Spray Dried Animal Plasma as an Alternative to Antibiotics in Weanling Pigs *-A Review -

Piglet health at weaning is compromised due to several stress factors. Following the ban of antibiotic growth promoters new alternatives are required to control these problems. This paper reviews the evidence available for the use of spray dried animal plasma (SDAP) as an alternative to antibiotics in weaning pigs. Data from 75 trials in 43 publications involving over 12,000 piglets (mean values) have been used to calculate the performance responses of piglets according to several factors including SDAP origin, protein source from the control diet being replaced, dose of inclusion, age and weight of the piglets at weaning, sanitary conditions and simultaneous use or not of medication. Although the use of SDAP of all origins results in positive responses, it appears that plasma from porcine origin has the highest efficacy. This could be explained by the specificity of its IgG against porcine pathogens. During the first week post-weaning the response to plasma appears to increase with the inclusion dose, although over the two-week pre-starter period an optimal inclusion level of 4-8% is suggested. SDAP improves feed efficiency more markedly when the piglets are challenged with an experimental infection or when feed does not contain medication, which could be indicative of a lower expenditure of energy and nutrients to build an immune response against the challenge. There is evidence supporting that SDAP IgG and other bioactive substances therein prevent the binding of pathogens to the gut wall and reduce the incidence of diarrhoea in the post-weaning phase. Overall, plasma can be postulated as an excellent alternative to in-feed antimicrobials for piglets in the post-weaning phase. (


INTRODUCTION
Weaning is the most critical stage in pig production.Pigs at weaning are suddenly removed from the sow into a new environment where they have to change from suckling to eating dry feed from a hopper and drinking water from a drinker.Additionally at the same time they are mixed with pigs from other litters and fighting to establish new hierarchies occurs.As a result piglets go through a period of anorexia that compromises the functionality and integrity of the intestinal mucosa (van Beers-Schreurs et al., 1998).Once this period of starvation is over, the hungry piglet eats more feed than its gastrointestinal tract can cope with.The mucosa has lost its integrity and has not yet adapted to producing enzymes for digesting feed of vegetable origin (Miller et al., 1986).All this may increase the amount of undigested feed in the gut and increase the animal's susceptibility to pathological disorders.These problems have been traditionally controlled with the use of antibiotics at subtherapeutical doses as growth promoters.However the use of antimicrobial growth promoters has been banned in the European Union due to concerns about the development of antimicrobial resistance that could be transferred to human pathogenic bacteria.However as observed the Danish antibiotic use monitoring programme (DANMAP, 2004), since their ban antimicrobial growth promoters have increasingly been replaced by prescribed therapeutic antibiotics.Veterinary prescription of antibiotics in Denmark has increased continously from 48 tonnes in 1996 to 112.5 tonnes in 2004.In the case of pigs in particular, it increased by 10% between 2003 and 2004.Thus several products, including spray dried animal plasma (SDAP), have been postulated as alternatives.The effect of SDAP on piglet performance has been previously reviewed (Coffey and Cromwell, 2001;van Dijk et al., 2001a).The objective of this paper is to review the evidence available to support the use of spray dried animal plasma as an alternative to antibiotics in weaning pigs.

