Disease in pork production is generally caused by multiple factors. Microbial pathogens are rarely the sole cause of a health problem on a pig farm. Clinical disease is usually the interaction of a pathogen with errors in management and a variety of contributing influences such as environment and host factors. Many pathogens are endemic in the swine population and yet some farms suffer heavy losses from disease, whereas the impact on other farms is much less because of management differences.
Economic considerations influence all decisions regarding health care on pig farms. Health management and disease prevention programs tend to be prioritized based on financial return, although other important considerations include animal welfare, food safety, and risk management. Pigs are omnivores extremely well suited to convert low-cost feedstuffs (often including waste products) into meat. Pigs efficiently convert feed to muscle while growing at a rapid rate. In addition, their fecundity is remarkable. Female pigs reach sexual maturity, breed, and produce a litter of ≥10 piglets at ~1 year of age and then continue to produce litters 2.5 times per year.
Health management plays a key role in profitable farming enterprises. Profit is based on maximizing income and minimizing costs. Income is a function of the price received per kg of pork produced times the total amount of pork produced. Therefore, aspects of health management that ensure good reproductive performance and consistent pig numbers at all stages of production and rapid growth contribute to a steady, high income. Production parameters, such as pigs 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 are well over half the cost of production, health management initiatives to reduce feed costs or improve feed efficiency tend to be areas of high priority. Some of the most economically important swine diseases may present with little in the way of clinical signs. For example, porcine proliferative enteropathy (Lawsonia intracellularis infection, see Porcine Proliferative Enteritis) might cause thickening of the bowel and reduced feed efficiency without signs of diarrhea or obvious illness, but the cost of the resulting increase in feed consumption likely makes this disease a high priority with regard to a health management program. Cost-benefit of programs should be evaluated to avoid spending so much to reduce the risk of the disease that it outweighs the benefits in improved performance. Therefore, a producer might choose to ignore a management protocol directed at disease control to save money longterm; however, this decision may change with time and circumstances, so medication and vaccine programs, for example, should be constantly assessed.
The use of production records is an essential part of a herd health program. Records are used to assess performance and identify areas of concern. When a general issue is identified, records also can be used to help pinpoint the problem as well as assess whether the intervention strategy was successful. Records can be used to set targets and motivate staff to achieve these targets as well as help the veterinarian develop partial budget scenarios to justify health care expenses.
In breeding herds, the parameter most commonly used to assess overall herd performance is average number of pigs weaned per sow per year. On many North American farms, this number is ≥25; some producers achieve a number >30. When this number is lower than the target, it is useful to examine the specific components of the parameter to determine the source of the problem. Pigs weaned per sow per year is a product of "pigs weaned per sow per litter" and "number of litters per sow per year." With a 3-wk lactation, it is possible to achieve ~2.5 litters per sow per year. If the number of litters per sow per year is low, it may be because of a poor farrowing rate (sows bred but failing to farrow; generally 80%–85% is achievable) or it may be because of prebreeding problems such as a long weaning-to-breeding interval (<7 days is typical). A low number of litters per sow per year will probably lead the herd veterinarian to investigate the breeding management; in contrast, if litters per sow per year is close to 2.5 but pigs weaned per sow per year is low, the veterinarian can begin to investigate the farrowing room to determine why litter size at weaning is low (10 pigs per litter is commonly achieved, but this number is increasing because of larger litter sizes being born). The cause of low numbers weaned may be because of small litters being born or a high preweaning mortality.
Record analysis can be used to focus attention so that resources can be concentrated on solving a specific problem. In the post-weaning until market period, the main production records include mortality, growth rate, and feed efficiency. Because feed cost contributes substantially to the cost of producing a market hog, measuring feed consumption and monitoring feed costs are extremely important. During the early growing period, pigs are extremely efficient in using feed to produce muscle. Feed:gain ratios of 1.5:1 or better are expected during the nursery phase (3–10 wk of age). As the pig approaches market age, metabolism changes and the pig begins to produce more fat, which shifts the feed:gain ratio so that >3 kg of feed is needed to produce 1 kg of gain. Overall through the grower-finisher phase, most herds achieve a feed:gain ratio of better than 3:1. Mortality records throughout the production stage are possibly the most useful parameter to identify a health problem. In general, preweaning mortality of ≥10% can be achieved on most farms, with nursery mortality of 3%–4% and grower-finisher mortality of 2%–3% achievable.
As in most agricultural businesses, pig farming has an economy of scale; hence, pig farms have grown larger and 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 (see Biosecurity). Great efforts are warranted to keep new diseases from entering an immunologically naive population, in addition to programs to restrict the spread of endemic diseases within the herd and especially between one production stage and another. The general strategy to prevent outbreaks of clinical disease is to minimize the level of pathogen challenge while maximizing herd and individual immunity. Poor management might result in a population of pigs with naive immune systems encountering novel pathogens, or in stress leading to a weakened immune system in vulnerable pigs. Alternatively, management errors might result in an overwhelming pathogen challenge in the case of an endemic disease or the entry of a new pathogen into a population of pigs without specific immunity.
One of the most effective management techniques to minimize the challenge from endemic diseases and possibly to eliminate a disease from a swine operation is the use of all-in/all-out pig flow through the various production stages. Commonly a group of sows are moved together into a clean and disinfected farrowing room, and later their piglets are weaned and the sows moved out as a group; all the weanling pigs enter a clean, empty nursery possibly on a separate site from the sow herd. Similarly, the pigs leave the nursery as a group and enter a clean, empty grower-finisher barn, possibly at a different site. This type of flow reduces the chance of endemic disease continuing to cycle in the population.
