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Immunosenescence: when are horses ‘aged’?

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01 March 2014, at 12:00am

“WHAT chronological age is ‘aged’?” was one of the questions posed by Dr Dianne McFarlane in her presentation at the 59th annual American Association of Equine Practitioners’ Convention, held in December.

Dr McFarlane, associate professor in physiological sciences at Oklahoma State University Center for Veterinary Heath Science, reviewed what veterinary surgeons know and what they’re still trying to learn about immunosenescence in horses.

“Immunosenescence is the effects of ageing on the immune system,” she explained, while noting the lack of published data on it for horses.

One challenge faced when studying immunosenescence is the age of horses, she said. “Age-induced changes may be missed if the study group is too young; but a study population that is too old may cause age-related immune deficits to be missed because the selected population may have survived to extreme old age as the result of exceptional immune function.”

In evaluating older horses, it is difficult to know if changes that occur are due to age or a subclinical disease, she said. “In an aged population, this can be difficult because of the high prevalence of co-morbidities that contribute to chronic, low-grade inflammation.

“It is also important to consider hormonal status during the study period because several of the pituitary and adrenal hormones are strong modifiers of immune function.”

Turning to infectious disease, Dr McFarlane said although there was not a lot of evidence of the effect of age on disease in horses, aged equines could be more susceptible to contracting West Nile virus; and in experimental settings, aged mares contracted neurologic equine herpesvirus-1 more frequently than young animals.

No solid data

“But we just don’t have any solid data suggesting older horses are significantly more susceptible to infectious disease than younger horses,” she said.

Age has no effect on faecal egg counts before or up to 12 weeks after anthelmintic administration, according to researchers, but, she added, aged horses with pituitary pars intermedia dysfunction (PPID) had higher faecal egg counts both before and after administration, meaning these horses shed more eggs than healthy aged horses.

Researchers have also studied cytokine and neutrophil function in middle-aged, old, and PPID-affected horses, she said. Among their findings: older horses have increased chemotaxis levels compared to middle-age and PPID horses; and PPID horses have a lower oxidative burst than middle-age and old horses, and the oxidative burst is negatively correlated to melanocyte- stimulating hormone and insulin concentrations.

Stating that although there had been little work on vaccine responses in aged horses, in one study researchers had found no difference in response to rabies vaccines, Dr McFarlane said. She also referred to a study showing that influenza titres appeared less robust in aged horses compared to younger horses.

She went on to give a number of recommendations about caring for ageing horses, particularly those aged 20 or over, to reduce disease risk. These included: using the same biosecurity techniques when managing them as for immature ones; follow the same vaccination protocols; perform routine faecal egg counts and worming programmes based on the results; monitor weight, body condition score and hair coat regularly for changes; test for endocrine disorders at least annually; and be vigilant.

Dr McFarlane said that with optimal care and good genetics, horses could remain actively performing into their late 20s and later.

“Many horses will maintain a good quality of life into their 30s and 40s when attention is paid to early recognition and intervention of health issues,” she said.