Materials and Methods

Animals—All Thoroughbred foals that were born to Thoroughbred mares from which presuckle colostrum samples were collected at a commercial horse-breeding farm in Victoria, Australia, from 2007 to 2008 and had sIgG concentration measured at 24 hours after parturition were eligible for inclusion in the study. Information for each mare, including age, parity, and history of giving birth to foals with NI, was obtained from records included in the Australian Studbook Database.^a Foals born to mares that had a history of offspring with NI were excluded from the study. Mares that did not reside at the facility were transported to the farm at least 4 weeks prior to the expected parturition date. Pregnant mares were vaccinated against equine herpesviruses 1 and 4 in the fifth, seventh, and ninth month of gestation and Clostridium tetani 4 weeks prior to the expected parturition date. Mares were housed on pasture with access to alfalfa-grass hay and grain mix. Mares that developed signs of imminent parturition were confined in 0.25-acre paddocks. Signs included enlargement and rounding of the udder, appearance of wax at the teat ends, relaxation of the vulva and elongation of the opening, decrease in appetite, and change in behavior or attitude of the mare. All births were observed by farm staff, and veterinary medical intervention was sought as necessary.

Owner consent was obtained prior to inclusion of mares and foals in this study. All procedures were conducted in accordance with the relevant legislation and codes of veterinary practice including the Veterinary Surgeons Act of Australia and the Australian code of practice for the care and use of animals for scientific purposes.

Colostrum collection, evaluation, and storage—Postpartum presuckle colostrum samples were obtained from all mares and analyzed by use of a sugar refractometer^b (cRI upper limit of detection, 32%). For all multiparous mares that produced high-quality colostrum with a cRI value > 23% and had no history of NI in previous offspring, 300 mL of colostrum was collected, placed in a labeled 300-mL plastic bottle, and frozen at −20°C (−4°F). Bottles containing colostrum remained frozen for a variable period (storage duration, ≥ 7 days to < 2 years) prior to use for treatment of foals at risk for FTPI. Volumes of colostrum were not collected from primiparous mares for storage, regardless of the cRI values.

Treatment of foals at risk for FTPI with stored colostrum—Standard guidelines¹² were followed for the treatment of all foals of dams that produced postpartum presuckle colostrum samples with a cRI value < 20%. Additionally, other foals that were considered to be at risk for FTPI as a result of weakness or poor suckling ability of the foal or because the mare did not allow the foal to nurse were treated with stored colostrum. Frozen colostrum was thawed in a 38°C (100.4°F) water bath prior to administration via a nasogastric tube. A volume ≥ 300 mL of thawed colostrum was administered 1 to 4 times within the first 6 hours after parturition in an attempt to ensure an adequate sIgG concentration in foals at risk for FTPI.

Management of NI—Pregnant mares were not tested for Aa or Qa antigens, which are commonly associated with cases of NI. Any pregnant mares that had a history of NI-affected offspring were not included in the study; additionally, their colostrum was stripped and discarded during a 24-hour prepartum period. After parturition, foals born to those mares were treated with 500 mL of stored colostrum that was thawed and administered via a nasogastric tube (every 2 hours for a 24-hour period) and muzzled prior to reintroduction to the mare. After collection, evaluation, and storage of colostrum from multiparous mares without a history of NI-affected offspring, colostrum was stored for a period of not less than 7 days prior to use to allow for the identification of foals with NI. If NI developed in a foal of a mare with an unknown history of NI, then the colostrum from that dam was removed from storage and discarded.

Assessment of sIgG concentration in foals—Blood samples were collected from foals for sIgG concentration measurement at approximately 24 hours after parturition. A 10-mL blood sample was collected from a jugular vein by use of a syringe and 21-gauge, 25-mm hypodermic needle and transferred into an evacuated tube containing no anticoagulant.^c Each sample was allowed to clot to separate the serum and cellular fractions without centrifugation. The sIgG concentration in each sample was measured on-site by the same technician by use of a spectrophotometer^d (upper limit of detection, 3,000 mg/dL). All sIgG concentrations were measured ≤ 1 hour after sample collection. Measurement of sIgG concentration was performed according to the manufacturer's recommendations.

