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Keywords:

  • horse;
  • selenium;
  • white muscle disease;
  • vitamin

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

Reasons for performing study

Selenium and vitamin E deficiency have been associated with nutritional myopathy, more commonly known as white muscle disease (WMD) in horses. However, correlations between selenium concentrations and presenting clinical signs, age, breed, gender, serum vitamin E, creatine kinase (CK) and final diagnosis, have not previously been evaluated.

Objectives

To determine the number of hospitalised horses in 3 age groups that were selenium tested and the proportions of horses with categorised presenting clinical signs; the association/odds risk of final diagnosis with selenium deficiency and to examine the association between selenium status, vitamin E status and serum CK in adult horses.

Methods

Two hundred and seventy-one hospitalised horses with a selenium concentration evaluated between 1996 and 2011 were examined retrospectively. Records were examined in order to ascertain selenium and vitamin E concentrations, age, breed, gender, CK values, presenting clinical signs and final diagnosis. Data were analysed with proportions, Fisher's exact t test, odds ratios and multivariate linear regressions.

Results

Within the <30 day old age group, 13/20 animals had low selenium concentrations. There were 18/42 horses in the 30 days to 2 years old age group with low selenium and 77/209 horses more than 2 years of age with low selenium. There was an association between low selenium and myopathy in the <30-day-old animals (P = 0.017), all of which were classified as having WMD. No associations were identified between nutritional myopathy and selenium status in horses between 30 days and 2 years of age or in horses more than 2 years of age.

Conclusions and potential relevance

This study indicates that WMD occurs most commonly in foals <30 days old and is associated with low selenium concentrations (7 out of 8 affected foals had blood Selenium levels <1.26 μm/l). Low serum selenium concentrations are common in hospitalised adult horses while nutritional myopathy is rare in these animals.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

There have been many reports of selenium deficiency myopathy, also known as nutritional myopathy and white muscle disease (WMD), in the equine veterinary literature [1-11]. Selenium deficiency has been documented in horses in 46 states within the United States and in many countries worldwide. In the United States, predispositions to development of WMD are greater in the northeast, northwest and eastern coastal areas due to selenium deficient soils [5, 12].

Selenium is a crucial component of the selenium dependent anti-oxidant, glutathione peroxidase. This enzyme has covalently linked selenomethionine incorporated into its structure that acts to catalyse the formation of oxidised glutathione in the face of free radicals, thus making it important in the cytosolic reduction of free radicals and detoxification of lipoperoxides and hydrogen peroxides [13, 14].

Due to its lipophilic nature, vitamin E acts as a potent free radical reducer within cell membranes, where it protects unsaturated lipids and other susceptible membrane components against oxidative damage. Vitamin E donates a hydrogen atom from its phenolic group to lipid peroxyl radicals produced during auto-oxidation of membrane polyunsaturated fatty acids. The more stable lipid peroxides formed by this process are further quenched by selenium dependent glutathione peroxidase. Consequently, vitamin E and selenium work synergistically to reduce oxidative damage in the mammalian body [15].

Few studies have been performed assessing selenium concentrations in horses presenting to a veterinary hospital and associated clinical signs or presenting complaints associated with the decision to evaluate selenium status in these horses. The results of a single study that examined the selenium status of 201 farm-owned horses indicated that approximately 29% of horses tested were selenium deficient and 8% vitamin E deficient, suggesting that selenium deficiency was a significant problem in this northern Canadian province; clinical signs were not examined in this study [11]. Another case study examined the findings of 11 foals with clinical and pathological signs of WMD in which serum glutathione peroxidase concentrations, but not selenium concentrations, were determined [5]. Because horses with low selenium and/or vitamin E status may not present with primary myopathic signs [11, 14], we decided to perform a retrospective analysis to examine all horses presenting to the Cornell University Equine and Farm Animal Hospital that had whole blood selenium concentrations performed as part of diagnostic evaluation from 1 January 1996 to 20 July 2011. The goals of this study were 3-fold: 1) to determine the number of horses in 3 distinct age groups that were tested for selenium concentration (≤30 days, 30 days to 2 years, >2 years of age) and their proportions based on categorised presenting clinical signs that have been previously associated with the deficiency; 2) to determine the association/odds risk of a final diagnosis with selenium adequacy and 3) to examine the association between selenium, vitamin E and creatine kinase (CK) status.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

