Pigs

References and resources for this tutorial

Bollen, Peter J.A., Hansen, Axel Kornerup, Rasmussen, Helle J., Bollen P.J.A., The Laboratory Swine, the Laboratory Animal Pocket Reference Series (Boca Raton, CRC Press, 1999)

Laber, Kathy, et. al. "Biology and Diseases of Swine," Chapter 15 in Laboratory Animal Medicine, 2nd Edition, James G. Fox, et. al., editors (New York: Academic Press, 2002)

"Ungulates" in The UFAW Handbook on the Care and Management of Laboratory Animals, 7th Edition (Malden: Blackwell Science, 1999)

Overview

The current state of knowledge and interest in swine spans a wider range than perhaps any other domestic animal since they are used for food, biomedical research and are also, increasingly, a pet as well. They have been domesticated for several thousand years and differ somewhat from other laboratory animals in that they have been primarily used as a food source. There is voluminous research about swine production, beginning as efforts to increase their viability as a food source and in recent years, to improve their welfare conditions.

One can look at the history of pigs in research as a barometer of attitudinal changes toward animals, both by the public and the professions that make use of them. Once considered the least respected of the domestic animals, they are now valued. A recent article in the New York Times, Animal Welfare’s Unexpected Allies, describes a Purdue University project that looks at animal preferences. "We want to get the animal's perspective, to see what they prefer," explains Edmond A. Pajor, assistant professor of animal behavior and welfare at Purdue. "We want to know: How important is social contact and space? What do they like and need." Over the last fifteen years, there has been an increase in the amount of research done on pigs for welfare determinations for pigs in agriculture production. How they are handled, since they are so easily stressed, has an impact on the quality of the meat; careful handling at slaughter is critical. Temple Grandin has posted a series of her research articles on pigs relating to handling and environment when they are being raised as a food resource. She notes:.

Australian scientist Paul Hemsworth found that inconsistent handling stressed pigs and reduced weight gains. If the handler occasionally slapped or shocked the pigs when he entered the pens, the animals became stressed each time he approached. Even if the handler is gentle and nice most of the time, the pigs learned that he is not to be trusted. How an animal perceives a situation during handling can have a big effect on stress levels. Procedures such as weekly weighing will usually have little effect on weight gain if the animals have learned that nothing bad is going to happen. Weekly weighing may have a detrimental effect on weight if the pigs see the "mean" man who shocked them with a prod. Most animals are smart enough to associate certain people with painful experiences. (Temple Grandin, Livestock Conservation Institute, “Environmental Enrichment for Confinement Pigs,” Proceedings of the 1988 Annual Meeting, Kansas City, Missouri)

They are sociable animals; in the wild they live in small matriarchal groups composed of a sow and offspring. Males, when young, live in bachelor groupings; as they age they become more solitary except for mating times. There are strict social hierarchies; newborn piglets establish a teat order within the first few days of birth and within a group of females a dominance ranking is established quickly. Thus, when kept in the laboratory, they do better with group housing; if they must be singly housed then the caging should accommodate some sort of direct contact between animals. For general information on husbandry see Comfortable Quarters for Pigs in Research Institutions by Temple Grandin.

They are omnivorous and most behavior is directed towards eating and rooting (the vigorous shoving about with the snout, manipulating soil and brush--to find food.) Their eyesight is good but it is their sense of smell that is the most highly developed and rooting behavior is the primary means of food searching.

Although most of the time pigs are maintained on concrete or grid type flooring, if given additional space and supplied with straw for bedding and foraging behavior they will organize their area into separate regions for sleeping and for elimination and will manipulate the straw into nesting and bedding configurations.

They are sensitive to restraint and procedures, environmental changes and social settings: “Recommendations for Investigators Using Pigs in Research”, by Temple Grandin, describes some of their basic needs. As with all animals, sensitivity to procedures and environment can affect physiological values; although there is increasing concern about quality of life for animals raised for food, fluctuations in physiological values generally do not affect their value as a food resource, but can affect research results.