Statistical analysis—Data analyses were performed by use of a software program.^e The proportions of foals with an sIgG concentration < 800 mg/dL at 24 hours after parturition among offspring of primiparous and multiparous mares were compared by use of the Fisher exact test and calculation of relative risk with 95% confidence intervals. For the calculation of descriptive statistics, colostrum samples with a cRI measurement > 32% were assigned a value of 32%, and serum samples from foals with sIgG concentration > 3,000 mg/dL were assigned a value of 3,000 mg/dL. A Pearson correlation coefficient was calculated between sIgG concentration (response variable) and cRI value based on foals that were not treated with stored colostrum and mares that had a cRI value not greater than the upper limit of detection of the sugar refractometer.^b A value of P < 0.05 was considered significant for all analyses.

Results

Mares and foals—During the study period, 232 Thoroughbred foals were born to 191 Thoroughbred mares. Seven foals were excluded from the study because a postpartum presuckle colostrum sample was not obtained from each of their dams. One additional foal was excluded from the study because its dam had a history of offspring with NI; however, NI was not a condition observed in foals enrolled in the study reported here. Thus, 224 Thoroughbred foals and 184 Thoroughbred mares were included in the analyses; 41 of these foals were born to primiparous mares (17 of which were from primiparous mares in 2007). Forty mares each gave birth to 1 foal in 2007 and another in 2008. Parity was not recorded for 1 mare.

Postpartum presuckle cRI values—The range of cRI values for 224 samples was 11 to 32 (mean ± SD, 25 ± 5%; median, 25%). The minimum recorded cRI value was 11%, which was detected in 3 colostrum samples (from 2 primiparous and 1 multiparous mare; Table 1). In 22 (9.8%) samples, the cRI value exceeded the upper limit of detection of the spectrophotometer (> 32%); for the purposes of analysis, these samples were assigned a value of 32%. The quality of 27 (12%) colostrum samples was considered inadequate (cRI value, < 20%).

Treatment of foals at risk for FTPI—Among the 224 foals, 27 were considered at risk for FTPI on the basis of the quality of their dams' colostrum. Another 3 foals were considered at risk for FTPI because of poor suckling ability of the foal or because the mare did not allow the foal to nurse. Overall, 30 (13%) foals were treated with stored colostrum. All foals (n = 5) of dams that produced colostrum with a cRI value < 15% were individually treated with 800 to 1,500 mL of colostrum that had a cRI value of 30%. Although standard guidelines were to treat all offspring of dams that produced colostrum with a cRI value < 20%, only 20 of 22 foals of dams that produced colostrum with cRI values ranging from 15% to 19% were treated. Those foals each received 300 to 1,000 mL of colostrum that had cRI values ranging from 24% to 32%. Two of the foals were not treated by farm staff; subsequently, PFTPI was diagnosed in those 2 foals. Furthermore, 5 foals born to mares that produced colostrum with cRI values ranging from 20% to 32% were treated; those foals received 400 to 500 mL of stored colostrum that had cRI values ranging from 24% to 30%.

sIgG concentrations in foals—At approximately 24 hours after parturition, the range of sIgG concentrations for all (treated and nontreated) foals was 340 to 3,000 mg/dL (mean ± SD, 1,597 ± 574 mg/dL; median, 1,491 mg/dL). The range of sIgG concentrations for nontreated foals (n = 194) was 340 to 3,000 mg/dL (mean, 1,624 ± 560 mg/dL; median, 1,551 mg/dL). For foals (n = 172) that were born to mares that produced colostrum with a cRI value ≤ 32% and not treated with stored colostrum, a significant correlation (r = 0.41; P < 0.001) was detected between the cRI value of mare colostrum and sIgG concentration of foals and was described by an equation as follows:

View Expanded

where the SEs for the values −65.0 and 64.6 were 285 and 11.1, respectively. The quadratic coefficient for this equation was not significant (P = 0.38).

