Common diseases related to IVD degeneration in dogs include DLSS, CSM, and Hansen type I and II IVD herniation.1,2 The lifetime prevalence of IVD herniation in dogs (ie, the proportion that will develop IVD herniation at some point during their lifetime) has been conservatively estimated at 2%.3,4 Intervertebral disk degeneration–related diseases are generally more common in chondrodystrophic breeds than in nonchondrodystrophic breeds and in older dogs than in younger dogs.5–7 Degeneration of the IVD is generally considered to be multifactorial,7,8 but a genetic influence has been reported in some breeds.9–11 Intervertebral disk degeneration is not synonymous with IVD disease. Although IVDs that cause clinical signs are typically degenerated, IVD degeneration can also be a common incidental finding.2,5,12–15
Some countries have introduced radiographic screening programs in an effort to reduce the occurrence of IVD herniation by excluding dogs that have a high number (≥ 5) of detectably calcified IVDs from use for breeding purposes.16–18 This may not be an efficient screening method because IVD calcification can develop in dogs without subsequent herniation, and IVD herniation can occur in the absence of IVD calcification.19 A more appropriate approach would be the use of DNA testing to screen dogs that have a high risk of developing IVD degeneration–related diseases; however, genetic markers for these diseases are not currently known.
In the study reported here, we used insurance claim records for veterinary health care and life insurance to identify dogs that had IVD degeneration-related disease and deaths attributed to this cause. The insurance companya that provided the records insures approximately 40% (200,000 to 250,000) of the Swedish dog population each year (estimated total, 500,000 to 625,000), and these dogs are considered to be fairly representative of the entire population of dogs in the country.20–22 In general, dogs are insured until 12 years of age for veterinary health-care purposes, and life insurance coverage (paid to owners upon death of the dog due to disease or accident) is provided until 10 years of age. Life insurance coverage is obtained for most dogs for which veterinary health-care insurance is obtained. The insurance process has been described in detail elsewhere.21
The objective of the study reported here was to determine the incidence and distribution of common IVD degeneration–related diseases in dogs of various breeds, ages, and sexes and to calculate mortality rates among dogs with these diseases. This information could potentially facilitate early diagnosis and preemptive treatments in dogs considered to have a high risk of developing IVD degeneration–related diseases. We also sought to lay a foundation for future genetic studies of IVD degeneration–related disease markers in dogs.
Materials and Methods
Sample—The study sample included the insurance records of all dogs for which veterinary health-care insurance, life insurance, or both were in effect at 1 insurance companya in Sweden between January 1, 1995, and December 31, 2006. Computer database records were reviewed for health insurance and life insurance claims for IVD degeneration–related diseases, which were included in the calculation of disease incidence rates and mortality rates.
Records review and data management—Data collected from the records included postal code of the owner (urban vs rural), beginning and end dates for which the dog was insured, type of insurance (veterinary health care, life insurance, or both), diagnostic codes, and the dog's breed (according to the Swedish Kennel Club standards), sex, and dates of birth and death. Crossbred dogs were excluded from the study. Dogs were further classified as chondrodystrophic or nonchondrodystrophic. Because a comprehensive categorization of breeds was not available, the following breeds were classified as chondrodystrophic on the basis of previously published information1,4,7,12,23–31: Basset Hound, Beagle, Standard Dachshund, Miniature Dachshund, English Bulldog, French Bulldog, American Cocker Spaniel, English Cocker Spaniel, Bichon Frisé, Jack Russell Terrier, Drever, Pug, Miniature Poodle, Cavalier King Charles Spaniel, Shih Tzu, Papillion, and Tibetan Spaniel.
Only the first reimbursed claim for veterinary health care that included the diagnosis of an IVD degeneration–related disease for each dog was used in incidence rate calculations; thus, the number of occurrences reflected the number of dogs newly identified as having IVD degeneration–related disease. Life insurance claims were used in calculations of mortality rate, regardless of whether or not reimbursement was made. Insurance claims, consisting of ≥ 1 receipt of payment for veterinary health care, had to be accompanied by a diagnostic code for primary clinical problem assigned by the attending veterinarian by use of a standardized diagnostic registry.32 Dogs with IVD degeneration–related diseases, including DLSS, CSM, or IVD herniation (with or without anatomic location specified), were identified on the basis of 19 codesb used to retrieve the records. Dogs were subsequently grouped in 5 ways on the basis of diagnostic classifications. The IVD degeneration–related disease (all) group included dogs with any of the 19 diagnostic codes. The subgroup of IVD herniation (all) included all dogs with diagnostic codes for IVD herniation, without consideration of the anatomic site involved. Three other subgroups (cervical [IVD herniation and CSM], thoracolumbar [IVD herniation], and lumbosacral [IVD herniation and DLSS]) were assigned on the basis of diagnostic codes related to anatomic site.
