Roundworms
The Way
Of Horses:
Controlling roundworms
(Parascaris spp./ascarids) has
been an on-going challenge
for horse breeders.
The 1916 United States Department
of Agriculture book “Diseases of the
Horse” describes how the roundworm
affects the growth and health of horses
– unthriftiness, diarrhea or constipation,
colic and respiratory problems. Recommended
treatment compounds
were tartar emetic, turpentine or
carbon bisulphid for the removal of
the parasite. Foals were treated with
a mixture of the chemical of choice
and honey which was placed on the
back of the tongue twice a week.
Older horses were fasted for 18 hours
and then administered the poison with
a stomach tube. The toxic chemicals
basically caused severe diarrhea and
purged the intestinal tract. The book
warns that the treatments are dangerous
to the horse and can cause death
or permanent injury to the kidneys or
other organs.
Roundworms are primarily a young
horse’s nemesis. The newborn foal is
free from parasites, but it doesn’t take
long for the youngster to be exposed.
Ascarid eggs can live for years in pastures
and turnouts on plants, buckets,
fences and stall walls – just waiting for
the exploring foal to find and swallow.
The eggs can also become stuck to
the mare’s udder and swallowed when
the foal nurses.
The eggs travel to the small intestine
where they hatch. After hatching,
the larvae work through the intestinal
wall into the blood stream, reaching
the liver and eventually ending up in
the lungs. The larvae are coughed
up, swallowed and return to the small
intestine where they complete their
lifecycle - maturing into adult egg-laying
Health
worms. The new eggs are passed
out of the horse in the manure where
they wait to be ingested by the next
EQUINE horse. It is estimated the eggs can live
for close to 10 years in a suitable environment.
Extreme heat and cold does
not effectively kill the eggs, which are
encased in a hard protective capsule.
60 www.EliteEquestrianMagazine.com Third and fourth treatments should be
considered at about 9 and 12 months
of age, respectively, and treatment
should primarily be targeting strongyles.
Tapeworm treatment should be included
in the 9-month treatment.” https://
aaep.org/sites/default/files/Guidelines/
AAEPParasiteControlGuidelines_0.pdf
Dewormers are ineffective on roundworms
until the parasites become
adults, so deworming foals before
the age of 2 months is not advised.
Deworming foals at an early age sets
up resistance – resulting in dewormers
being ineffective later when they can
be of use.
Care must be taken when administering
the first dose of dewormer. If
the foal has a large worm burden the
sudden die-off of adult worms can
cause a blockage, resulting in impaction
colic. It should be done under the
supervision of a veterinarian.
By the time the young horse is two
years old resistance to roundworms has
developed. Occasionally adult horses
will have a low roundworm burden.
In 1916 managing manure was the key
to keeping the exposure down. The
book recommended stalls and pastures
be cleaned daily and the manure
not spread on paddocks where mares
and foals were kept. Feed was to be
offered off the ground.
This is true today - management is key.
Overstocking pastures and turnout
areas should be avoided. Isolation of
new horses with dubious health history
should be implemented until a fecal
count can be taken and treatment
completed before introduction to the
herd. Manure removal and pasture
rotation can also help in managing
internal parasites.
It is impossible to totally remove the risk
of exposure to roundworms, but it can
be decreased.
As the 1916 book stated roundworms
can cause poor growth, coughing,
nasal discharge, inflammation and
colic. But foal owners must realize
that youngsters without symptoms will
still have roundworms. Total eradication
of roundworms in the environment
is impossible.
Over the last 100 years our choice
of chemicals and administration has
improved but the effectiveness of our
modern dewormers is decreasing.
A study done in September 2002 on
a farm in Ontario, Canada showed
that foals which had been regularly
dewormed with ivermectin since birth
had poor roundworm control – they
were resistant to ivermectin. Patrick
D Hearn, F & Peregrine, Andrew.
(2003). Identification of foals infected
with Parascaris equorum apparently
resistant to ivermectin. Journal of the
American Veterinary Medical Association.
223. 482-5, 455. 10.2460/javma.
2003.223.482.
To prevent resistance to dewormers
the American Association of Equine
Practitioneers recommends for foals
and weanlings: “During the first year
of life foals should receive a minimum
of four anthelmintic treatments. First
deworming should be carried out
at about 2-3 months of age, and a
benzimidazole drug is recommended
to ensure efficacy against ascarids.
Second deworming is recommended
just before weaning (approximately 4-6
months of age). An extra treatment
can be justified before weaning if the
time period between the two treatments
exceeds 3 months. At weaning
a fecal egg count is recommended
to determine whether worm burdens
are primarily strongyles or ascarids, to
facilitate the right choice of drug class. EE
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