Worming and Parasite Management 101

We get asked for advice by horse owners all the time on which dewormer they should buy for their horse, how often they should worm, should you they wormers etc... These are all great questions that start out with the popular answer of: it depends. We do not know which worms are affecting your horse, or what your horse's health, age, feeding program, and stabling situation is like. All of these factors need to be involved when you are having a discussion about controlling parasites in your horse.

First, you need to know the basic life cycle of a parasite. This is quite simple. Most start out as an egg, mature into a larvae, and then into an adult who lays eggs and starts the life cycle all over again. Eggs or larvae are usually found in the ground in the manure of an infected horse, your horse then swallows said eggs and larvae while grazing. These parasites then mature into adults in your horse's gastrointestinal systems. Some species migrate into other organs (such as the lung or the liver) and return once they are mature to the intestines to lay eggs. These species can cause more damage to the horse than those that spend their life in the intestines.

In order to control parasites, you must disrupt the life cycle. Now there are several ways to disrupt the life cycle of a parasite before it even gets to your horse. The most important way to disrupt the life cycle would be manure management. Picking pastures and keeping stalls clean and relatively manure free is the most effective way for decreasing the amount of parasites in your horse (and it will decrease your fly population too!). Manure that has been removed can be placed in a compost pile, or spread on cropland or pastures not being grazed by horses. The eggs and larvae in composted manure will die if sufficient heat is built up. Also, Spreading manure by dragging pastures will decrease incidence of infective larvae if the climate allows for drying of manure. Another option is to alternate grazing pastures for your horses with cattle or sheep. This will disrupt the parasite life cycle because most internal parasites are host specific. Mowing or dragging pastures to break up manure piles will expose eggs and larvae to the heat, which can help to reduce their numbers.

Now that you have managed parasites as best as you could before they ever reached the horse, the next step is to control the parasites within the horse. A fecal analysis is often not something horse owners think of before starting a worming schedule, but it is a very effective tool when controlling parasites. Worms are rarely visible to the naked eye in manure, so it is hard to determine exactly which parasites are found the most on your farm. Sending a few fecal balls into your vet for analysis will give you an egg count of various parasites present in the fecal sample and give the owner a good idea about the degree of parasite infestation on the farm. Once you have the results, you are able to make a better purchasing decision when it comes to which type of wormer you should buy. It will also help you to determine how often your horse needs to be wormed.

Listed below for your convenience are some common parasites, what they are, how they effect your horse, and which wormer can control them.

1. Large Strongyles

   Life Cycle: Start out as eggs, hatch into larvae and consumed by horses while grazing. Larvae mature in the intestinal tract.  One type of large strongyles called the blood worm migrates into the blood vessels of intestines. These bloodworms have a life cycle of 6-7 months. The other two types migrate into the liver and have a life cycle of 8-11 months.

  Effects: Heavy infestation of bloodworms can cause severe or fatal colic or blood vessel ruptures. Other large strongyles cause less severe damage including weight loss, anemia, or colic.

Dewormers: Ivermectin based wormers give a broad spectrum control of Large Strongyles. Fenbendazole based wormers also offer additional control of certain mature stages. Frequent deworming is important (consult your veterinarian to determine how frequently your horse needs to be wormed for large strongyles).

2. Small Strongyles

Life Cycle: These start out similar to large stronglyes. However, instead of traveling to other body parts, small stronglyes burrow into the wall of the large colon. They stay there for months or years until proper conditions trigger them to emerge. While they are encysted (burrowed) in the wall of the large colon, they are usually the most damaging to internal parasites and are resistant to most wormers.

Effects: If large numbers emerge at once, small strongyles can cause severe health problems including diarrhea, weakness, muscle wasting, and colic.

Control: Ivermectin based wormers give a broad spectrum control of Small Strongyles. Fenbendazole based wormers also offer additional control of certain mature stages. Moxidectin can control some encysted small stronglyes. Frequent deworming is important (consult your veterinarian to determine how frequently your horse needs to be wormed for Small strongyles).

