(This paper was originally published in the NC Pork Report)
Introduction
Failure to eat is an obvious explanation for slow growth of sick pigs. Unfortunately, the answer is not that simple. Complex interactions between infection, disease and the animal's immune system interfere with growth, despite resumption of "normal" feed intake after illness. Consequently, optimal feed conversion and average daily gain are difficult to achieve with disease problems in nursery and grow-finish pigs.
Pig growth is controlled by a complex interaction of genetic, hormonal and nutritional factors. Growth hormone (somatotropin) is essential for normal growth in the pig; however, it is evident that specific growth factors also are necessary for growth, particularly skeletal muscle. In pigs, insulin-like growth factor-I (IGF-I) is critical in the growth-promoting actions of growth hormone. Growth rates in most food animals, including the pig, are correlated with IGF-I status. In fact, pigs selected for fast gain express greater concentrations of circulating IGF-I than their contemporaries selected for slow gain.
A growing number of reports indicate that disease and/or the immune response interfere with the relationship between growth hormone, IGF-I and growth. It is well documented that nutrition affects the regulation of IGF-I in healthy individuals; however, factors, other than nutrition and somatotropin concentrations, alter IGF-I concentrations during disease or after immunological challenge (Fan et al., 1995). This paper will describe some of the interactions between disease and growth and the potential implications of these interactions.
Attempts to Minimize Disease
Various strategies are used to minimize pathogen transmission between pigs of different ages, stages of production or farms. These strategies were designed to reduce problems with diseases and, thus, improve pig growth and performance. All-in-all-out (AIAO) movement of pigs, segregated/medicated early weaning, Isowean7 and three site production are typical examples of management systems for disease control. Antibiotics often are useful to treat bacterial diseases, but cost, quality assurance and other considerations limit the use of antibiotics in pork production. Disease prevention commonly involves vaccination programs, but vaccine efficacy and cost-effectiveness are variable.
Producers continue to observe the lack of uniformity in growth of nursery and finishing pigs despite the use of new technologies, antibiotics and vaccines. Slow-growing pigs are commonly held for additional time in either the nursery or finishing facilities, while the majority of pigs are moved to the next stage of production or to slaughter. Mixing the slow- growing pigs with younger, healthy pigs has some degree of risk, specifically, certain diseases are perpetuated and spread by the mixing of pigs. Rather than holding the slow-growing pigs in the nursery, some producers maintain AIAO pig flow and move the affected pigs to the finishing floor as lightweight pigs. The long term success of either management procedure is debatable. It is clear that pork production continues to be hampered by diseases, despite many perceived improvements in disease control.
Can Nutrition Overcome the Impact of Disease?
Numerous studies have been conducted that relate the incidence and severity of gross lesions at slaughter to the variability in pig growth. However, examination of sub-standard pigs does not consistently reveal manifestations of previous disease problems. Since slow-growing pigs often are provided with the same nutrition and facility design as their rapidly growing counterparts or littermates, it often is difficult to identify the underlying causes of the slow growth. Furthermore, the overall performance of entire pens or buildings of pigs are often impaired in the absence of chronic diseases or nutritional deficiencies.
Recent investigations provided valuable insight into the provision of optimal dietary regimens to pigs following medicated early weaning (MEW). The general concept underlying the rapid and lean growth of MEW pigs is that the pigs' immune systems are minimally activated. In contrast, reduced growth of immunologically challenged pigs, i.e. conventional pigs exposed to common pathogens, may not be alleviated by altering amino acid nor energy supply (van Heugten, 1992; van Heugten et al., 1994). This dichotomy poses a challenge to swine producers; improved performance can be achieved with healthy pigs but dietary manipulation does not overcome the impaired growth and feed conversion associated with disease or immune stress.
Complex Interactions Between Immunity and Growth
The influence of immune activation/stimulation on growth was well documented in the poultry industry. In chicks, immune stimulation reduced weight gain, increased muscle protein degradation, decreased protein synthesis, reduced muscle protein accretion and impaired accretion of cartilage and bone. In contrast, the precise mechanisms of decreased growth in conventionally raised pigs or "sick" pigs in facilities designed for high health are poorly understood.
