ILAR Journal Online, Volume 38(1) 1997: Unusual Mammalian Models

Online Issues

<< All Back-issues

<< This Issue's Table of Contents

ILAR Journal V38(1) 1997
Unusual Mammalian Models

The SPF Pig in Research
Joe Safron and Janet C. Gonder

Joe Safron, DVM, MS, is Associate Director and Janet Gonder, DVM, is Director of Comparative Medicine, Corporate Research and Technical Services, Baxter Healthcare Corporation, Round Lake, Illinois.

INTRODUCTION

The domestic (meat type) pig, Sus scrofa domestica, has become a common research animal used in biomedical research due to similarities to humans in many of the organ systems. So similar, in fact, that axenic, gnotobiotic, and specific pathogen-free pigs are being promoted as potential sources of organs available for xenotransplantation (Ye and others 1994). These 3 levels of microbial status, while having important differences, are commonly confused because many of the derivation and maintenance practices are similar.

Pigs, like other laboratory animals, may be produced using 4 basic levels of care. Conventional animals are produced without standardized controls for microbial status. Domestic pigs and miniature pigs produced under conventional practices vary in health status, known genetic background, and level of available herd historical information. Axenic, or germ-free, pigs are produced and maintained free of any known microbial agents. Gnotobiotic animals are produced with a known flora of microbial agents. Axenic and gnotobiotic pigs are surgically delivered and maintained under strict isolation. Gnotobiotic pigs are subsequently inoculated with a known microbial flora. True gnotobiotic and axenic pigs are not generally available commercially as they are very expensive to produce and maintain. Specific pathogen-free (SPF) animals are produced free of specific microbial agents. These animals are derived using gnotobiotic procedures but are then maintained in more conventional facilities. With SPF animals, management and care procedures are geared towards restricting entry of specific "indicator'' disease syndromes. In swine the designation SPF has a proprietary commercial meaning as well. The purpose of this paper is to clarify the definition of domestic SPF pigs, and discuss their appropriateness for biomedical research.

THE NATIONAL SPF SWINE ACCREDITING AGENCY, INC.

In the United States and in several foreign countries, formal organizations exist that have developed acceptance criteria for SPF domestic pigs. In the United States the major organization for this purpose is the National SPF Swine Accrediting Agency, Inc.¹ (National SPF Agency). This organization began in the late 1950s, when early swine production investigators recognized that surgically removing a pig from its mother lessened the chance of disease transmission. Several state SPF organizations were formed in the early 1960s, which were combined in 1964 to form the National SPF Agency (Alexander and Harris 1992).

The Accreditation Process

The primary purpose of the National SPF Agency is to improve swine production efficiency through disease prevention. The accreditation system involves an initial depopulation of all pigs on a farm, repopulation with surgically derived pigs, formal documented farm inspections by a trained veterinarian every 90 days, and slaughter inspections of market weight pigs (6 to 8 months of age) at 90-day intervals. Suppliers that comply with these criteria may be accredited.

The National SPF Agency tracks, at minimum, indicator disease syndromes (see Table 1) in member herds through farm health checks and slaughter inspections of market weight animals. Herds must not have used vaccines or insecticides to achieve their negative disease status.

Origin of SPF Herds

Accredited SPF herds must be derived from a laboratory approved by the National SPF Agency, an accredited primary SPF herd, or an accredited secondary SPF herd. There are currently 8 approved laboratories in the Midwest. In the SPF laboratories, pigs are surgically delivered by hysterectomy or cesarean section and reared using gnotobiotic techniques. Primary herds are closed herds; new additions to the herd must come from an approved laboratory. Secondary herds are also closed, however, additions can come from an approved laboratory, an accredited primary herd, or an accredited secondary herd. Embryo transfer techniques can be used in accredited herds, but the laboratory performing the transfer must be approved by the National SPF Agency. Artificial insemination can be used within accredited herds, but offers some risk of disease transmission. The National SPF Agency maintains inspection reports on each SPF accredited herd, containing specific information on the origin of the herd and whether techniques of embryo transfer and artificial insemination are used.