Incidence, mortality, and case fatality calculations—Incidence rates with 95% CIs were determined on the basis of reimbursed veterinary health-care insurance claims that included a first-time diagnosis of IVD degeneration–related disease for dogs aged < 12 years and were calculated as the number of disease occurrences/DYAR. Mortality rates with 95% CIs were determined on the basis of life insurance claims of dogs aged < 10 years that had died (including those that were euthanized) because of IVD degeneration–related diseases and were calculated as the number of deaths/DYAR. Dog-years at risk was determined as the product of the number of dogs and the mean time dogs were insured (in years); thus, 100 dogs insured for 8 years each would contribute 800 DYAR.
Incidence and mortality rates for IVD degeneration–related diseases were determined according to breed, sex, and location of residence (urban vs rural) for the overall group and for each diagnostic subgroup and are expressed per 10,000 DYAR.33 Breed-specific incidence rates were calculated for breeds with > 12,000 DYAR for veterinary health-care insurance purposes. Breed-specific mortality rates were calculated for all breeds with > 9,000 DYAR for life insurance purposes for which incidence rates were calculated.
An approximate case fatality rate (ie, the percentage of dogs that died of the IVD degeneration–related disease that was diagnosed) was calculated by dividing the mortality rate by the incidence rate of IVD degeneration–related diseases.
Age-related measures—The overall risk of developing IVD degeneration–related diseases, relative to age, was calculated for the entire study population as well as for the 3 breeds at highest risk. For this analysis, the veterinary health care and life insurance data sets were combined. The date used was that of the first diagnosis of an IVD degeneration–related disease, regardless of the type of claim. The dog's age at the time of insurance enrollment and on the date it was censored from the age-related analysis (ie, the date that insurance was stopped, the first claim was reimbursed for veterinary health care related to IVD degeneration, or the dog died) were used as time variables. Each dog was entered on the first day a record appeared in the data set, and if it did not have a relevant health-care claim filed and was not censored for any other reason, it was censored on the last day of the study period.
Statistical analysis—The data were not considered normally distributed; this was taken into account for all statistical methods used in this study. A proportional hazards regression procedurec was used for Cox regression to determine baseline survival curves that reflect the probability, related to age, that dogs in the population would develop an IVD degeneration–related disease. Age-specific hazards (for veterinary health-care and life insurance combined) were constructed by use of a macro for data smoothingc that produced a smoothed estimate of the hazard curve from the survival function determined by proportional hazards regression34; 95% CIs were also calculated and included in the graphs. The width parameter was set to one-tenth of the range of event times.
A Spearman rank correlation was used to evaluate the relationship between the incidence of IVD degeneration–related diseases and breed classification (chondrodystrophic or nonchondrodystrophic). Correlations were calculated by use of statistical software.d
For all incidence and mortality rates, a 95% CI was calculated. Values were considered significantly different when the 95% CIs of compared variables did not overlap. Case fatality rates were approximated by calculating the ratios between mortality rates and incidence rates. Differences in case fatality rates were not statistically evaluated.
Results
The sample comprised 677,057 dogs, including 665,249 dogs for which veterinary health-care insurance was provided and 552,120 dogs for which life insurance was provided during the 12-year period (2,772,423 and 2,055,261 DYAR, respectively). Most dogs had both veterinary health-care and life insurance (n = 540,312 dogs), but some dogs had only health care (124,937) or only life insurance (11,808). Intervertebral disk degeneration–related diseases were reported in 186 of the 308 breeds represented in the study. Fifty of these breeds met the inclusion criteria for calculation of incidence rates by use of veterinary health-care claims (> 12,000 DYAR), and 52 breeds met the inclusion criteria for calculation of mortality rates (> 9,000 DYAR).
Incidence rates of IVD degeneration–related diseases—The overall incidence rate of IVD degeneration–related diseases for all dogs in the study was 27.8 (95% CI, 27.2 to 28.4) occurrences/10,000 DYAR. This was determined on the basis of reimbursed first-time veterinary health-care insurance claims for IVD degeneration–related diseases in 7,708 dogs. Males (33.6 [95% CI, 32.6 to 34.6] occurrences/10,000 DYAR; n = 4,578) were more commonly affected than were females (22.2 [95% CI, 21.4 to 23.0] occurrences/10,000 DYAR; 3,130). The male-to-female ratio was 1.5:1. The incidence rate was higher in dogs from urban areas (34.7 [95% CI, 33.3 to 36.1] occurrences/10,000 DYAR; n = 2,401) than in dogs from rural areas (25.5 [95% CI, 24.8 to 26.2] occurrences/10,000 DYAR; 5,307). Miniature and Standard Dachshunds had the highest risk for IVD degeneration–related diseases, and 9 of the 10 breeds at highest risk for these diseases were chondrodystrophic. Doberman Pinscher was the only nonchondrodystrophic breed among these 10 (Table 1).