WEANING
At weaning, the supply of the beneficial factors provided by sow's milk to the piglet is suddenly stopped.Among these epidermal growth factor, polyamines, insulin and insulin like growth factor have been reported to contribute to the development of the intestinal tract (Pluske et al., 1997).Sow's milk also contains protective antibodies, the main immunoglobulin in milk being IgA (Husband and Bennell, 1980).
After the first days of age, the IgA and other milk immunoglobulins are not absorbed at the intestinal level; they provide continuous defence throughout the intestinal lumen against infectious organisms to which the sow is resistant (Svendsen and Larsen, 1977).This protection is essential while the piglet's own IgA production has not completely developed; this does not occur until 6-8 weeks of age (Svendsen and Brown, 1973).Therefore piglets weaned at 3-4 weeks of age lose the protection from maternal antibodies and cannot efficiently fight infections.It has been estimated that by 3 to 4 weeks of age the piglet receives approximately 1.6 g of IgA/d (Svendsen and Brown, 1973).Ideally the piglet should be offered sow's milk for the first days post-weaning.Unfortunately this is not possible, but an attempt to use ingredients with similar characteristics is feasible.The sow's mammary epithelium transforms precursors from blood or interstitial fluid into milk constituents by different processes.Among these transcytosis is of special interest; by this mechanism, intact proteins, immumoglobulins, hormones and growth factors can cross the mammary epithelium and be secreted intact in the milk (Hunziker and Kraehenbuhl, 1998;Monks and Neville, 2004).Many of the components in sow's milk may also be present in SDPP, and therefore SDPP may be a good substitute for sow's milk.
At weaning, piglets are producing enzymes that are adapted to the digestion of the nutrients contained in sow's milk.Appreciable quantities of lipase, amylase and other enzymes required to digest the nutrients in dry feed are not produced until 3-4 weeks of age and an adequate production is not achieved until 8 weeks of age (Kidder and Manners, 1980;Jensen et al., 1997).In addition, the piglet's ability to produce hydrochloric acid in the stomach at weaning is limited (Easter, 1988).During lactation, the pH of the stomach is kept low due to conversion of lactose to lactic acid by the lactic acid bacteria (Lactobacillus and Bifidobacteria).At weaning, the lactose supply is suddently reduced and a feed with high buffer capacity is provided, which results in an increased pH of stomach digestive contents.The increased pH causes the loss of the acidic protective barrier against the entry of germs into the small intestine, and an inefficient protein digestion may occur (Easter, 1988).Furthermore, at weaning the piglet has to learn to eat a new dry feed and to drink water.As a result piglets take on average about 15 h to initiate feed intake, but in some cases they may take up to 48 hours (Bruininx et al., 2001).The use of highly palatable ingredients may reduce the time to initiation to eating and ameliorate many of the problems associated with post-weaning anorexia.The poor feed intake associated with weaning results in a reduction of the height of the intestinal villi to almost half of their height before weaning.The mitotic activity in the crypts increases to compensate for this loss of enterocytes, their depth increases and the maturity of the enterocytes is reduced (Hampson, 1986;Miller et al., 1986;Kelly et al., 1991;Pluske et al., 1991).As well as the reduction in the small intestinal absorption area, the high proportion of immature enterocytes also compromises the enzymatic activity at the tips of the villi (Miller et al., 1986).The longer the piglet takes to iniciate eating, the hungrier it is and so eats in excess.As enzyme production is not appropiate due to the previous low feed intake (Makkink et al., 1994) feed cannot be properly digested and it may give undesirable microflora the chance to proliferate and cause GIT disorders.

SPRAY DRIED ANIMAL PLASMA AS A PROTEIN SOURCE
Animal plasma is a by-product from the abattoir, obtained from animal blood.The cellular fraction of blood is separated by centrifugation with the use of an anticoagulant.Plasma is then concentrated by vacuum evaporation or by filtration with inverse osmotic membranes or ultrafiltration and is finally dried by the spray technique, thus obtaining the so-called "spray dried animal plasma" (SDAP).During spray drying, plasma proteins are exposed to high temperatures for a very short period of time, and this has the advantage over conventional drying that the proteins are not denaturalised and preserve their biological activity (Gatnau et al., 1989;Borg et al., 2002).According to its origin we can classify SDAP into "spray dried porcine plasma" (SDPP) and "spray dried bovine plasma" (SDBP), which are of porcine and bovine origin, respectively.It has been suggested that the beneficial effects of spray dried plasma are related to its immunoglobulin content (Gatnau and Zimmerman, 1991), and for this reason, some SDAP sources have an standardised IgG content.In addition, sources of plasma that are enriched for some specific immunoglobulins, have also been obtained from pigs that have been vaccinated against specific pathogens for this purpose.The plasma obtained with this procedure is known as spray dried immune porcine plasma (SDIPP).Plasma sources can also differ according to the use of technological processes (e.g.ultrafiltration) to reduce the salt content before spray drying.Studies comparing plasma with high and low ash content have not found differences (Gatnau and Zimmerman, 1994;Rantanen et al., 1994;Russell, 1994).However these plasma sources have been tested at inclusion levels between 2-7.5%.It is possible that the high ash content could be detrimental at higher plasma inclusion levels.
A hygienic collection and processing of the blood is essential to ensure a good quality of plasma.Irradiation or antibacterial chemical treatment with formaldehyde of plasma has been shown to improve the efficacy of feed grade plasma (DeRouchey et al., 2004).However the same authors could not find an effect in plasma of low bacterial contamination (food grade).The safety of plasma as possible vector for transmissible diseases of pigs must be also taken into consideration (van Dijk et al., 2001a).Polo et al. (2005) have demonstrated that the spray drying process is effective in inactivating pseudorabies and porcine respiratory and reproductive syndrome viruses.The same authors have also shown that functional antibodies against porcine parvovirus in SDAP fed to naïve pigs did not promote their seroconversion.
Plasma is composed of protein, minerals and water.Proteins constitute the most important fraction; 95% of these are albumins and globulins (Tumbleson et al., 1986).The albumins are proteins whose function is the maintenance of plasma osmotic pressure and provision of the buffering capacity of the blood.Within the globulins, the gamma globulins have an immune function, of which IgG are the most important functional fraction of blood plasma.The other types, IgM, IgA, IgD and IgE are minor in blood plasma.The protein content in plasma ranges between 70 and 80% depending on the technological processes applied during its production.High protein levels are usually related to low ash contents and vice versa.Plasma sources concentrated by ultrafiltration have a higher protein concentration than those concentrated by reverse osmosis or vacuum drying.The amino acid composition of different spray dried plasma sources and their amino acid ileal digestibilities for weaning piglets are reported in Table 1.There are no substantial differences in amino acid composition between the plasma from porcine or bovine origin.Relative to the requirements of weaning pigs, the amino acid composition of plasma protein is well balanced, except for methionine and isoleucine which are limiting (Figure 1; NRC, 1998).The nutritive value of SDAP is similar to that of other high quality proteins, and cannot explain on its own the positive effects observed on the performance of weaning pigs.For example Kim and Easter (2001) did not find differences in lysine availability between SDAP and fishmeal, and Chae et al. (1999) observed that SDAP had a lower ileal protein digestibility than dried skim milk, isolated soy protein, and wheat gluten.

