Introduction
Because of increasing public concern regarding the environment, agricultural has been looking for ways to eliminate, reduce, or control potential pollutants from manure (Summers, Reference SUMMERS1993; Carey et al., Reference CAREY, LACEY and MUKHTAR2004; Ritz et al., 2004). The poultry industry has been faced with environmental challenges related to manure disposal, as in the U.S. alone, more than 13 billion kg of poultry manure and litter are produced annually (Moore, Reference MOORE1998; Cabrera and Sims, Reference CABRERA and SIMS2000; Jackson et al., Reference JACKSON, SEAMAN and BERTSCH2006). Poultry manure is considered to be an optimal fertilizer because it contains large quantities of plant nutrients. Fifty percent of dietary nitrogen and 57% of phosphorous intakes from feed are excreted in broiler manure (Patterson et al., Reference PATTERSON, LORENZ E.S., WEAVER and SCHWARTZ1998), and commercial White Leghorn hens excrete approximately 65% of their nitrogen and 70% of phosphorous intakes (Patterson and Lorenz, Reference PATTERSON and LORENZ1996). Both nitrogen and phosphorous are important pollutants affecting surface and ground water quality.
Moore (Reference MOORE1998) indicated that potential environmental problems associated with poultry manure include leaching of substances, surface runoff of pollutants, and ammonia gas (NH3) volatilisation, which increases acid deposition in the air. NH3 volatilisation in poultry manure can cause human and animal health problems, air pollution, and eutrophication in water courses. The majority of NH3 from poultry manure is lost to the air while the litter and manure are retained in the chicken house (Moore, Reference MOORE1998). Sims and Wolf (Reference SIMS and WOLF1994) reported that over 50% of poultry manure N might be volatilised via NH3. Patterson and Lorenz (Reference PATTERSON and LORENZ1996) also reported that approximately 40% of feed N is lost to the atmosphere.
Atmospheric ammonia accumulation in poultry houses reduces growth performance, egg production, resistance to various diseases, and immune responses (Kling and Quarles, Reference KLING and QUARLES1974; Reece et al., Reference REECE, LOTT and DEATON1980; Caveny et al., Reference CAVENY, QUARLES and GREATHOUSE1981; Nagaraja et al., Reference NAGARAJA, EMERY, JORDAN, SIVANANDAN, NEWMAN and POMEROY1983; Deaton et al., Reference DEATON, REECE and LOTT1984; Nagaraja et al., Reference NAGARAJA, EMERY, JORDAN, SIVANANDAN, NEWMAN and POMEROY1984; Baker et al., Reference BAKER, VANHOOSER, SWARTZLANDER and TEETER2004; Miles et al., 2004; Yahav, Reference YAHAV2004; Olanrewaju et al., Reference OLANREWAJU, MILLER, MASLIN, THAXTON, DOZIER, PURSWELL and BRANTON2007; Wang et al., Reference WANG, MENG, GUO, WANG, WANG, YAO and SHAN2010). Yahav (Reference YAHAV2004) evaluated the effects of different atmospheric ammonia levels at high ambient temperature and relative humidity on the performance and body temperature regulation in broiler chickens. The results revealed that body weight and feed intakes significantly decreased as atmospheric NH3 concentrations increased from 16 to 54 ppm, and that broiler chickens at the lowest NH3 concentration were more able to maintain their body temperature compared to those at the highest concentrations. These results suggest that atmospheric ammonia concentrations affect growth performance and thermoregulation of broiler chickens. Miles et al. (2004) evaluated the effects of different atmospheric ammonia levels (0, 25, 50, and 75 ppm) on broiler performance. Body weight at seven weeks of age was significantly reduced by the 50 and 75 ppm levels of ammonia compared to 0 ppm, and mortality was significantly increased at the 75 ppm ammonia level (13.9%) compared to the 0 ppm level (5.8%). This study showed that atmospheric ammonia is detrimental to the growth performance of broiler chickens. Wang et al. (Reference WANG, MENG, GUO, WANG, WANG, YAO and SHAN2010) reported that atmospheric NH3 depressed not only growth performance but also immune responses. Both body weight and feed efficiency were greatly reduced when broiler chicks were exposed to 52 ppm ammonia. Furthermore, the haemagglutination inhibition antibody titre for Newcastle Disease Virus (NDV) was significantly lower when broiler chickens were exposed to the 26 or 52 ppm ammonia. Therefore, reducing atmospheric ammonia volatilisation is critical to maintain animal health and growth performance.