Case selection

Inclusion criteria were all horses examined at the Cornell University's Equine and Farm Animal Hospital from 1996 through 2011 that had a selenium concentration determined by the Cornell University New York State diagnostic laboratory. Data from horses with whole blood selenium concentrations evaluated included gender, age and breed. The clinical signs and diagnosis reported in the medical record were reviewed by 3 of the authors (R.S., T.D and A.K.) for completeness of data. Additional data collected on horses, when available, were concurrent serum vitamin E concentration and serum CK values. The horses were stratified based on age groups determined as ≤30 days, 30 days to 2 years and >2 years of age. The age groupings (30 days to 2 years and >2 years) were selected based on the potential differences in energy requirements and feeding patterns of young growing horses vs. adults. Selenium concentration was considered to be adequate >0.76 μm/l in foals <10 days of age, and ≥2.15 μm/l for horses >10 days of age [16]. Vitamin E concentration was considered to be adequate at 4.18 μm/l for <10 day old foals, 2.8 μm/l for 10 to 30-day-old foals, 3.5 μm/l for 30-day to 2-year-old horses and 4.6 μm/l for horses ≥2 years [17]. The reference range considered appropriate for CK was 142–548 μ/l.

The most popular breeds presenting to our clinic were categorised as Quarter Horses (n = 55), Thoroughbreds (n = 54), mixed-breed horses (n = 27), Standardbreds (n = 24), Warmbloods (n = 16), Miniature horses (n = 15), Draught horses including Clydesdales, Percherons, Fjords and Belgians (n = 15), Paints (n = 12), Arabians (n = 9), Ponies (n = 5), Appaloosas (n = 4) and other more rare breeds with fewer than 5 represented (n = 35). Fisher's exact test and odds ratios were performed on data for any breed that had greater than 10 animals. Since this did not encompass all horses and since larger horses tend to consume less calories per kg body weight potentially leading to less selenium and vitamin E consumption, we also categorised the breeds based on size with horses with mature bodyweights <200 kg in one group, 200–600 kg in a second group and >600 kg in the third.

We determined which clinical signs prompted clinicians to examine selenium status and when available, vitamin E concentrations, by creating categories describing the clinical signs that have historically been associated with selenium and/or vitamin E deficiency. These categories were: neurological abnormalities, musculoskeletal abnormalities, malaise/poor weight gain or loss, gastrointestinal disorders, cardiac insufficiencies, endocrinopathies, hoof and hair coat abnormalities, immune dysfunction and poor performance [3, 8, 11, 14, 16-18]. Horses were placed in these categories based on the associated clinical signs as outlined in Table 1.

Table 1. Clinical signs and prevalence associated with whole blood selenium testing
Clinical categoryAssociated clinical signs or clinical pathology abnormality<30 days30 days to 2 years>2 years
  1. AST= aspartate transaminase; CK= creatine kinase; GO= gastrointestinal.

NeurologicalAtaxia6/2012/4264/209
Central or peripheral neuropathy
Dysphagia
MusculoskeletalMuscle atrophy11/2024/42111/209
Muscle tremors or fasciculations
Increased AST or CK
Orthopaedic abnormalities
Dysphagia/nasal discharge
Malaise/weight loss/poor gainLethargy7/2015/4271/209
Inappetance
Weight loss
Poor growth
Hoof/hair coat abnormalitiesCracked or brittle hooves0/203/4227/209
Dull or poor hair coat
Conjunctivitis
GI disordersRolling, flank kicking, abdominal pain6/209/4234/209
Diarrhoea
Reproductive disordersPyometritis1/201/429/209
Repeat breedings
Early embyonic death
Abortion or sudden death of foals
Cryptorchid
Immune dysfunctionPoor immune response5/202/420/209
CardiacHeart murmur2/205/4216/209
Arrythmia
EndocrineHypothyroidism0/201/4210/209
Hyperadrenocorticism
Poor performanceRecent poor performance0/200/4237/309

All cases were then categorised by diagnosis, which included neurological disease, myopathic disease, endocrine disease, gastrointestinal disorders, cardiac insufficiency, infectious and/or immune compromise, orthopaedic, congenital or open. Diagnoses that accounted for fewer than 5 cases were placed in the ‘other’ category.