Total number of DYAR for veterinary health-care insurance purposes, incidence rates of IVD degeneration–related diseases, and mortality rates of dogs for which death was attributed to an IVD degeneration–related disease.
| Variable | DYAR | Incidence rate* (95% CI) | No. of affected dogs | Mortality rate† (95% CI) | No. of deaths |
|---|---|---|---|---|---|
| All dogs | 2,772,423 | 27.8 (27.2–28.4) | 7,708 | 9.4 (8.9–9.8) | 1,924 |
| Sex | |||||
| Male | 1,363,175 | 33.6 (32.6–34.6) | 4,578 | 11.5 (10.8–12.2) | 1,174 |
| Female | 1,409,248 | 22.2 (21.4–23.0) | 3,130 | 7.2 (6.7–7.8) | 750 |
| Location | |||||
| Rural | 2,079,943 | 25.5 (24.8–26.2) | 5,307 | 9.4 (8.9–9.9) | 1,484 |
| Urban | 692,480 | 34.7 (33.3–36.1) | 2,401 | 9.2 (8.3–10.1) | 440 |
| Breeds at high risk | |||||
| Miniature Dachshund | 15,433 | 237.1 (212.9–261.4) | 366 | 56.7 (43.1–70.3) | 67 |
| Standard Dachshund | 155,240 | 141.5 (135.5–147.4) | 2,196 | 35.7 (32.4–39.0) | 446 |
| Doberman Pinscher | 12,411 | 88.6 (72.1–105.2) | 110 | 56.0 (41.5–70.6) | 57 |
| Beagle | 20,509 | 68.3 (56.6–80.0) | 140 | 15.0 (9.3–20.8) | 26 |
| American Cocker Spaniel | 13,495 | 60.8 (47.6–73.9) | 82 | 14.1 (7.0–21.2) | 15 |
| English Cocker Spaniel | 42,059 | 52.1 (45.1–59.0) | 219 | 15.6 (11.2–20.1) | 48 |
| Cavalier King Charles Spaniel | 55,124 | 49.9 (44.0–55.8) | 275 | 10.3 (7.3–13.3) | 46 |
| Tibetan Spaniel | 14,994 | 48.7 (37.5–59.9) | 73 | 8.5 (3.0–14.1) | 9 |
| Shih Tzu | 18,938 | 39.6 (30.6–48.6) | 75 | 9.2 (4.0–14.5) | 12 |
| Papillon | 28,085 | 39.5 (32.2–46.9) | 111 | 6.8 (3.1–10.4) | 13 |
| Rottweiler | 40,191 | 38.3 (32.3–44.4) | 154 | 18.3 (13.7–22.9) | 61 |
| Dalmatian | 17,347 | 36.9 (27.9–45.9) | 64 | 8.6 (3.5–13.6) | 11 |
| German Shepherd Dog | 188,356 | 36.3 (33.5–39.0) | 683 | 23.5 (21.0–25.9) | 350 |
| Miniature Schnauzer | 27,957 | 35.8 (28.8–42.8) | 100 | 5.9 (2.6–9.2) | 12 |
| Bernese Mountain Dog | 18,606 | 31.7 (23.6–39.8) | 59 | 18.9 (12.3–25.6) | 31 |
| Breeds at low risk | |||||
| Finnish Spitz | 15,329 | 2.0 (0.3–4.0) | 3 | 0.0 (0.0–0.0) | 0 |
| Finnish Hound | 19,229 | 3.1 (0.6–5.6) | 6 | 5.5 (2.1–8.9) | 10 |
| Swedish Elkhound | 48,176 | 3.3 (1.7–4.9) | 16 | 1.6 (0.4–2.8) | 7 |
| Collie | 41,172 | 3.4 (1.6–5.2) | 14 | 1.3 (0.0–2.2) | 4 |
| Samoyed | 15,514 | 3.9 (0.8–7.0) | 6 | 1.7 (0.0–4.0) | 2 |
All dogs in the study were insured by a single insurance company in Sweden. Incidence rates were determined on the basis of the first reimbursed veterinary health-care claim that indicated diagnosis of an IVD degeneration–related disease. Mortality rates were determined on the basis of life insurance claims for dogs with death attributed to IVD degeneration–related disease, regardless of whether reimbursement was made. Breed-specific incidence rates were calculated for breeds with > 12,000 DYAR for veterinary health-care insurance purposes. Breed-specific mortality rates were calculated for the same breeds if they had > 9,000 DYAR for life insurance purposes. Results are shown for the 15 breeds at highest risk and the 5 breeds at lowest risk for occurrence of an IVD degeneration–related disease.
Incidence rate per 10,000 DYAR among 665,249 insured dogs < 12 years old.
Mortality rate per 10,000 DYAR among 552,120 insured dogs < 10 years old.
