23 December 2022
What is pig herd immunity in swine production?
Herd immunity is a concept that refers to the effect of vaccination programmes applied to the whole animal population of a pig herd. This massive action serves as a tool for population control of certain diseases since its objective is not only to protect individual animals.
It is a relative term, also defined as the ability of an individual to resist or limit the severity of infection or disease. It describes the collective immunological status of a host population concerning a specific microorganism.
How does pig herd immunity work?
Herd immunity can also be described as the resistance of a group of individuals to the invasion and spread of an infectious agent and can be expressed as the proportion of resistant individuals in the population.
The proportion of resistant individuals in the population will depend on the exposure status before the entry of the specific agent (vaccine or natural infection) and movements in the population (births, deaths, discards, or introductions).
Vaccinating more individuals reduces the proportion of susceptible pigs on the farm and, consequently, the number of individuals that can be infected and become infectious.
Why does pig herd immunity matter for pig health and farm performance?
As Lager and Buckley (2019) said, pork has become the most consumed meat worldwide, implying that new approaches are needed to improve herd immunity on pig farms.
The high levels of productivity required by modern pigs, the appearance of new diseases, and the existence of increasingly large and dynamic farms have stimulated the development of control strategies based on the herd population and not on the individual.
Consequently, it has generated significant changes in pig production regarding pig flow, medication or vaccination strategies, the introduction of replacements, and biosecurity programmes.
How does pig herd immunity affect piglet health during lactation?
Colostrum provides newborn piglets with energy and passive immunity and is essential for survival. Due to the epitheliochorial placentation of the sow, piglets are born with almost no serum antibody and absorb colostrum that is three- to fourfold higher in IgG, IgG2, and IgA than serum.
For the first 24–48 hours of life, the pig intestine can absorb macromolecules, including immunoglobulins, by pinocytosis, providing the neonate with passive immunity from colostrum. Although this process commences prepartum, the primary absorptive function occurs postnatally. This specific maturational process is timed to maximise immunoglobulin uptake shortly after birth. Piglets born prematurely have a lower capacity for protein absorption than piglets born at full term; therefore, fetal maturity is an essential factor in successful immunoglobulin uptake from the colostrum.
A good level of herd immunity is also required to reduce piglet mortality since it is one more point to take into account during piglet lactation in addition to the environment, sow crushing prevention measures, colostrum management, biosecurity, batch farrowing, or iron supplementation.
What is the economic impact of pig herd immunity?
Calderón Díaz et al. (2020) conducted a study to examine the economic impact of vaccination status in porcine reproductive and respiratory syndrome virus (PRRSv), Mesomycoplasma hyopneumoniae (MHYO), and swine influenza virus (SIV) positive farms.
Their results showed that herds with positive status to respiratory disease pathogens:
- Suffered financial losses compared to negative herds
- Had an increase in feed costs in different production stages
- Had lower annual mean profit
- For a given level of profit, faced an increased risk of financial losses
Moreover, positive status for MHYO was associated with the most significant loss in annual mean profit compared to PRRSv, mainly attributed to lower sales. It resulted in a loss of profit of €5 per pig produced.
How does piglet mortality impact farm profits?
Neonatal mortality remains one of the significant problems for pig production worldwide, and the greater heterogeneity due to selection based on prolificacy results in a higher number of weak piglets during the lactation phase.
Devillers et al. (2011) published an article in which results showed that piglets that died before weaning had lower:
- Birth weight and weight gain in the first 24 hours
- Colostrum intake
- Rectal temperature
- Plasma IgG and glucose concentrations at 24 hours of age
Stygar et al. (2022) concluded that the losses due to piglets dying before or during farrowing, as well as before weaning, can result in a loss of revenues and extra production costs between €12 and €23 per litter.
Which factors influence pig herd immunity?
To positively impact herd immunity on the farm, farmers can work on biosecurity, animal management, or vaccination, for example.
Biosecurity
Modern housing and farming methods that tend to segregate age groups, allow for cleaning of the environment between production groups, and minimise the risk of disease introduction through strict biosecurity measures, are the most important methods of reducing the total use of antimicrobials and other therapeutic products as well as improving the efficacy of those treatments used.