Although several strategies have been suggested to reduce ammonia volatilisation in poultry manure, production and utilisation of egg yolk antibodies (IgY) specific to uricase represents a potential approach. Therefore, the focus of this review is to discuss approaches to inhibiting microbial uricase activity by an egg yolk antibody (IgY) specific to microbial uricase in order to reduce NH3 volatilisation in poultry manure.
Strategies for reducing ammonia volatilisation from poultry manure
The process of ammonia volatilisation is enhanced by microbial activity. There are five enzymatic steps in the aerobic degradation of uric acid (Bachrach, Reference BACHRACH1957; Bongaerts and Vogels, Reference BONGAERTS and VOGELS1976; Bongaerts et al., Reference BONGAERTS, UITZETTER, BROUNS and VOGELS1978; Hunde et al., Reference HUNDE, PATTERSON, RICKE and KIM2012). Firstly, uric acid in poultry manure is decomposed to allantoin by microbial uricase. Secondly, allantoin is converted into allantoic acid by allantoinase. Thirdly, allantoic acid is converted into ureidoglycolate by allantoate amidohydrolase, and then ureidoglycolate is converted into glyoxylate and urea by ureidoglycolase. Finally, urea is hydrolyzed into ammonia and CO2 by urease. Inhibiting any one of these microbial enzymes could be a useful block to NH3 formation in poultry manure.
After ammonia is generated by microbial processes, pH is a major factor regulating the equilibrium between ammonium ion (NH4+) and ammonia (NH3) (Kirchmann and Witter, Reference KIRCHMANN and WITTER1989). When manure pH increases, the ammonium ion is converted to ammonia gas (Moore, Reference MOORE1998) and easily volatilised to the air. Thus, the conversion of ammonia to other chemical forms, which are not volatilised, could reduce ammonia emission from poultry manure.
Blocking critical enzymes in bacterial uric acid degradation to reduce ammonia volatilised from poultry manure is the critical step for limiting ammonia production and for producing antibodies specific to the key microbial enzymes as feed additives.
Antibody inhibition of microbial enzymes in animal hosts
Previous research has shown that it is possible to inhibit microbial enzymatic activity in animal hosts with antibodies. Immunisation of rats, chicks, or pigs against urease was found to promote growth (Dang and Visek, Reference DANG and VISEK1960; Visek, Reference VISEK1962; Glimp and Tillman, Reference GLIMP and TILLMAN1964). Ammonia produced by microbial urease in the gastrointestinal tract acts as a toxin that can suppress growth performance of the animal (Visek, Reference VISEK1962). Often, urease-immunised animals have greater body weight than the controls. Dang and Visek (Reference DANG and VISEK1960) reported that rats and chicks immunised against crystalline jackbean urease were significantly heavier and had better feed efficiency compared to controls. Immunised animals in an in vivo study of 14C urea metabolism expired 2.58% of radioactivity as 14CO2, whereas control animals expired substantially more (4.31%). These results indicated that the antibody specific to urease inhibited the activity of microbial urease, reducing ammonia production in the gastrointestinal tract.
Similarly, these antibodies have the potential to inhibit uricase activity in poultry manure and reduce ammonia volatilisation. There are several ways to utilise an uricase-specific antibody, including direct treatment of poultry manure with the antibody, supplementation of the antibody to poultry diets, or direct uricase immunisation of birds. Choosing which approach fits best depends upon several factors. Primary issues that influence choice are the ease of specific and effective antibody generation as well as the economics of mass production of large quantities and the shelf life stability of the antibody preparation. In addition, delivery of the antibody preparation to the target site must be management friendly, if it is to be commercially feasible. Given these concerns direct incorporation of antibodies into manure represents several logistical problems that preclude routine implementation. Immunisation of the birds represents a potential control measure at the intestinal tract level, but does not ensure inhibition in the manure. Supplementation of antibodies into the feed solves both problems by initial limitation of microbial activity in the bird, as well potential inhibition of uricase activity in the manure by birds excreting antibodies. However, for antibody supplementation in feeds to be effective, the source of antibody must be sufficiently stable to withstand environmental conditions associated with feed processing and storage as well as the potentially destructive conditions associated with the digestive processes in the intestinal tract of the bird (Berghman et al., Reference BERGHMAN, ABI-GHANEM, WAGHELA and RICKE2005).