In total, 7,708 dogs received veterinary care for IVD degeneration–related diseases. Anatomic location of the of the abnormality was known for only 3,463 of the 7,708 dogs, and in the remaining 4,245 dogs, a general diagnosis of IVD herniation was given without anatomic localization, with an overall incidence rate of 16.7 (95% CI, 16.2 to 17.1) occurrences/10,000 DYAR. The male-to-female ratio was 1.6:1 for IVD degeneration–related diseases without specified anatomic localization. Dachshunds were overrepresented in this group; incidence rates were 188.0 (95% CI, 166.5 to 209.4) occurrences/10,000 DYAR for Miniature Dachshunds and 108.0 (95% CI, 102.8 to 113.1) occurrences/10,000 DYAR for Standard Dachshunds.
Cervical IVD herniation or CSM was diagnosed in 844 dogs, yielding an overall incidence rate of 3.0 (95% CI, 2.8 to 3.2) occurrences/10,000 DYAR. The male-to-female ratio was 1.6:1. Cervical IVD herniation was diagnosed in both chondrodystrophic and nonchondrodystrophic dogs, and Doberman Pinschers were overrepresented (Table 2).
Incidence rates of cervical IVD herniation and CSM among dogs and mortality rates of dogs for which death was attributed to these diseases.
| Variable | Incidence rate* (95% CI) | No. of affected dogs | Mortality rate† (95% CI) | No. of deaths |
|---|---|---|---|---|
| All dogs | 3.0 (2.8–3.2) | 844 | 0.9 (0.8–1.0) | 187 |
| Sex | ||||
| Male | 3.8 (3.5–4.1) | 518 | 1.2 (1.0–1.5) | 127 |
| Female | 2.3 (2.1–2.6) | 326 | 0.6 (0.4–0.7) | 60 |
| Location | ||||
| Rural | 2.7 (2.5–2.9) | 570 | 1.0 (0.8–1.3) | 137 |
| Urban | 3.9 (3.5–4.4) | 274 | 0.9 (0.7–1.0) | 50 |
| Breeds at high risk | ||||
| Doberman Pinscher | 58.6 (45.2–72.1) | 73 | 40.3 (28.0–52.7) | 41 |
| Miniature Dachshund | 24.4 (16.8–31.9) | 40 | 5.1 (1.0–9.1) | 6 |
| Beagle | 13.5 (8.5–18.4) | 28 | 2.9 (0.4–5.4) | 5 |
| Standard Dachshund | 13.0 (11.3–14.8) | 210 | 1.7 (1.0–2.4) | 21 |
| Rottweiler | 8.9 (6.0–11.8) | 36 | 4.2 (2.0–6.4) | 14 |
| Cavalier King Charles Spaniel | 8.8 (6.3–11.2) | 49 | 1.6 (0.4–2.7) | 7 |
| Whippet | 7.0 (2.4–11.5) | 9 | 0.0 (0.0–0.0) | 0 |
| Dalmatian | 6.87 (3.0–10.8) | 12 | 1.6 (0.0–3.7) | 2 |
| American Cocker Spaniel | 6.56 (2.3–10.8) | 9 | 1.9 (0.0–4.5) | 2 |
| English Cocker Spaniel | 5.64 (3.4–7.9) | 24 | 1.3 (0.0–2.6) | 4 |
| Breeds at low risk | ||||
| Swedish Elkhound | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Collie | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Münsterländer | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Golden Retriever | 0.2 (0.0–0.4) | 3 | 0.0 (0.0–0.0) | 0 |
| Papillon | 0.4 (0.0–1.0) | 1 | 0.0 (0.0–0.0) | 0 |
Incidence rates were determined on the basis of the first reimbursed veterinary health-care claim that included a diagnosis indicative of cervical IVD herniation or CSM. Mortality rates were determined on the basis of life insurance claims for dogs with death attributed to either disease, regardless of whether reimbursement was made. Results are shown for the 10 breeds at highest risk and the 5 breeds at lowest risk for occurrence of cervical IVD herniation or CSM.
See Table 1 for remainder of key.
Thoracolumbar IVD herniation was diagnosed in 1,045 dogs, with an overall incidence rate of 3.7 (95% CI, 3.5 to 4.0) occurrences/10,000 DYAR. The male-to-female ratio was 1.5:1. Of the 7 breeds at highest risk, all were chondrodystrophic, and Dachshunds again were overrepresented (Table 3).
