Neutering is associated with developing hemangiosarcoma in dogs in the Veterinary Medical Database: An age and time-period matched case-control study (1964–2003)

Abstract

The hypothesis that neutered dogs in the Veterinary Medical Database (VMDB) are at increased risk for developing hemangiosarcoma (HSA) was tested. Dogs (n = 5736) were diagnosed with HSA from a population of 2 106 324 dogs in the VMDB from 1964 to 2003. A case-control design matched on age and time period was created for general, cardiac, and splenic HSAs. A logistic regression analysis was performed including breed. Spayed females had an odds ratio (OR) of 1.59 for splenic, 1.47 for cardiac, and 1.72 for HSA in general. Castrated males had an OR of 1.26 for splenic and 1.14 for HSA in general compared to intact males. Controlled for historical time period and patient age, VMDB data support that neutering is associated with development of splenic HSA and HSA in general in both male and female dogs, but not cardiac HSA with an apparently lower than previously described magnitude of association.

Key clinical message:

This case-control design confirms an association between neutering and development of HSA and splenic HSA, but not cardiac HSA, in both male and female dogs. By controlling for time period at diagnosis, the bias of recent early neuter practices is eliminated, suggesting early neuter is not a principal driver of this effect.

Introduction

Hemangiosarcoma (HSA) is a common neoplasm of dogs that accounts for 45% to 51% of all splenic malignancies and is associated with a poor prognosis (1–9). Besides the spleen, HSA commonly occurs in the right atrium of the heart, skin and subcutis, and liver (8,10–14). Studies have identified that neutering is associated with risk of developing splenic and cardiac HSA, in particular (9,14). It has been reported that spayed female dogs have 5× the risk of intact female dogs for cardiac HSA (14), and 2.2× the risk for splenic HSA (9). Hormonal influence on the cause, prevention, and therapy of several canine cancers has been well-documented. Past studies have shown an association between neuter status and the development of mammary carcinoma, prostatic carcinoma, transitional cell carcinoma, lymphoma, mast cell tumors, perianal adenomas, and bone sarcomas (15–25).

In humans, only females routinely experience large changes in sex hormone levels over their lifetimes, which is thought to have variable effects on cancer development. Information on similar effects in males is scant. The Veterinary Medical Database (VMDB) describes a dog population with large numbers of both intact and neutered individuals, allowing altered hormone status to be studied in a large sample. The VMDB is the oldest and largest widely accessible database in North America. The VMDB contains individual patient records from 27 North American academic veterinary hospitals, including more than 6 million records of various animal species in the SNVDO-coded database from 1964 to 2004. The VMDB data permit comparisons between sexually intact and neutered dogs of both sexes with a diagnosis of canine HSA, including anatomic location of the primary tumor and age at time of diagnosis.

The purpose of this study was to evaluate the association between neutering status and risk of HSA in a case-control study that minimizes the effect of age bias and time period of diagnosis. Our hypothesis was that neutered dogs are at an increased risk for developing HSA, especially in the spleen and heart, across the 40-year history of the VMDB.

Materials and methods

Data retrieval

For this study, data from 1964 to 2003 were obtained from the VMDB. Queries were designed to retrieve all unique entries of dogs affected by HSA in any anatomic location with known gender status as well as a control population matched for age group and time period within 3 y of presentation of the case animal. Query parameters for affected dogs were limited strictly to distinct initial visits of all dogs with a diagnosis of HSA of any anatomic location. Query data returned included the unique identifying case number, gender, breed, age, and diagnosis code. Parameters for unaffected dogs were limited strictly to distinct initial visits of all dogs matched for age of the dogs with HSA, presenting within 3 y of the matched case, but without any of the above diagnoses. For the purpose of this study, we specifically looked at cardiac HSA and splenic HSA. We also looked at HSA as a general diagnosis, so this category included HSA of all anatomic locations (i.e., spleen, heart, liver, skin, subcutis, other).

Data analysis

The population of HSA-affected dogs was compared to the matched control population using Chi-square analysis for differences in proportion of gender status distribution. Within each diagnosis (cardiac HSA, splenic HSA, and general HSA), Chi-square analysis was also used to evaluate distribution of breeds compared to the general population of the VMDB to identify over-represented breeds with HSA. For each gender, an odds ratio (OR) was calculated with a 95% confidence interval (CI) for neutered:intact by diagnosis. The same groups were also compared as a binary logistic regression analysis with diagnosis as the dependent variable and gender and breed as the covariates. All statistical analyses were performed using commercial software (GraphPad Prism; GraphPad, San Diego, California, USA or SAS; SPSS, Chicago, Illinois, USA). Differences at the 95% CI (P < 0.05) were deemed significant.

