Developmental Orthopaedic Disease
in Thoroughbreds

RIRDC

Background – what is the disease?

Developmental orthopaedic disease (DOD) refers to a group of problems occurring in growing thoroughbred horses. The term encompasses a diffuse range of conditions such as osteochondritis dissecans (OCD), cervical vertebral malformation ("wobblers"), angular limb deformities, subchondral bone cysts, epiphysitis (involving the growth plate) and flexural deformities (e.g. contracted tendons).

There is concern that the incidence of DOD on Australian stud farms has increased in recent years. This is a view shared by stud managers, veterinarians and farriers. The Rural Industries Research and Development Corporation (RIRDC) was contacted by key industry representatives to conduct a workshop which aimed to establish a coordinated approach to the problem. This report details the outcomes of the meeting held in Scone on the 14th March, 1997.

1. Defining the problem

The different forms of DOD were listed in order of importance under the following three phases of development:

A. Newborn foals

1. Rotational and angular limb problems

2. Tendon problems

weak tendons
contracted tendons
3. "Windswept" foals (bone misalignment)

4. Fractured sesamoids (most commonly seen at the end of this period)

B. Late foals (older than 90 days)

1. Acquired angular limb problems

2. Contraction problems

club foot
forward at the knees
3. Epiphysitis (usually at 4-5 months of age)

C. Weanling to yearling

1. Epiphysitis (seen before OCD)

2. OCD

3. Flexural/contracted tendons

4. Hoof development problems e.g. differing sizes, overload

The incidences of these conditions appear to vary from year to year. Often the first foals born in the season provide an indication of the type of problem that will predominate for the remainder of the season.

Acquired problems were regarded as being more difficult to correct than those present at birth, and concern was expressed that acquired DOD was becoming less responsive to treatment. Concern was also raised about the possible detrimental effects the interventions may have, such as over-correction of the original problem.

2. Extent of the problem

As many as 10% of foals are not eligible to be sold through yearling sales because of developmental orthopaedic disease. Estimates of the incidence of OCD ranged from 0.4% to over 10%.

Based on clinical findings and radiographs, veterinarians diagnosed OCD in 4% of the foal population. With regard to angular limb deformities, approximately 8% of foals required surgery and 40% needed corrective hoof trimming whilst up to 80% of foals showed some degree of angular deviation.

Contracted tendons affected between 3 and 5% of foals and the incidence of "wobblers" was estimated at 6%.

In addition to establishing the current level of DOD on Hunter Valley studs, it is important to determine whether or not this problem has increased in recent years.

Consideration of the population of mares and foals for each year must be taken into account if the latter issue is to be evaluated accurately. The number of mares in the Hunter Valley during the stud season reached a peak of approximately 10,000 in 1987.

This number has since decreased to between 6,000 and 8,000 mares, representing 22-24% of the national brood mare population.

3. Economic significance

Various estimates of the monetary costs of DOD were made based on the following assumptions:

8% of the 3,000 Hunter Valley foals require surgery involving a periosteal strip ($200), corrective shoeing and trimming ($100) and labour, radiographs, cost of boxing etc. ($200) - cost $120,000

40% of foals require only corrective shoeing and trimming ($100) - cost $120,000

5% of foals develop OCD and half of these (75) will not be sold as yearlings ($90,000 average price for an elite sale) - cost $6,750,000

Cost of diagnosing OCD ($200 for radiographs, clinical examination and transport to the veterinary clinic) in 4% of foals - cost $24,000

Of those diagnosed with OCD, approximately 50% will be treated ($1,000 average surgical and associated costs) - cost $60,000

2.5% of the foals will be trained as two-year-olds ($36,000 in costs from birth until the 1st race start as a two-year-old) but never race because of OCD - cost $2,700,000
Total costs arising from angular limb deviations and OCD: $9.8 million

4. Factors involved in DOD

DOD is regarded as a multifactorial problem. Possible contributing causes include:

Nutrition of the brood mare, foal, weanling and yearling
energy levels
copper deficiency (primary)
zinc excess (causing a secondary copper deficiency)
calcium and phosphorus levels
feed size and frequency of feeding
time of weaning (early vs late)
creep feeding (detrimental?)
Genetics
Environment
season
locality
paddock vs box
Exercise
Pasture quality and area
Ground hardness
Diseases resulting in catch-up nutrition and growth spurts
Biomechanics-abnormal limb loading
Body mass and growth rate (body score)
Endocrine factors/administration of external steroids
5. Approach to the problem

Angular limb deformities and OCD were identified as priorities for investigation. Outcomes were best divided into short term and long term goals.

A. Short term goals: retrospective studies

i) Retrospective audit

Initial efforts should be directed towards establishing current levels of DOD on Hunter Valley studs and determining whether or not the incidence of disease has increased in recent years. An audit of existing records held by veterinary practices and studs will determine the quality of data available and the ease with which they can be retrieved. Information on the number of foals born on each stud also needs to be collected for accurate calculation of incidence rates. The success of such a study relies on the full cooperation of the studs and the assurance of confidentiality.