Many clinical signs in sick pigs are the result of endotoxins. Endotoxins are part of the cell wall of common pathogenic bacteria, such as E.coli and Salmonella. In addition to bacterial infections, pigs are chronically exposed to endotoxins, which are well recognized as aerosol contaminants in nursery and finishing facilities. A common research method, used to stimulate an immune response and mimic natural exposure, is to treat pigs with purified endotoxin. Endotoxin treatment reduces food and water intake, elevates body temperature, heart rate and respiration rate in the pig and changes blood levels of several hormones.
Specific factors, called cytokines, are produced and secreted by the pig's white blood cells as a defense mechanism in response to endotoxins. Cytokines suppress the secretion of the significant growth promoting hormones, affect glucose balance, increase protein oxidation, increase muscle proteolysis and alter other metabolic processes. Thus, immunological challenge impairs metabolism intended for growth and skeletal muscle accretion in order to enhance metabolic processes that support the immune response. The alteration in metabolism involves a decrease in IGF-I concentrations. Therefore, it is not surprising that dietary manipulation fails to improve pig growth after immunological challenge.
Recent increases in the incidence of bacterial diseases have been attributed to Porcine Reproductive and Respiratory Syndrome virus (PRRSV) infection. It is evident that increased mortality is associated with concurrent PRRSV and bacterial infections and many producers claim that growth is impaired in pigs surviving the infection. Immune suppression by PRRSV was assumed to be responsible for the increase in bacterial diseases; however, recent studies indicate that PRRSV infection exerts a paradoxical effect on the pig's immune system. The virus infects and destroys certain immune cells, while enhancing antibody and cell-mediated function in the systemic circulation (Molitor et al., 1996). At present, the influence of PRRSV-immune system interactions on growth and IGF-I concentrations has not been evaluated in weaned pigs.
Strategies to Minimize Immunological Stress
As previously mentioned, the pork industry has adopted strategies, such as SEW and three site production, to minimize the influence of disease on pig growth. Unfortunately, diseases and impaired growth occur despite the use of these strategies. The role of immunologic stress on growth and feed efficiency is well recognized in other species. Various pharmacological agents have been developed to block the immune response and the actions of cytokines. However, it should be recognized that the cytokines are necessary and beneficial for preventing infection in the animal. Also, it is unlikely that these pharmacological agents will be developed for commercial application in the pork industry.
A common recommendation is to minimize immune stress. The vast majority of producers already possess the management tools and knowledge to reduce immune challenge and disease in pigs - the problem is to use these techniques on a routine and cost-effective basis. Isolation and testing of replacements, biosecurity and other methods are available to reduce the risk of introducing new pathogens to herds. Producers are cognizant that drafts, cold, heat, crowding, air quality, etc. affect pig performance. These basic management factors affect the immune system of pigs. Consequently, mistakes in fundamental animal husbandry create ideal opportunities for infectious agents to overwhelm the immune system of pigs.
Summary
The relation between growth, immunity and disease appears very complex; however, diet formulation, artificial insemination and other technologies were once considered complex and difficult to understand or implement on a practical basis. As biotechnology continues to evolve, it will not be surprising to observe new methods or treatments to overcome the influence of disease on pig growth. Meanwhile, pork producers currently know a fundamental solution to improve pig performance. The best research, antibiotics, vaccines can not replace nor are effective substitutes for sound STOCKMANSHIP!!!!
Take -Home Message
Suggested Reading
Stahly T. Impact of immune system activation on growth and optimal dietary regimens of pigs. Proc. 10th Ann. Carolina Swine Nutrition Conf. Raleigh, NC. pp. 74-82. 1994.
Van Heutgen E, Spears JW, Coffey MT. The effect of dietary protein on performance and immune response in weanling pigs subjected to an inflammatory challenge. J. Anim. Sci. 1994;72:2661.