Farm Inspections

Accredited farms must have facilities and management practices that ensure sound herd health and isolation programs. Farm inspections, which are conducted at 90-day intervals by an independent veterinarian trained and certified by the National SPF Agency, ensure that these requirements are being met. Inspections cover the following areas:

Herd health. Herd health is evaluated based on clinical evidence of any and all diseases, not solely the indicator diseases. All swine, at all age levels, are evaluated for clinical signs of turbinate atrophy and snout distortion; Actinobacillus pleuropneumonia; other upper respiratory infections and pneumonias; porcine reproductive and respiratory syndrome; transmissible gastroenteritis; salmonellosis; swine dysentery and other diarrheas; lice, mange, and other skin conditions; and poor production efficiency. Any evidence of an indicator disease is followed up by definitive diagnostic testing.

Breeding swine must be serologically negative for pseudorabies and brucellosis. Further serologic testing may be conducted by the farm and is noted in the inspection report. Common agents tested for include porcine reproductive and respiratory syndrome, Actinobacillus pleuropneumonia, mycoplasmosis, and transmissible gastroenteritis.

Further testing such as skin scrapings for external parasites, fecal examination for internal parasites, and blood testing for porcine stress syndrome may be conducted and become part of the inspection record. Parasite control programs, vaccination programs, and drug use programs are evaluated and documented. The program of on-farm and consultative veterinary support is also documented in the report.

Farm management practices. Inspectors evaluate farm isolation or "biosecurity" practices. The farm must post signs prohibiting casual trespass. Procedures must be in place so that all personnel and visitors must shower when entering and leaving the facility. Clean protective apparel such as boots and coveralls must be available and used. Disinfectant footbaths must be placed at key entrances to facilities and should be changed frequently. The farm must be physically isolated from other farms by appropriate distances and security fencing. The food and water supply and storage facilities must be designed to avoid contamination. Waste removal and storage procedures must be adequate. Vehicle traffic must be restricted from the farm, and where appropriate, trucks and tractors should be disinfected. An effective vermin control program must be in place. Herd additions, embryos, and semen must come from approved suppliers and are noted on the inspection report.

The general husbandry practices are reviewed, including population densities at all growth stages, cleanliness of all facilities, facility maintenance procedures, temperature and ventilation control, traffic patterns, and special production programs such as early weaning programs or multi-site production systems. Finally, production and herd health records are evaluated, including the disease history of herds receiving pigs from the inspected farm (National SPF Agency 1994).

Gross necropsy or slaughter inspections. Slaughter inspections are conducted every 90 days by a certified veterinarian. Inspections focus on gross evidence of nasal turbinate atrophy and signs of pneumonia. Herds where animals exhibit any signs of these diseases are not recommended for accreditation. Herds found free of all indicator diseases, but are found to have sign of turbinate atrophy, are classified as a "National SPF Health Controlled Herd". Health controlled herds must pass 2 negative 90-day slaughter inspections before accreditation is granted.

Accreditation status is determined by a formal review of the farm and slaughter inspection reports by officials from state and the National SPF Agency. Accreditation may be denied by either organization if diseases other than the indicator diseases are found during the farm and slaughter examinations. All records of inspections and accreditation status information are available to interested parties from the national organization.

Acceptability for Research

SPF practices, while limiting the indicator diseases, also tend to limit many other clinical and subclinical diseases that may have an impact on biomedical research. Herds of long-term accreditation status are likely to have no or very low titers to other diseases such as swine influenza virus, encephalomyocarditis virus, leptospirosis, porcine respiratory and reproductive syndrome, transmissible gastroenteritis virus, erysipelas, trichinellosis, and toxoplasmosis. Serologic testing can be obtained through state veterinary diagnostic laboratories. There are however, potential variations in SPF farm management practices that may affect disease status. These variations should be evaluated prior to acquiring animals for use in the research laboratory. Examples include the location of the farm (farms geographically distant from other dwellings or in states where pigs are not commonly raised are less likely to be exposed to disease), degree of outdoor exposure (outdoor housing systems increase the chances of exposure carried by birds, rodents, and insects), type of feed used (pelleted feed may reduce contamination), water quality (shallow wells are more easily contaminated), and exposure to waste (slatted floor systems reduce fecal exposure when compared with solid floor systems).