Antibody production in the hen
Sources of antibody typically used by research laboratories include mammalian monoclonal and polyclonal antibodies. Frequently, rabbits have been used to produce useful antibodies. However, in recent years, the production of antibodies in chickens and the extraction of specific antibodies from egg yolk are increasingly attracting the interest of the scientific community (Schade et al., Reference SCHADE, HLINAK, MARBURGER, HENKLEIN, GERL, ZOTT, PFISTER and ERHARD1997, Reference SCHADE, CALZADO, SARMIENTO, CHACANA, PORANKIEWICZ-ASPLUND and TERZOLO2005).
Hens transfer antibodies to their egg yolk to provide immunity to their offspring similar to the placental transfer of antibodies in mammals (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993; Mahdavi et al., Reference MAHDAVI, RAHMANI, NILI, SAMIE and SOLEIMANIAN-ZAD2010). The kind of immunoglobulin isotype found in the egg yolk is IgY, equivalent to IgG in mammalians (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993; Chalghoumi et al., Reference CHALGHOUMI, THEWIS, PORTETELLE and BECKERS2008; Mahdavi et al., Reference MAHDAVI, RAHMANI, NILI, SAMIE and SOLEIMANIAN-ZAD2010). While all antibody isotypes are secreted into the egg, only IgY is found in the follicle. The serum IgY is transferred to the egg yolk by active transport during oogenesis (Rose and Orlans, Reference ROSE and ORLANS1981; Mahdavi et al., Reference MAHDAVI, RAHMANI, NILI, SAMIE and SOLEIMANIAN-ZAD2010). Although, egg yolk antibodies immunologically represent the same pattern as the serum antibodies, they will have a delayed appearance of approximately one week (Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986). IgG, not IgM or IgA, is selectively deposited in the egg yolk from the circulating plasma pool (Malkinson, Reference MALKINSON1965; Orlans, Reference ORLANS1967; Faith and Clem, Reference FAITH and CLEM1973). Egg yolk IgY retains its plasma characteristics, and occurs in concentrations similar to those of plasma IgY (Patterson et al., Reference PATTERSON, YOUNGNER, WEIGLE and DIXON1962a; Reference PATTERSON, YOUNGNER, WEIGLE and DIXON1962b; Rose and Orlans, Reference ROSE and ORLANS1981). IgM and IgA are also found in the egg but are only present in the albumin (Tizard, Reference TIZARD1992).
Avian antibodies have been recognised for several decades and offer several biochemical advantages over mammalian antibodies. These advantages are due to phylogenetic differences between avian and mammalian species and can result in increased sensitivity as well as decreased background in immunological assays (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993). Egg yolk IgY is a major immunoglobulin from birds, amphibians and reptiles. Interestingly, IgY does not activate mammalian complement system, and does not bind to mammalian rheumatoid factors, protein A or G. In addition, it has a high yield, and is easy to extract and purify (Cai and Chen, Reference CAI and CHEN2009). IgY plays a major role in antibody-mediated defence mechanisms of tissue and skin surfaces; they can agglutinate and precipitate antigen (Tizard, Reference TIZARD1992).
Immunisation and antibody purification
Numerous studies have reported that hens are excellent model animals for producing specific antibodies against different protein sources. Since immunisation and preparation of antisera tends to be expensive and not always routine, the presence of IgY in the egg has led different researchers to propose chicken egg yolk antibodies as an alternative source of diagnostic polyclonal antibodies (Bar-Joseph and Malkinson, Reference BAR-JOSEPH and MALKINSON1980; Polson et al., Reference POLSON, VON WECHMAR and REGENMORTEL1980; Ricke and Schaefer, Reference RICKE and SCHAEFER1990; Biswas et al., 2010). Gassmann et al. (Reference GASSMANN, THOMMAS, WEISER and HUBSCHER1990) immunised six 22 to 24-wk-old laying hens with proliferating cell nuclear antigen. The authors reported that 20 to 30 μg of antigen were enough to produce specific polyclonal antibodies 20 d after immunisation. Plateau antibody levels in the egg were reached after 30 d and maintained at high levels to 81 d. One hen produced approximately 4 g of antibodies and 30 mg of specific antibodies isolated from 62 eggs.