Data analysis

All presenting clinical signs for the specific age groups are represented as proportions and percentages. Due to the small populations of horses less than 30 days old and horses between 30 days and 2 years of age, Fisher's exact test was performed for each diagnosis categorisation (neuropathy, myopathy, endocrine, infectious and immunity, congenital and open/other diagnosis) and selenium adequacy was designated as either one for adequate or 0 for inadequate. Fisher's exact tests were also performed for gender and breed categorisation for associations with selenium adequacy in the <30 days and 30 days to 2 year age groups. For adult horses over 2 years of age, odds ratios and 95% confidence intervals (CI) were calculated for final diagnoses, breed categorisation and gender, as they relate to selenium adequacy. Simple linear regressions were used to compare vitamin E and selenium concentrations for all adult horses that had a measurable selenium and vitamin E status. Multivariate logistical regressions were performed to determine associations between selenium, vitamin E concentrations and CK as indicators of muscle damage and multivariate logistical regression was performed to determine associations between a myopathic diagnosis and selenium and vitamin E status.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

Whole blood selenium concentrations were examined in 271 horses from 1 January 1996 through 20 July 2011. In the <30-day age group there were 20 horses, the 30-day to 2-year-old age group contained 42 horses and >2-year-old age group had 209 horses. Within these age groups, 13/20 (62%), 18/42 (43%) and 77/209 (37%) horses had low whole blood selenium concentrations, respectively. White muscle disease was a clinical diagnosis for a total of 14 horses with 8 foals <30 days old (40%), 2 horses in the 30 day to 2-year group (4.8%) and 4 horses in the >2-year age group (1.9%). Of the WMD foals in <30-day group, 5 of the 8 horses had a selenium below 0.32 μm/l, 2 were between 0.63 and 1.0 μm/l and only one horse had a normal selenium (2.8 μm/l); however, records indicate possible selenium injection immediately before presentation to our referral facility. Selenium concentrations were below 0.32 μm/l in 2 adult horses, 1.0 μm/l in one horse and 2.0 μm/l in another with a diagnosis of WMD. There were 3 foals <30 days and 6 adult horses with a whole blood selenium value <0.63 μm/l and no diagnosis of WMD. Overall, selenium status of horses with WMD ranged from <0.32 to 2.7 μm/l in foals <30 days of age while in horses aged 30 days to 2 years selenium concentrations ranged from 2.6 to 2.8 μm/l and <0.32 to 2.0 μm/l in horses >2 years old. Animals diagnosed with WMD and their corresponding age, breed, selenium and vitamin E status are outlined in Table 2. After treatment, 6 out of 8 foals under 30 days of age recovered, as did one of 2 horses aged 30 days to 2 years and all horses (4 out of 4) over 2 years of age (Table 2). Information on the number of selenium and vitamin E injectable supplements for 2 foals were available and these foals were found to need at least 4 injections until whole blood selenium levels reached reference range (one foal) or near reference range (one foal). The clinical signs that prompted clinicians to assess selenium status are listed in Table 1 with myopathic, neurological and lethargy with weight loss being the top 3 clinical signs prompting selenium evaluation (Table 1). After evaluation of the final diagnosis for each case, a Fisher's exact test was performed on all diagnosis and selenium adequacy for age groups. In the <30 days age group, the only diagnosis associated with inadequate selenium status was WMD (P = 0.017); no other diagnosis was associated with inadequate selenium in either the 30 days to 2 years or >2 year age groups (Table 3).