EFFECTS OF SDAP ON THE GROWTH PERFORMANCE OF NURSERY PIGS
A large amount of work on the efficacy of SDAP has been published since it was first proposed as a protein source for piglets in the late eighties (Zimmerman, 1987;Gatnau and Zimmerman, 1990).Table 2 summarises the results of 75 trials in 43 publications involving over 12,000    piglets that have been used to prepare this review.The mean differences (vs.control) in each trial for weight gain, feed intake and feed to gain due to the use of SDAP have been used to calculate the performance responses of piglets according to several factors using the GLM procedure of SAS.The mean improvements over control for each group considered were compared using Tukey's Studentized Range (Honestly Significant Difference) test and each mean value was compared to a value of zero (control) using a Student's t-test.Factors considered are: nature of protein being replaced by SDAP, source of spray dried plasma used, weaning age of the pigs, presence or absence of a pathogenic challenge and presence or absence medication in the feed, and the corresponding results are presented in Table 3 to 7.
To study a possible relationship between the positive effects observed for the diets containing SDAP and its nutritional value, it can be of interest to compare the magnitude of the responses according to the nature of the protein being replaced.A positive response should be expected if SDAP replaced a protein sources with poorer nutritive value whereas no response or a negative effect  should be expected if the protein source being replaced had the same or a better nutritive value, respectively.Table 4 shows the productive response to plasma according to the nature of the protein being replaced.For all the protein sources being replaced, SDAP improved weight gain and feed intake which suggests that factors other than nutritive value may also be involved in the positive effects of SDAP.
It is worth noting that feed to gain ratio changed very little for most protein sources.Only the replacement by SDAP of fishmeal (during the first week post-weaning) and casein (two weeks post-weaning) resulted in significant feed to gain ratio improvements.The feed intake mediated effect supports the hypothesis of an improved palatability of the feeds containing SDAP.Ermer et al. (1994) observed that piglets had a higher preference for SDPP than for dried skim milk containing feeds.This contrasts with the observations of Torrallardona and Solà-Oriol (2009), who evaluated the pig's preference for SDPP and another fourteen additional protein sources in relation to a common reference diet.SDPP preference was intermediate among the protein sources tested and did not differ significantly from them except for potato protein that had a significantly lower preference.Numerically, fishmeal, lupine, soybean meal-44, extruded soybeans, soybean meal-48, dried porcine solubles and dried skimmed milk at 10% of inclusion had higher preference values than SDPP.The higher feed intakes observed for the diets containing plasma could also result from an improved health status and higher body weight of the piglets (Torrallardona et al., 2003).In pair fed piglets plasma has also improved performance which suggests a specific biological effect independent of feed intake (Jiang et al., 2000b).
The response of weaning piglets to spray dried plasma from porcine (SDPP), bovine (SDBP), and mixed or unknown (SDAP) origin are presented in Table 3.All three sources significantly improve the performance of the piglets.It appears that plasma from porcine origin has a higher efficacy than the other sources, although statistically significant differences are only obtained for feed intake in the 0-14 d period.It is possible that the specific IgG against porcine pathogens in SDPP are advantageous.This is in good agreement with studies in which the direct comparison of the two sources resulted in higher productivities for SDPP than for SDBP (Hansen et al., 1993;Gatnau and Zimmerman, 1994;Rantanen et al., 1994;Smith II et al., 1995;Pierce et al., 2005).However, higher responses for SDBP have also been reported (Russell, 1994).It has also been suggested that SDPP is better than SDAP of mixed origin (Owusu-Asiedu et al., 2002).However, in a study in which SDPP was compared with two sources of SDAP  (with or without standardised IgG content), a better efficacy for the SDAP standardised in IgG content over the other sources was observed, indicating the relevance of the IgG content of the different sources of plasma (Bosi et al., 2001).
The advantadge of a standardised IgG content of SDAP has also been suggested by other authors (Campbell et al., 1998;Conde, 2005).This is supported by studies in which the comparison of SDAP with its albumin, immunoglobulin and low molecular weight fractions show that the beneficial effects are associated with the immunoglobulin fraction (Gatnau et al., 1995;Owen et al., 1995;Weaver et al., 1995;Pierce et al., 2005).Autoclaving SDPP decreases efficacy and this has been explained by an inactivation of its specific antibodies (Owusu-Asiedu et al., 2002), although other bioactive components may have also been destroyed.Two SDPP sources, obtained from pigs exposed to either high or low antigenic exposure were compared, in an attempt to check if the high antigenic exposure resulted in more efficient plasma, but no advantage was obtained (Stahly et al., 1995).Similarly, spray dried immune porcine plasma (SDIPP) obtained from pigs vaccinated against specific pathogens resulted in little productive advantages to piglets experimentally challenged with the same pathogen.This is probably due to an already existing "natural" antigenic load in the conventional non-vaccinated SDPP (Conde, 2005;Niewold et al., 2007).