Results

Query results returned 5736 dogs that were diagnosed with HSA in any anatomic location (426 intact females; 2190 spayed females; 1786 intact males; 1334 castrated males) from a total population of 2 106 324 dogs retrieved from the database. Of the dogs with HSA, 1828 were diagnosed with splenic location (146 intact females; 710 spayed females; 537 intact males; 435 castrated males) and 631 were diagnosed with cardiac location (45 intact females; 249 spayed females; 185 intact males; 152 castrated males).

Query results returned 5729 control dogs that were matched by time-period and age group without HSA in any anatomic location (756 intact females; 2258 spayed females; 1643 intact males; 1072 castrated males with 7 dogs having unknown gender status and discarded) from a total population of 2 106 324 dogs retrieved from the database. As controls for the dogs with splenic location HSA, 1825 were returned (239 intact females; 729 spayed females; 521 intact males; 336 castrated males). As controls for the dogs with cardiac location HSA 631 were returned (70 intact females; 263 spayed females; 155 intact males; 143 castrated males).

There was an overall increased risk for HSA associated with being neutered for the disease in general and for the splenic location, compared with their intact counterparts. Spayed females were at significantly higher risk than intact females (P < 0.0001) for HSA in general (OR = 1.72; 95% CI: 1.51% to 1.97%) and splenic location HSA (OR = 1.59; 95% CI: 1.27% to 2.01%). The association of neutering with cardiac location of HSA was not significant on Chi-square analysis (P = 0.0785) (OR = 1.47; 95% CI: 0.98% to 2.23%). Castrated males were also found to have a significantly higher risk than intact males (P = 0.0122) for HSA in general (OR = 1.14; 95% CI: 1.03% to 1.27%) and splenic location HSA (P = 0.0177) (OR = 1.26; 95% CI: 1.04% to 1.52%). The association of neutering with cardiac location of HSA was not significant (OR = 0.89; 95% CI: 0.65% to 1.22%).

Over the time period of the VMDB queried, the proportion of spayed and neutered dogs increased (17). The proportion of intact female dogs in the control group was consistently higher than the proportion of intact females in the HSA group by year (Figure 1). The proportions of males in the disease and control groups were more equal (Figure 2).

Figure 1.

The proportion of intact female dogs in the case-control data set (HSA solid line; control dashed line). The proportion of intact females in the control set is consistently higher over the 40 years of VMDB cases queried.

Figure 2.

The proportion of intact male dogs in the case-control data set (HSA solid line; control dashed line). The magnitude of difference in the proportion of intact males in the control set is not as great as that of females over the 40 years of VMDB cases queried.

Mixed-breed dogs and 20 purebred dogs with HSA from any anatomic location were represented by at least 50 individuals in the VMDB (Table 1). Using Chi-square analysis, breed type was associated with the development of general, splenic, and cardiac HSA (P < 0.0001). The golden retriever had the strongest association with the development of HSA in general, followed by the German shepherd, American cocker spaniel, miniature poodle, and boxer. The German shepherd had the strongest association with the development of splenic HSA, followed by the golden retriever, American cocker spaniel, and miniature poodle. The golden retriever had the strongest association with the development of cardiac HSA (P < 0.0001) with the remaining 19 pure breeds and mixed breed dogs having a significant association with a P-value < 0.05 (Table 2).

Table 1.

Most commonly identified breeds by diagnosis of hemangiosarcoma (HSA).

Breed	General HSA	Splenic HSA	Cardiac HSA
Golden retriever	713	177	134
Boxer	172	48	10
German shepherd	820	300	71
English setter	76	23	9
Scottish terrier	52	15	6
Labrador retriever	449	156	49
German shorthaired pointer	52	17	3
Standard poodle	68	27	6
Siberian husky	67	8	15
Basset hound	58	15	3
Mixed breeds	1560	494	148
Miniature schnauzer	100	40	5
Shetland sheepdog	82	15	15
English springer spaniel	63	29	0
Great Dane	57	17	1
Irish setter	78	24	5
Rottweiler	58	16	10
Doberman pinscher	110	39	6
Beagle	68	21	4
American cocker spaniel	86	27	4
Miniature poodle	77	25	3

Table 2.