Incidence rates of thoracolumbar IVD herniation among dogs and mortality rates of dogs for which death was attributed to this disease.
| Variable | Incidence rate* (95% CI) | No. of affected dogs | Mortality rate† (95% CI) | No. of deaths |
|---|---|---|---|---|
| All dogs | 3.7 (3.5–4.0) | 1,045 | 0.6 (0.5–0.7) | 131 |
| Sex | ||||
| Male | 4.5 (4.2–4.9) | 622 | 0.8 (0.6–1.0) | 81 |
| Female | 3.0 (2.7–3.2) | 423 | 0.5 (0.3–0.6) | 50 |
| Location | ||||
| Rural | 3.5 (3.1–4.0) | 798 | 0.7 (0.5–0.8) | 104 |
| Urban | 3.8 (3.5–4.1) | 247 | 0.6 (0.4–0.8) | 27 |
| Breeds at high risk | ||||
| Miniature Dachshund | 41.0 (31.1–50.8) | 67 | 4.2 (0.5–7.9) | 5 |
| Standard Dachshund | 27.2 (24.7–29.8) | 437 | 3.8 (2.9–4.9) | 48 |
| American Cocker Spaniel | 11.7 (6.0–17.4) | 16 | 0.9 (0.0–2.8) | 1 |
| Beagle | 8.6 (4.6–12.6) | 18 | 0.6 (0.0–1.7) | 1 |
| Papillon | 8.1 (4.8–11.5) | 23 | 0.0 (0.0–0.0) | 0 |
| Miniature Schnauzer | 7.1 (4.0–10.2) | 20 | 0.5 (0.0–1.5) | 1 |
| English Cocker Spaniel | 7.0 (4.5–9.6) | 30 | 3.3 (1.2–5.3) | 10 |
| Breeds at low risk | ||||
| Finnish Hound | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Petit Basset Griffon | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Finnish Spitz | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| Belgian Tervuren | 0.0 (0.0–0.0) | 0 | 0.0 (0.0–0.0) | 0 |
| English Springer Spaniel | 0.2 (0.0–0.5) | 1 | 0.0 (0.0–0.0) | 0 |
Incidence rates were determined on the basis of the first reimbursed veterinary health-care claim that included a diagnosis indicative of thoracolumbar IVD herniation. Mortality rates were determined on the basis of life insurance claims for dogs with death attributed to this disease, regardless of whether reimbursement was made. Results are shown for the 7 breeds at highest risk and the 5 breeds at lowest risk for occurrence of thoracolumbar IVD herniation.
See Table 1 for remainder of key.
Lumbosacral IVD herniation and DLSS were diagnosed in 1,574 dogs, with an overall incidence rate of 5.6 (95% CI, 5.4 to 5.9) occurrences/10,000 DYAR. The male-to-female ratio was 1.5:1. The 8 breeds at highest risk were all nonchondrodystrophic, and German Shepherd Dogs were overrepresented (Table 4).
Incidence rates of lumbosacral IVD herniation and DLSS among dogs and mortality rates of dogs for which death was attributed to these diseases.
| Variable | Incidence rate* (95% CI) | No. of affected dogs | Mortality rate† (95% CI) | No. of deaths |
|---|---|---|---|---|
| All dogs | 5.6 (5.4–5.9) | 1,574 | 2.6 (2.3–2.8) | 586 |
| Sex | ||||
| Male | 6.7 (6.2–7.1) | 916 | 3.3 (2.9–3.6) | 333 |
| Female | 4.6 (4.3–5.0) | 658 | 1.9 (1.6–2.1) | 193 |
| Location | ||||
| Rural | 5.1 (4.8–5.4) | 1,072 | 2.8 (2.3–3.3) | 393 |
| Urban | 7.2 (6.6–7.8) | 502 | 2.5 (2.2–2.7) | 193 |
| Breeds at high risk | ||||
| German Shepherd Dog | 27.9 (25.5–30.3) | 526 | 18.1 (15.9–20.3) | 270 |
| Doberman Pinscher | 17.5 (10.2–24.9) | 22 | 7.9 (2.4–13.3) | 8 |
| Rottweiler | 15.9 (12.0–19.7) | 64 | 8.1 (5.0–15.1) | 27 |
| Bernese Mountain Dog | 15.5 (9.9–21.2) | 29 | 10.4 (5.4–15.3) | 17 |
| Boxer | 14.0 (8.7–19.3) | 27 | 6 (2.1–9.9) | 9 |
| Dalmatian | 13.8 (8.3–19.3) | 24 | 1.6 (0.0–3.7) | 2 |
| Irish Setter | 11.3 (6.3–16.2) | 20 | 1.5 (0.0–3.5) | 2 |
| Labrador Retriever | 9.3 (7.7–11.0) | 128 | 2.1 (1.2–3.0) | 22 |
| Breeds at low risk | ||||
| Yorkshire Terrier | 0.0 (0–0) | 0 | 0.0 (0–0) | 0 |
| Petit Basset Griffon | 0.0 (0–0) | 0 | 0.0 (0–0) | 0 |
| Finnish Spitz | 0.0 (0–0) | 0 | 0.0 (0–0) | 0 |
| Tibetan Spaniel | 0.0 (0–0) | 0 | 0.0 (0–0) | 0 |
| Drever | 0.2 (0.2–0.5) | 1 | 0.2 (0.2–0.5) | 1 |
Incidence rates were determined on the basis of the first reimbursed veterinary health-care claim that included a diagnosis indicative of lumbosacral IVD herniation or DLSS. Mortality rates were determined on the basis of life insurance claims for dogs with death attributed to either disease, regardless of whether reimbursement was made. Results are shown for the 8 breeds at highest risk and the 5 breeds at lowest risk for occurrence of lumbosacral IVD herniation and DLSS.
