Infectious disease in pork production is generally polymicrobial; however, microbial pathogens are rarely the sole cause of health problems on pig farms. The onset and magnitude of clinical disease is usually a function of the interaction between a pathogen and conditions created by management deficiencies and other contributing factors, such as environmental and host/population factors. Understanding the "epidemiological triad" of pathogens, environmental factors, and swine population factors, and how these factors interact, is essential for achieving the full health and productivity potential of the herd.
Many pathogens are endemic in the swine population; yet, while some farms suffer heavy losses from disease, others are much less affected. This disparity is due to differences in management practices, variation in pathogen virulence, and variation in susceptibility of swine populations to challenges (ie, lack of appropriate pathogen-specific herd immunity).
Management and Economic Considerations for Pigs
Economic considerations influence most health care decisions on pig farms. Health management and disease prevention programs tend to be prioritized on the basis of financial return; however, other important considerations include animal welfare, food safety, antimicrobial use, and risk management.
Pigs are omnivores that are extremely well suited to converting low-cost feedstuffs (often including waste products) into meat. Pigs efficiently convert feed to muscle and grow rapidly. In addition, their fecundity is remarkable when compared to other food animals. Female pigs reach sexual maturity, breed, and produce a litter of ≥ 14 piglets before they are 1 year old and continue to produce litters approximately 2.5 times per year.
Health management plays a key role in profitable farming enterprises. Profit is based on maximizing income and minimizing costs that fail to provide substantial returns. Income is a function of the price received per kilogram of pork produced multiplied by the total amount of pork produced. Therefore, aspects of health management that ensure good reproductive performance, consistent pig numbers at all stages of production, and rapid growth contribute to a steady, high income.
Production parameters, such as kilograms produced per sow per year and measures of throughput, should be carefully monitored to ensure the farm is achieving a high level of productivity. At the same time, production costs must be evaluated to assess the health or profitability of a swine operation. Because feed costs account for well over half the cost of production, health management initiatives to lower feed costs or improve feed efficiency tend to be areas of high priority. In addition, pig livability (the capacity to survive, thrive, and remain healthy throughout all production phases) is a key factor for economic efficiency, because pigs that die fail to provide any return on the feed they consumed, medications they received, space they occupied, and all other resources they used.
Some economically important swine pathogens can cause diseases that produce limited clinical signs in affected animals. For example, infection with Lawsonia intracellularis (porcine proliferative enteropathy) can lead to bowel thickening and decreased feed efficiency without any apparent clinical signs of diarrhea or obvious illness in affected pigs; however, the cost of the resulting increase in feed consumption and slower average daily weight gain likely makes L intracellularis infection a high priority in a health management program. Infection with porcine reproductive and respiratory syndrome virus, porcine circovirus 2, Salmonella, or other pathogens might result in similar outcomes, depending on the specific pathogen, environmental factors, and population factors.
Pig farmers should conduct cost-benefit analyses of health assurance programs to avoid spending so much on decreasing specific disease risk that the cost outweighs the benefits in improved performance. When costs outweigh benefits, a producer might choose not to implement selected management protocols directed at disease control. This decision, however, can change with time and circumstances, so farm-specific biosecurity, medication, and vaccine programs should be constantly assessed to take into account changes in the epidemiological triad.
Production Records for Pigs
Production records are an essential part of a pig herd health program. They capture vital information used to do the following:
assess herd performance
identify general areas of concern and pinpoint specific problems requiring intervention
assess the effectiveness of implemented intervention strategies
set production targets and motivate farm staff to achieve those targets
help the herd veterinarian develop economic analyses to justify health care expenses
In breeding herds, the most common parameter for assessing overall herd performance is average number of pigs weaned per sow per year. On many farms, this number is ≥ 30; some farms achieve > 36.
Average number of pigs weaned per sow per year is a product of "number of pigs weaned per sow per litter" and "number of litters per sow per year." When the average is lower than the target, it is useful to examine both of these components to determine the source of the problem.