The minimum sIgG concentration detected was 340 mg/dL in 1 foal, and 2 foals each had an sIgG concentration > 3,000 mg/dL (Figure 1). Overall, 13 (5.8%) foals had an sIgG concentration < 800 mg/dL. The incidence of FTPI in all foals (treated and nontreated) was 0.4% (1/224); in the foal with FTPI, the sIgG concentration was 340 mg/dL. The incidence of PFTPI was 5.4% (12/224); sIgG concentrations in those foals ranged from 605 to 774 mg/dL. Because their dams produced colostrum of inadequate quality (cRI, 15% to 19%), 2 foals were treated with stored colostrum (400 to 450 mL); however, PFTPI was diagnosed in both foals. A foal of a dam that produced high-quality (cRI, 32%) colostrum was allowed to suckle; a diagnosis of PFTPI was subsequently made for that foal.

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Scatterplot of the sIgG concentration at 24 hours after parturition in 224 Thoroughbred foals versus cRI value of postpartum, presuckle colostrum samples collected from their respective Thoroughbred dams (184 mares; 40 Thoroughbred mares each gave birth to 1 foal on 2 occasions during the study period). Foals born to mares that produced poor-quality colostrum (cRI value, < 20%) or that were considered at risk for FTPI because of other foal- or mare-related factors were administered ≥ 300 mL of stored equine colostrum (cRI value, > 23%) via nasogastric tube 1 to 4 times in the 6-hour period after parturition (crosses). Other foals were not treated with stored colostrum (circles). Serum samples were evaluated with a spectrophotometer.^d The upper and lower horizontal lines indicate sIgG concentrations of 800 and 400 mg/dL, respectively.

Citation: Journal of the American Veterinary Medical Association 236, 10; 10.2460/javma.236.10.1085

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Foals born to primiparous mares were not found to be at increased risk for an sIgG concentration < 800 mg/dL at 24 hours after parturition. An sIgG concentration < 800 mg/dL at 24 hours after parturition was detected in 3 of 41 (7.3%) foals born to primiparous mares and in 10 of 182 (5.5%) foals born to multiparous mares. The relative risk was 1.3 (95% confidence interval, 0.4 to 4.6; P = 0.71).

Five foals were treated for lethargy that was thought to be related to sepsis. Five foals were treated for lameness, and 1 of these foals was hospitalized for lavage of the metacarpophalangeal joint of the left hind limb. Two foals were treated for diarrhea and 1 for omphalophlebitis. Concurrent disease was not detected in any mare.

Discussion

The most common immunodeficiency disorder of foals is FTPI. This disorder develops secondary to ingestion of insufficient quantities of colostrum, ingestion of colostrum with insufficient immunoglobulin content, failure to absorb immunoglobulins from the gastrointestinal tract, or catabolism of circulating immunoglobulins in neonatal foals that are ill.⁹ The frequency of FTPI and PFTPI in foals will vary depending on management conditions.^2,13 The reported incidence of FTPI in Australia is approximately 10%³ to 16%² and is similar to the rates of approximately 13%^14,15 and 18%¹⁶ in the United States and 15%¹⁰ in the United Kingdom.

Because of the range of reported^{2,3,10,13–15} incidence rates for FTPI in foals (2.9% to 16%), we investigated a farm-use protocol for minimizing FTPI. The comparatively low incidence of FTPI in the present study may be a reflection of the early detection of foals at risk for FTPI and treatment of those foals with stored colostrum. Other investigators have identified^12–14,17 a significant relationship between the IgG content of colostrum and the sIgG concentration of foals following ingestion of that colostrum; additionally, this relationship was also significant in the study reported here.

Although management procedures on the farm in the study reported here were considered excellent, a diagnosis of FTPI was made in 1 of the 224 foals, and a diagnosis of PFTPI was made in each of 12 foals. Of these 13 foals, 9 were not considered to be at risk for FTPI; therefore, they were not treated with stored colostrum. According to the protocol set by standard guidelines,¹² 2 foals should have been treated because their dams each produced colostrum of inadequate quality (cRI value, 15% to 19%). Furthermore, 2 foals were treated with an insufficient quantity of the stored colostrum. The occurrence of FTPI and PFTPI could be further minimized through more careful implementation of the practice of storing high-quality colostrum and treatment of at-risk foals with a sufficient quantity of stored colostrum. Furthermore, foals of dams that produce high-quality colostrum (based on cRI value) may remain at risk for the development of FTPI or PFTPI as a result of other factors that influence the ingestion and catabolism of immunoglobulins in the early neonatal period. In the present study, FTPI and PFTPI were diagnosed in a subset of foals, despite the fact that their dams produced colostrum with a cRI value > 20%; it is possible that ingestion of an insufficient quantity of colostrum or catabolism of immunoglobulins secondary to disease resulted in FTPI and PFTPI in those foals.