The 20 Madec principles are the basis that all farms should take into account when working on their internal and external biosecurity:
Farrowing unit:
- Strictly enforce the all-in/all-out system. Empty, clean, and sanitise between batches.
- Shower the sows and treat them with dewormers before farrowing.
- Carry out adoptions only and exclusively when necessary and during the first 24 hours after farrowing, but not before 12 hours after birth.
Post-weaning:
- Use small pens (<13) with solid partitions.
- Strictly enforce the all-in/all-out system. Empty, clean, and sanitise between batches.
- House pigs at low densities (3 pigs/m²).
- Increase the space per feeder by 7 cm/piglet.
- Improve air quality (NH₃ 10 ppm, CO₂ 0.15%).
- Improve temperature control.
- Do not mix batches.
Growth/fattening phase:
- Use small pens with solid partitions.
- Strictly enforce the all-in/all-out system. Empty, clean, and sanitise between batches.
- Do not mix pigs from weaning pens.
- Do not mix animals in fattening.
- House at densities higher than 0.75 m²/pig.
- Improve air quality and temperature.
Others:
- Carry out an appropriate vaccination programme.
- Adequate air and animal circulation.
- Carry out strict hygiene during tail docking, injections, etc.
- Transfer to infirmaries or immediately euthanise sick animals.
Piglets in nursery pens on a pig farm, showing how farm biosecurity measures help improve herd immunity and reduce disease pressure.
Piglet management
Minimising exposure of piglets to pathogens using all-in/all-out systems or a good biosecurity programme can provide enhanced protection and give an extended window before vaccination.
However, those swine management systems where continuous flow and lower biosecurity are practised may require a more intensive or adapted vaccination programme.
Replacement sow management and its influence on pig herd immunity
A good acclimatisation of replacement sows is needed. The exposure of new females to the infectious agents present on the farm to prevent them from infection when entering gestation is one of the most relevant management actions that can be carried out on the farm.
This exposure should ideally be through vaccinations (e.g. vaccination against neonatal diarrhoea); however, the absence of adequate vaccines for some diseases makes it necessary to use alternatives such as intentional exposure to serum, tissues, farm animals, or faeces. All these procedures should be managed based on consultation and recommendation of a veterinarian and must respect all local and country regulations.
The procedure consists of an exposure phase and a recovery or "cooling" phase to guarantee that the sows entering the population are immunised but have also stopped excreting viruses or bacteria. This is a way to keep introducing replacements without increasing the number of susceptible sows in the population.
Farrowing unit management
The farrowing unit is the most crucial stage for the piglets' future and for the development of herd immunity.
Therefore, excellent farrowing unit management is needed. It includes:
- Tail docking
- Teeth clipping
- Castration
- Iron administration
- Coccidiosis treatment
These procedures should only be carried out under appropriate veterinary guidance, and in line with local regulations and best practices.
A sow with nursing piglets in a clean farrowing unit, showing how proper farrowing management and biosecurity help improve herd immunity and piglet survival.
How to build an effective pig vaccination programme
Vaccines are an essential tool for the health management of swine herds as they increase animal resistance to infections and improve pig herd immunity.
They are extensively used in swine production as primary preventive measures and as elements of more holistic programmes designed to eradicate pathogens. The usefulness of vaccination varies among diseases and even from herd to herd.
What should a pig vaccination programme address?
Disease eradication remains an important goal, although it is rarely achieved. Fortunately, through trying to eliminate disease, its transmission and frequency (and often its severity) are reduced. And, in this process of improving herd immunity on pig farms, vaccines have a lot to say.
Zimmerman et al. (2012) emphasise that a good vaccination programme must begin by defining in detail these two aspects:
- Identify the particular disease risks in a pig farm. Before recommending a vaccination programme, a careful review of endemic disease agents and the risk of external introduction of agents is essential.
- Consider the effect of maternal immunity (the younger the pig, the better protection because of high levels of maternal antibodies) and the age of the pigs at the time of vaccination (the older the pig, the better the vaccine response).