Table 2. Final diagnosis of white muscle disease and association with low whole blood selenium using Fisher's exact t test (under 30 days and 30 days to 2 years) or odds ratio (>2 years)
CaseAge groupBreedSelenium (μm/l)Survived
 1≤30 daysThoroughbred2.7No
 2≤30 daysWarmblood<0.32Yes
 3≤30 daysStandardbred<0.32Yes
 4≤30 daysQuarter Horse6.4Yes
 5≤30 daysQuarter Horse<0.32No
 6≤30 daysQuarter Horse<0.32Yes
 7≤30 daysMixed horse<0.32Yes
 8≤30 daysMiniature horse1.0Yes
 930 days to 2 yearsQuarter Horse2.8Yes
1030 days to 2 yearsPaint2.6No
11≥2 yearsMiniature horse<0.32Yes
12≥2 yearsWest Phalian2.0Yes
13≥2 yearsMixed horse1.0Yes
14≥2 yearsAmerican Saddle Horse<0.32Yes
Table 3. Final diagnosis and association with low whole blood selenium (Se) using Fisher's Exact t test (under 30 days and 30 days–2 years) or odds ratio (>2 years)
Final diagnosisUnder 30 daysWhole blood Se association.30 days–2 yearsWhole blood Se association>2 yearsSe odds ratio (95% CI)
  1. NA = not applicable.

Neuropathy2/20NA5/42P = 0.37132/2090.428 (0.18–1.04; P = 0.09)
Myopathy8/20P = 0.0175/42P = 0.37144/2091.389 (0.71–2.73; P = 0.43)
Endocrinopathy0/20NA1/42NA5/2092.03 (0.43–12.1; P = 0.54)
Autoimmune/infection10/20P = 0.5618/42P = 0.26133/2090.60 (0.26–1.36; P = 0.29)
Gastrointestinal disease1/20NA5/42P = 0.14622/2090.83 (0.34–3.03; P = 0.85)
Congenital disorder3/20P = 0.478/42P = 0.4380/209NA
Skeletal/joint disease0/20NA4/42NA31/2090.362 (0.14–0.93; P<0.05)
Open/Other diagnosis2/20NA1/42NA35/2092.37 (1.14–4.96; P<0.04)

Due to the large population of adult horses >2 years of age that had a diagnosis, odds ratio testing and 95% CIs were reported for the association between selenium deficiency and diagnosis of neuropathy, myopathy, gastrointestinal disorders, infection and immunity problems, endocrine disorders, orthopaedic disease and an open/other diagnosis (Table 3). The odds ratio was not significantly increased for any disease entity other than ‘open/other’. Therefore, horses were over twice as likely to have an open diagnosis if they had a low selenium concentration on presentation. In contrast, horses with orthopaedic diseases were over twice as likely to have selenium concentrations within the reference range. There was no relationship between selenium and myopathy in the group of horses >2 years of age. However, there were many cases of polysaccharide storage myopathy in this group. Consequently, confirmed cases of known polysaccharide storage myopathy were removed from analysis to determine whether this affected the relationship between clinical myopathies and low selenium concentration. After removing horses presenting with this disease from the analysis, the odds ratio for horses presenting with myopathy was not significant (P = 0.451; OR = 1.473, 95%, CI = 0.66–3.25).

There were no associations identified between gender, breed, or size categorisations in the >2-year-old group. Simple linear regressions were used to examine the relationship between selenium and vitamin E status in all adult horses for which both vitamin E and selenium were measured. Of the 132 horses that had both tests performed, Pearson's correlation revealed a significance (P = 0.031) in the regression, but with a weak association (r2 = 0.18; Fig 1). Regression analysis of selenium and CK values in 144 horses revealed no significant correlation (P = 0.549; r2 = 0.04). Considering the correlation between selenium and vitamin E, a general linear model incorporating the independent variables of selenium and serum vitamin E and their significance on the dependant variable of serum CK was performed that revealed no significance (P = 0.38) for the 105 horses with all 3 parameters measured.

figure

Figure 1. Linear regression depicting the correlation between whole blood selenium and serum vitamin E concentrations. (P = 0.031; r2 = 0.18).