In summary, many pig diseases are controlled by this combined strategy of minimizing disease challenge and maximizing individual and herd immunity. For example, the common methods used to prevent neonatal diarrhea caused by enterotoxigenic Escherichia coli (ETEC) include all-in/all-out farrowing room management, the use of slatted flooring constructed of nonporous material that is easily cleaned, and washing and disinfection protocols to ensure minimal bacteria will be present in the environment to challenge the newborn piglets. At the same time, it is common practice to vaccinate the sows against ETEC before farrowing so they have high levels of specific immunoglobulins present in colostrum and milk to provide passive immunity to the piglets. In addition, it is necessary to ensure the piglets receive these immunoglobulins, so steps such as cross-fostering are important. Disease can occur if either the challenge becomes too great or the immune protection waivers, so both approaches are important and complementary.
Because herd size is often very large, ensuring that the population as a whole has immunity and that pockets of naive animals are not present within the herd can be a challenge. A subpopulation of susceptible animals is likely to act as a reservoir for endemic diseases. This is particularly true for viral diseases for which vaccination is not very effective. For example, it is a common practice to purposely infect all sows in a herd with porcine reproductive and respiratory syndrome (PRRS) virus (see Porcine Reproductive and Respiratory Syndrome) using a live field strain of the virus and at the same time closing the herd to new introduction if the herd is experiencing an outbreak of PRRS. The goal of such a program is to expose all the sows to the PRRS virus so that the entire herd develops immunity. When all the sows are immune, they will pass antibodies and not the virus in colostrum to the piglets. In this way, PRRS can be eliminated from the herd.
Health strategies can be divided into three categories. First, there are strategies designed to live with endemic diseases. Generally, these are caused by pathogens that survive in the environment and are too difficult to eliminate, or they are ubiquitous organisms that generally cause little problem. The former are handled by maximizing immunity and minimizing the challenge, as outlined above. In the latter, disease flare-ups are often triggered by environmental-management deficiencies, which if corrected will restore the healthy state in the herd. Second, some pathogens can be eliminated. For example, Sarcoptes scabiei var suis can be eliminated from a herd using the strategic administration of agents such as ivermectin and doramectin. Alternatively, diseases such as transmissible gastroenteritis and PRRS can be eliminated from a herd 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 longterm to eliminate the diseases, if possible and if it results in savings from reduced routine medication or vaccination. Third, there are strategies to prevent pathogens from entering the herd. As herd size has increased, the emphasis in maintaining the population of animals free of certain diseases has increased in importance. Some key components of a biosecurity program include careful management of replacement stock with quarantine and monitoring; preventing entry of rodents, birds, and other animals; precautions to prevent disease transfer from trucks and fomites; and the restricted entry of people (also see Biosecurity).
Vaccination is a key health management tool to enhance individual and herd immunity. Commercial vaccines are available for most of the important swine diseases, and when commercial vaccines are not available, autogenous vaccine may be a possibility. Generally, only a small number of vaccines are used in most herds. The decision to use a vaccine depends on a number of factors and needs to be assessed and frequently reassessed on an individual herd basis. To use all of the available vaccines would be cost prohibitive. Criteria used to decide which vaccines to incorporate into a herd vaccination program include cost (including labor) and efficacy of the vaccine, cost of the disease or possibly the risk of the disease occurring in the herd, and availability of alternative measures that might be more useful than vaccination. On many farms, gilts and sows are vaccinated before breeding to protect from reproductive failure caused by Leptospira sp, Erysipelothrix rhusiopathiae, and parvovirus infection. It is also common practice on many farms to vaccinate sows during gestation with an enterotoxigenic E coli vaccine to boost antibodies in the colostrum and milk to protect piglets from diarrhea via passive immunity. Common vaccines given to weaned pigs include porcine circovirus and Mycoplasma hyopneumoniae. Several other vaccines warrant important consideration for most farms, including L intracellularis, swine influenza virus, and PRRS virus.
The decision to vaccinate for a particular pathogen or develop specific control strategies sometimes depends on whether the disease is present on the farm. Knowing the disease status of a herd is an important consideration for a number of reasons. For example, it is critical if two sources of pigs need to be mixed or sources of replacement stock need to be chosen. To know what diseases are present in a herd, monitoring needs to be regularly performed and multiple sources of information incorporated into the assessment. On herd visits, animal inspection can identify clinical signs of disease. Pigs that are scratching may cast suspicion that mange is present in the herd. The presence of coughing and sneezing might prompt further investigation of respiratory diseases. It is common practice to euthanize and conduct postmortem examinations on unthrifty pigs to screen for the presence of diseases such as enzootic pneumonia or ileitis. Analysis of blood or oral fluid from a representative sample of animals to monitor for disease is an important part of herd health evaluations. In addition, production records and drug use records can help to assess health status. Another information source may be abattoir reports, or if possible following pigs through the slaughterhouse floor and assessing lesions such as milk spots on liver that indicate roundworm migration.
The herd visit is also important to determine possible housing-environmental-management shortcomings. Animal inspection may reveal signs of stress or mismanagement affecting animal welfare and productivity. Behavioral vices such as tail biting may indicate underlying environmental problems such as crowding, or insufficient resources such as waterers or feeding space. A facility inspection should spot damaged penning and flooring that could lead to injury. Air quality, room temperature, and presence of drafts can all be assessed. Stocking density is possibly the most obvious potential stressor that should be investigated during a herd visit. Published guidelines for space requirements are being used more and more in animal welfare audits. Space requirements (see Table: Space Recommendations for Growing Pigs) vary according to age and weight of pigs as well as for flooring type and other considerations such as season, ventilation or cooling systems, and group size. Investigation of disease outbreaks may require a team approach, with a veterinarian interacting with experts in nutrition, building design or engineering, and other fields to determine the triggering factors. The longterm solution to a disease problem often depends on a change in management.