Chicken antibody production brings great benefit concerning the welfare of the immunised animals, due to non-invasive antibody harvesting with the added convenience of simple egg collection. After immunisation, the specific antibodies are transported to the egg yolk, from which the IgY then can be separated without sacrificing chickens. An additional advantage is the rapid and simple IgY isolation from egg yolk. IgY technology is a great improvement and should be considered as a good alternative to conventional polyclonal antibody production in mammals. (Tini et al., Reference TINI, JEWELL, CAMENISCH, CHILOV and GASSMANN2002). With these advantages, IgY has been widely used in veterinary science, functional food and bio-products, and proved potential application for disease control and treatment (Cai and Chen, Reference CAI and CHEN2009).
There are three inoculation methods for immunisation, which include intramuscular (IM), intraperitoneal (IP), and intravenous (IV) administration. IP and IV yield the faster titre response with peak immune response occurring between seven and nine days followed by a decline in concentration until minimal levels are reached on the 28th day. A booster response has not been noticed though a second injection will give essentially the same response as the first. IM injections show an increase in titre levels if there is a second injection on the 28th day (Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986). Moreover, improving the quality of the antigens and optimising immunisation protocols will make IgY more effective and feasible for research and commercial antibody applications (Schade et al., Reference SCHADE, CALZADO, SARMIENTO, CHACANA, PORANKIEWICZ-ASPLUND and TERZOLO2005).
Crude egg yolk may be used as an antibody source, but the lipids in the yolk may interfere with the antibody activity (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993). Because of this, antibodies are usually purified from the yolk. There are several published purification methods that can be used for large scale purification of chicken antibodies (Polson et al., Reference POLSON, VON WECHMAR and REGENMORTEL1980, Reference POLSON, COETZER, KRUGER, VON MALTZAHN and VAN DER MERWE1985; Fertel et al., Reference FERTEL, YETIV, COLEMAN, SCHWARTZ, GREENWALD and BIANCHINE1981; Jensenius et al.,1981; Hu et al., Reference HU, ROCHOW and DIETERT1985; Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986; Akita and Nakai, Reference AKITA and NAKAI1992; Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993; Meulenaer and Huyghebaert, Reference MEULENAER and HUYGHEBAERT2001; Kim and Patterson, Reference KIM and PATTERSON2003).
There are numerous advantages of egg yolk polyclonal antibodies over other sources of antibody. They can be obtained daily without bleeding the immunised hen (Polson et al., Reference POLSON, VON WECHMAR and REGENMORTEL1980; Karlsson et al., Reference KARLSSON, KOLLBERG and LARSSON2004). The advantages to working with IgY antibodies include new insights into the IgY transfer mechanism from blood to yolk as a biological basis for the technology; and the high efficacy of the technique, in view of the extraordinary amount of IgY produced by one hen in one year (between 30 g and 40 g in total) (Schade et al., Reference SCHADE, CALZADO, SARMIENTO, CHACANA, PORANKIEWICZ-ASPLUND and TERZOLO2005). A chicken usually lays about 280 eggs in a year, and egg yolk contains 100-150 mg of IgY per yolk, suggesting that more than 40 g of IgY per year can be obtained from each chicken through eggs (Mine and Kovacs-Nolan, Reference MINE and KOVACS-NOLAN2002). To establish this kind of system for the efficient production of IgY, it is important to consider that hen strains/breed affects the total production of specific IgY because the weight of egg yolk and number of egg production are different between the strains (Li et al., Reference LI, NAKANO, SUNWOO, PAEK, CHAE and SIM1998). In addition, egg yolk antibodies, when stored as an intact egg retain activity for at least six months (Brambell, Reference BRAMBELL1970; Brandly et al., Reference BRANDLY, MOSES and JUNGHERR1946).
In addition, untrained personnel can easily collect eggs at little expense (Polson et al.,1980; Karlsson et al., Reference KARLSSON, KOLLBERG and LARSSON2004) and minimal physiological stress or pain is involved in egg gathering compared with blood sampling of mammals (Schade et al.,1991; Karlsson et al., Reference KARLSSON, KOLLBERG and LARSSON2004). Finally, because commercial laying hens could provide economic and high amounts of valuable antibodies, commercial egg production facilities potentially offer attractive options for the commercialisation of antibody production (Polson et al., Reference POLSON, VON WECHMAR and REGENMORTEL1980; Hassl et al., Reference HASSL, ASPOCK and FLAMM1987; Swendsen et al., Reference SWENDSEN, CROWLEY, OSTERGAARD, STODULSKI and HAU1995; Meulenaer and Huychebaert, 2001).
