Sarcoids on horses have been described since historical times (19
) and have been recognized as a clinical entity since 1936 (1
). Although there has been abundant research on sarcoids in Europe and the United States there has not been, until now, an attempt to describe the epidemiology of sarcoids in horses in western Canada. In this study, 802 horses with sarcoids were identified based on biopsy submission records to multiple veterinary diagnostic laboratories in western Canada.
As sarcoids are the most common skin tumor of horses (10
) it is not surprising that horses with sarcoids were present in large numbers in each of the 3 provinces. Submissions from the largest number of horses were from British Columbia (378), followed by Alberta (231), and Saskatchewan (193). According to a 2003 report on horse numbers in Canada, Alberta had the largest number of horses with 299 753, British Columbia had 141 410, and Saskatchewan had 126 252 (21
). While results from this study may suggest that sarcoids are more common on a per animal basis in British Columbia, the data were derived from submission of biopsies and the results may be biased based on the willingness of owners to spend money on veterinary diagnostic services. The same report states that owners in British Columbia spend an average of $379 annually on veterinary care per horse compared with $255 in Alberta; this may explain the apparent difference in frequency of diagnosis per capita (21
As noted in other studies on sarcoids, affected horses frequently have multiple sarcoids at the time of biopsy (22
). In this study, horses with multiple sarcoids at the time of biopsy represented 28.7% of the submissions and sarcoids composed 41.9% of all biopsies involving equine skin at Saskatchewan veterinary diagnostic laboratories, making them the most frequently diagnosed skin condition.
Of the 6 clinical classifications of sarcoids suggested by Knottenbelt (14
), 5 were found (). No malevolent sarcoids were found in this study and this clinical type has only been reported by 1 group (24
). All other clinical types were present in relatively equal proportions with the exception of the fibroblastic type which was present at approximately twice the frequency of other types. While this may represent the actual clinical distribution of cases, it may also represent the greater frequency with which these larger and more aggressive tumors are likely to receive diagnostic attention (20
Most sarcoids occurred on the head, followed by the limbs, and the neck and shoulder (). This distribution differs from those in other studies which found the lower abdomen to be the most common region for sarcoids to develop (22
), but is similar to that of another study (27
). Interestingly, all studies in which the abdomen was the most frequent location of sarcoid development were European, while the study with findings similar to the current study was from the Pacific Northwest of the United States. The variability in sites may be due to differences in the use of horses in different areas. It has been postulated that horses in North America are more likely to injure their legs on rough ground than are those in Europe (25
), but this remains speculative and has not been scientifically examined.
Twenty-three equine breeds were diagnosed with sarcoids in this study. Several studies have examined relative risk for the development of sarcoids among breeds (8
). Two of these studies found that among horses diagnosed with sarcoids in New York State, Quarter horses were more than twice as likely to develop sarcoids compared with Thoroughbreds, which in turn were twice as likely to develop sarcoids as Standardbreds (8
). In one of these studies, Arabians were identified as the breed with the highest risk of sarcoid development, more than 3 times higher than Thoroughbreds (9
). This breed disposition for development of sarcoids was not present in the current study. Only donkeys were at a statistically significantly higher risk to be diagnosed with a sarcoid (). The increased risk of donkeys developing sarcoids has not been previously identified. Warmbloods, Standardbreds, and Arabians were among the breeds at the lowest risk of developing sarcoids in the current study; however, the risk reduction was not statistically significant in any of these breeds. This difference in results may represent a variation based on the geographic location and genetic heritage of the horses involved, but a more likely explanation may be the reference populations used to make these comparisons. The reference population of the earlier study was comprised of horses with corneal ulcers, indigestion, carpal bone chip fractures, and tendonitis (8
), while the current study’s reference population was comprised of horses with skin conditions other than sarcoids.