Five foals, which were offspring of mares that produced high-quality colostrum, were treated with stored colostrum. In those instances, poor suckling behavior was observed because the foals were weak and unable to suckle or because the mare did not allow the foal to nurse. Results of our investigation support the treatment of foals with poor suckling behavior given that foals of dams that produced colostrum of adequate quality may still develop FTPI or PFTPI. Foal vigor, as determined by maturity, congenital defects, general health, and conformation, has been identified¹⁶ as a critical factor in the development of FTPI; to further minimize the occurrence of FTPI and PFTPI, foaling attendants should be trained to objectively assess foal vigor criteria in neonatal foals.

It has been suggested^9,18 that 250 to 2,000 mL of high-quality colostrum, which contains 60 to 90 g of IgG, must be ingested within the first 6 hours after birth to prevent FTPI in foals. Ideally, because this corresponds to a cIgG concentration of 60 g/L,¹⁹ high-quality colostrum that is stored and used for treatment of atrisk foals should have a cRI value > 23%. The guidelines for the present study were to administer ≥ 300 mL of stored colostrum (cRI value, > 23%) 1 to 4 times (during the first 6 hours after parturition) to foals at risk for FTPI. Generally, this quantity of ingested colostrum was sufficient to achieve sIgG concentrations > 800 mg/dL in foals that were vigorous and suckling well, but 2 foals that received treatment with 400 to 450 mL of colostrum each had a sIgG concentration < 800 mg/dL. This finding suggests that the colostrum administered to these foals was inadequate in volume or IgG concentration or that the immunoglobulins were rapidly catabolized as a result of concurrent illness. No foal treated with > 450 mL of stored colostrum developed FTPI or PFTPI. Therefore, it is our recommendation that foals at risk for FTPI or PFTPI be treated with a volume of a high-quality colostrum that is proportional to the perceived risk for the development of FTPI in that foal. Foals born to mares that produce poor-quality colostrum (cRI value, 0% to 15%) could require treatment with 1,000 mL of high-quality colostrum to prevent the development of FTPI or PFTPI.¹² Foals born to mares that produce colostrum that is considered slightly less than adequate (cRI value, 16% to 20%) could require treatment with 500 mL of high-quality colostrum. Treatment with an additional 500 mL of high-quality colostrum could be required in foals that are weak, unable to suckle, or at risk for other neonatal disease.

The active assessment of passive immunity in neonatal foals has been associated with a decrease in morbidity.^4,20 The incidence of disease in the study reported here was similar to the expected incidence of disease on commercial breeding farms.^4,20,21 Although the assessment of the incidence of disease was not a primary goal of the study reported here, it is expected that minimizing the incidence of FTPI would reduce the risk of infection-related disease in foals.

In the study reported here, the sIgG concentration of foals was measured at 24 hours after parturition. In another study,⁸ investigators demonstrated that sIgG concentration can be measured in foals as early as 8 hours after parturition. Therefore, an alternative approach to minimizing the development of FTPI would be to measure sIgG concentration at 8 hours after parturition and subsequently treat at-risk foals with high-quality colostrum prior to discontinued absorption of immunoglobulins through the intestinal tract at approximately 18 hours after parturition.⁸

Colostrum that was selected for storage was evaluated by use of a sugar refractometer^b prior to freezing; however, this practice was not repeated after thawing of the frozen colostrum. When frozen, the bacteriostatic activity of colostrum is preserved for up to 2 years.²² The immunoglobulin content of colostrum is preserved after a single freeze-thaw cycle, but the repeated cycle of freezing and thawing will cause a reduction in immunoglobulin content.^22–24 It is the recommendation of the authors that high-quality colostrum is frozen in small volumes and thawed immediately prior to administration; after thawing, colostrum should be refrigerated, and any colostrum that is not used for treatment within 12 hours should be discarded.

