How herd immunity improves pig farm productivity

23 December 2022

The use of antibiotics and other products, such as zinc oxide, is being affected by new legislation, especially in the European Union. In this context, strengthening the immune system of pigs (among others with the use of biological products such as vaccines) and, above all, considering the importance of herd immunity has become essential for the entire swine sector worldwide.

But why is herd immunity important? What relationship does it have with productivity? And at an economic level, how does it affect the pig sector? In this article, we are going to analyze the importance of herd immunity at production level and its impact on economic aspects.

What is herd immunity?

Herd immunity is a concept that refers to the effect of vaccination programs 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 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.

The importance of herd immunity on pig farms

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 programs.

How does herd immunity work?

Herd immunity definition could also be 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.

Disease eradication or control?

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) emphasize that a good vaccination program must begin by defining in detail these two aspects:

Identify the particular disease risks in a pig farm. Before recommending a vaccination program, 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).

Herd immunity and its impact on the economic aspects of pork production

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, the risk of financial losses increases.

Moreover, if herd immunity is affected by the 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.

Herd immunity and 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 maximize 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 piglets lactation in addition to the environment, sow crushing prevention measures, manage colostrum, biosecurity, batch farrowing or iron supplementation.

Picture 1-1

With a good level of herd immunity, we will also be capable of reducing piglet mortality during lactation

How does the piglet's health impact 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 the first 24h.
Colostrum intake.
Rectal temperature
Plasma IgG and glucose concentrations at 24h 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.

Factors that 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 minimize 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:

1. Strictly enforce the all-in/all-out system. Empty, clean, and sanitize between batches.
2. Shower the sows and treat them with dewormers before farrowing.
3. 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:

4. Use small pens (<13) with solid partitions.
5. Strictly enforce the all-in/all-out system. Empty, clean, and sanitize between batches.
6. House pigs at low densities (3 pigs/m²).
7. Increase the space per feeder by 7 cm/piglet.
8. Improve air quality (NH₃10 ppm, CO₂ 0,15%).
9. Improve temperature control.
10. Do not mix batches.

Growth/fattening phase:

11. Use small pens with solid partitions.
12. Strictly enforce the all-in/all-out system. Empty, clean, and sanitize between batches.
12. Do not mix pigs from weaning pens.
14. Do not mix animals in fattening.
15. House at densities higher than 0,75 m₂/pig.
16. Improve air quality and temperature.

Others:

17. Carry out an appropriate vaccination program.
18. Adequate air and animal circulation.
19. Carry out strict hygiene during tail docking, injections, etc.
20. Transfer to infirmaries or immediately euthanize sick animals.

Maximizing the level of biosecurity on our farms positively influences herd immunity.

Piglets management

Minimizing exposure of piglets to pathogens using all-in/all-out systems or a good biosecurity program can provide enhanced protection and give an extended window before vaccination.

However, those swine management systems where continuous flow and lower biosecurity are practiced may warrant a more aggressive vaccination program.

Replacement sows management and its influence on herd immunity

A good acclimatization 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 that can do 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 feces. All these procedures should be managed based on consultation and recommendation of veterinarian and 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 immunized 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 becoming of herd immunity.

Therefore, excellent farrowing unit management is needed. It includes:

Tail docking.
Teeth clipping.
Castration
Iron administration.
Coccidiosis treatment.

A well-managed farrowing unit helps to improve herd immunity.

Vaccination improves pig herd immunity

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 programs designed to eradicate pathogens. The usefulness of vaccination varies among diseases and even from herd to herd.

There are some important considerations that a veterinarian needs to evaluate to include a vaccine in the treatment program for a particular herd:

The cost–benefit, including labor to administer the vaccine.
The improvement one would expect from the vaccination program requires knowledge of vaccine efficacy and an understanding the disease costs present in the herd.

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.

Optimizing the timing of a vaccination program is often a problem. To maximize compliance and minimize labor, 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.

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 herd immunity in the farrowing unit?

The evaluation of pig herd status and immunity has to be done using laboratory techniques for analyzing the antibody immune response in the pigs. The most used techniques are enzyme-linked immunosorbent assay (ELISA) and haemoagglutination (HI) test. In both cases, blood samples are needed from the sows in the farrowing unit.

ELISA

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.

Conclusion

In short, improving biosecurity, applying for good vaccination programs, and a suitable replacement acclimatization protocol are vital aspects if we want to maintain a high level of herd immunity in our pig farms, which helps us improve at a productive level and, consequently, economically.

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.

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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.

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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.

How herd immunity improves pig farm productivity

What is herd immunity?

The importance of herd immunity on pig farms

Herd immunity and piglet health during lactation

Factors that influence pig herd immunity

How to evaluate herd immunity in the farrowing unit?

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