Number of litters per sow per year: With a 3-to-4-week lactation period, it is possible to achieve approximately 2.5 litters per sow per year. If the number of litters per sow per year is low (< 2.5), it could be due to a poor farrowing rate (sows bred but failing to farrow; generally, a farrowing rate of > 90% is achievable). Alternatively, prebreeding problems, such as a long weaning-to-breeding interval (for approximately 94% of sows, within 7 days is typical), could be the source of the problem. A low number of litters per sow per year generally prompts the herd veterinarian to investigate breeding management.
Number of pigs weaned per sow per litter: If the number of litters per sow per year is close to 2.5 but the number of pigs weaned per sow per year is low, the herd veterinarian might investigate the farrowing room to determine why litter size at weaning is low. Many commercial lines achieve 13 pigs weaned per litter; however, this number is increasing because larger litter sizes are being born. Low numbers weaned could be due to small litters being born or to a high preweaning mortality rate.
Once the specific problem has been identified, records analysis can inform the solution. Subgroup analysis might also reveal the root cause of low breeding productivity in a herd. Two common subgroups include parity order (when parity 1 productivity is lower than that of older sows, there are opportunities to improve gilt development and acclimation practices) and female genetics (some herds are composed of different genetic lines, which might have different productivity potential and/or health status). In the period between postweaning and market, the key production records are those relating to feed efficiency, growth rate, and mortality rate.
The cost of feed contributes substantially to the overall cost of producing a market hog; therefore, measuring feed consumption and monitoring feed costs are extremely important recordkeeping tasks:
During their early growing period, pigs are highly efficient at turning feed into muscle.
During the nursery phase (age 3–10 weeks), approximately 1.5 kg of feed produces ≥ 1 kg of weight gain (a feed:gain ratio of ≥ 1.5:1).
As pigs get to reach the finishing stage, their metabolism changes, and they begin to produce more fat, which shifts the feed:gain ratio to approximately 2.8:1.
Overall, throughout the whole growing phase (from nursery age until the end of finishing), most herds achieve a ratio of ≥ 2.6:1. The top 10% of US producers achieved 1.3, 2.6, and 2.4 for nursery, finishing, and weaning-to-finishing feed conversion rates, respectively (1).
For growth, pigs should gain, on average, at least 473 g, 927 g, and 782 g of weight per day in the nursery, growing-finishing, and whole weaning-to-finishing stages of production, respectively (2).
Pearls & Pitfalls
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Mortality records throughout production are possibly the most useful for identifying health problems. In general, a preweaning mortality rate of ≤ 10% can be achieved on most farms; a nursery mortality rate of 1.5–3% and a growing-finishing mortality rate of 2–3% are achievable. Thus, producers should aim for a pig livability from weaning to harvest of > 95% (2).
Pearls & Pitfalls
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An important development has been the adoption of automated, ongoing monitoring of key productivity indicators. Pig producers can store their production data remotely on servers accessed via the internet (ie, "in the cloud") and use readily available algorithms, such as statistical process control (using charts and analytical tools to analyze production data over time to monitor, control, and improve herd performance). With such ongoing monitoring, pig producers and their veterinary professionals can detect deviations in productivity early and act swiftly to determine their cause. If deviations are demonstrated to be caused by infectious agents (per diagnostic investigation), producers and veterinarians can rapidly implement appropriate health interventions to mitigate losses and prevent further transmission of pathogens to epidemiologically connected herds.
Herd Size and Disease Challenge for Pigs
As with most agricultural businesses, pig farming has an economy of scale; hence, pig farms have grown increasingly larger over the past few decades, with a trend toward vertical integration in the industry. The management of health issues in large populations requires a major focus on biosecurity, which has three main components:
bioexclusion
biomanagement
biocontainment
Bioexclusion focuses on preventing new pathogens from being introduced into the herd. The goal of biomanagement is to prevent within-herd pathogen transmission and thereby decrease the infection risk. Finally, biocontainment aims to prevent pathogen transfer from infected herds to nearby or epidemiologically connected herds. Great efforts are warranted to keep new diseases from entering an immunologically naive population and to restrict the transmission of endemic diseases within a herd, especially between production stages.