Pigs and science

Pigs were one of the earliest research animals: in ancient Greece Erasistratus (304-250 B.C.) used them to investigate the mechanics of breathing. In Rome, Galen (130-200 A.D.) used them to demonstrate blood circulation. As scientific research became a large and successful endeavor, pigs, like rats, became increasingly popular since most people lack the emotional attachment to swine—they became in many protocols the replacement animal for dogs and cats. And since historically, both rats and pigs are animals devalued by society, their use as tools of science is generally more acceptable by the public. Just as the white rat was genetically modified to suit research needs, the miniature pig has become, like the rat, a research tool par excellence with breeding programs selecting for specific traits that can be utilized in research protocols. And given the state of current pig genome research there are increased studies using mice and swine in tandem for biomedical information that has application to humans.

Miniature pigs are purpose bred for research (and have become popular pets as well--see the British Micropig Society); they are smaller than those bred for production, with a much slower growth curve, but physiologically in all other ways they are similar to agricultural pigs. The exponential growth curve begins at 3 months; for research lasting longer than 3 weeks, miniature swine are preferable both for handling ease and welfare considerations. Because pigs have been bred for weight gain, restricting their feed causes them emotional distress. But, since they tend toward obesity, for long term projects a diet low in calories and high in fiber, with plenty of straw and other items for rooting is necessary for their welfare. 

Miniature pigs have a convenient body size for surgical procedures and given their anatomical similarities to humans particularly in terms of skin, skeleton, teeth, gastrointestinal tract, pancreas, liver, kidney, lung, and immune status they are often used as models for humans. In cardiac research, they are often the model of choice, particularly for studies on cholesterol. They are widely used in pharmacology research and diabetes research. During recent years, pigs have become utilized for their biological parts, e.g. cardiac valve replacement; the area of xenotransplantation, a recent development, has resulted not only in increased scientific research but also new and difficult ethical questions.

A recent book on intellectual property issues included the ethical questions raised in a patent application for development of a genetic marker for pig litter size. Here the ethical questions were not about animals in research, but rather about two other complicated ethical dilemmas. First there is the moral question about patenting life forms and second, the conflict of interest problem of researchers in academia who joint with business. The authors note that “Farm animal research was catching up with the rest of biology….” (M.F. Rothschild and G.S. Plastow, “Development of a Marker for Litter Size,” in Intellectual Property Rights in Animal Breeding and Genetics, CABI Publishing, 2002, p. 189.)  Given the exponential increase in biotechnological research using pigs, along with genomic knowledge in general, there are ethical concerns now that could never have been foreseen.

Since genetic engineering has the potential to alter the uses to which domestic animals are put, it also can lead to fundamental changes in the relationship between 1) individuals of the same species or population, 2) different species, 3) engineered animals and their products, and 4) products and humans. There currently are major research efforts underway to develop the use of genetically engineered animals as sources for production of nontraditional materials for human use. Such uses can be divided into three major categories: biopharmaceutical for animal or human use; live cells, tissues or organs for xenotransplantation use; and raw materials for processing into other useful end products.” National Academies Press, Animal Biotechnology: Science Based Concerns (2002) p. 51.

There is a wealth of information about pigs since they have become a critically important animal in biomedical research: Information Resources on Swine in Biomedical Research 1990-2000 contains resources as well as informative articles as to their biology.

Biology and organ systems

The Swine Study Group Notes (from the NetVet website) gives a summary of basic information, ranging from glossary, breed taxonomy, body systems and illnesses. Veterinarians Michael Swindle and Allison Smith’s online article, “Comparative Anatomy and Physiology of the Pig” contains a detailed review of all the systems, including histological photographs of the major organs.

The skin is similar to human skin. The bones are short and strong; the thorax is small and the skull is very strong with a large frontal sinus.

Gastrointestinal:They have 44 teeth; newborns have 8 needle teeth, 5 more deciduous teeth erupt during the first 2 months and the first permanent teeth erupt by 6 months. The pig is 1 ½ years old before the permanent teeth are all in. Swine are monogastric and they are prone to peptic ulcers. The bile duct and pancreatic duct enter the duodenum–-they are used as models for pancreatic secretion studies since the anatomy is similar to humans. The small intestines contain numerous lymph node areas and the large intestines are extensive and very coiled. Lymph nodes can be found along all the major arteries in the abdomen. The liver is very fibrous and the kidneys are similar to humans in structure and size.