Recruitment of other stud areas such as Oakey (Queensland) and Shepparton (Victoria) to participate in the study would provide valuable data for comparison. Such information is needed to determine whether DOD is a regional problem confined to the Hunter Valley or a national problem.

ii) Case-control study

A retrospective case-control study would provide a relatively inexpensive means of identifying preliminary risk factors for DOD and could be completed in a relatively short time. However, retrospective case-control studies are limited by the detail and quality of existing records. Cases of angular limb deformity and OCD identified from the previous three years could be compared with control (unaffected) horses randomly selected from stud records. Available information includes: sex and date of birth of foal, mare (resident or visitor), stallion, season, age at onset of clinical problem and body condition score of the mare (not all studs).

B. Long term goals: prospective study

A prospective study extending over three to five years would be an expensive and logistically demanding undertaking. However, it has the distinct advantage of providing detailed information on all variables of interest, rather than being limited by existing records. The use of standardised recording sheets on all participating studs in conjunction with seminars for involved personnel would ensure consistency of the data. The following factors were regarded as important variables to record:

Mare
monthly condition score during pregnancy
nutrition
serial milk samples
resident mare vs visiting mare
Foal
birth weight
date of birth
body condition score
age at disease onset
sire
exercise
blood samples to measure calcium, phosphorus, zinc, copper and endocrine levels
Nutrition
scoring system of the appearance of the pasture
quantitative analysis of pasture (energy, protein etc.)
quantity of grain fed
Any interventions arising from the results of the studies must then be assessed to evaluate the reduction in disease incidence and the cost-effectiveness of implementing the changes.

Workshop participants

W. Howey, R. Rose, D. Hodgson, J. Stewart, S. Reid, C. Chicken, J. Freestone, M. Wylie, T. Lowis, P. O'Brien, J. Rodger, J. White, D. Evans, J. Aldred, W. Neville, P. Flynn, J. Halt, C. Bailey, D. Racklyeft,

Additional reading

Leo B. Jeffcott, MA, BVetMed, PhD, DVSc, FRCVS
"Osteochondrosis - An International Problem For The Horse Industry" J Equine Vet Sci; (1996); 16; 1:32

Eduardo Baccar Varela, DVM
"Control of the Growth Rate of Young Foals in a Pasture-Feeding Program in Argentina" Proc Annu Conv Am Assoc Equine Pract; (1995); 40; :274

Andris J. Kaneps, DVM, PHD, Susan M. Stover, DVM, PHD, Timothy R. O'Brien, DVM, PHD, Roy R. Pool, DVM, PHD, Neil H. Willits, PHD
"Radiographic Characteristics of The Forelimb Distal Phalanx and Microscopic Morphology of The Lateral Palmar Process In Foals 3-32 Weeks Old - Vet Radiol & Ultrasound; (1995); 36; 3:179-187

Laude A. Mitten, DVM; Lawrence R. Bramlage, DVM, MS, Diplomate ACVS; and Rolf M. Embertson, DVM, Diplomate ACVS
"Racing Performance of Thoroughbred Foals with Angular Limb Deformities Treated by Hemicircumferential Periosteal Transection" Proc Annu Conv Am Assoc Equine Pract; (1994); 39; :109

George D. Mundy
"Equine Nutritional Needs" Vet Clinical Nutrition;(1994);1;4:181

C. Wayne Mcllwraith, BVSc, PhD, FRCVS
"What is Developmental Orthopedic Disease, Osteochondrosis, Osteochondritis, Metabolic Bone Disease?" Proc Am Assoc Equine Pract; (1993); 38; :35-44

S. G. Jackson, J.D. Pagan
"Developmental orthopedic disease, multiple causes - no sure cures" J Equine Vet Sci; (1993); 13; 1:9

"A practitioners approach to herd problems with developmental orthopedic diseases in the horse." Summ Annu Conv Am Assoc Equine Pract;(1992);38;:239

D. D. O'Donohue, F. H. Smith, and K. L. Strickland
"The Incidence of Abnormal Limb Development in the Irish Thoroughbred From Birth to 18 Months" Equine Vet J; (1992); 24; 4:305

Kronfeld, D. S. Virginia Polytechnic Institute and State University, Blacksburg VA. Meacham, T. N. Donoghue, S.
"Dietary aspects of developmental orthopedic disease in young horses." Vet Clin North Am Equine Pract; (Aug 1990); 6; 2:451-465

Pool R R
"Developmental orthopedic disease in the horse: normal and abnormal bone formation." Proc Annu Conv Am Assoc Equine Pract; (1987); 33:143-158

Hintz H F
"Factors which influence developmental orthopedic disease." Proc Annu Conv Am Assoc Equine Pract; (1987); 33:159-162

Gabel A A, Knight D A, Reed S M, Pultz J A, Powers J D, Bramlage L R, Tyznik W J
"Comparison of incidence and severity of developmental orthopedic disease on 17 farms before and after adjustment of ration." Proc Annu Conv Am Assoc Equine Pract; (1987); 33:163-170