The general strategy for preventing disease outbreaks in pig herds is to minimize the level of pathogen challenge and maximize herd and individual immunity. Poor herd management can result in a population of pigs with naive immune systems encountering novel pathogens, or it can create stress that leads to weakened immune systems in vulnerable pigs. Alternatively, management errors can lead to an overwhelming pathogen challenge, in the case of endemic disease, or to the entry of a new pathogen into a population of pigs without specific immunity.
One of the most effective management techniques for minimizing the challenge from endemic diseases and possibly eliminating a disease from a swine operation is the use of single-source, unidirectional, all-in all-out pig flow through the various production stages. Commonly, the flow proceeds as follows:
A group of sows are moved together into a clean, disinfected farrowing room.
Later, their piglets are weaned, and the sows are moved out as a group.
The weanling pigs enter a clean, empty nursery, possibly on a separate site from the sow herd.
The pigs leave the nursery as a group and enter a clean, empty growing-finishing barn, possibly at a different site.
This flow decreases the likelihood that endemic disease will continue to cycle in the population.
Many pig diseases are controlled by this combined strategy of minimizing disease challenge and maximizing individual and herd immunity. For example, common practices for preventing neonatal diarrhea caused by enterotoxigenic Escherichia coli (ETEC) include all-in all-out farrowing room management, the use of slatted flooring made of easy-to-clean nonporous material, and washing and disinfecting to minimize environmental bacteria that could challenge newborn piglets. In conjunction, sows are typically vaccinated against ETEC before farrowing to ensure that they have sufficient levels of specific immunoglobulins in colostrum and milk to provide passive immunity to their piglets. In addition, steps such as cross-fostering help to ensure that piglets receive these immunoglobulins. Disease can occur if either the disease challenge becomes too great or the immune protection wavers; therefore, both approaches are important and complementary.
Because pig herds are often very large, ensuring that the entire population has immunity can be a challenge. A subpopulation of susceptible animals in a herd might act as a reservoir for endemic pathogens, amplifying or perpetuating infection dynamics over time. One common strategy to minimize the risk of subpopulations of mixed immune status is to purposely infect all sows in a herd with a live strain of porcine reproductive and respiratory syndrome virus (PRRSV) and simultaneously close the herd to new introductions if the herd is experiencing an outbreak of PRRS. The goal is to expose all the sows to PRRSV, ideally with a commercially available modified live virus (MLV) vaccine, so that the entire herd develops immunity. Once all sows are immune, they will pass antibodies (and not the virus) in colostrum to piglets. In this way, PRRSV can be eliminated from the herd.
When a breeding herd has continuous introduction of susceptible gilts, or when a herd is continuously exposed to wild-type virus (as in highly pig-dense areas), some veterinarians choose to immunize the herd 2–4 times a year and make sure incoming gilts are also immunized and nonshedding.
Health Strategies for Pigs
Health strategies for managing disease on pig farms fall into three general categories:
strategies for living with endemic pathogens
strategies for eliminating pathogens from the herd
strategies for preventing pathogens from entering the herd
Generally, endemic diseases are caused by one of two types of pathogens: 1) those that survive in the environment and are too difficult to eliminate, and 2) those that are ubiquitous even in high-health genetic multiplier herds (a swine production unit whose purpose is to multiply superior, high-health breeding stock under strict biosecurity to prevent disease).Examples of endemic pathogens include Lawsonia intracellularis, Glaesserella parasuis, porcine circovirus 2, and Escherichia coli. Endemic pathogens are handled by maximizing immunity and minimizing the challenge (see Herd Size and Disease Challenge). Disease flare-ups are often triggered by disruptions in the epidemiological triad that result in, for example, deficiencies in environmental management; in such cases, strategies are directed at correcting those deficiencies to restore herd health.