Cardiac:The heart is small compared with the overall body size and the coronary arteries and veins are similar to those in humans. They also develop atherosclerosis of the major arteries in a relatively short time if maintained on a high fat diet, so are often used in studies on cholesterol related issues. The trachea has a separate branching of the bronchus for the right lobe lung area; when intubating stop short (and above) of the tracheal bifurcation.

Biochemistry values can vary depending on breed, genotype, environment and procedures; match the values to age rather than size.

Major illnesses

There are a large number of swine diseases; to avoid outbreaks or subclinical levels of infection, most research swine are obtained from established vendors with specific pathogen free (SPF) status or from herds maintained at the research facility and/or via cesarean section on site. Standard health monitoring procedures with sample submission to a veterinary diagnostic laboratory are part of routine animal care. If animals from the same population are consistently used, one can take advantage of herd immunity as well.

Although bacterial illnesses accounts for most symptomatic disease in research pigs, an asymptomatic virus load can interfere with research in less obvious but critical ways.

The immune system may be modulated by spontaneous infections in the absence of clinical disease, an effect which may be suppressive or activating or both at the same time, but on different parts of the immune system. …Some microorganisms have a specific effect on enzymatic, hematological, and other parameters monitored in the animal during an experiment…Streptococcus suis, for instances, decreases single dose clearance but increases multiple dose clearance of sulfadimidine….Spontaneous infections in an experimental infection model may compete with the experimental infection, which in the worst cases may fail. …opportunistic pathogens may become pathogenic in animals with cancer, e.g. swine with lymphosarcomas are more likely to die from colibacillosis than swine without lymphosarcomas….Microorganisms present in the animal may contaminate samples and tissue specimens such as cells, sera, etc. (Peter Bollen, et. al., The Laboratory Swine, CRC Press, 2000, p. 36-37)

As an example, one of the major diseases of concern for swine production is atrophic rhinitis; this can exist in sub-clinical levels in a herd and poor husbandry is often a factor. Although research has shown that low levels may not affect pork quality, in a research setting, this respiratory illness can affect results in the manner noted above since the immune system will be compromised and the pigs stressed.

Illnesses with zoonotic potential

Most of these illnesses carry a low level of risk for staff; transmission from fecal contamination of food is the most common path for zoonotic diseases such as Campylobacter jejuni, Eschericia coli. Brucella suis, which can cause severe fever and spine lesions and even death can occur from direct contact with tissues and body fluids of infected pigs. Inhalation of Mycobacterium spores can cause tuberculosis. The two zoonotic diseases carrying a high risk from contact with diseased pigs to staff are Erysipelothrix rhusiopathiae (symptoms similar in both pigs and people are fever, arthritis, possible damage to heart valves or sudden death) and Salmonella Spp. (symptoms of diarrhea in both pigs and man); the former is transmitted via direct contact and the latter via fecal contamination of food.

Nutrition

The National Academies Press’ Nutrient Requirements of Swine, 10th revised edition is available online. Although young pigs are generally given ad lib feeding, miniature swine or those in longer term studies need some sort of feed restriction to prevent obesity. In this case, a low calorie diet is usually used so as to give them a more comfortable volume. Water can be delivered either through automatic devices or in the feeding trough with the food; after the food is gone the trough can be filled with water. Pigs are usually given a standard pre-formulated diet which can account for nutritional needs without supplement. Newborn piglets do have a need for iron supplement:

The baby pig is born with a limited supply of iron, and because the sow’s milk is also low in iron, supplemental iron is a must. The most commonly used sources of iron to prevent anemia in newborn pigs are injectable and oral products. Injectable iron is the preferred method of anemia prevention. An intramuscular injection of 200 mg of iron dextran given at 1 to 3 days of age will prevent the anemia problem. Most producers will give an iron injection within the first 3 days of life. Need for a second injection depends on the amount of iron available to the baby pigs during the lactation period and how much was given in the first injection. The baby pigs can receive iron orally from consuming creep feed or sow feed or from the sow’s feces. Over 90 percent of the injected iron from the initial treatment is utilized over the first 3 weeks. If less than 200 mg of iron is given in the first injection, a second iron shot may be needed. Need for a second injection also depends primarily on blood hemoglobin concentration, a rapid and reliable indicator of the iron status of the pig. Blood hemoglobin levels of 10 mg/100 ml or above indicate adequate iron status. Hemoglobin levels of 8 to 9 mg/100 ml indicate a borderline anemia condition, whereas a value of 7 or below indicates an anemic condition. If blood hemoglobin levels fall below the 10 mg/100 ml level, a second iron shot is advisable. (Kansas State University Agricultural Experiment Station and Cooperative Extension Service, General Nutrition Principles for Swine, p. 17)