Significantly overrepresented breeds with hemangiosarcoma (HSA).

	X2	P-value
General HSA
Golden retriever	680.97	< 0.0001
German shepherd	334.71	< 0.0001
American cocker spaniel	123.29	< 0.001
Miniature poodle	111.82	< 0.001
Boxer	84.953	< 0.001
Splenic HSA
German shepherd	190.94	< 0.001
Golden retriever	91.35	< 0.001
American cocker spaniel	38.97	< 0.01
Miniature poodle	34.23	< 0.05
Cardiac HSA
Golden retriever	380.99	< 0.0001

Analysis with a binary logistic regression analysis with diagnosis as the dependent variable and gender and breed as the covariates supports the prior analyses and suggests that breed is an important co-variate. Males were more likely to develop HSA (OR: 1.495) compared to females, and spayed or neutered dogs were more likely to develop the disease (OR: 1.4) (P < 0.001) compared with intact dogs. Breed was also significantly associated with the development of HSA in general. Golden retriever and German shepherd breeds had the highest association (P = 0.001) with boxers having a significant association as well (P = 0.004). The effect of spaying (OR: 1.72) was greater than the effect of castration (OR: 1.14) in association with HSA.

Discussion

Supporting previous literature, this study found a significant association between neutering and the development of HSA in companion dogs, especially in females. Our findings were consistent with previous studies that reported spayed females having an increased risk of developing splenic HSA compared with intact females. The large risk for cardiac HSA in dogs from the VMDB, however, was not identified when a case-control analysis was performed controlling for age and time period at diagnosis (9,14). The larger population considered over the longer time period of the current study may more accurately reflect the risk associated with neutering in the North American population of dogs. In addition, this is the first study to establish an OR for the risk of developing HSA in general in castrated males compared with intact males and in spayed females compared with intact females, controlling for age and time-period at diagnosis in such a large population.

The goal of the present case-control study was to minimize comparisons of neutering status of cancer patients to younger populations of dogs. The impression of intact dogs having lower risk of an older-dog disease is therefore minimized. The finding that the cardiac location of HSA is not associated with neutering status is interesting. This may suggest that breed plays a strong role in both risk of disease and impact of neutering on disease development. The difference from the previous VMDB study evaluating cardiac tumors likely reflects the use of the matched controls in this study compared to the general database parameters in the previous study (14). Furthermore, the magnitude of effect appears to be greater in female dogs than in male dogs.

The risk for developing HSA was significantly higher than background for several breeds. The golden retriever, German shepherd, American cocker spaniel, miniature poodle, and boxer were at the greatest risk. It should be noted that German shepherds, golden retrievers, Labrador retrievers, and boxers have been previously identified to have a significantly increased risk of HSA (2,8–10,26). Interestingly, in our study, there was a significant association between Labrador retrievers (P < 0.05) and the development of HSA; however, unlike previous studies, it was not one of the most common breeds. In our study, the golden retriever was the breed most strongly associated with the development of cardiac HSA (P-value < 0.0001), which is consistent with the findings of Ware et al (14) who evaluated cases from the VMDB. In a retrospective study looking at the neuter status of German shepherd dogs and the development of cancer, it was found that neutering at any age was not associated with an increased risk of developing cancer in both male and female dogs (27); however, the age cut-off for analysis in that study was 8 y, which is younger than the age range in which most dogs with HSA appear in the VMDB.