See Table 1 for remainder of key.
Mortality rates for dogs with IVD degeneration–related diseases—In total, 1,924 dogs died of IVD degeneration–related diseases, with an overall mortality rate of 9.4 (95% CI, 8.9 to 9.8) deaths/10,000 DYAR. More male dogs (11.5 [95% CI, 10.8 to 12.2] deaths/10,000 DYAR; n = 1,174) than female dogs (7.2 [95% CI, 6.7 to 7.8] deaths/10,000 DYAR; 750), died of these diseases. Mortality rates were not significantly different between dogs from urban areas (9.2 [95% CI, 8.3 to 10.1] deaths/10,000 DYAR; n = 440) and those from rural areas (9.4 [95% CI, 8.9 to 9.9] deaths/10,000 DYAR; 1,484; Table 1).
Case fatality rates associated with IVD degeneration–related diseases—The overall case fatality rate (mortality rate [deaths/10,000 DYAR]/incidence rate [occurrences/10,000 DYAR]) of dogs with IVD degeneration–related diseases was 34% (9.4/27.8). These rates were generally higher (although statistical significance could not be tested) in nonchondrodystrophic breeds than in chondrodystrophic breeds; case fatality rates for Doberman Pinschers and German Shepherd Dogs were 63% (56.0/88.6) and 65% (23.5/36.3), respectively, compared with 24% (56.7/237.1) in Miniature Dachshunds and 25% (35.7/141.5) in Standard Dachshunds.
Associations of IVD degeneration–related disease with age and breed—The hazard (combined incidence and mortality rate) of IVD degeneration–related diseases increased with age (Figures 1 and 2). Evaluation of combined veterinary health-care and life insurance claims revealed that the 3 breeds at highest risk of developing these diseases before the age of 12 years were (in decreasing order) Miniature Dachshund, Doberman Pinscher, and Standard Dachshund. According to these calculations, of dogs that lived up to 12 years of age, 20% of Miniature Dachshunds, 17.5% of Doberman Pinschers, and 15% of Standard Dachshunds were at risk for ≥ 1 occurrence of IVD degeneration–related disease. The proportion of dogs < 12 years old with this same risk in the entire population was 3.5%. The risk of IVD degeneration–related disease increased with age. German Shepherd Dogs had the highest risk of lumbosacral IVD degeneration–related disease in this study and had a 7% lifetime prevalence of any IVD degeneration–related disease before the age of 12 years.
Associations between age and occurrence of IVD degeneration–related disease among dogs for which only veterinary healthcare insurance (124,937 dogs < 12 years old), only life insurance (11,808 dogs < 10 years old), or both (540,312 dogs) were in effect during a 12-year study period. Dogs were censored with the first qualified insurance claim that included diagnosis of an IVD degeneration–related disease. Hazards were calculated by use of combined incidence and mortality rates. The IVD degeneration–related disease (all) group included all dogs with a diagnostic code related to IVD degeneration. The subgroup of IVD herniation (all) included all dogs with diagnostic codes for IVD herniation (including dogs with CSM and DLSS), regardless of the anatomic site involved. Subgroups assigned on the basis of diagnostic codes related to anatomic site included cervical (IVD herniation and CSM), thoracolumbar (IVD herniation), and lumbosacral (IVD herniation and DLSS).
Citation: Journal of the American Veterinary Medical Association 240, 11; 10.2460/javma.240.11.1300
Survival curves indicating the age-related probability of dogs developing an IVD degeneration–related disease. The proportion of dogs without IVD degeneration–related diseases (black lines) was plotted against age on the basis of combined veterinary health-care and life insurance claim data for the same dogs as in Figure 1. The gray lines indicate 95% CIs. Breed-specific data were evaluated for Miniature Dachshunds (A), Standard Dachshunds (B), and Doberman Pinschers (C), in addition to the overall dog population (D).
Citation: Journal of the American Veterinary Medical Association 240, 11; 10.2460/javma.240.11.1300
Correlation between IVD degeneration–related diseases and chondrodysplasia—The incidences of IVD degeneration–related disease (all), thoracolumbar IVD herniation, and IVD herniation (all [IVD herniation, CSM, or DLSS, without consideration of the anatomic site involved]) were significantly (P < 0.01 for all correlations) correlated with the chondrodystrophic breed classification (R = 0.47, 0.77, and 0.73, respectively). The incidence of cervical IVD degeneration–related disease was not significantly correlated with breed classification, and that of lumbosacral IVD degeneration–related disease was significantly (P < 0.01) and negatively correlated with chondrodystrophic breed classification (R = −0.73).