The other consideration would be if the instead of ad lib feeding, a restricted amount of the formulation were given to prevent obesity. In this case it is necessary to review the levels of vitamins/minerals and balance of protein/carbohydrates/starch to be sure that a reduction in food volume does not result in low levels of a specific nutrient. And, as stated previously, “….more often restricted feeding with a low energy diet is applied to allow the animals a satisfying food volume intake.” (Bollen, op.cit. p. 28) If feed is restricted, there will be need for supplemental environmental enrichment such as straw or objects to manipulate as pigs have been bred for appetite and weight gain curves in addition to their normal food drives.

Husbandry and welfare considerations

Comfortable Quarters for Pigs in Research Institutions, has specific details. In general, pigs are usually kept on some sort of grid flooring and not always supplied with bedding. It is important to be sure that the grids are wide enough for feces to fall through (their hooves will also push stools through) but the grids need to be small enough to prevent their feet from getting caught. Their hooves will wear down if kept on concrete but if kept on grids, they will need their hooves trimmed regularly. Plastic coated grids work well, with a 10-15 mm spacing. As said in the Overview, pigs are social animals and need to be either group or pair housed; if singly housed, they must be able to have some direct contact with each other, e.g. through openings between the bars. Care must be taken that the floor is roughened to prevent slipping and contains no sharp edges or protrusions; pens should be very sturdy with solid foundations. Outdoor housing is not suitable for some research due to lack of control over temperatures and the chance of infection

Since pigs have short and sparse hair and no sweat glands they are sensitive to temperature levels and fluctuations as well as to air drafts. They are unable to regulate their body temperature well so if they are kept on concrete they need to be protected from temperature fluctuations and drafts. For young, straw should be provided to prevent heat loss. Pigs will hesitate to lie down on cold concrete and may take a “sitting” or “dog” posture. With straw, they will organize nest areas and tolerate temperature fluctuations better.

Pens should be checked daily and excreta removed; grid floors should be cleaned of feces in the corners. If straw is used, soiled bedding should be removed. Pens should be washed daily and quarters sanitized on a schedule: research institutions tend to sanitize more frequently than in agricultural situations. Pigs should not come in contact with disinfectants; they should be removed when the pens are washed and returned only when the area is dry.

Pigs are intelligent and when the space is available will choose separate areas for urinating and defecating and will construct nest areas when given straw. Straw provides comfort; extended time on bare concrete can result in pressure sores. The straw is also a rooting medium and they will eat some of it for roughage. Some sort of environmental enrichment will keep them occupied and their stress level down; this will help them tolerate procedures better and help stabilize physiological values. Items such as rubber balls, hanging chains, brushes attached to the walls, or plastic crates or turf mats on the floor will all work; keep any enrichment items clean as the pigs will not use them if they are soiled with feces.

Although it may seem costly and time consuming on the part of support staff, environmental enrichment and personal attention to the pig used in a research study leads to more rigorous data and thus, in the long run, is practical (cost-effective) as well as ethical.

"Pigs are," says Hampshire, "next to humans, the best animals for physiological studies. They can be 'chronically instrumented' with a tube, called an indwelling catheter, that is connected to a blood vessel. Like patients who are receiving chemotherapy, their blood can be sampled and they can receive medicines through the catheter." A valve that works like a two-way spigot on a sink is attached to the catheter, so that repeated injections and measurements can be made without causing discomfort to the animal. The animals wear spandex sweaters that hold the tubing in place. "Pigs are not stupid," says Hampshire, "they are just tolerant of the instruments."

"The pigs are happy when their comfort needs are met," she says, "and their progress is tremendous when they trust the people working with them." They enjoy food, attention, and toys and like to pile up next to each other in the pen. Most days, staff members play with them, and the pigs pick treats from the treat cart. Hampshire says that the pigs like the company of humans better than they like being with other animals. As we talked, a pig passed by the office on a stroll with a technician." (People Doing Science)

Temperature, humidity and ventilation

Newborns do well at 86-100 degrees Fahrenheit (they are usually provided a heated floor and heat lamp area to crawl to); 84 degrees for juveniles (6-8 weeks); 75 degrees (14-16 weeks) and 63 degrees for adults.