The etiology of HSA in dogs is currently unknown; however, the findings of our study and others suggest that hormones may play a role in its development in companion dogs. To the authors’ knowledge, there are 2, breed-specific studies that investigated the role of timing of estrogen alteration by neutering and the development of HSA. In 1 such retrospective study by Torres de la Riva et al (28), it was found that the percentage of HSA cases in late-neutered (> 12 mo of age at time of neuter) female golden retriever dogs was 4× that of intact and early neutered (< 12 mo of age at time of neuter) females. In a survey-based analysis of Vizsla neuter status and the development of cancer, there were similar results in that female Vizslas neutered before or after 1 y of age have 6× or 11.5× the risk, respectively, of developing HSA compared to intact females (29). These findings raise the question of the role of timing of estrogen removal and its association with the development of HSA. The identified effect of neutering across 40 y of cases suggests that early neutering is not a major driver, as early neutering is a more recent practice. One possible correlation for this phenomenon from human medicine is microsatellite instability after estrogen removal leading to colon cancer development in women. Based on clinical outcomes, it is thought that estrogen is protective by inhibiting the microsatellite instability pathway that leads to colon cancer (30). As women age, it becomes more likely that they will have a microsatellite instability-positive tumor due to their lack of estrogen (30). Applying this concept to the role of neutering on HSA in female dogs, the Torres de la Riva study (28) suggests that with early neutering, before the first estrus, the potentially neoplastic cells are not exposed to estrogen, so neutering would not lead to tumor occurrence. Conversely, in females that have undergone several estrous cycles, the potentially cancerous cells have been sensitized to estrogen, making estrogen protective as long as the female remains intact. However, if the female is neutered after several estrous cycles, thereby removing the influence of estrogen, the estrogen-sensitized cells can then become neoplastic, leading to a higher rate of HSA in late-neutered compared to early-neutered females (28). In the present study, we were unable to assess the age at which each dog was neutered using the VMDB, but it is worth investigating in the future to see if the findings of Torres de la Riva et al (28) can be extrapolated to the general dog population, at least in a breed-specific manner.

In the present study, it was found that castrated males are at increased risk for developing HSA compared to intact males. The reason for this has yet to be investigated; however, studies looking at canine prostate cancer and the increased risk in neutered male dogs compared to intact male dogs have discussed the possibility of the loss of androgen receptors after castration favoring tumor progression, independent of age at castration (17,19). Such a mechanism may play a similar role in males that develop HSA after castration. In one study of humans with hepatic angiosarcoma, one of the known etiologies was discovered to be treatment with androgenic-anabolic steroids, further supporting hormonal involvement in the genesis of this cancer (31). Further investigation of the effects of hormonal changes after castration is warranted.

In the United States, the majority of companion dogs are neutered with the common recommendation of neutering between the ages of 6 and 9 mo and before a female’s first estrus (32). In a large 2007 study in of the United States, 64% of the dogs were neutered, with males being more likely to be neutered than females and mixed breeds more likely than purebred dogs (33). The findings of the present study should be considered when owners are deciding on timing of elective gonadectomy, especially if the dog is a high-risk breed for developing HSA, since the disease carries such a poor prognosis.

As was discussed in a previous study using the VMDB, the VMDB contains some inherent biases that must be considered, particularly referral bias (17). Drawn from veterinary teaching hospitals, most of the cases are referred for secondary or tertiary care, and, as such, distribution of age, gender, breed, and socioeconomic status of the owner may not represent the population at large. Additionally, there is visit misclassification bias that exists when querying all initial visits to avoid recheck visits. Consequently, the same dog can appear multiple times in a query output if the entry was not coded correctly as a recheck to be excluded. These were carefully and entirely eliminated from this dataset using conservative queries based on a unique identifier followed by manual curation of the data in a spreadsheet, and automated screening using SAS software. No note of immunohistochemistry being performed was included in the VMDB entries, so it must be assumed that the diagnoses reflect the best judgment of the pathologists based on hematoxylin & eosin (H&E) light microscopy. Finally, data from recent years may not represent all cases from the participating institutions, as there is a lag in submission, processing, and incorporation of the data into the database tables. Nonetheless, this database represents a significant accumulation of information over the past 4 decades of veterinary teaching hospital experience. In addition, the VMDB minimizes geographical bias due to the large number of cases from multiple veterinary teaching hospitals across North America. It offers the greatest potential to answer important epidemiologic questions in veterinary medicine.

In conclusion, neutering is significantly associated with risk of developing HSA for both male and female dogs. The previously reported increased risk for cardiac HSA does not appear to hold in an analysis using controls matched for age and time period of diagnosis. The risk associated with being neutered is highest for spayed females. Breeds in the VMDB most diagnosed with HSA are typical of those previously reported; however, the Labrador retriever was not previously among the highest associated breeds. The findings of this study support our hypothesis, and data from previous studies, suggest the need for evaluating hormonal contributions to the genesis of HSA. In the future, performing a large population study investigating the age at neuter and the development of HSA is warranted to get a better idea as to the role of sex hormones. Also, since elective gonadectomy is considered unethical and highly discouraged by professional veterinary associations in some countries around the world (34), it would be interesting to evaluate the incidence of HSA in their companion dog populations and compare them to these findings. CVJ