There are some important considerations that a veterinarian needs to evaluate to include a vaccine in the treatment programme for a particular herd:
- The cost–benefit, including labour to administer the vaccine.
- The improvement one would expect from the vaccination programme requires knowledge of vaccine efficacy and an understanding of the disease costs present in the herd.
Why do pig vaccination programmes sometimes fail?
Swine practitioners are sometimes faced with an unexpected vaccination failure when using a product that has worked well under similar circumstances. Possible causes of a failure include:
- Improper storage and handling of the vaccine, such as failure to refrigerate or protect from light.
- Incorrect administration, such as subcutaneous injection, when an intramuscular injection is required.
Optimising the timing of a vaccination programme is often a problem. To maximise compliance and minimise labour, the swine industry prefers to use combination vaccines that require a single injection to be given at a time when animals are ordinarily handled (such as at weaning). However, there may still be high levels of passive immunity present to interfere with the stimulation of immunity from vaccination. Assessment of such cases (vaccination of breeding animals, levels of Ig at the time of vaccination) needs to be done and proper timing of the administration should be proposed.
What role does mass vaccination play in pig herd immunity?
Mass interventions on the sow nucleus include strategies such as simultaneous vaccination or inoculation of all individuals in the population. Positive experiences have been reported with mass vaccination against PRRSv, Aujeszky's Disease, and Foot-and-Mouth Disease.
How to evaluate pig herd immunity: the importance of herd evaluation in swine medicine
The evaluation of pig herd status and immunity has to be done using laboratory techniques for analysing the antibody immune response in the pigs. The most used techniques are enzyme-linked immunosorbent assay (ELISA) and haemagglutination inhibition (HI) test. In both cases, blood samples are needed from the sows in the farrowing unit.
ELISA for pig herd immunity assessment
ELISA is an extensively used assay for quantitatively detecting antibodies against pathogens in biological fluids. It has high specificity and sensitivity.
Indirect ELISA detects total anti-viral or anti-bacterial/mycoplasma antibodies. In that case, a pathogen antigen is attached to the solid phase, and the sample is incubated until it forms an antigen-antibody complex.
HI test for the evaluation of pig herd immunity
HI is a serological test that is used for the detection of antibodies. Its mechanism is based on the principle that the haemagglutinin of viruses has a haemagglutinating effect on red blood cells.
It is used as the standard method for the detection of anti-SIV antibodies. This means that blood will clot in addition to these viruses because the red blood cells bind to the haemagglutinin on the virus's surface.
If the sample contains specific antibodies, the addition of the SIV produces an antigen/antibody reaction visible by the subsequent addition of erythrocytes.
In short, improving biosecurity, applying good vaccination programmes, and a suitable replacement acclimatisation protocol are vital aspects for maintaining a high level of herd immunity in pig farms, which helps improve production performance and, consequently, economic outcomes.
Key Takeaways
- Herd immunity protects the whole population, not just individuals: Vaccination programmes applied across an entire pig herd reduce the proportion of susceptible animals, limiting disease transmission and severity at population level.
- Respiratory disease status affects herd performance: Herds positive for PRRSv, MHYO, or SIV may face higher feed costs, lower annual mean profit, and increased risk of financial losses compared to negative herds.
- Piglet survival depends on colostrum and maternal immunity: Piglets born with almost no serum antibody rely on colostral immunoglobulin uptake in the first 24–48 hours, making sow herd immunity critical to pre-weaning survival.
- Biosecurity, vaccination, and gilt acclimatisation work together: No single intervention is sufficient — the 20 Madec principles, targeted vaccination programmes, and controlled replacement sow exposure collectively strengthen pig herd immunity.
- Laboratory monitoring confirms herd immune status: ELISA and haemagglutination inhibition (HI) tests on sow blood samples provide objective data for herd evaluation in swine medicine.
References
Anderson, R. M. (1992). The concept of herd immunity and the design of community-based immunization programmes. Vaccine, 10(13), 928-935. https://doi.org/10.1016/0264-410X(92)90327-G.