SPRAY DRIED ANIMAL PLASMA AT DIFFERENT DOSES AND AGE OF THE PIGLETS AT WEANING
The response of piglets according to the inclusion doses of plasma in the feed is presented in Figure 2 and 3.It appears that during the first week post weaning (Figure 2) weight gain and feed intake responses to plasma increase with its level of inclusion in the feed.Over the two-week pre-starter period (Figure 3), however, an optimal inclusion level of 4-8% is suggested.It is well known that during the first week post weaning piglet's feed intake is very low (Pluske et al., 1997), and therefore, the higher inclusion levels of SDAP may facilitate an adequate intake of IgG.
Over the two week period, however, feed intake is normalised and the higher SDAP doses may result in nutrient imbalances (i.e.methionine, isoleucine or salt) that reduce the productive responses.Most dose-response studies have shown an optimum inclusion level for the first two weeks post-weaning, that falls within this range of 4-8%.Thus, an optimum inclusion of 6% was observed compared to inclusion levels of 2 and 4% (Rantanen et al., 1994) or to inclusion levels of 2, 4 and 8% (Gatnau et al., 1991;Burnham et al., 1995).Gatnau and Zimmerman (1992) concluded that performance was maximised at 6-8% of inclusion after testing doses of 2, 4, 6, 8 and 10%.Similarly, higher performances have been observed for 6% over 3% SDPP (Angulo and Cubiló, 1998;Lawrence et al., 2004), for 5% over 2.5% SDAP (Grinstead et al., 2000) and for 7% over 3.5% SDAP (Kerr et al., 1998).Other dose response studies, however, have shown that performance was optimised at levels of inclusion around 3% (Coffey and Cromwell, 1995;Grinstead et al., 2000).Studies with optimised responses at higher inclusion levels have also been reported.Kats et al. (1994) demonstrated that weight gain and feed intake (but not feed efficiency) increased linearly up to inclusion levels of 10% if diets were supplemented with methionine.They concluded that supplemental methionine is required to maximise piglet performance with SDAP diets.It is likely that the maximum SDAP inclusion level will depend on the isoleucine content of the diet as this will become the most limiting amino acid after methionine supplementation (Figure 1).The salt content in some SDAP sources can also be relatively high and could be a limiting factor for the use of high levels of inclusion.Salt concentration can vary depending on the concentration procedure used before drying and on the anticoagulant used.Thus SDAP sources concentrated by reverse osmosis or vacuum drying may have a higher salt content than SDAP concentrated by ultra filtration (10.9 vs. 4.1%).Similarly, the use of tri-sodium citrate as anticoagulant also results in a higher salt content than the use of tri-sodium phosphate.It can be concluded that spray dried plasma can be used in piglet diets up to 10% of inclusion, as long as a correct nutrient balance is maintained.
The response to plasma according to the age of the piglets at weaning was also studied (Table 5).Three age groups were considered 10-17, 18-24 and 25-32 days.Rooke et al. (2003) have shown that the concentrations in blood of specific colostral antibodies transferred from the sow to the piglets decline quadratically during the first weeks of life.They observed a rapid reduction in their concentration between 2 and 14 days of age, and a much slower decline thereafter until day 34.These authors also described that the total IgG concentration did not change between days 7 and 28 of age, but increased between days 28 and 35 suggesting an activation of the pig's own immune system at around four weeks of age.Therefore for the three age groups considered it could be assumed that: i) piglets of 10 to 17 days of age still have some degree of protection from the colostral antibodies, but their own immune system is immature; ii) that piglets of 18 to 24 days of age have a substantially reduced protection from colostral antibodies and an immature immune system; and iii) that the piglets of 25 to 32 days of age also have a reduced protection from colostral antibodies, but their own immune system is at the initial stages of development.No statistically significant differences between weaning age groups on the effect of SDAP on weight gain or feed efficiency were observed.However, a significantly higher feed intake for the 18-24d age group was observed during the first week post-weaning, and numerically, the response in weight gain and feed efficiency for this age group was also higher.The higher response to SDAP for this age group, coincides with their supposed poorest immune protection, and agrees with a possible support of SDAP to the immune system of the piglets via its IgG content or other mechanisms.In a trial by Torrallardona et al. (2002) although no interaction between weaning age and SDAP could be observed in piglets weaned at 22 or 32 days, a higher response for the younger animals is suggested, which supports the above observations.The importance of SDAP in the very young piglet is also supported by the observation that the inclusion of SDPP in creep feed improves the subsequent postweaning performance (Van Dijk et al., 2001b).

