British Breeder February 2022

Feature Article - Worming | Towcester Vets
Why worry
about worming
young stock?
It is common practice for equine
premises to have worming protocols in
place but this often focussed on the adult
population. Do we need to worry about
the young stock too? Should the same
protocols exist for them? The fact that
foals are born with no natural immunity
against worms and that we are seeing
ever increasing resistance to commonly
used wormers mean foals and young
stock are most at risk of serious and
potentially fatal disease from worms.
This article runs through some clinical
cases highlighting the importance of
farm/ yard management and the
appropriate use and timing of wormers
in these most at risk individuals.
Topics covered:
Ascarid Impactions
Tapeworm and intussusceptions
A resistance problem
A potentially fatal
ascarid impaction
A four month old foal presented with ill
thrift, a poor coat, nasal discharge and
an elevated breathing rate. The foal had
not been wormed. Ultrasound of the
chest revealed changes consistent with
pneumonia and infectious changes were
present on bloods, with a high white
blood cell count. Ascarids can cause
respiratory signs as well as
gastrointestinal signs due to the
extraordinary route around the body the
larval stages take (migrating from the
intestine to the liver, into the circulation,
into the lungs before being swallowed
and ending back in the gut to mature
further). Minimal response to antibiotics
was seen in this foal, as is common with
respiratory signs associated with
ascarids (Clayton and Duncan 1978).
The foal received a wormer and the
following morning was found showing
severe colic signs that did not improve
with pain relief. Abdominal ultrasound
revealed multiple loops of distended,
amotile small intestine. The foal was
referred for emergency colic surgery.
A huge ascarid impaction blocking the
small intestine was diagnosed.
Fortunately, this foal made a full
recovery.
Abdominal ultrasound revealing multiple loops of amotile small intestine
The risk of acarid infection is greater on
premises that are breeding foals
regularly, as youngsters continually
burden the land. However, the size of the
burden is hard to quantify as there is no
correlation between worm numbers and
egg counts (Nielsen et al. 2010)
Ascarid infections can be fatal if they
become impacted in the small
intestinal. This is most common in foals
aged 4-8 months (Tatz et al 2012). To
avoid this aim to treat from 2 months of
age (treating prior to this is less
effective and encourages development
of resistance). Resistance to ivermectin
and moxidectin is seen widely in ascarid
populations. Treating with five days of
Fenbendazole (Panacur ®) can help
target the younger/ larval stages and
has a slower onset of activity so can be
useful if heavy burdens are suspected.
Treating with pyrantel is also beneficial.
Once horses reach maturity and their
innate immunity develops targeted
treatment for ascarids becomes less
necessary.
Tapeworm and
intussusceptions
An eight-month-old weanling presented
with ill thrift and intermittent low-grade
colic signs of a few days’ duration,
which had worsened over time. The
weanling was from a long-established
stud farm that foals a large number of
mares each season. The weanling was
yet to be wormed for tapeworm.
Ultrasound revealed significantly
oedematous large intestinal wall and a
suspected intussusception. This weanling
underwent surgery, a diagnosis of an
intussusception was made and
tapeworm were identified at the
junction between the ileum and the
caecum. Thankfully, despite the gut
initially appearing severely
compromised, once the intussusception
was resolved and the blood supply
returned, no gut had to be removed
and the yearling made a good and full
recovery.
Abdominal ultrasound revealing oedematous
(thickened large intestinal wall) and suspected
intussusception.
Several studies have found an
association between the presence of
tapeworms and colic (Nielsen 2016b),
particularly in horses under three years
of age (Sembrat 1975). Large numbers
of the developing stages of these worms
attach to part of the gut wall, particularly
the ileum, caecum and the ileo-caecal
junction. Colic can be associated with a
physical obstruction of the lumen or due
to damage of the gut lining (mucosa)
and can be associated with
intussusceptions
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