In horses, ponies, and donkeys with PPID, ACTH concentrations in the blood are higher than in animals without this disease. Therefore, the veterinarian measures the relative amount of ACTH as part of their diagnosis. They always interpret the results within the context of the medical history and clinical examination. It is impossible to say whether a horse has PPID based solely on blood tests or only on the clinical picture.
The importance of context in diagnosing PPID: medical history and clinical examination
The most common clinical signs can also have a cause other than PPID. Conversely, ACTH levels can be elevated even if PPID is not present. ACTH is produced in two parts of the pituitary gland: the anterior lobe and the intermediate lobe. In PPID, the intermediate lobe is dysregulated, causing the production of ACTH to skyrocket. Unfortunately, a veterinarian cannot determine with an ACTH test which part of the pituitary gland the ACTH originates from or to what extent the elevated value is caused by something other than PPID. Stress, pain, illness, food intake, exertion, age, season, and even geographic latitude are all factors that can cause higher ACTH levels.
ACTH and breed variations
A 2022 study found that ponies, particularly Shetland and Welsh ponies, and Arabians have higher ACTH levels in the fall compared to horses of other breeds [1]. These findings, regarding ponies, were confirmed a year later by Australian researchers. They concluded that ponies have higher ACTH levels in the autumn compared to Thoroughbreds [2].
Ponies and seasonal influences on ACTH
The study examined 399 healthy horses and ponies, none of which showed clinical signs of PPID or had been previously treated with pergolide. Factors known to increase ACTH levels were excluded as much as possible.
The research team analyzed two blood samples from each animal: one taken during the peak of the seasonal rise and one six months later, when values are typically at their lowest. Results showed that ponies, especially Shetland ponies, had significantly higher ACTH levels in the fall compared to Thoroughbreds, while this was not the case in the spring.
Importance of clinical picture in PPID diagnosis: interpretation of results
The researchers explicitly link this conclusion to the need for veterinarians to exercise extra caution when interpreting test results in pony breeds during the autumn. ACTH measurement should only be performed when the clinical picture clearly indicates PPID. A single PPID blood test, conducted in the autumn as part of a general health check in a pony without clinical signs, increases the likelihood of a false-positive result.
Re-testing
If a Shetland pony has elevated ACTH levels in the autumn and clinical signs of PPID are present but inconclusive, a repeat test in the spring could be considered to assess if the levels remain elevated. However, if the clinical picture is strongly indicative of PPID, the situation is different. What remains clear is that the extent to which ACTH levels increase in the autumn should likely be interpreted differently.
(photo: Maëlys Algans)
Let the cobbler stick to his last
These findings underscore the importance of considering breed-specific factors when establishing reference ranges for ACTH in the diagnosis of PPID in ponies. The researchers emphasise that adjusted reference ranges cannot simply be inferred from the data they provided. Therefore, it is crucial to not try and make a diagnosis yourself based on interpreting blood values independently. The fact that your pony is a Shetland does not automatically mean that high ACTH levels in autumn exclude a PPID diagnosis. On the other hand, of course, there is no harm in drawing your vet’s attention to the existence of this research:
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