SPRAY DRIED ANIMAL PLASMA UNDER INFECTIOUS CHALLENGE AND COMBINATION WITH MEDICATED FEED
Table 6 shows the effect of spray dried plasma according to whether the piglets had been subjected to an experimental health challenge or not.It appears that plasma improves feed to gain ratio more markedly when the piglets are exposed to an experimental challenge.This could be indicative of a lower expenditure of energy and nutrients to build an immune response against the challenge.Studies in which SDPP has been tested under different health environments conclude that the benefits from plasma are significantly higher under lower health conditions, which supports this view.Coffey and Cromwell (1995) observed that weight gain and feed intake were clearly enhanced by SDPP under a conventional environment, but that the response to SDPP was much smaller when pigs were kept in a very clean environment, which resulted in a significant SDPP by environment interaction.Stahly et al. (1995) also tested the effect of adding 6% of SDAP in the diet with piglets that had been either reared under a conventional environment (high antigen exposure) or via a medicated early weaning scheme (low antigen exposure).They also observed a significant interaction between SDAP and environment so that SDAP improved weigh gain, feed intake and feed efficiency in high antigen exposed pigs but not in low antigen exposed pigs.Similarly, Bergstrom et al. (1997) also concluded that high health segregated early weaning pigs responded less to SDAP than pigs with a lower health status conventionally reared in an on-site nursery.Bregendahl et al. (1998) also observed that the addition of 5% SDAP in pre-starter feeds improved weight gain by 34% under a dirty environment, but only by 10% under a clean environment.For antibiotics, it has also been observed that they have little or no effect on growth when the animals are kept under clean conditions (Roura et al., 1992), which supports the hypothesis that the modes of action of antibiotics and SDAP may share similar mechanisms.
A more marked improvement in FGR in response to SDAP is also observed if feed does not contain medication (Table 7).It has to be noted, however, that in many of the trials considered in this review, animals submitted to an experimental challenge were also offered feed that was not medicated, and therefore both factors may be confused to some degree.Trials evaluating plasma and presence or absence of medication together have shown no interaction between the two factors, so that both effects are additive (Rojas et al., 1994;Coffey and Cromwell, 1995;Torrallardona et al., 2002;Bosi et al., 2004).Cain and Zimmerman (1997) observed that plasma was effective against rotavirus but not against E. coli, which could explain the additivity of plasma and antimicrobials.Other studies however, have shown tendencies or significant interactions between SDAP and antimicrobials (Torrallardona et al., 2003;Bikker et al., 2004).In most studies in which SDAP has been compared directly against antibiotics (Rojas et al., 1994;Coffey and Cromwell, 1995;Torrallardona et al., 2002Torrallardona et al., , 2003Torrallardona et al., , 2007;;Bikker et al., 2004;Bosi et al., 2004;Conde, 2005), organic acids (Owusu-Asiedu et al., 2003b;Torrallardona et al., 2007), other sources of immunoglobulins (Owusu-Asiedu et al., 2002, 2003a,b), plant extracts (Nofrarias et al., 2006), zinc oxide (Owusu-Asiedu et al., 2003b) or carbadox (Owusu-Asiedu et al., 2003b) SDAP results in superior or equivalent results.Srichana et al. (2004) observed that whereas the effect of SDAP occurs mainly in the first week post-weaning that of antibiotics persists for many weeks.The reduction of SDAP  efficacy beyond the third week post-weaning may be explained in part by the reduction in its level of inclusion.

