What's new with old horses--geriatric medicine update (Proceedings)

Geriatric horses are the fastest growing segment of the equine population, and represent an increasing percentage of all equine patients seen by veterinarians and referred to specialty hospitals. This increase is composed of retired horses, breeding animals whose reproductive years have been extended by modern techniques, and performance animals.

Regardless of their use, owners of geriatric horses who seek veterinary care uniformly desire that their animals enjoy a pain-free, quality life. Geriatric horses present a unique challenge to attending veterinarians. One of the two largest sets of problems faced by practitioners and owners is managing chronic lameness or other painful conditions, while the other is identifying and managing age-specific problems such as dental disease, recurrent airway obstruction, and pituitary pars intermedia dysfunction.

What's old?

A geriatric individual is defined as one that has lived at least 75% to 80% of its normal lifespan. Because there is a large variation in aging changes between different horses, this definition is of limited use individually. By and large, the smaller the horse, the longer it lives. Ponies are much longer-lived than horses. Light breeds such as Arabians and Morgans may be productive well into their twenties and thirties, while draft breeds and warm bloods exhibit age-related changes at an earlier chronologic age. When surveyed, owners report that their horses exhibit what they consider age-related changes at an average of 20 years. 

A recent NAHMS study revealed that 7.5 % of horses were older than 20 years of age. Lameness was the most common ailment indentified as a cause of death in aged horses. Paradis and co-workers divided the older horse into three groups; 15-20 years of age were termed mature, 20-29 years of age were defined as old, while 30+ years of age were defined as very old. The oldest horses that were studied were over 40 years. Greater than half the horses older than 20y were still involved in some form of physical activity and many were competing in athletic events.

Phenotypic signs of aging

A horses is a continuous erupter, and its teeth undergo continuous changes that are well-defined through its lifecycle. Altered dentition and an increasing slope to the incisors are consistent changes of aging horses. Loss of peri-orbital fat and elongation of the muzzle due to change in incisor angle contribute to the “aged appearance” seen in many older animals. Loss of pigment accompanied by graying of the hair coat is another consistent finding.

Older animals lose muscle mass and muscle tone. This may lead to a loss of top line and the impression that the horse is too thin. When evaluating an aged horse for weight loss, it is important to perform body condition scoring and judge whether loss of muscle mass is responsible for the impression. Development of a thick, wavy hair coat that does not shed is not a normal aging change, but rather indicative of pituitary disease.                          

Physiologic changes associated with aging

Standard clinical chemistry testing such as CBC and clinical chemistry values are generally similar between aged and younger horses although subtle differences do exist. Red cell indices such as MCV and MCHC are increased in aged horses.  There is a tendency for older horses to have higher blood globulin concentrations and increased lymphocyte numbers. Within lymphocyte populations, an increased CD4:CD8 ratio in aged horses suggests a nonspecific inflammatory state.

Although aged horses do respond to vaccination with antibody production, the response is not as marked as in younger animals.  Resting arterial oxygen concentrations were lower in aged horses when compared to younger animals, suggesting decreased diffusion across the alveolar membrane. Aged horses had lower aerobic capacity, although exercise training does improve fitness level and cardiovascular parameters in horses of every age.

Nutrition and GI tract

Despite the growing number of prescription feeds targeted at aged horses, very little research has been published that specifically examines the nutrient requirements of aged horses.  Many geriatric horses thrive on the same diet they enjoyed as younger animals, and there is no reason to make changes arbitrarily once a certain birthday is passed. As with horses of other ages, good quality forage should be the backbone of any feeding program. Addition of a supplement that balances macrominerals, supplies trace elements, and supplies adequate protein and energy can be fed as needed.

There are many chronic conditions in aged horses that are improved with dietary management. It is preferable to target dietary alterations to the specific condition the horse may suffer from rather than simply recommend a generic senior feed. For example, horses with allergic respiratory disease should be fed low-dust, low-mold feeds. This often precludes dry hay as a feed source. Insulin-resistant horses have decreased risk of laminitis when fed a low starch diet. Additional calories can be supplied as fat if needed to maintain body condition.

The common observation that many aged horses lose weight has led to the assumption that their digestive efficiency is decreased when compared to younger animals. Increasing available calories, often in the form of easily digested feeds and fat. may be useful to maintain condition score. It is important that decreased muscle mass, a normal feature of aging, is not mistaken for generalized weight loss. Condition scoring and careful examination should help differentiate between these two conditions. Apparent digestibility of protein may be less in horses over 20y, and diets containing 14 to 16% crude protein have been recommended.

Serum concentrations of ascorbic acid are lower in aged horses than in younger animals. Although the significance of this finding is not well understood, Vit. C supplementation is routinely recommended.

Many aged horses develop intractable diarrhea. The reason why this occurs is poorly understood, and treatments are generally symptomatic. In the majority of cases, no bacterial or parasitic infection can be detected, and the diarrhea does not respond to larvicidal worming or antibiotic therapy. Transfaunation with either fresh manure or cecal contents can be helpful at times, suggesting that alterations in gut flora may cause this syndrome.