Stability of IgY against gastrointestinal enzymes
When egg yolk antibodies are included in feed, the stability of IgY in the gastrointestinal tract is very important. The feasibility of IgY delivery by oral administration has been evaluated by several laboratories. Shimizu et al. (Reference SHIMIZU, FITZSIMMONS and NAKAI1988) found IgY to be stable up to 70°C and determined that freezing and freeze-drying did not influence its activity and IgY was stable at pH 4 or above, whereas it rapidly decreased below pH 4. Heavy and light chains of IgY were more labile to pepsin compared to bovine IgG, however IgY was more resistant to chymotrypsin and trypsin exposure than bovine IgG. Although IgY has good stability for temperature and some intestinal enzymes, it is still susceptible to pepsin digestion and low pH. However, some studies have shown that dietary IgY supplements retained activity after consumption and inhibited the harmful effects of antigen proteins (Bartz et al., Reference BARTZ, CONKLIN, TUNSTALL and STEELE1980; Hatta et al., Reference HATTA, MABE, KIM, YAMAMOTO, GUTIERREZ and MIYAZAKI1994).
Bartz et al. (Reference BARTZ, CONKLIN, TUNSTALL and STEELE1980) evaluated the prevention of a murine rotavirus infection with chicken egg yolk antibodies. IgY was isolated from the eggs of rotavirus-immunised hens by water extraction, and then the water-soluble fraction was fed to three-day-old mice twice a day to protect them from infection with the murine rotavirus. The results indicated that the murine rotavirus infection rate decreased to 15% among mice fed the IgY fractions.
Pimentel (Reference PIMENTEL1999) reported that the pigs fed a diet containing spray-dried eggs from urease-immunised hens had a significant improvement in feed conversion, and broilers fed freeze-dried eggs containing anti-urease antibodies increased weight gain significantly more so than the controls. The data suggest that IgY could still retain its activity in the gastrointestinal tract and impart benefits in animal performance and disease prevention.
Commercialisation potential of egg yolk antibodies
A modern commercial laying hen almost produces an egg per day. With each egg yolk containing a volume of approximately 15 ml and retaining similar antibody reactivity and concentration to serum antibodies (Patterson et al., Reference PATTERSON, YOUNGNER, WEIGLE and DIXON1962a; Reference PATTERSON, YOUNGNER, WEIGLE and DIXON1962b; Rose and Orlans, Reference ROSE and ORLANS1981), this is equivalent to almost 100 ml of serum per week. If each egg yolk yielded approximately 10 ml of a purified antibody preparation, and 100 µl of a 100-fold dilution of this preparation is needed for each reactant well in a standard enzyme-linked immunosorbant assay (ELISA), then one egg would supply sufficient antibodies for 10,000 wells (Ricke et al., Reference RICKE, SCHAEFER, COOK and KANG K.H.1988). This could be compared to an immunised rabbit, which yields approximately 20 ml whole blood per week when repeatedly bled, and thus comparatively limited amounts antibody can be produced by each rabbit (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993). Furthermore, if initial cost of the animal, daily cost of feed and care, and time required to generate the desired antibodies are taken into consideration, the cost of feeding and handling is considerably less for hens than rabbits (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993). Specifically, rabbits are only useful at peak antibody production because of the stress induced by repeated bleeding and for this reason they are often exsanguinated (the collection of all blood to the point of death) at their titre peak. Eggs provide a constant source of antibodies not only for daily monitoring of when peak titres occur, but have been shown in limited studies to retain this peak for up to 200 days after inoculation (Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986). The possibility of purifying large amounts antibodies is of particular interest for oral administration of specific antibodies to prevent bacterial infections (Larsson et al., Reference LARSSON, BALOW, LINDAHL and FORSBERG1993). Egg antibodies can be utilised for disease determination and structure determination as effectively as mammalian antibodies (Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986). Historically, eggs have been widely reputed to be effective as passive antibody sources when used in a prophylactic manner; e.g. farmers using raw eggs as an antidiarrheal in young calves (Losch et al., Reference LOSCH, SCHRANNER, WANKE and JURGENS1986). More recently, oral administration of egg yolk antibodies has also been successively used to prevent rotavirus infections in mice (Ebina et al., Reference EBINA, TSUKADA, UMEZU, NOSE, TSUDA, HATTA, KIM and YAMAOTO1990), Escherichia coli infections in rabbits (O'Farrelly et al., Reference O'FARRELLY, BRANTON and WANKE1992) and pigs (Wiedemann et al., Reference WIEDEMANN, KUHLMANN and LOSCH1990) and caries in rats (Hamada et al., Reference HAMADA, HORIKOSHI, MINAMI, KAWABATA, HIRAOKI, FUJIWARA and OOSHIMA1991; Otake et al., Reference OTAKE, NISHIHARA, MAKIMURA, HATTA, KIM, YAMAOTO and HIRASAWA1991).