MODE OF ACTION OF SPRAY DRIED ANIMAL PLASMA
Plasma is a protein source of high interest for pre-starter piglet diets.Most studies evaluating SDAP have shown to improve growth, feed intake and in some cases feed conversion (Table 2).Some authors claim that the beneficial effect of plasma results from the associated increase in feed intake, whereas others maintain that they are the result of the action of its specific bioactive components.Those supporting that the effect is mediated by an increase in the palatability of the diet and the corresponding increase in feed intake base their hypothesis on the observations of Ermer et al. (1994) for a higher preference for SDAP over dry skim milk feeds.However, Torrallardona and Solà-Oriol (2009) in a series of double free-choice tests, observed that SDPP preference was intermediate among fourteen common protein sources tested and in some cases SDPP's preference was much lower than that of protein sources (e.g.fishmeal) shown to result in worse productive results.This suggests that the increased feed intake observed for the SDAP containing diets is not a palatability mediated effect.It is possible that the higher feed intake is a consequence of the effect of these diets on the health of the piglets (Torrallardona et al., 2003).It is well known that macrophages secrete cytokines in response to antigenic stimuli, and that cytokines act in the brain to reduce feed intake (Johnson, 1997).Therefore the increased feed intake observed for the SDAP containing diets could be due to a reduction in the pro-inflammatory cytokines produced as a consequence of a lower antigenic load with SDAP containing diets.In pair feeding trials, piglets offered the same amount of feed with or without plasma have shown that the effects of plasma are independent of intake, which suggests a specific biological effect (Jiang et al., 2000b).In weaning pigs, feed intake has been shown to be closely correlated with the integrity of the gut mucosa (Pluske et al., 1997).Figure 4 shows the changes in villi and crypt morphology due to SDAP measured at different days postweaning obtained form different studies (Jiang et al., 2000b;Owusu-Asiedu et al., 2002, 2003a,b;Torrallardona et al., 2003Torrallardona et al., , 2007;;Conde, 2005;Nofrarias et al., 2006).SDAP clearly increases villus height (particularly in the first two weeks post-weaning), but its effect on crypt depth is not as clear.
The most widely accepted hypothesis for the mode of action for plasma is probably that its immunoglobulin content is biologically active against pathogens and enterotoxins.This was initially supported by studies in which plasma was proven to reduce the incidence in postweaning diarrhoea (Gatnau and Zimmerman, 1991;Peet Schwering et al., 1995;Cain and Zimmerman, 1997).Furthermore, studies testing SDPP under environments of different health status have shown that the benefits of plasma are significantly higher under poor sanitary conditions (Coffey and Cromwell, 1995;Stahly et al., 1995;Bergstrom et al., 1997;Bregendahl et al., 1998).Gatnau et al. (1995), Owen et al. (1995), Weaver et al. (1995) and Pierce et al. (2005) have separated plasma into high, medium and low molecular weight fractions, representing globulins, albumin and fibrin, respectively and compared their efficacy with that of SDAP.Their results show that the high molecular weight (immunoglobulin) fraction is the responsible for the beneficial effects of plasma.Godfredson-Kisic et al. (1999) showed that a diet with 2% porcine globulin concentrate, is as effective in maintaining growth and intake of recently weaned piglets as a diet with 8% plasma.Immunoglobulins keep their biological activity after processing the plasma (Gatnau et al., 1989).They can reach the proximal intestine of recently weaned piglets without being digested, most likely preserving their ability to bind to bacteria and virus (Morel, 1995).The destruction of SDAP antibodies by autoclaving has been shown to reduce its efficacy (Owusu-Asiedu et al., 2002), although it cannot be ruled out that other bioactive components in plasma could also be damaged with this procedure.Plasma contains antibodies against pathogenic bacteria such as enterotoxic E. coli (Owusu-Asiedu et al., 2002).Therefore, plasma immunoglobulins may provide antimicrobial protection, reduce the intestinal immune system activation and prevent mucosal damage by pathogenic bacteria in particularly susceptible animals such as the newly-weaned pig.It has been shown that plasma with guaranteed high levels of immunoglobulins is superior to conventional plasma (Bosi et al., 2001;Conde, 2005).In Bosi et al. (2001)'s study, piglets challenged with E. coli K88 and fed plasma with high IgG levels had a lower concentration of specific IgA against K88 in plasma and saliva.This suggests a protective effect against the adhesion E. coli K88, since IgA production against a specific bacteria, requires adhesion of this bacteria (using its fimbriae) to the enterocytes in the mucosal villi.Since IgG cannot be absorbed through the intestinal wall in 3-4 week old piglets, they have to act in the intestinal lumen.IgG bind to the virus or bacteria, avoid their union to the enterocytes and prevent the colonisation and damage of the intestinal wall.This is supported by the observations of Perez-Bosque et al. (2004) in challenged rats, who observed a lower activation of the intestinal immune system with the use of SDAP.It has also been hypothesised that the IgG against soyabean proteins present in SDAP, could help weaning piglets to adapt better to the weaning diet rich in highly antigenic soybean proteins.However this could not be demonstrated in a trial designed for this purpose (Hartke et al., 2003).
Glycoproteins have been proposed for being responsible for SDAP's effects instead of (or in addition to) immunoglobulins, by impeding antigen binding.Thus Sanchez et al. (1993) showed in vitro that certain glycoproteins obtained from plasma could act as a binding site for E. coli fimbriae, and Mouricout and Julien (1986) observed that bovine plasma glycoproteins inhibited E. coli adhesion to enterocytes.Subsequently in a study in calves, Mouricout et al. (1990), saw that glycoproteins obtained from bovine plasma inhibited intestinal adhesion of E. coli and protected the calves which had been deprived of colostrum against lethal doses of the bacteria.Nollet et al. (1999), in studies of piglets, used a plasma source without specific immunoglobulins for E. coli F18, and showed that it impeded E. coli F18 binding to enterocytes by receptor competition, which suggested a non-specific protection mechanism.Challenge studies in which diarrhoea was evaluated have shown that SDAP improves faecal scores (Owusu-Asiedu et al., 2003a,b;Conde, 2005), and body condition (Van Dijk et al., 2002a).Torrallardona et al. (2003) observed increased Lactobacilli counts in ileum and caecum of piglets fed SDAP, suggesting that SDAP promotes a beneficial microbiota.This observation however, could not be confirmed in posterior studies by the same group (Conde, 2005;Torrallardona et al., 2007).In experiments with piglets, Jiang et al. (2000b) saw that plasma increased efficiency in dietary protein utilisation and reduced intestinal mass, cell density between villi in the lamina propia and circulatory urea concentration, which the authors attributed to a minimisation in amino acid catabolism by the microflora and an increase in dietary amino acid availability for growth.According to Jiang et al. (2000a), SDAP consumption reduces dietary amino acid catabolism in the intestine, and increases the efficiency of dietary protein utilisation, enabling greater availability of dietary amino acids for growth of lean tissue.Touchette et al. (2002) showed a lower basal activation of the immune system in piglets fed SDAP than in piglets without SDAP.The same group of researchers described an increased activation of the hypothalamic-pituitary-adrenal (HPA) axis in SDAP-fed piglets following a challenge with LPS or E. coli (Touchette et al., 1999;Carroll et al., 2002).Carroll et al. (2002) proposed two mechanisms by which plasma prevents HPA axis activation.The first is a direct effect of plasma, preventing growth and colonisation of antigenic bacteria in the small intestine, by immunoglobulins.The second is an indirect effect of plasma on mucosal integrity, promoting intestinal growth, improving villi height and the villus height: crypt depth ratio as shown in previous studies (Spencer et al., 1997;Touchette et al., 1997).This would enable the animals to have a better barrier, which would prevent potential pathogens from crossing the intestinal wall.Both mechanisms would reduce HPA axis activation in piglets, reducing immunological stress, and immune system stimulation.Immune system activation has been shown to be associated with reduced intake and growth (van Heugten et al., 1994), due to the increase in pro-inflammatory cytokines which inhibit growth and intake (Johnson, 1997), and with a change in nutrient distribution, no longer used for skeletal muscle growth but for supporting the energy expenditure necessary for the immune system (Klaising and Johnstone, 1991).Bosi et al. (2004) confirmed a reduction in the production of the pro-inflammatory cytokines (tumour necrosis factor-α, interleukin-8 and interferon-γ) in the jejunum of SDAP-fed piglets challenged with enterotoxigenic E. coli (ETEC) K88.Therefore the reduction in immune system activation, which results from the consumption of diets with SDAP by piglets, would improve their intake and growth (Touchette et al., 2002).