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

White muscle disease is a distinct entity in young horses with numerous case reports documenting the lesions and association with low selenium and/or vitamin E concentrations [1, 3, 6, 8, 9]. In contrast, few case reports and studies have documented any associations between selenium or glutathione peroxidase status, (a direct reflection of selenium status) and WMD in adult horses [6, 11, 18, 19]. One such case report describes a 6-year-old Quarter Horse with severe masseter myonecrosis associated with low selenium concentration (<0.32 μm/l) [7]. Ludvikova et al. described 3 horses with masseter muscle myositis with associated low selenium (1.0 and 0.67 μm/l) or glutathione peroxidase concentrations (2.3 μ/l) [20]. In another case report, a hyperlipidaemic, pregnant mare with suspected masseter myositis was documented to have low glutathione peroxidase, while selenium was not measured [10]. A more extensive series of case reports summarised clinical and pathological findings on 8 horses with signs of masseter muscle myositis <5 months of age. Three of these animals had low blood selenium concentrations [21]. Another report describes dystrophic myodegeneration in adult horses as related to vitamin E and selenium status [2], yet no studies have examined selenium adequacy as it relates to a variety of clinical signs or diagnoses in a large number of horses.

Selenium deficiency occurs most often in areas with selenium deficient soils. Low soil selenium concentrations exist throughout the northeastern region of the United States where our veterinary referral hospital is located. Because of this low selenium environment, WMD is often considered as a differential diagnosis in horses with suspected myopathy and whole blood selenium concentrations are often determined. Due to the well documented normal range for whole blood selenium concentrations in the literature and the fact this test has less restrictions on storage time and temperature than glutathione peroxidase concentrations, whole blood selenium concentration is the preferred test to utilise when considering selenium deficiency [15, 16, 22, 23].

Some studies have described selenium status in particular populations of animals but did not report on any association with clinical signs or diagnosis in their reports [11, 19]. Due to the number of selenium concentrations determined at our institution, this retrospective analysis was performed to examine the prevalence of WMD in 3 different age groups of horses for which selenium concentrations were available. The findings for each age group were fairly consistent with the top 5 general clinical presentations prompting selenium concentration testing being: musculoskeletal conditions, neurological deficits, weight loss/malaise/poor growth, gastrointestinal disorders and hair and hoof abnormalities. Although these disorders were identified most commonly, the only disease that was strongly associated with low selenium concentration in the <30 days age group was WMD. An association between low selenium concentration and a diagnosis of myopathy, in general, was not seen.

White muscle disease is primarily a disease of young foals. The development of this disease in foals less than one week of age would suggest that the deficiency was associated with poor maternal selenium status in nursing foals [5, 6, 14]. Higuichi and colleagues reported that selenium concentrations tended to be lower in mares of affected foals, however mares with affected and nonaffected foals have a large range in selenium concentrations [6]. Glutathione peroxidase concentrations tended to be greater in mares of affected foals than in mares of unaffected foals. However, since this study did not examine selenium concentrations in mare's milk or absorption of selenium by the foals, little can be deduced about the pathogenesis regarding the low selenium status observed in the affected foals. While it seems prudent to supplement pregnant mares in low selenium areas, further studies regarding selenium concentrations of milk, gastrointestinal absorption of selenium and faecal excretion of selenium in foals with WMD would be required to better define the pathogenesis of this disease. Interestingly, one mature mare with selenium deficiency and masseter myositis in the current study, had a healthy 4-day-old foal. However, we also had nearly an equal number of foals that did not have WMD but had selenium concentrations <0.63 μm/l (3/8) suggesting that selenium deficiency may not be the only factor involved in the pathogenesis of this disease.