There are several other considerations when using SPF pigs in the laboratory.

The National SPF Agency accredits farms producing commercial domestic pigs. The program is not designed for miniature pigs. Domestic pigs up to 35 kg can be used easily for acute and subchronic studies. Special handling and housing are required for chronic models, as domestic pigs reach market weight (100 kg) at around 6 months of age, and can reach 300-400 kg by 2 years of age. Further, most pigs produced in accredited herds tend to be marketed as breeding animals. Advanced notice to farms and suppliers is usually required to obtain younger pigs for research laboratories.
Farms are not generally accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care, International (AAALAC) nor are they registered with the U.S. Department of Agriculture.
Herds of domestic pigs may be routinely provided with antibiotics, growth promotants, or other medications in the feed or water. A listing of these medications can be obtained from the supplier, the herd's attending veterinarian, and the accredited herd's Farm Inspection Report.
SPF pigs are not necessarily free of internal parasites. Light infections of roundworms, nodular worms, and coccidia may be found.
SPF pigs are available in several breeds and as crossbreeds and therefore will vary in color. Some research laboratories may prefer the white-skinned breeds to facilitate ear vein access.
The method of transport from the farm must be evaluated. Movement in open-type stock trailers increases the chances of exposure to disease agents.
SPF pigs are naive to many diseases. Agents such as Bordetella bronchiseptica and Pasteurella multocida can pass between species in the laboratory. To maintain the SPF status of pigs, they must be isolated from other animals, including other strains of pigs such as miniature pigs. Most miniature pig herds are produced under conventional practices and include vaccinations for B. bronchiseptica and P. multocida in their herd health program, while SPF herds do not. Vaccinated animals are free of the disease, not necessarily free of the infectious agent and therefore may transfer disease to the naive SPF pigs.
Several lines of domestic pigs are specifically bred to carry the gene (Halothane gene) responsible for malignant hyperthermia or porcine stress syndrome. If pigs are heterozygous for this trait, they partially express the trait and become more heavily muscled and show higher feed efficacy and improved meat quality. Pigs homozygous for this trait may suffer the clinical disease characterized by high fever and muscle fasciculations if unduly stressed or exposed to certain inhalant and injectable anesthetics. Farms must carefully control breeding practices to avoid breeding carriers. Use of carrier animals should be carefully evaluated by investigators, as the partial expression of this gene in carriers may adversely alter research data.

Steps for Use of SPF Pigs in Your Laboratory

Contact the National SPF Agency for swine herds in your area. Several commercial suppliers also market SPF pigs. Request information on the herd from the supplier, the farm's attending veterinarian, and the national organization on breeds available, herd origin, and current health status. If possible select SPF herds with long-standing accreditation.
Request further health-related information if necessary, such as other tests that have been conducted on the herd. Many herd owners will supply samples for further diagnostic testing if requested.
Review the test status of the herd for porcine stress syndrome or malignant hyperthermia.
If vein access is required, select white-colored pigs.
If possible, transfer animals in enclosed, temperature-controlled trucks without exposure to other pigs.
Housing systems designed for dogs work well for small domestic pigs. Pigs have a natural rooting instinct and like to play with waterers and feeders. Make sure accessories are fastened well to the unit. Smooth sharp edges on caging. Food and water bowl holders, and sharp, die-cut ventilation holes in stainless steel caging panels can result in severe nasal and oral lacerations on curious pigs. Plastic water piping should be out of reach. Minor lameness may be seen in newly arrived pigs if placed on flooring that is different from the herd of origin.
Perform fecal examination and worming if necessary.
Do not change diets rapidly. Rapid change to laboratory type non-medicated diets can result in clinical diarrhea. It is best to obtain a 2-week supply of feed from the supplier and make the conversion slowly. Most pigs are fed ad libitum on the farm and grow rapidly. Restricting the feed in the laboratory to 3 to 4% of body weight slows growth and works well with young domestic pigs.
Re-evaluate your facility traffic patterns, with consideration of the microbial status of each group of animals in the facility.