CONCLUDING REMARKS
Plasma is a widely accepted ingredient for pre-starter piglet diets that has been shown to improve growth, feed intake and/or feed conversion.There is evidence suggesting that the improvement in feed intake is due to a reduction in the production of pro-inflammatory cytokines as a consequence of a lower antigenic load when SDAP is provided, rather than to an improvement in feed palatability.The superiority of SDAP over all the other protein sources it has been compared to (even those that have been shown to have equivalent or better nutritional value) is against a nutritionally driven effect.Most of the evidence provided suggests a specific action of the IgG or other bioactive substances in SDAP against porcine pathogens.IgG and other bioactive substances may prevent the binding of pathogens to the gut wall, reduce the activation of the immune system and reduce the incidence of diarrhoea in the post-weaning phase.The specificity of the IgG could explain the better efficacy of plasma from porcine origin over that of plasma from bovine or mixed origin.The suggested higher efficacy of SDAP for piglets weaned at 18-24 days of age (coinciding with a low protection from colostral antibodies and an immature immune system of the piglet), also supports an IgG mediated mode of action.Finally, as for antibiotics, SDAP efficacy is much higher under poor sanitary conditions (i.e.experimental challenge or absence of medication in the feed).In conclusion spray dried plasma appears to prevent the adhesion of pathogenic bacteria to the gut wall, reduces de activation of the immune system and the production of pro-inflammatory cytokines (thus avoiding a reduction in feed intake), and reduces the needs for energy and nutrients to build an immune response against the challenge, which results in better growth.For all this plasma can be postulated as an excellent alternative to in-feed antimicrobials for piglets in the post-weaning phase.