Relative humidity levels should be at 50-70%.

Ventilation is usually maintained at 10-15 air changes an hour. Depending on how many animals are housed, the air flow might need to be adjusted; too low and the noxious gases could build up, but if set too high for the stocking density, the environment might end up being drafty.

Space recommendations as per the Guide for the Care and Use of Laboratory Animals vary considerably:

Recommended Space for Swine

Animals/Enclosure

Weight, kg

Floor Area/Animal, ft2
1

Up to 25

12.0

Up to 50

15.0

Up to 100

24.0

Up to 200

48.0

>200

>60.0

2-5

<25

6.0

Up to 50

10.0

Up to 100

20.0

Up to 200

40.0

>200

>52.0

>5

<25

6.0

Up to 50

9.0

Up to 100

18.0

Up to 200

36.0

>200

>48.0

Handling and Procedures

The University of Minnesota’s excellent Research Animal Resources website has detailed information on Restraint and Handling of Swine. Scroll down through the different animals until you reach the section on pigs. Pigs are notorious for being easily stressed and sensitive to rough handling. Approach them gently, speaking softly, crouching near them if possible rather than standing over them. Whenever possible, food should be used as an enticement and stress reliever. A handful of chow sprinkled into a holding pen or area will encourage the pigs to enter and keep them happily rooting. Since they don’t take restraint well, it is common practice to keep them in slings for procedures or to lightly anesthetize them. It is worth taking the time to train pigs to become used to minimal, non-invasive procedures since they can be very difficult to restrain when stressed. If possible, they can be distracted by being fed a snack. Sedation is usually via the IM route and pre-anesthetics and analgesics via IV with access at the ear veins. Inhalation anesthesia via face mask is stressful and should only be used if already pre-operatively sedated. Intubation requires trained personnel. It is important to monitor the pig continuously to be sure the anesthetic depth remains appropriate and continuous.

Monitoring

As with all animals a thorough knowledge of normal appearance and behavior is the place to start; with practice it is easy to spot an ill or uncomfortable pig. If possible, observe them without handling since this stresses them.

Checklist:

It is best to decide on a formal repeatable exam procedure. For example:

Overall appearance: coat should be clean, dry, with white-pink skin.

Urine and feces of amount and consistency and color as usual and similar to other pigs, no obvious weight gain or loss, normal activity level, expression BAR (bright, alert, reactive).Check the group first to see that all are similar and then focus on individuals.

Normal behavior is one of responsiveness, curiosity, tail wagging, interest in being approached, reaction to being handled, and active interest and noise at feeding time. In general, a pig that is reluctant to eat is in distress of some sort.

Breathing: 20 bpm; miniature swine up to 55 kg: 20; up to 90 kg; 25

Nose: pink, dry

Mouth: no discharge, teeth clean

Eyes bright, eyelids clean, not swollen

Ears: clean, veins and surface on inside clean, dry, smooth, not reddened.

Tail/Anus: clean, no evidence of mucous or feces

Genitalia: clean, no exudates (if female in estrus the vulva will be reddened and inflamed)

Legs: no stiffness or damage or swelling, check hooves for trimming needs, normal gait and stance

Assessing pain

Always look at overall demeanor first: the pig may be quieter than normal with reduced activity and social interactions. A roughened coat, or hair standing up indicates discomfort. Look for any abnormality in postures or positions, favoring of leg or area, resistance to changing position. Look for either increased aggressiveness or apathy. In general, if a procedure would cause pain in humans, assume that it will do so for the pig. And again, any reluctance to eat in a pig is abnormal. The Animal Welfare Information Center has a useful online publication, “Recognition of Pain in Farm Animals” (scroll down).

Current research using pigs

Given the wide range of breeds used in research studies, publications ask that authors of papers specify exactly the breed and the derivation.