Discussion
In the study reported here, a conservative lifetime prevalence for IVD degeneration–related disease before the age of 12 years for the overall sample population was 3.5%. Combined with the estimated case fatality rate for dogs < 10 years of age (34%), it can be concluded that these diseases caused death in approximately 1 in 100 dogs. However, the incidence rate of IVD degeneration–related diseases (determined on the basis of reimbursed first-time veterinary health-care insurance claims) was much higher in some breeds, particularly in chondrodystrophic dogs, with a lifetime prevalence of 20% in Miniature Dachshunds (the breed at highest risk for these diseases). The risk of IVD degeneration–related diseases was found to increase with age. In the authors' opinions, this finding does not confict with previous reports4,5,7 indicating that middle-aged dogs are most prone to developing IVD degeneration–related diseases but suggests that the longer a dog lives, the more likely it is to have signs of these diseases.
Although IVD degeneration–related diseases developed in several breeds, the incidence of these diseases was typically high in chondrodystrophic breeds, mainly Dachshunds. Moreover, IVD degeneration–related diseases were not diagnosed at all in some breeds. The large amount of variation in incidence among different breeds is suggestive of a genetic component and indicates that screening programs, such as those currently conducted in Denmark to reduce the occurrence of IVD herniation in Dachshunds,16 could be successful in reducing the occurrence of these diseases, provided that the exclusion criteria used is linked with increased heritability of disease. However, Rohdin et al19 have suggested that excluding dogs from breeding programs on the basis of the number of radiographically visible IVDs might not be the most successful approach to disease prevention. The results of the present study support the need for future genetic studies to identify DNA markers of IVD degeneration, which may aid future screening programs intended to reduce the incidence of disease.
In addition to a potential genetic component, evidence in some reports suggests that external factors, such as physical activity or environmental variables, influence the development of IVD degeneration–related diseases. For example, there are currently approximately 4,500 active service dogs in Sweden that are used mainly by police or military services or serve as guide dogs.35 Most of the police dogs (75%) and military dogs (65%) are German Shepherd Dogs, the breed at highest risk for DLSS.36,37 Of all Swedish police dogs (regardless of breed), 16.5% are reported to be retired early (< 8 years of age) because of unspecified problems related to the spinal cord or vertebrae.35 The lifetime prevalence of IVD degeneration–related disease in German Shepherd Dogs < 12 years of age in the present study was 7%. The apparently higher percentage and younger age of German Shepherd Dogs used for police work that have clinical signs of IVD degeneration–related diseases suggest that factors other than genetics, such as high physical workload, may potentially accelerate the incidence of IVD degeneration–related diseases in these dogs.
The overall case fatality rate (mortality rate [deaths/10,000 DYAR]/incidence rate [occurrences/10,000 DYAR]) of dogs with IVD degeneration–related diseases in the present study was 34% (9.4/27.8), with an indication of higher rates in nonchondrodystrophic than in chondrodystrophic dogs (63% [56.0/88.6] in Doberman Pinschers and 65% [23.5/36.3] in German Shepherd Dogs, compared with 24% [56.7/237.1] in Miniature Dachshunds and 25% [35.7/141.5] in Standard Dachshunds). However, case fatality rates were likely underestimated because dogs > 10 years old were not included in the life insurance data, whereas veterinary health-care data included dogs < 12 years old. The higher case fatality rates of nonchondrodystrophic dogs could be dependent on a number of factors. The authors considered that nonchondrodystrophic dogs were more often large-breed dogs (compared with chondrodystrophic dogs), and these generally have a shorter life expectancy than do small-breed dogs; additionally, the management of convalescence in paraparetic and paraplegic dogs is more difficult and physically demanding for owners of larger dogs.
Mortality rates of dogs with IVD degeneration–related diseases were similar between those from rural areas (9.4 [95% CI, 8.9 to 9.9] deaths/10,000 DYAR) and those from urban areas (9.2 [95% CI, 8.3 to 10.1] deaths/10,000 DYAR), yet the incidence rate for these diseases was significantly higher in urban dogs (34.7 [95% CI, 33.3 to 36.1] occurrences/10,000 DYAR) than in rural dogs (25.5 [95% CI, 24.8 to 26.2] occurrences/10,000 DYAR). Possible explanations for this difference include environmental differences between dogs living in urban and rural areas, genetic differences within breeds, or the possibility that distribution of breeds may be different between the 2 types of regions. Additionally, it is possible that dogs in rural areas might have been less likely to receive a thorough diagnostic workup and care because of a relative lack of veterinary specialty facilities or that the costs of diagnosis and treatment may have been too low in some rural areas for owners to claim veterinary health-care insurance.
