Your Horse Riding Equestrian Reference

A guide to the Horse Worms Life Cycle

Horse Health … A look at the equine intestinal parasite or horse worms life cycle.

Always use a good worming programme that includes a range of wormers and always consult your vet if you suspect worms are a problem.

Modern de-wormers and the excellent education of horse owners over the past few decades has reduced the dangers from hore worms, but it is all to easy for horse owners to lapse in their duty of care to the domesticated horse.

In This Section …

Understanding how the horse worms life cycle repeats and how the different species of horse worms affect the horse in your care, is crucial if you are to appreciate why you should operate an effective worming program.

It also helps you understand why good pasture management is an integral part of that worming program.

The life cycle of Roundworms, Bloodworms, Bot Fly, Tapeworms and Pin Worms

The Round Worms Life Cycle :

Round worms life cycle

Roundworms – generally affect young horses much more than older horses.

There are several reasons for this higher rate of infection in young horses and more specificially 2 year olds.

Firstly foals and yearlings don’t seem to be as particular about where or what they eat and they have a tendency to eat in areas where the number of round worm eggs may be greater.

It also appears that as a horse gets older it developes some immunity to the eggs or larva as they migrate through the tissues.

And that keeps them from becoming adult worms in the intestines.

  • Mature round worm eggs are eaten by the horse and hatch when swallowed
  • The larvae pass through the intestinal wall and into the horse’s liver
  • The blood from the liver carries the larvae to the horse’s lungs, where the round worm larvae get into the airways
  • The larvae are then coughed up and swallowed to become adults in the horse’s small intestine
  • The adult females lay around 200,000 eggs each day, in the small intestine
  • The round worm eggs are passed in the horse’s droppings and develop when temperatures are right.
  • The mature eggs are then eaten by the horse and the round worm life cycle repeats.
Roundworm eggs can remain dormant in the soil for many years, and can withstand great variations in temperature

This can be well below freezing for long periods of time, or very hot and still not kill the larvae inside the eggs.

  • The adults look very much like some of their close relatives, earth worms.
  • They can grow to 5 inches or nearly 20 inches in length and are generally white to light grayish blue in color.
  • They are usually described as looking like spaghetti and can cause the young horse to appear mal-nourished (rough hair coat and pot bellied).
  • The larva are often responsible for fairly severe respiratory problems in weanlings and yearlings as they migrate through the lungs.
  • Large numbers of these worms can also cause impaction colic in young horses.

The Blood Worms Life Cycle :

Bloodworms life cycle

Bloodworms – Strongyles are responsible for the most damage done to horses by parasites.

  • Blood worm larvae are eaten by horse and go through the intestinal wall to migrate through the horse’s tissues
  • The migration can cause damage to the horse’s liver, arteries and intestinal lining
  • The blood worm larvae remain encysted in the intestinal lining before growing to adults in the horse’s large intestine
  • Adult blood worms in the large intestine lay eggs that are passed in the horse’s droppings
  • The eggs hatch and develop, passing through two larval stages before becoming infective ensheathed third stage blood worm larvae
  • Infective larvae are eaten by horse and blood worm life cycle repeats
There are over 34 species of strongyles divided basically into 2 groups. The large and the small.

The large strongyle group has been the most damaging.

However, the education of horse owners about the effects of internal parasites over the last 50 years, plus the development of new and more effective de-wormers has greatly reduced the harmful effects that strongyles have within the equine industry.

But, the prolific nature of the strongyle worms has allowed them to withstand the effects of these new worming medications.

Blood worm infestation can cause colic and anemia or death if left untreated

Some horses pass as many as 5000 strongyle eggs per gram of stool and a drop of dew taken from a blade of grass in an equine pasture may contain as many as 100 dormant larvae.
  • Studies have also indicated that 80%-90% of colics in horses are caused by strongyles.
  • Both large and small strongyles have been proven to cause colic.