The pathogenesis of nutritional myopathy in adult horses appears to have an even weaker association with selenium status, as there failed to be any association between selenium status in horses >2 years of age with any definitive diagnosis except the open category. Even when horses with polysaccharide storage myopathy were eliminated from the statistical models, there was no increased risk of a myopathy associated with a low selenium status. This corroborates the long-held clinical impression that WMD is primarily a neonatal disease and that low selenium concentrations in adult horses may be considered insignificant, particularly as >37% of adult horses were considered to be selenium deficient. If, however, selenium concentrations are <0.63 μm/l, clinical selenium deficient myopathy should be considered as a possible diagnosis depending on the clinical presentation, particularly in foals. Our findings regarding the prevalence of low selenium status, are higher than the 29% value reported by Muirhead et al. who tested healthy horses in the selenium deficient northeast of Prince Edward Island [11]. The higher prevalence in the current study may be due to the clinical nature of this study, as many horses may have been anorectic or had gastrointestinal issues that may have led to inadequate intake or absorption of selenium, prior to presentation. This is further supported by our significant odds risk ratio for an open diagnosis (undiagnosed animals) in which problems such as inadequate dietary intake, poor gastrointestinal absorption or anorexia may be a part of the reason for presentation. These current findings show no association between selenium status and myopathies in horses >2 years of age, suggesting the need to redefine selenium adequacy in adult horses as well as a need to develop other tests for selenium adequacy.

Besides gluthione peroxidase activity, selenium functions in deiodinase enzyme activity transitioning T4 to T3 [11, 24]. The results of one field study failed to identify an association between selenium status and T3 concentrations, but indicated that selenium status and T4 concentrations increased in parallel, contrary to findings in other species [11, 25]. This discrepancy may be multifactorial in nature and may include poorly defined reference ranges whereby selenium status may not represent physiological function and potential differences in equine deiodinase activity. It is also important to note that this was not a controlled experimental study to examine selenium deficiency and adequacy but rather a population field examination with many variables that could have influenced serum T4 concentrations.

The interplay between vitamin E and glutathione is important for maintaining appropriate anti-oxidant balance as both have been associated with myopathic conditions [1, 8, 14, 26]. Vitamin E status in dried forage in the northeast region of the United States is dependent on temperature, climate, storage and drying time. This often results in marginal to deficient vitamin E concentrations in forages, particularly during the winter and spring months [15]. When vitamin E and selenium were examined in all horses in the present study, a weak association between the 2 was apparent, suggesting that this may be due to the regionally low selenium status coupled with potentially low vitamin E in forages in the northeast. Although a stronger association between selenium and vitamin E was expected, the common use of selenium and vitamin E supplements among horse owners in this region may have confounded this. Consequently, the lack of historical information regarding the animals’ feeds and supplementation status was a weakness of this study. The current study also failed to identify significant associations between CK, as an indicator of muscle-related damage, and selenium status nor did multivariate models that incorporated vitamin E, selenium concentration and CK.

In summary, the results of this study indicate that more than 35% of horses presented to our teaching hospital with clinical signs associated with suspected neuromuscular deficits, gastrointestinal disorders and lethargy/weight loss, will have low serum selenium concentrations. The only clinical diagnosis that was consistently associated with selenium deficiency was WMD in foals ≤30 days old. No other associations were identified between any diagnosis and low selenium. A weak but significant correlation was identified between serum vitamin E and whole blood selenium concentration, suggesting a potential abnormality with dietary intake of both of these antioxidants. Due to the high rate of selenium deficiency and lack of myopathies in many of the horses in the current study, further studies are needed to better determine parameters that more accurately reflect selenium adequacy.

Authors' declaration of interests

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

Dr Streeter's residency program was funded by P & G Pet Care and she has received a research grant from Nestle Purina. Dr Wakshlag has received grants from Nestle Purina and is on their advisory board. Dr Mittel, Dr Divers and Alexandra Korn have no conflict of interests.

Authorship

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References

Dr Streeter performed all manuscript preparation, statistical analysis and interpretation with the help of Dr Joseph Wakshlag, Dr Linda Mittel and Dr Tom Divers. Dr Divers, Alexandra Korn and Dr Streeter compiled the data and performed the interpretive summaries. Drs Mittel, Divers and Wakshlag initiated study design.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Authors' declaration of interests
  8. Authorship
  9. References
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    Pearson, E.G., Snyder, S.P. and Saulez, M.N. (2005) Masseter myodegeneration as a cause of trismus or dysphagia in adult horses. Vet. Rec. 156, 642-656.
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