Veterinary care data for DLSS, calculated on the basis of information from the same insurance database and for the same time period used in the study reported here, were recently published.11 In that study,11 the incidence ± SD of lumbosacral disease and DLSS was 6.2 ± 0.1 occurrences/10,000 DYAR, compared with 5.6 (95% CI, 5.4 to 5.9) occurrences/10,000 DYAR in the present study. There were some differences in the data extraction and presentation, which likely explain the discrepancies between results of the 2 studies. An important difference is that, in the present study, we determined the incidence of IVD degeneration–related disease on the basis of the first claim filed only, whereas in the previous study,11 > 1 incidence of an IVD degeneration–related disease could be reported for an individual dog (with a once-yearly maximum), which resulted in a slightly higher incidence rate in that study.11 Moreover, we did not include dogs ≥ 12 years old, whereas the previous study11 had no upper age limit. Finally, we used a cutoff value for inclusion of specific dog breeds of 12,000 DYAR, whereas the previous study11 used 3,600 DYAR; thus, we excluded less common breeds in the present study.
Although approximately 40% of dogs in Sweden are insured by the company from which data were obtained in the present study, and these dogs are generally considered representative of the entire Swedish dog population,20–22 the incidence of IVD degeneration–related diseases in the overall Swedish dog population could differ from that of dogs for which insurance was obtained through this source. Hence, results reported here are applicable only to the investigated dog population.
A limitation of the present study was the assumption that veterinarians make the correct diagnosis and use the correct diagnostic codes. We used a strict selection of the diagnostic codes to identify dogs with IVD degeneration–related disease. This selection may have led to a number of false-negative results because dogs assigned nonspecific diagnostic codes by the attending veterinarian were not considered to have IVD degeneration–related diseases for purposes of the study. Two of these nonspecific diagnostic codes (back pain without specification and signs of pain on palpation of the back) had been assigned to > 3,000 dogs during the study period. In addition, the diagnosis of IVD degeneration–related diseases via clinical examination with subsequent conservative treatment would likely have an associated cost lower than the veterinary health-care insurance deductible for these dogs; thus, claims may not have been filed for some affected dogs. The strict selection of diagnostic codes, in combination with the likelihood of dogs with IVD degeneration–related diseases for which no insurance claims were filed, is likely to have resulted in some degree of underestimation of the true incidence of these diseases in the Swedish dog population.
The use of 12,000 DYAR as inclusion criteria for breed-specific analysis of data in the present study excluded many less common dog breeds. Although the high cutoff increases the precision of incidence rates, it may also potentially lead to the exclusion of some breeds at high risk for disease.
Another potential confounding variable was breed-related bias among owners. In the study reported here, hunting dogs such as Finnish Hound, Finnish Spitz, and Swedish Elkhound were found to be among the breeds with the lowest risk of IVD degeneration–related diseases. It could be argued that these dogs are bred for hunting and are therefore generally healthier than some other breeds. It could also be argued that the owners of hunting dogs might be less likely to bring their dogs to the veterinarian for treatment, and thus, the insurance data might not reflect the true disease incidence among these dogs. However, it is logical to assume that owners who choose to have insurance will use it when relevant and that owners of insured hunting dogs are as likely to make use of this insurance as are other dog owners.
Another type of breed-related bias that could have affected the data used in the present study is associated with diagnosis. Veterinarians may be more inclined to code for IVD degeneration–related diseases when the signalment suggests a breed at risk; in other words, it is possible to ascribe clinical signs of disease to an IVD degeneration–related disease when in fact they may be due to another problem. Similarly, veterinarians may be less likely to code for IVD degeneration–related disease in breeds where this finding is generally not expected.
Although previous studies7,35,38 of the epidemiology of IVD degeneration–related diseases involved smaller cohorts of patients treated at academic or referral clinics, the outcomes of those studies were similar to the results of the study reported here. Another large study4 was performed > 30 years ago in a population of dogs in North America and findings were similar (8,117 cases with 356,954 DYAR). In contrast to results of the present study and 2 previous studies,4,39 results of a retrospective case series40 involving 105 dogs indicated that cervical IVD disease was more common in female dogs than in male dogs. The differences suggest that large population-based epidemiological studies may reduce the risk of biased results.
Although IVD degeneration–related diseases develop in dogs of various breeds, chondrodystrophic breeds and a small number of nonchondrodystrophic breeds such as German Shepherd Dogs and Doberman Pinschers were overrepresented in the present study, indicating a genetic influence in the pathogenesis of these diseases. Intervertebral disk degeneration–related diseases were associated with case fatality rates of 34% in the overall population and > 50% in the German Shepherd Dogs and Doberman Pinschers.
ABBREVIATIONS
| CI | Confidence interval |
| CSM | Cervical spondylomyelopathy |
| DLSS | Degenerative lumbosacral stenosis |
| IVD | Intervertebral disk |
| DYAR | Dog-years at risk |
Agria Pet Insurance, Stockholm, Sweden.
A list of the diagnostic codes used is available upon request from the corresponding author.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
SPSS, version 17.0, SPSS Inc, Chicago, Ill.
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