As experimental subjects, swine have a number of advantages over other animal species.  They have a favorable reproductive capacity with a short gestation period, large litter size, and a polyestrus cycle.  Furthermore, swine anatomy and physiology are similar to humans, especially in cardiovascular, pulmonary, skeletal, and integumentary systems. These similarities have made the swine the primary species of interest as organ, tissue, and cell donor species for xenograft transplantation procedures.  Genetically altered pigs for potential use in clinical xenotransplantation are now technically feasible as precise genetic modifications in the pig genome are achieved.  Thus, establishing a resource to supply needed swine organs would greatly support and advance these specific research goals.  The development and increased use of purpose-bred swine strains (i.e., inbred strains, various hybrid lines, and lines with genetically selected or altered genes) in biomedical research by investigators from around the world generate increased demands on animal breeding programs, specifically for specific-pathogen free (SPF) quality animals.  (National Swine Research and Resource Center. 

The Animal Welfare Information Center has published an extensive database that works as a literature survey for current research models. It is organized according to organ system. They note that:

Swine have been utilized as a biomedical research model in a wide variety of disciplines; the most notable being cardiovascular research. Swine exhibit many similarities to man in cardiovascular anatomy and physiology including: a comparable heart to body size ratio, spontaneous development of atherosclerosis, coronary arterial anatomy, lipoprotein structure, propensity towards obesity, and susceptibility to a variety of stressors. Pigs are nonruminant omnivores and exhibit similarities to man in digestive physiology. Piglets and human infants share similar nutrient requirements and a susceptibility to a host of gastrointestinal pathogens.

The pig has been used as a model for diabetes, alcoholism, peptic ulcers, and liver transplant studies. Swine have multilobar kidneys similar in size, number, and structure to human kidneys, which makes the pig an excellent model for kidney obstruction and reflux neuropathies. In the field of dermatology, swine have been utilized to study wound healing, plastic surgery procedures, the formation of human melanomas, and the effects of photoaging. Swine also have been utilized as models in skeletal research, toxicology, and pharmacology.

Pigs are an intelligent and versatile species that can adapt to a variety of experimental conditions, provided their environmental and behavioral needs are met (533, 537, 539). As swine continue to gain popularity as a biomedical research model, there is a need for more information on the management, handling, care, and welfare of swine in laboratory settings.   (Animal Models in Biomedical Research: Swine)

Given the state of the art of genomic sequencing, a new area has opened up involving working with two species at once. For example, porcine genomic sequencing along with mouse genomic sequencing can be used along with human genomic information. This raises multiple sets of complex ethical questions. For example,

Dr. Emmanuel Opara and a team of scientists at Duke University recently announced a novel treatment that could potentially eliminate the need for insulin in type I or Type II Diabetics. Nine months ago his team injected a baboon named Babs-92 with specially encapsulated islet (insulin producing) cells from a pig's pancreas. (Baboon + Pig: And the Potential Future of Diabetes Control!)

Online resources from academic institutions

Iowa State University College of Veterinary Medicine http://www.vetmed.iastate.edu/departments/vdpam/swine/

Kansas State University Swine Information. www.oznet.ksu.edu/dp_ansi/swine/swine.htm

Michigan State University Swine Resources www.msue.msu.edu/msue/iac/animswin.html

North Carolina State University Extension Swine Page http://mark.asci.ncsu.edu/

Oklahoma State University Swine www.ansi.okstate.edu/exten/swine/

Purdue University Swine Index www.ansc.purdue.edu/swine/index.htm

University of Kentucky (Swine Research & Education) www.uky.edu/Agriculture/AnimalSciences/swine/swine.html

University of Minnesota Swine Housing & Equipment http://www.extension.umn.edu/topics.html?topic=4&subtopic=86

University of Missouri Swine Publications www.muextension.missouri.edu/xplor/agguides/ansci/swine.htm

University of Nebraska Swine Publications www.ianr.unl.edu/pubs/swine/index.htm

Study/discussion question

The housing standards for pigs raised in agriculture for food are very different from those for pigs used in research institutions. (See Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching) Why do you think the standards are so different? For example, the practice of giving straw to pigs for bedding and enrichment, cited in the Comfortable Quarters for Pigs in Research Institutions is generally not standard in research institutions and rare in agriculture, although the Guide noted above states, "When supplemental zone-heating is not provided, farrowing houses should be bedded with a suitable material such as straw." (Guide, p. 74) Do you think there should be three different levels of welfare, 1 for food production, 1 for agricultural research and 1 where the pig is a used as a biomedical model for humans? Why or why not? What is different, or the same, for the three types of research?