19May 2009 CumbriaHorse
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Ignore worms at your peril
Sam Galloway, of Frame, Swift and Partners, says that understanding
worm life-cycles can help to reduce the burden for horses and donkeys
O
ne of the most essential
mantras of veterinary
diagnostics is that com-
mon things occur com-
monly. Despite the advent of
fantastic new anthelmintics,
the pernicious effects of inter-
nal parasites may well be the
cause of a thin looking horse or
a horse that will not gain
weight despite good nutritional
support.
The common causes of weight
loss 50 years ago, such as poor
dentition, parasitism or liver dam-
age due to ragwort, are still the
common problems today.
Worm egg counts will be needed
to help with the diagnosis and
they are hugely helpful after treat-
ment as well.
Post-treatment counts help
detect cases of wormer resistance,
although they are fairly rare. A
positive count shortly after worm-
ing is more likely to indicate that
an inadequate dose has been given
due to underestimating the
horse's weight or that the horse
spat a proportion of the wormer
on the ground!
Worm egg counts from faecal
sampling will only detect round-
worms, including the family of
large strongyles, pinworms and
small redworms.
Faecal worm egg counts do not
detect tapeworms or lungworms.
Tapeworms are detected via blood
sampling to demonstrate the pres-
ence of anti-bodies. Lungworms
may be detected in the faecal sam-
ples of donkeys, but this is not
reliable in horses or ponies. This
is because donkeys are the `nat-
ural' host to dictyocaulis arnfieldi
and horses are not as host-specif-
ic.
Horses and ponies put up a ter-
rific resistance to the lungworm,
preventing the adult from laying
any eggs. Because donkeys are
more host-specific, they do not
immunologically resist the lung-
worm as much.The life cycle can
consequently be completed so that
eggs are passed and the pasture
contaminated.
The huge inflammatory effort
that horses and ponies put into
resisting lungworm is why they
have a persistent cough; donkeys
often won't cough at all despite a
significant burden.
With good monitoring and mod-
ern wormers, there is no reason
why donkeys and horses cannot
co-exist. A horse with a chronic
cough would be investigated with
a fibre-optic examination of the
airways and a sample of inflam-
matory fluid taken.
Lungworm cannot be demon-
strated on the lung-wash sample,
but the presence of a particular
inflammatory cell (eosinophils)
allows the diagnosis of lung-
worm.
Eliminating lungworm used to
be a huge problem, but modern
anthelmintics are vastly more
effective than their predecessors.
We should not rely totally on
anthelmintics to control internal
parasites though. They are also
vulnerable through their often
bizarre life-cycle and their need to
spend some of that life-cycle out-
side the host.
It is worth knowing a bit about
various life-cycles to help reduce
equine worm burdens.
The large roundworms
(stongyles) lay their eggs on the
pasture. Even a moderate faecal
worm egg count of 1,000 eggs per
gram means that a horse can
deposit millions of eggs daily
onto its grazing pasture.
It is difficult to give a set
regime for worming as foals and
yearlings have little innate resis-
tance to parasites. They can be
real multipliers of a worm prob-
lem and quickly contaminate a
pasture. Adult horses have
acquired resistance and pass
many less eggs.
Checking the worm egg counts
will reveal the degree of pasture
contamination.
The eggs hatch and the larvae
are ingested. The larvae of S.
edentatus migrate through the
bowel wall and chew their way
through the peritoneum, liver and
pancreas before finding their way
back to the gut to develop as egg-
laying adults. The larvae of S. vul-
garis leave the bowel and find
their way back through the cra-
nial mesenteric arteries that sup-
ply the intestine with blood. They
damage the walls of the blood ves-
sels and cause aneurysms and
thromboses.
There has been a massive
reduction in the incidence of colic
in the equine population and, in
my opinion, bringing these par-
asites under control has played a
massive part.
Small strongyles (strongy-
loides) are known as red worms.
The larvae can gain access to the
body either by ingestion or by
skin penetration. These larval
stages can arrest their develop-
ment in the bowel wall and over-
winter there. In the spring-time
they all erupt at the same time
causing huge, sudden damage to
the bowel wall.
These inhibited larvae are
known as cyathostomes and can
kill the host when they simulta-
neously erupt. Regular worming
with appropriate wormers can
eradicate cyathostomes, but not
all wormers are effective.
Tapeworms such as anoplo-
cephala are also implicated with
colic. The adults congregate at the
junction between the small and
large intestine. The immature
stages live in the intermediate
host on the pasture in the shape of
the oribatid mite. The mites are
unknowingly eaten and the cycle
begins again.
Bots are the larval stage of the
gastrophilus fly. The fly lays her
eggs on the horse and are licked
off. Once in the mouth, the devel-
oped larvae are not directly swal-
lowed but migrate through the tis-
sues of the mouth and neck and
enter the stomach where they
stay for 10 to 12 months before
being passed out to develop as
flies.
All these parasites are implicat-
ed in causing ill thrift, colics and
even death. Knowing their life-
cycles helps us use the correct
wormers at the right time to
defeat this insidious enemy.
Diagnosis: Endoscopy can be used to look for lung worm

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