CONCLUSION

In many laboratories, pigs are replacing other larger species of animals as prime research candidates. Similarities in the cardiovascular, urogenital, integumentary, skeletal, and digestive systems of swine to humans has contributed to increased use of pigs in research. Further, because of these similarities, pig tissues and organs are currently being used and studied as human xenografts, with the potential for increased use in this area in the future. The growing need for swine of research quality demands higher health quality standards not generally available in commercial swine herds. Like other laboratory animals, improved health quality has been associated with advances in care and production practices derived from advances in gnotobiotic technology. Pigs are well suited for the gnotobiotic techniques: they breed easily (often 2-3 times per year) and they produce large litters (8-12 pigs per litter). Pigs have a 6-layer, diffuse epitheliochorial placentation, which resists transmission of agents in utero, subsequently the young are less likely to acquire maternally derived diseases and do well under human foster care. There are many SPF herds across the United States, Canada, and Europe that have improved health status using this technology. However, as when procuring any SPF animal, each supplier must be evaluated. Herds having long-term accreditation status by the National SPF Agency are free of the most common pathogens of pigs, but it is essential to understand specifically what care and standards have been used for each individual group or herd prior to their use in the biomedical laboratory. Following these general guidelines can help to provide porcine models with minimal disease-induced variability.

¹Further information about the National SPF Agency can be obtained from The National SPF Swine Accrediting Agency, Inc., PO Box 280, Conrad IA 50621. Tel: 515/366-2124; Fax: 515/366-2232.

REFERENCES

Alexander TJL, Harris D. 1992. Methods of disease control. In Diseases of Swine, 7th ed. Leman AD, editor. Ames: Iowa State University Press. p 808-836.

National SPF Swine Accrediting Agency, Inc. Rules and regulations. 1994. Conrad, Iowa 50621.

Ye Y, Niekrasz M, Kosanke S, Welsh R, et. al. 1994. The pig as a potential organ donor for man, a study of potentially transferable disease from donor pig to recipient man. Transplantation 57:694-703.

TABLE 1 SPF indicator syndromes

Disease Syndrome	Agent(s)	Clinical signs
Turbinate Atrophy and Snout Distortion	Bacterial agents Bordetella bronchiseptica Pasteurella multocida	A disease of young growing pigs, which results in atrophy and distortion of the bones of the nasal cavity. Commonly called Atrophic Rhinitis
Pneumonia	Numerous viral and bacterial agents Swine influenza virus Porcine reproductive and respiratory syndrome virus Actinobacillus pleuropneumonia Pasteurella multocida Mycoplasma hypopneumoniae Haemophilus parasuis Bordetella bronchiseptica	Signs can range from clinically inapparent to severe respiratory difficulty. Severe infections are characterized by head down posture and forced breathing called "thumping"
Swine Dysentery	Bacterial agent Serpulina hyodysenteriae	A muco-hemorrhagic colitis causing varying degrees of diarrhea
Lice	Sucking Louse Haematopinus suis	Infestation with this louse can cause severe skin irritation and pruritus. Agent can spread blood borne diseases between pigs
Mange	Burrowing mite Sarcoptes scabiei var. suis	Causes severe pruritus and loss of condition
Pseudorabies	Viral agent Herpesvirus suis	Usually inapparent in chronically infected herds. In naive young pigs may see nervous and respiratory signs. Like other herpesviruses can cause death in aberrant hosts such as dogs, cats, and cattle.
Brucellosis	Bacterial agent Brucella suis	Usually a reproductive disease of swine, leading to decreased herd reproductive performance.