Bernaerdt, E., Maes, D., Van Limbergen, T., Postma, M. & Dewulf, J. (2022). Determining the characteristics of farms that raise pigs without antibiotics. Animals, 12(10), 1224. https://doi.org/10.3390/ani12101224.
Calderón Díaz, J. A., Fitzgerald, R. M., Shalloo, L, Rodrigues da Costa, M., Niemi, J., Leonard, F. C., Kyriazak, I. & García Manzanilla, E. (2020). Financial analysis of herd status and vaccination practices for porcine reproductive and respiratory syndrome virus, swine influenza virus, and Mycoplasma hyopneumoniae in farrow-to-finish pig farms using a bio-economic simulation model. Frontiers in Veterinary Science. https//doi.org/10.3389/fvets.2020.556674.
Calderón Díaz, J. A., Shalloo, L., Niemi, J. K., Kyriazakis, I., McKeon, M., McCutcheon, G., Bohan, A. & Manzanilla, E. (2019). Description, evaluation, and validation of the Teagasc Pig Production Model. Journal of Animal Science, 97(7): 2803–2821. https://doi.org/10.1093/jas/skz163.
Devillers, N., Le Dividich, J. & Prunier, A. (2011). Influence of colostrum intake on piglet survival and immunity. Animal, 5(10), 1605-1612. https://doi.org/10.1017/S175173111100067X.
FAO (2010). Good practices for biosecurity in the pig sector. https://www.fao.org/3/i1435e/i1435e.pdf.
Fitzgerald, R. M., O'Shea, H., García Manzanilla, E., Moriarty, J., McGlynn, H. & Calderón Díaz, A. (2020). Associations between animal and herd management factors, serological response to three respiratory pathogens and pluck lesions in finisher pigs on a farrow-to-finish farm. Porcine Health Management, 6(34). https://doi.org/10.1186/s40813-020-00173-z.
Kirkden, R. D., Broom, D. M. & Andersen, I. L. (2013). INVITED REVIEW: Piglet mortality: Management solutions. Journal of Animal Science, 91(7), 3361–3389. https://doi.org/10.2527/jas.2012-5637.
Lager, K. M. & Buckley, A. C. (2019). Porcine anti-viral immunity: How important is it? Frontiers Immunology, 10, 2258. https://doi.org/10.3389/fimmu.2019.02258.
Poonsuk, K. & Zimmerman, J. (2018). Historical and contemporary aspects of maternal immunity in swine. Animal Health Research Reviews, 19(1), 31-45. https://doi.org/10.1017/S1466252317000123.
Postma, M., Backhans, A., Collineau, L. Loesken, S., Sjölund, M., Belloc, C., Emanuelson, U., Beilage, E., Nielsen, E. O., Stärk, K. D. C. & Dewulf, J. (2016). Evaluation of the relationship between the biosecurity status, production parameters, herd characteristics and antimicrobial usage in farrow-to-finish pig production in four EU countries. Porcine Health Management, 2(9). https://doi.org/10.1186/s40813-016-0028-z.
Rose, N. & Andraud, M. (2017). The use of vaccines to control pathogen spread in pig populations. Porcine Health Management, 3(8). https://doi.org/10.1186/s40813-017-0053-6.
Stygar, A. H., Chantziaras, I., Maes, D. Moustsen, V. A., De Meyer, D., Quesnel, H., Kyriazakis, I. & Niemi, J. K. (2022). Economic feasibility of interventions targeted at decreasing piglet perinatal and pre-weaning mortality across European countries. Porcine Health Management, 8(22). https://doi.org/10.1186/s40813-022-00266-x.
The Pig Site (2002). UPDATE: Practical methods for controlling PMWS. https://www.thepigsite.com/articles/update-practical-methods-for-controlling-pmws.
The Pig Site (2020). Swine it #41 - What does herd immunity really mean? - Dr. Christopher Chase (podcast). https://www.thepigsite.com/news/2020/06/swine-it-41-what-does-herd-immunity-really-mean-dr-christopher-chase.
Zimmerman, J. J., Karriker, L. A., Ramírez, A., Schwartz, K. J. & Stevenson, G. (2012). Disease of Swine, 10th edition. ISBN 978-0-8138-2267-9.