If transfaunation is performed, care should be taken to ensure the maximum number of living organisms are delivered to the recipient's cecum and colons. A 3-day course of omeprazole will increase stomach pH in the recipient. Donor feces should be collected from as deep in the rectum as possible, then kept warm as they are diluted in water to the consistency that will pass through a stomach tube and given as quickly as possible.

Musculoskeletal disease

Many aged horses suffer from some degree of lameness due to chronic degenerative joint disease, chronic laminitis, tendon or ligament injury. In most instances, exercise that provides for range of motion delays progression of lameness. Turn out in a small paddock that allows the horse continuous low impact exercise while it ambulates at its own pace often keeps the horse the most comfortable. Complete stall rest can be counterproductive.

Shoeing to increase ease of breakover and supply support and cushioning may improve comfort. This is particularly true with laminitis and suspensory problems. There is a spectrum of analgesics available, ranging from dietary supplement to daily medication. Non-steroidal anti-inflammatory drugs are the most commonly used analgesic in the horse. There is a maximum amount of analgesia one can expect from NSAID drugs, so increasing amounts above label recommendations will greatly increase the risk of complications such as GI ulceration without providing significant improvement in pain relief.

Despite the recognized problems with administering high levels of NSAID drugs, there is no evidence that suggests giving lower amounts for prolonged periods of time produces the same detrimental effects. Many horses receive one gram of phenylbutazone daily for years without incident.

The Cox2 inhibitor, firocoxib (Equioxx®) at a dose of 0.1 mgm/kg SID ,  only has action on the prostaglandins produced in response to inflammation. For this reason it may produce fewer side effects than classic NSAID therapy, particularly if higher doses are needed. Response to cox2 inhibitors can be individual, and some horses may not receive relief from their administration.

If pain is limited to a small number of joints, local therapy may be preferable to more generalized treatment. Intra-articular corticosteroids are extremely potent, although repeated injections result in cartilage degeneration and may lead to severe degenerative joint disease. Topical DMSO, NSAID preparations, or capsaicin can provide temporary relief as well.

Chondroitin and glucosamine are commonly used as chondroprotectants. Experimental studies on a commonly used preparation of chondroitin, glucosamine, and manganese ascorbate indicate that it was beneficial in relieving pain in horses with osteoarthritis and navicular disease.  Another short-term study found no beneficial effect in an experimental synovitis model. That study may have been terminated too soon, however, as it can take up to 4 to 6 weeks before beneficial effects are noted.

There are a large number of oral supplements and neutraceuticals available to the horseman who is managing a chronic painful condition in an aged horse. Most contain ingredients that have been found to be beneficial in other species, or for which there is some theoretical reason to believe they might be beneficial. These supplements are poorly regulated, and in most instances there are no controlled studies that indicate any benefit.

The geriatric well horse examination

Practitioners should consider modifying their well horse examinations for older horses to both screen for the effects of normal aging and identify diseases common in older horses. A careful dental examination should be performed annually, if not more frequently. A speculum should be used in order to visualize the entire dental arcade on both the maxilla and mandible. If tooth loss or malocclusion suggests that chewing may be decreased, a proactive diet change to an easily chewed feed such as pelleted or cubed forage may prevent choke or GI upset.  If tooth loss or other permanent alteration in dentition is identified, the owner should be notified that an accelerated dental examination schedule is warranted. Floating or other dental manipulations may be needed more frequently than every 6 months to prevent wave mouth or choke.

Body condition scoring and weighing by either scale or weight tape should be recorded so that trends toward loss of condition can be identified. The horse should be jogged to assess whether lameness is present so that an appropriate analgesic regime can be instituted. A careful dermatologic inspection, including all mucous membranes, can identify skin tumors while they are still small and easily treated.

 Screening asymptomatic horses for PPID is warranted given the high number of aged horses that suffer from the disease and the fact that the first clinical sign may be laminitis or infection. The normal seasonal increase in endogenous ACTH must be taken into account when testing. In the northern hemisphere, blood ACTH concentration is higher from August through November than during the rest of the year. There are also exaggerated responses to evocative testing such as the TRH and domperidone response tests as well as the possibility of false positive results to dexamethasone suppression testing in the fall.

For these reasons, past recommendations have warned practitioners against testing for PPID in the fall. Recent data has shown, however, that while all horses exhibit a degree of increase in ACTH in the fall, horses with abnormal pituitary function exhibit hugely exaggerated increases. Finding an extremely elevated resting endogenous ACTH concentration ( greater than 2X the upper limit of normal) in September is extremely suggestive of PPID, even in the absence of any other clinical signs, and further testing is warranted. Alternatively, a normal endogenous ACTH concentration between August and November makes the presence of PPID highly unlikely.

The decision to euthanize

No horse lives forever, and, despite the best care, euthanasia must be considered.  It is often easier for everyone involved if the subject of euthanasia is brought up as part of a well horse visit when the horse is not in immediate distress. The AAEP guidelines for euthanasia can be helpful when discussing euthanasia with owners. Exploring future scenarios and asking the “what if..” questions will help clarify for owners when it is no longer humane to continue.