Immunosenescence: when are horses ‘aged’? - Veterinary Practice
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Immunosenescence: when are horses ‘aged’?

“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

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

“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

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.

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