Production of the uricase-specific egg yolk antibody
Kim and Patterson (Reference KIM and PATTERSON2003) evaluated the production of the uricase-specific egg yolk antibody from immunised hens, the purity of IgY isolated and purified by the different purification methods and the inhibitory effects of the uricase-specific IgY on the activity of uricase. A total of 12 single comb White Leghorn hens were injected intramuscularly with uricase from Arthrobacter globiformis. The hens were immunised a second and third time at one and two weeks, respectively, after the initial injection. The production of uricase-specific IgY was first detected at two weeks after the initial immunisation, and levels increased more than three-fold at four weeks. The purification methods included water extraction, ammonium sulphate precipitation, and ethanol precipitation exhibited the most acceptable purity with over 97% uricase-specific IgY recovery. Finally, the effects of IgY specifically on the activity of microbial uricase were determined by an in vitro assay. The IgY reduced uricase activity by 58% compared to the control treatment. These results suggested that eggs from uricase-immunised hens are a good source for producing high levels of uricase-specific antibodies, and uricase-specific IgY has the potential to be used as an inhibitor in order to reduce ammonia emissions in poultry manure. Further research is needed to demonstrate the effect of uricase-specific IgY on ammonia volatilisation in poultry manure by supplementing isolated uricase-specific IgY into poultry diets.
Adrizal et al. (Reference ADRIZAL, PATTERSON and CRAVENER2007) examined whether immunisation of broiler breeder hens with uricase, urease, or combination of uricase and urease on IgY titres against these antigens and nitrogen retention in hen manure could occur. Hens were immunised with uricase, urease, or both in the breast muscle on days 0, 7, and 14. Serum and egg yolk IgY titres were measured at 4, 9, 12, 17, 21, and 24 days after immunisation. Increased uricase-IgY levels in egg yolk were detected after the 2nd injection and maintained throughout the analysed period. Egg yolk urease-IgY levels were enhanced on days 17, 21, and 24 compared to the control and high levels of uricase- or urease-IgY in serum were observed as well. In addition, the combination of immunisation against uricase and urease significantly enhanced IgY levels compared to the control, but were less than the individual antigen immunisation. Moreover, they found that manure total nitrogen levels were significantly increased when hens were immunised with the uricase and urease in combination.
Conclusions
The use of hen egg yolk antibodies should be an easily attainable, practical alternative and/or substitute for rabbit serum polyclonal antibodies. Egg yolk antibodies are not only inexpensive to acquire, they are more ethically correct in that they cause little or no harm to the animal during collection. However, they are not currently in wide use among researchers and have not been considered commercially. In order to gain acceptance and achieve viable commercialisation, several objectives need to be addressed:
1) Can egg yolk antibodies be made that are as specific as other polyclonal antibodies?
2) What is the duration of specific antibody production after immunisation?
3) How much does the immune response vary from hen to hen?
In addition, further studies are necessary to prove the effects of feed-grade specific uricase antibody on microbial uricase activity, ammonia emission, and nitrogen retention in in vitro assays with poultry manure and feeding trials with broilers and hens.
Acknowledgements
This review was supported by Manitoba Egg Farmers, Natural Science and Engineering Research Council (NSERC-discovery), and Agri-Food Research and Development Initiative (ARDI) and in part (S.C. Ricke) by Hatch Grant H8311 administered by the Texas Agricultural Experimental Station and Texas Advanced Technology program grant #000517-0220-2001 (Texas Higher Education Board, Austin, TX) as well as a USDA Food Safety Consortium grant (University of Arkansas).