The larval stages of large strongyles migrate through many tissues in the body of the horse, such as the liver, arteries, and intestinal lining.

Small strongyle larvae excyst in the lining of the intestine and cause colic when they exist in large numbers.

Bot fly  worms life cycle

The Bot Fly and its Worms Life Cycle :

Bot flys infect horses in large parts of the country.

Bots have a varied and interesting life cycle and the adults resemble a honey bee.

  • Female adults bot fly’s lay eggs that they attach to the horse’s coat
  • Horse’s groom and lick the eggs.
  • The eggs hatch and penetrate the horse’s tongue and gum tissue
  • The bot fly larvae migrate through the horse’s tissues to the stomach where they attch to the lining and can cause small ulcers.
  • The bot fly larvae mature in the stomach and are passed in the horse’s droppingsin the spring, where they harrow into the ground.
  • Later in the spring the bot fly larvae mature, emerge from the ground as adult flies and repeat the bot fly worms life cycle

There are three main species of bot fly differing in size and colour. Each species lays it’s eggs in a differnet region of the horse’s body and in it’s stomach.

The larvae in the horses stomach look similar to many free living grubs.

The larvae attach themselves to the lining of the stomach and can cause a very small ulcer.

When there is a heavy infection and a great many larvae present then the individual ulcers can join and form one large ulcer.

If the resulting ulcer is large enough it can cause horse colic.

Tape worms life cycle

The Tape Worms Life Cycle :

Tapeworms are generally not thought to be much of a problem to horses.

However, in the last few years there seems to have been an increase in the number of horses being infected with tapeworms.

  • Horse’s eat grain mites, commonly found on permanent pasture, with developing tapeworm eggs inside.
  • Eggs become the head of the tapeworm and attach to horse’s intestinal lining
  • Tapeworm head begins to grow segments filled with eggs
  • The adult tapeworm continues to live in the large intestine of the horse and consists of the head and the egg filled segments.
  • Eventually the segments break off and are passed in the horses’s stool
  • The horse’s stool is eaten by grain mites (obatid mites) commonly found on permanent pasture.
  • The horse eats the grain mites and worms life cycle repeats.
The three common tape worms that affect horse’s are…
  • Paranopplocepha la Mamillaria – averaging 3cm in length
  • Anoploceplala Perfoliata – averaging 8cm in length
  • Anoploceplala Magria – growing up to 80cm in length
Normal de-worming medications are generally not very effective at eliminating tapeworms.

Therefore, in areas where tapeworms are known to be exist, special deworming programmes and specific wormers must be used to specifically control tapeworm.

Fortunately at present tapeworms do not cause horses a great amount of difficulty.

However, in certain situations the worm can cause colic and determined efforts should be made to control them.

Pin worms life cycle

The Pin Worms Life Cycle :

Pin worms – when you hear someone say

"My horse must have worms, it’s rubbing it’s tail"

then pin worms are usually the main culprit .

  • Female adult Pin worms lay eggs abd attach them the the horse’s anus
  • Eggs with glue eggs are attached to the horse’s anus causing extreme irritation
  • Horse rubs tail and eggs drop off into feed or pasture and are eaten by the same and other horse’s
  • Eggs hatch and spend a short time in the horse’s mucosa of intestine, become adults.
  • Adult pin worms feed on intestinal contents and begin laying eggs
  • Worms life cycle is repeated

The glue that the adult female pinworm uses to attach her eggs to the horses external rectal tissue causes severe irritation to the horse.

This irritation is a major cause of tail rubbing.

However, due to major improvements in recent worming medications and the modern overall cleanliness of todays stables and yards pinworm problems have been greatly reduced.

Remember : Treat horse worms and intestinal parasites by always using a de-worming programme that includes a wide range of commercial de-wormers.

Never use the same wormer continually as this will build the worms immunity. Use Intelligent Worming if you suspect your de-worming program is not as effective as it should be.

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