I have just sent my first samples of the year off to be
tested for worm eggs! A couple of weeks
ago I wrote a blog about Worm Egg Counts and I thought today it would be worth
reposting a blog I wrote a few years ago about worms ....
What are worms?
They are parasitic worms which affect the health and well
being of horses and ponies. There are
several different types which affect horses and some are so small that they can
only be seen under the microscope. Some
types are very damaging, others are just a nuisance.
Lifecycle
Most of the worms follow a similar lifecycle, there are 2 stages.
Free living, on pasture - Adult worms live in the intestines
and lay eggs which are then passed out in the droppings. The eggs hatch into Larval stage 1 and then
develop into Larval stage 2. These
stages remain in the droppings and are not infectious.
Larval stage 3 move out of the droppings into the grass where they can then be
eaten by the horse. Moisture is
essential for the larvae to move out of the droppings!
Inside the Horse - Once eaten the stage 3 larvae will pass
through into the stomach and the intestines.
They develop here and eventually become adults. During the summer this
whole lifecycle is about 8 weeks. As
autumn approaches an increasing number stop developing and hibernate in the
wall of the intestine (hypobiosis). In
spring these larval stages will emerge.
Each type of worm has a specific lifecycle and the larval
stages of some migrate within the body, passing through the liver, lungs and
blood vessels causing damage.
It is worth knowing that most parasitic worms are 'host specific'
and so those that infect horses won't infect sheep or cattle and vice
versa. Donkeys and horses have the same
parasites though!
The free living stage is affected by the climate. Warm and damp conditions will mean the
lifecycle is quicker than if it is cold or dry.
Worm Types
Roundworms (Nematodes)
Large Redworms = Strongylus vulgaris
Lifecylce 6 – 11 months
2 – 5 cm long, reddish brown worms not common but
potentially very dangerous. These have a
much longer life cycle than the small redworm and it takes roughly 6 – 7 months
development time from ingesting infective larvae to adult worms laying eggs
which pass out in the faeces.
Infection is acquired by ingesting infective larvae over the
spring and summer. The larvae spend the
majority of their time in the walls of arteries, particularly the major artery
supplying the gut, the cranial mesenteric artery, and its branches. They move in and out of the artery wall
causing severe damage, and blood clots form.
These clots can become dislodged and block lesser blood vessels.
Adults feed off the plugs of intestinal wall.
Small Redworms = Cyathostomes
Lifecycle 5 – 18 weeks
Forming between 90% and 100% of the worm burden, these worms
are very small and live in the large intestine.
They are the most important parasite of horses and the encysted larvae
can become dormant for several months or even years. Adults feed off plugs of
intestine interfering with digestion and nutrient absorption but the larvae are
more damaging. If not properly controlled
during the previous summer the seasonal emergence of huge numbers of larvae
from the gut wall during late winter and early spring can cause acute
diarrhoea, rapid weight loss, colic and sometimes death.
Pinworms or Seatworms = Oxyuris equi
Lifecycle 4 – 5 months
Females can be up to 10 cm long. Adult worms migrate to and lay their eggs
around the skin on the outside of the anus.
The egg laying causes intense irritation. The shed eggs are then picked up off pasture
and stabling. Scratching and rubbing of
the anal region results in the opening of wounds and loss of hair on the dock.
Large Roundworm = Parascaris equorum
Ascarids
Lifecycle 10 – 12 weeks
Affecting foals these are very long worms (50cm) when mature. They produce large numbers of tough coated
adhesive eggs, the females are capable of laying as many as a million eggs per
day. These eggs which stick to the
environment eg: mare’s coat and udder, stable walls and floors and can persist
for many years.
The larvae migrate through the bloodstream to the liver and
lungs before returning to the small intestine to develop into adults and start
to lay eggs. Egg laying adults can be present in the gut of foals at
around 12 weeks of age. By 18 months – 2 years of age young horses acquire a solid
immunity to ascarid infection.
Lungworms = Dictyocaulus arnfieldi
Lifecycle 2 – 4 months
Infected larvae are picked off the pasture. The larvae migrate through the blood stream
to the lungs where they develop into adults.
Eggs are laid, coughed up, swallowed and pass out in the dung.
Only rarely is the lifecycle complete in
adult horses. Lungworm causes
respiratory problems, persistent coughing and ill thrift in adult horses. Symptoms are rarely seen in foals or donkeys
(which can act as carriers for the disease).
Threadworms = Strongyloides westeri
Lifecycle 8 – 14 days
Very small worms.
Foals are usually only susceptible up to 6 months of age, after which
they develop immunity. Classically
causes diarrhoea in foals of 1-2 weeks of age, although cases have been
reported in foals up to 16 weeks of age. This parasite passes to the foal
mainly via the mare’s milk after foaling and, as the diarrhoea often occurs
when the mare first comes into season, it has been called ‘foaling heat
scours’. The worm also has a free living stage which can penetrate the skin.
Large Mouthed Stomach Worms = Habronema muscae
The adults live in the stomach. Eggs are passed out in the droppings, hatch
and are picked up by fly maggots feeding in the dung. The larvae are carried in the mouth parts of
the fly. As the flies feed they infect
the horse which swallow the larvae.
Stomach Hairworm = Trichostrongylus axei
Lifecycle 3 weeks
These worms live in the stomach causing irritation and
damage to the stomach lining. They can also infect sheep and cattle. Heavy infestations can cause loss of appetite
and diarrhoea.
Neck Threadworm = Onchocerca
Adults live in the tendons and ligaments. The larvae live under the skin and are taken
up by midges as they feed. They can also
congregate in the eye tissue.
Bots = Gastrophilus
Lifecylcle 1 generation per year
Bots are not worms but fly larvae. They are the most common and important
parasite of the stomach. The flies lay
eggs on the legs and face which hatch before entering the mouth. The larvae live in the tissue of the mouth
and tongue for several weeks before reaching the stomach. The larvae attach to the stomach wall where
they remain until the following spring when they pass out in the
droppings. They mine underground where
they pupate. The adult flies emerge
during the summer months and start to lay eggs, the first hard frost kills the
adults.
Bot Eggs
Tapeworm = Anoplocephala perfoliata and others
Lifecycle 3 – 5 months
Tapeworm eggs are passed out in the dung and consumed by a
free living intermediate host, the oribatid (forage) mite. The mites are eaten up with the grass by the
horse. The adult worms attach themselves
at the junction of the small and large intestine. Tapeworm has been associated with colic and
other gut disorders but this association is far from clear.
I am just hoping that Tommy and Basil have low
counts and so won't need worming!
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Until
next time!
Jo