More geldings than mares or stallions were diagnosed with sarcoids in this study; however, no differences in relative risk were noted (). In previous studies where a gender predilection has been shown, geldings were found to be at an increased risk compared to stallions and mares (5
). In these studies, it was suggested that castration surgery is a risk factor for the development of sarcoids, but the lack of a gender predilection and the relative rarity of paragenital location of sarcoids in this study would not support this conclusion.
Sarcoids have been reported to be a disease of young horses (20
), and the mean age of horses with sarcoids in the study group was significantly younger than those of the reference population, but affected animals ranged from 6 mo to 31 y old. Of particular note, is that unlike previous reports which suggested that sarcoids rarely develop in horses older than 7 y (22
), the median age of all horses in the current study with several clinical types of sarcoids (nodular, occult, and mixed) was 7 y, suggesting that half of the horses with this diagnosis were this age or older at the time of diagnosis. Of interest, is that the fibroblastic type classification, the most aggressive clinical type, was also the type with the youngest median age, statistically significantly younger than other types (). It has been suggested that sarcoids develop initially as one of the less aggressive types such as occult or verrucose sarcoids and then transform into the more aggressive fibroblastic form (24
). If this were the case, it seems counterintuitive that the most aggressive form of sarcoids is seen more often in younger horses than are the less aggressive forms from which it may derive.
Bovine papillomavirus has been reported as the cause of equine sarcoids (12
) and in this study BPV DNA was amplified from 77% of 96 samples. The amplification rates of BPV DNA from sarcoids in various studies has ranged from 73% to 100%, with studies with lower rates using formalin-fixed paraffin-embedded samples as in the current study (12
). Confirmation that BPV DNA was being recovered was obtained by sequencing 4 randomly chosen PCR products. The significance of the slight nucleotide change in one sequence is uncertain, but nucleotide variation in other regions within these genes has been previously found in equine sarcoids (18
Bovine papillomavirus was recovered from all clinical types of sarcoids examined (). The lowest recovery rates were in occult sarcoids with 12 of 19 positive (63%). Whether this represents lower levels of BPV in these types of sarcoids or some of these biopsies were not truly sarcoids is unclear. Occult sarcoids have the most subtle changes histologically of any of the sarcoid types and also have the smallest lesions by volume, often confined to a small proliferation of fibroblasts beneath the skin surface. It is possible that some of these lesions were misdiagnosed as sarcoids or that the smaller volume of neoplastic tissue in occult sarcoid as compared to other types of sarcoids reduced the success of DNA amplification resulting in a lower BPV positivity rate in this type of sarcoid.
Most sarcoids with amplified BPV DNA in this study contained solely BPV2 DNA based on RFLP (80% of the total). Bovine papillomavirus type 1 was found as the only amplified BPV type in 19% while a single sample had both BPV1 and BPV2 amplified. This is dramatically different from most studies of sarcoids. In Europe, BPV1 accounts for a vast majority of DNA detected with only a small number of sarcoids containing BPV2 (12
). Studies in the United States have found BPV1 and BPV2 in roughly equal proportions in the eastern United States (12
), while BPV2 predominates in the western United States representing 63% of the amplified DNA (13
). No previous studies have found as high a rate of BPV2 infection as this current study. Most of the work conducted on equine sarcoids was performed on horses infected with BPV1 rather than BPV2, the type much more commonly found in this study in western Canada. No clinical or pathogenetic differences between infection with BPV1 and BPV2 have been reported, and the applicability of studies performed on horses with BPV1 to horses with BPV2 is not known (13
In summary, the study herein is the largest study of equine sarcoids focused on western Canadian horses. Sarcoids appear to be a common lesion of the skin of horses in western Canada. There are several noteworthy features identified in this study. Specifically, donkeys appear to be at an increased risk of sarcoid development, older horses are more commonly affected by sarcoids than some previous reports suggest and, perhaps most importantly, BPV2, not BPV1 is the predominant viral type detected in sarcoids in these horses. More research is needed to determine the importance of BPV2 as a cause of sarcoids and to determine if there are different clinical implications in sarcoids caused by BPV2 versus those caused by BPV1 or differences in the pathogenesis of these lesions. CVJ