Some pathogens can be eliminated. For example, Sarcoptes scabiei var suis can be eliminated from a herd by strategic administration of antiparasitics such as ivermectin and doramectin. Other disease pathogens, including those that cause transmissible gastroenteritis and porcine reproductive and respiratory syndrome (PRRS), can be eliminated by closing the herd to new introductions and purposely exposing all animals to the disease to create herd immunity. It is usually desirable in the long term to eliminate diseases—that is, if elimination is possible and if it results in cost savings from decreased routine medication or vaccination.
Finally, preventing pathogens from entering the herd by implementing and maintaining bioexclusion practices is another approach to herd health. As herd sizes have increased, the emphasis on maintaining a herd free of certain diseases has increased in importance. Key components of a biosecurity program include careful management of replacement stock through quarantine and monitoring; preventing entry of rodents, birds, and other animals; taking precautions to prevent pathogen transfer from trucks and fomites; and restricting the entry of humans without proper biosecurity protocols.
For some pathogens, such as the PRRS virus or Mycoplasma hyopneumoniae, the chosen health strategy depends on the likelihood of 1) successfully eliminating the pathogen and 2) keeping the herd free of the pathogen for a reasonable period of time. For example, producers managing breeding herds with strong biosecurity measures might choose an elimination strategy, whereas producers in highly pig-dense areas with nurseries or finishers attached to their breeding stock might choose to keep herd immunity high through ongoing vaccination programs and maintaining unidirectional flows.
Vaccination and Disease Status for Pigs
Vaccination is a key health management tool for enhancing individual and herd immunity in pigs. Commercial vaccines are available for most of the important swine diseases, and when commercial vaccines are not available, autogenous vaccine might be a possibility. Generally, only a few vaccines are used in most herds. The decision to use a vaccine depends on several factors and should be assessed and frequently reassessed on an individual herd basis. To use all available vaccines would be cost prohibitive.
On many farms, gilts and sows are vaccinated before breeding to protect against reproductive failure caused by Leptospira sp, Erysipelothrix rhusiopathiae, and parvovirus infections. It is also common to vaccinate sows during gestation with an enterotoxigenic Escherichia coli vaccine to boost antibodies in colostrum and milk to protect piglets from diarrhea via passive immunity. Weaned pigs are commonly vaccinated against porcine circovirus 2 and Mycoplasma hyopneumoniae. Several other vaccines warrant consideration for most farms, including those for Lawsonia intracellularis, influenza A virus, rotaviruses, and porcine reproductive and respiratory syndrome virus.
The decision to vaccinate for a particular pathogen or develop specific control strategies might depend on whether the disease is present on the farm. Knowing the disease status of a herd can be critical in certain circumstances, such as when two sources of pigs need to be mixed or when sources of replacement stock need to be chosen.
Determining what diseases are present in a herd requires regular monitoring and multiple sources of information, including veterinary visits, postmortem evaluations, fluid analyses, and various records. On herd visits, veterinarians can look for clinical signs of disease, such as scratching (which could indicate mange in the herd) or coughing and sneezing (which might prompt further investigation for respiratory diseases). Euthanasia and postmortem examination of unthrifty pigs to screen for diseases such as enzootic pneumonia or ileitis is also common practice. Analysis of blood, processing fluids, postmortem tongue fluids, or oral fluid from a representative sample of pigs helps monitor for pathogen activity. In addition, production records and drug use records can be analyzed to assess herd health status. Abattoir reports are also useful, as is following pigs through the slaughterhouse floor to assess for lesions, such as milk spots on the liver that indicate roundworm migration.
The overarching goal of a vaccination program is to build immunity against targeted pathogens. Thus, it is important to ensure that animals being vaccinated are healthy enough to mount an immune response. In the acute phase of outbreaks, vaccine efficacy might not be as good as when vaccinating healthy herds. In the initial phase of outbreaks, pigs should be treated and good husbandry practices followed until pigs recover clinically. After that, vaccination may be used therapeutically or preventively.