Results of several studies^12,19 have indicated that the sugar refractometer^b is a highly accurate and practical method for measurement of cIgG concentration; furthermore, it is considered that colostrum with a cRI value from 20% to 30% (cIgG concentration, 50 to 80 g/L) are adequate for provision of a sufficient concentration of cIgG for the transfer of passive immunity in foals.¹² The ease of use of the sugar refractometer^b enables its use in the field.¹⁹ The upper limit of detection of the sugar refractometer^b that was used to measure cRI in the study reported here was 32%. Although the upper limit of detection restricts the usefulness of this device for accurately determining a cRI > 32%, this is rarely important in practical terms. In the farm setting described in the present study, the sugar refractometer^b was highly useful for the detection of mares that were producing colostrum of poor quality as determined by cRI values < 20%.^12,19

As described,²⁵ the spectrophotometer^d that was used to measure the sIgG concentration of foal serum in the present study was found to be the most sensitive and specific of all commercially available IgG assays; the assay uses immunoturbidimetric analysis to determine sIgG content. Quantitative measurement via radial immunodiffusion has determined¹⁷ that the mean ± SD sIgG concentration of healthy Thoroughbred foals is 1,400 ± 100 mg/dL, which is similar to the sIgG concentration (1,597 ± 574 mg/dL) reported in the present study.

Other studies^2,4 have revealed an association between immunoglobulin status of foals and neonatal foal morbidity. To prevent neonatal foal morbidity and financial losses incurred by the owners of equine farms, an objective of any horse-breeding operation should be to ensure adequate transfer of passive immunity in neonatal foals. On well-managed horse-breeding farms, sIgG concentration of foals should be evaluated at 24 to 36 hours after parturition; furthermore, the documentation of an sIgG concentration > 800 mg/dL is required prior to registration of foals for insurance against death in Australia.

The identification and prompt treatment of FTPI in neonatal foals with IV administration of plasma is recommended for increasing the foals' sIgG concentration to ≥ 800 mg/dL, thereby reducing the risk of infection and morbidity in neonatal foals. A considerable amount of expense is associated with plasma administration to foals with FTPI by veterinarians; additionally, prophylactic administration of antimicrobials to foals that have FTPI or PFTPI further adds to the expense of managing these foals.²⁶

In the present study, routine practices and standard guidelines were established on the farm to enable the development of a reserve of high-quality, frozen colostrum with minimal expenditure of additional resources. It is recognized that not all horse-breeding farms are able to justify the cost of measuring sIgG concentration in every foal. Our investigation highlighted some of the risk factors for FTPI and a means by which at-risk foals that require treatment with high-quality colostrum for prevention of FTPI or PFTPI can be accurately identified. Accurate and precise records for verification of the source of colostrum are essential for identifying mares that may be carrying the Aa or Qa antigens and to prevent prolonged storage of colostrum (ie, > 2 years). It has been determined⁸ that 250 mL of high-quality colostrum may be collected from mares and stored without affecting the sIgG concentration in those mares' foals. Colostrum was not harvested from primiparous mares in the present study because of the perceived risk of these mares producing an inadequate volume of colostrum for their own foals' consumption.

Ideally, colostrum that is used for treatment of foals at risk for FTPI should be derived from mares in the same geographic location as that of the foal to augment immunity against pathogens that are common to that location. Through the development of protocols for on-farm storage of colostrum, this objective may readily be achieved. Additionally, colostrum that is collected from mares housed within a breeding farm environment may confer additional benefits to foals that are treated with stored colostrum by providing immunoglobulins that prevent neonatal disease in addition to those provided by commercial equine plasma products.

The benefits of the enteral administration of colostrum beyond the provision of immunoglobulins to the foal are known. The gastrointestinal tract is the most important route of infection in neonatal foals.^27,28 Colostrum that is administered enterally enhances the protection of the gastrointestinal tract against ingested pathogens and provides nutrition to enterocytes.²⁹ Complement components and immunoglobulins found in colostrum also enhance opsonization capacity and neutrophil activities that are essential for immune function and protection in an immunologically naïve foal.^30,f

Results of this investigation suggest that the incidence of FTPI in at-risk foals can be minimized through on-farm intervention protocols; thus, these protocols have the potential to reduce morbidity in neonatal foals and mitigate financial losses. The education of farm staff and compliance with these protocols are critical factors for ensuring the administration of stored colostrum to foals at risk for FTPI.