3. Roundworms

Life Cycle: Horses swallow eggs of roundworms in contaminated hay or water. These eggs develop into larvae in the stomach, and then migrate to the live, heart, and longs where they are then coughed up and swallowed. Once back in the stomach they develop into adults. Their life cycle takes about three months.

Effects: The most damage occurs as the roundworms migrate through the body, causing coughing, pneumonia, liver damage, diarrhea, and colic. Large numbers of roundworms can cause intestinal blockage or ruptures. Signs include pot bellies, rough hair coats, slow growth, and unthriftiness.

Control: Use Ivermectin based wormers for broad spectrum control. NOTE: horses acquire some immunity to roundworms as they age. Horses under two years old are much more susceptible to roundworm infection than older horses. When first dewormed foals older than 3 months are more prone to colic. Consult your vet regarding worming for roundworms.

4. Lungworms

Life Cycle: Larvae are ingested while grazing, then they migrate through the body to the lungs where they mature and lay eggs. Eggs hatch in the longs, are coughed up and swallowed into the stomach and then passed out in manure. Life cycle takes about 28 days. Donkeys are typically the ones infected by lungworms, but can be found in horses pastured with them.

Effects: coughing or respiratory problems, especially when exercising.

Control: Use an Ivermectin based wormer. Consult vet on frequency of deworming.

5. Pinworms

Life Cycle: Female pinworms lay eggs in the skin around the horse's anus where they are rubbed off onto ground and eaten by the horse.

Effects: Eggs are extremely itchy. Horses will sometimes rub their tails until all the hair is pulled off. Adult pinworms are about 1-3/4 inches long and can sometimes be seen around the anal area with a clear discharge (egg masses)

Control: Use disposable wipes or paper towels to clean area under the tail to avoid spreading the eggs and infection. Use cormers containing ivermectin, fenbendazole, pyrantel pamoate, piperazine, moxidectin, or praziquantel to control.

6. Stomach Worms

Life Cycle: Transmitted to horses by flies which transfer the larvae when the land on the horse's legs, muzzle, or eyes and the larvae are licked off and ingested.

Effects: There are several types of stomach worms and usually don't cause much of a problem other than perhaps some mild diarrhea. However, if the larvae get into an open sore or in the moist areas of the eyes they can cause a serious skin condition called summer sores. The body responds to the irritation by forming a lesions (called a granuloma) which doesn't heal. It is very itchy and the horse often rubs and scratches the sore causing severe trauma to the area.

Control: Use an Ivermectin-based wormer. Consult a vet for frequency of deworming.

7. Bots

Life Cycle: Adult bot flies place eggs on horses' forelegs, shoulders, around the jaws, and around the lips. When larvae hatch, they move into the mouth and burrow into gum tissues and the tongue to develop. Eventually they move into the stomach and intestines where they live up to 12 months before passing out of the horse and develop into adult bot flies. Cold weather kills bot flies.

Effect: Can cause damage to the tissue of the horse's mouth and intestinal tract, but most horses don't show signs of serious diseases from bots. Very large numbers of bots have been associated with gastric ulcers.

Control: Use Ivermectin Based Wormer.Worm for bots during the spring/summer months.

8. Tapeworms

Life Cycle: A tiny mite acts as an intermediate host, carrying the tapeworm. Horses develop tapeworm infections when they eat this mite in grass, hay or grain.

Effect: Tapeworms can cause colic, ranging from mild cramping to severe colic requiring surgical treatment.

Control: Use a Praziquantel based dewormer annually to control for tapeworms.

Hopefully this helps you horse owners to make some informed worming decisions (I know I was happy to put my college notes to good use!). Remember, running a fecal analysis can help to manage parasites more effectively and could even let you know that you are doing a great job and don't need to worm as frequently (saving you money and decreasing the chance of parasite resistance to wormers!)