Figure 2 .Figure 3 .
Figure 2. Effect of feeding different doses of spray dried plasma to piglets on the improvement in average daily weight gain, average daily feed intake and feed to gain ratio over the control in the first week after weaning.Number of trials (n) = 95 for ADG, n = 96 for ADFI and n = 92 for FGR.

Figure 4 .
Figure 4. Effect of feeding spray dried plasma to piglets on the change in villus height, crypt depth and villus:crypt ratio relative to the control at different days post-weaning.Number of trials (n) = 30 for villi height, n = 30 for crypt depth and n = 22 for V:C ratio.

Table 1 .
Essential amino acid composition and apparent ileal digestibility of different spray dried plasma sources

Table 2 .
Summary of trials testing spray dried plasma as a protein source for piglets at weaning

Table 2 .
Summary of trials testing spray dried plasma as a protein source for piglets at weaning(Continued)

Table 2 .
Summary of trials testing spray dried plasma as a protein source for piglets at weaning(Continued)

Table 2 .
Summary of trials testing spray dried plasma as a protein source for piglets at weaning(Continued)

Table 2 .
Summary of trials testing spray dried plasma as a protein source for piglets at weaning(Continued)

Table 3 .
Performance improvement over the control diet of piglets fed with different sources of spray dried plasma for one or two weeks after weaning

Table 4 .
Performance improvement over the control diet according to the protein being replaced of piglets fed spray dried plasma for one or two weeks after weaning a, b Values in same column with different letters are significantly different as analysed by Tukey's (HSD) test (p<0.05).n: Number of trials.* p<0.05; † p<0.1.Statistical significance of improvement over controls without plasma.

Table 5 .
Effect of weaning age on the performance improvement over the control diet of piglets fed spray dried plasma for one or two weeks after weaning Values in same column with different letters are significantly different as analysed by Tukey's (HSD) test (p<0.05).n: Number of trials.* p<0.05.Statistical significance of improvement over controls without plasma.

Table 6 .
Effect of an experimental health-challenge to piglets on their performance response to spray dried plasma in the first one or two weeks after weaning

Table 7 .
Effect of feed medication on the performance response of piglets to spray dried plasma in the first one or two weeks after weaning Values in same column with different letters are significantly different as analysed by Tukey's (HSD) test (p<0.05).n: Number of trials * p<0.05; † p<0.1.Statistical significance of improvement over controls without plasma.