Worms and Worming - Part 1!

In January I posted an 'Essential Paddock Knowledge' blog which talked about fencing, gates and water options.  I mentioned that I would do a separate blog about Poisonous Plants (which will be in a couple of weeks) and a blog about worms and worming. 

Worming is  a HOT TOPIC at the moment because many of the worms which affect horses are becoming resistant to the chemicals.  Therefore we are being encouraged to use worm counts and reduce the amount of wormer we use... more about that next week.  This week I thought I would do the science bit!

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, 8-9mm.  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.

Next week I will talk about what this means and what we can do to limit the affect these parasites have on our horses!

Have you seen this week's video 'How to ...do a Tapeworm Test' 
Horse Life and Love.   Please check it out and SUBSCRIBE.

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Until next time!

Jo