow, biosecurity risks and routine vaccination and medication programs being used. This information allows your herd veterinarian to decide if the source herd is compatible with the destination herd. Knowing this information will allow for a customized vaccination program for the gilts that will be coming into the herd without overlaps or gaps in the program. Once this communication has been completed and the health status accepted by the herd veterinarian and owner, sourcing can proceed.

The vet-to-vet communication is not a one-time event. For the best success in maintaining health stability, seek ongoing communication about health changes in the multiplication herd, including its sources, health
status or flow.

Gilt Development Unit (GDU)

The age of gilts that enter the herd is dependent upon the pathogens that are present in the herd and the length of time needed for exposure of the animals to gain stability and be ready to enter the sow herd. Generally, the longer the exposure time, the better. The idea is that these animals will be immune and no longer shedding disease at a rate high enough to impact the sow herd for the downstream flow of the pigs.

Using PRRS as an example for a herd that chooses to remain positive, the longer the length of time between virus exposure and entry into the sow herd, the better to ensure that gilts are not entering the population shedding virus. In these types of herds, many operations have decided to receive gilts at weaning age or shortly after so they can be exposed to the herd’s viral strain of PRRS via live-virus introduction, with or without modified-live vaccination, and have as much time as possible to generate immunity prior to entering the herd (in this example, 5-6 months). The goal is to not be viremic (PRRS polymerase chain reaction or PCR negative) at the time of entry into the sow herd.

Mycoplasma is another agent with a long shedding period. The sooner gilts can be exposed to mycoplasma in the developer unit, the more likely sow herd stability can be maintained.

In some instances, producers have decided to “health match” the herds, bringing in replacements from mycoplasma-positive herds so the gilts are already positive coming from the source herd, and have started to develop their immunity prior to arrival. If gilts are sourced from a mycoplasma-negative herd, then gilts need
to be exposed to “seeder” animals
(recently infected animals that are still shedding the organism) so they can develop natural immunity prior to entering the sow herd.

There are other herd-specific diseases, primarily bacterial, such as Streptococcus suis, Haemophilus parasuis, Actinobacillus suis and Actinobacillus pleuropneumonia, that will need to be addressed in the gilt developer. A specific plan is needed to ensure the gilts have ample time to develop immunity before entering the sow herd.

In parity-segregated systems, this will go all the way up to the time of weaning so there is maximal immunity in the sow farms and pigs that flow downstream. These flows are very stable and the performance is very predictable until a new pathogen enters the system.

Isolation Timeframe

Isolation is the timeframe where animals are held as a separate population from the GDU or sow unit. Biosecurity measures are put in place to make sure there is no direct contact with the main herd population.

Chores in the isolation unit should be done at the end of the day. Most are shower-in/shower-out, with all equipment, medications and vaccinations stored in this area until isolation is cleared. The isolation period serves two purposes:

  • Allows for time to demonstrate any changes in health status that may have occurred at the source herd and are not yet detectable.
  • Allows time following transport for any possible infection during the transport process, via contaminated truck or aerosol, to be detected.

The isolation timeframe will vary with the pathogens involved, but generally will be 3-4 weeks.

Animals in isolation must be monitored for clinical signs of disease. If there are mortalities or severe disease conditions, necropsies can be performed to aide in the diagnosis. At the end of the isolation period, replacement gilts should be monitored serologically for any exposure to pathogens.


This is the process of exposing the gilts to specific pathogens that are present in the sow herd. The goal is for gilts to get exposed to the pathogen(s) and develop their own immunity during the acclimation period.

The means for introducing these pathogens during this period will vary depending on the desired exposure pathogens. One example is exposure to the live PRRS virus from the sow herd. Other examples are mycoplasma, APP or Staphylococcus hyicus, where seeder pigs (recently infected animals that are still shedding the pathogen) are used to expose gilts to herd pathogens. Generally, the best animals for this application are gilts from a previous group that are still recovering.

Feedback material is another means of introducing pathogens during the acclimation process. Using placentas, mummies and material from hot boxes are good for reproductive immunity. It is also a good time to get exposure to enteric pathogens, such as the farm-specific E. coli, rotavirus, Clostridium perfringens Type A, etc. This can be done using feedback that is used for prefarrowing for exposure.

Although isolation and acclimatization may occur in the same facility in some herds, they are separate processes, and the goals for each need to be distinguished from each other.

Customized Vaccination

Vaccination schedules need to be customized for each herd based on the pathogens present in that herd. The core prebreeding vaccinations for most herds include leptospira, parvovirus and erysipelas. The first injections should be given at 3-5 weeks prebreeding, and the second (booster)
injection given 1-2 weeks before breeding.

Circovirus vaccination is recommended for gilts in the late-development stage, even though they may have been vaccinated earlier in life to reduce porcine circovirus-associated disease (PCVAD) circulation.

Additional vaccinations that are standard in many herds include ileitis (Lawsonia intracellularis) and Mycoplasmal pneumonia. Some farms also vaccinate with an autogenous Haemophilus parasuis, Steptococcus suis and Actinobacillus suis.

It’s important to have a detailed vaccination schedule so the timing and products used are known by everyone, and to ensure there is no confusion about the proper dose and timing prior to gilts entering the herd. Table 1 lists the most common vaccination products, timing, product, dosage, injection site and withdrawal period.

Many herds use a standard form to clear a group of gilts to enter the herd. This form details any specific exposures of farm pathogens and diagnostic results, as well as vaccinations, so each can be checked off and initialed prior to the group entering the herd. (See Swine Vet Center form on page 22). The form helps ensure that all procedures for that group of gilts have been completed and nothing slips through the gaps prior to entry.


Monitoring the health status (cough, diarrhea, lameness, etc.) of the group in the isolation phase is important to ensure there are no new diseases being brought in. If there are unexplained mortalities or severe health problems, some of these animals should be submitted for complete necropsy and diagnosis.

If the isolation facility is in close proximity to the main sow farm, a screening sample within 2-3 days of arrival is important to make sure there are no infected pigs in these facilities for an extended period of time that could put the sow farm at risk. An example would be to test incoming pigs for PRRS and mycoplasma.

The decision to test at the end of isolation will depend upon the pathogens you want to monitor. For example, you might want to test for PRRS (both PCR and enzyme-linked immunosorbent assay [ELISA]) and mycoplasma (ELISA).

The number of animals to test will depend on the level of risk and the type of production. Most commercial farms take a 95/10 sample (95% confidence level that you will detect at 10% or higher infection rate). For most herds, this will be 30 animals. If the unit the pigs are entering is filtered and the GDU is not, then you may want to test all animals to be sure you don’t miss the “less than 10% infected” animals.

Customize Your Plan

The isolation and acclimatization of replacement gilts into the herd is a very important step to maintaining stable health within a sow herd. The development plan should be written and documented so that everyone on the farm is clear about what procedures must be completed prior to introduction into the herd.

Unfortunately, there is no one-size-fits-all approach. These plans need to be customized to the herd because there will be different health concerns and risks in each herd. We have outlined basic principles to consider as you put the plan together. It’s important to remember that the gilt developer unit and sow herd are responsible for the health of the pigs downstream, especially in a straight-line flow system.

TAGS: Reproduction