Herd Visit and Facility Inspection for Pigs
Herd visits by a veterinarian are important for determining potential housing, environmental, or management shortcomings on pig farms. During a herd visit, the veterinarian inspects the pigs for signs of stress or mismanagement that could be affecting animal welfare and productivity. Behavioral vices, such as tail biting, might indicate underlying environmental problems (eg, crowding) or insufficient resources (eg, inadequate waterers or feeding space).
The veterinarian also inspects the facility, looking for damaged penning and flooring that could lead to injury. Air quality, room temperature, and presence of drafts might also be assessed.
Stocking density is possibly the most obvious potential stressor warranting investigation during a herd visit. Published guidelines for space requirements are consulted more and more in animal welfare audits. Space requirements vary according to age and weight of pigs, as well as flooring type (see the table ). Additional considerations include season, ventilation or cooling systems, and group size.
Space Recommendations for Growing Pigs
Production Stage | Body Weight | Space Allowance |
|---|---|---|
Nursery | Weaning to 27 kg (30 lb) | > 0.34 m2 |
Growing | 27–68 kg (30–100 lb) | > 0.68 m2 for slatted floor, > 0.90 m2 for solid floor |
Finishing | 68 kg (150 lb) to market weight | > 0.75 m2 for slatted floor, > 1 m2 for solid floor |
Data from Pig Improvement Company. Wean to Finish Guidelines. PIC; 2019. https://www.pic.com/run-a-successful-wean-to-finish-operation | ||
Investigating disease outbreaks might require a team approach, in which the visiting veterinarian interacts with experts in nutrition, building design or engineering, and other fields to determine the triggering factors. The long-term solution to a disease problem often depends on a change in management.
During herd visits and facility inspections, veterinarians should watch for the following common risk factors for endemic disease onset:
poor ventilation and stagnant airflow
poor overall hygiene
improper flow of humans between different age groups in the herd (ie, from older pigs to younger)
holding back weaning piglets for quality (ie, weaning older-than-average pigs)
having too many nurse sows in newborn piglet rooms
poor colostrum management, especially in hyperprolific sows that produce more than two piglets per functional teat
poor vaccine management (eg, breaking the vaccine cold chain somewhere between storage warehouse, transport to the farm, farm storage, and the barn before administration)
poor vaccination delivery practices, including poor hygiene, improper vaccine dilution, and incorrect dosage and administration procedures (ie, not following the product label)
mixing too many sources of pigs with varying health statuses for key diseases
poor access to fresh feed
poor access to fresh, clean water (in terms of water pressure and flow per minute adequate for the production stage)
Key Points
Veterinarians are part of the production team in swine management operations, and they must consider cost-benefit factors in determining disease prevention and treatment strategies.
Swine disease is driven by the epidemiological triad: disease occurs when pathogen challenge, environment, and host/population factors align unfavorably—not from pathogens alone.
Most economically important pathogens are endemic, and their impact depends primarily on management quality and herd immunity, not mere presence or absence.
Effective disease control relies on minimizing pathogen challenge and maximizing immunity, with all-in all-out pig flow as one of the most powerful tools.
Biosecurity has three distinct goals: prevent pathogen entry (bioexclusion), limit within-herd spread (biomanagement), and prevent spread to other herds (biocontainment).
Health management is an economic decision: improving feed efficiency, growth rate, reproduction, and livability delivers the greatest return, supported by production records and continuous monitoring.
References
MetaFarms. Production Analysis Summary for U.S. Pork Industry: 2019–2023. MetaFarms, Inc.; 2024. https://www.porkcheckoff.org/research/production-analysis-summary-for-u-s-pork-industry-2019-2023
Pig Improvement Company. Wean to Finish Guidelines. PIC; 2019. https://www.pic.com/run-a-successful-wean-to-finish-operation
