Deworming your horse

Roundworms

Roundworms (Parascarus equorum) usually appear in horses 3 to 9 months old. A veterinarian or diagnostic laboratory can find eggs in the horse's feces.

Signs of roundworms in foals:

• Decrease in appetite
• Slow growth rate
• Dull, dry hair coat
• Potbelly appearance

Treatment

Use pyrantel pamoate or fenbendazole to kill adult roundworms. If a heavy burden is suspected, fenbendazole will work better. You can use ivermectin or piperazine to kill the larval stages of the worm.

Large strongyles

Large strongyles (Strongylus vulgaris) infects the cecum and ventral colon (large intestine). Fecal flotation tests can detect large strongyles. When large numbers of larvae invade the intestine, the horse may become clinically sick.

Signs of sickness from large strongyles:

• Fever
• Diarrhea
• Loss of appetite
• Weight loss
• Colic
• Death, if not treated

With chronic infections, random repeated colic is a major sign of disease.

Treatment

Use ivermectin and moxidectin to treat against larval stages. And use oxibendazole, fenbendazole, or pyrantel pamoate to treat against adult worms.

Small strongyles

Small strongyles (Cyathostome) form little growths in the lining of the cecum and colon. As they emerge, they damage the lining and cause swelling, which can hinder digestion and uptake of nutrients.  Fecal flotations can detect small strongyles.

Signs of small strongyles:

• Anorexia
• Weight loss
• Diarrhea
• Colic
• Severely underweight, if not treated  

Treatment

Use ivermectin, oxibendazole, pyrantel pamoate, or piperazine to treat against adult worms. You can use ivermectin, moxidectin, or pyrantel tartrate to treat against larvae.

Tapeworm

Tapeworms (Anoplocephala perfoliata) live at the end of the small intestine and in the large colon. Tapeworms are present in horses over 6 weeks old. You may see portions tapeworm in the horse's feces. Horses don’t consistently shed tapeworm segments or eggs in their manure so fecal test may not detect them. The rate of tapeworms in horses is highest in Minnesota so be sure to include products that target tapeworm in your deworming program.

Horses with tapeworms have no visible signs of parasitism. But tapeworms may cause:

• Slow growth
• Loss of body condition
• Diarrhea
• Colic

Treatment

Use twice the normal dose of pyrantel pamoate to treat against tapeworm.

Stomach bots

Horses with stomach bots (Gasterophilus) often show no symptoms. Stomach bots can cause lesions in the mouth, esophagus and stomach, which may make the horse reluctant to eat.

Treatment

Treat against stomach bots with ivermectin or moxidectin after the first hard frost.

Pinworms

Pinworms (Oxyuris equi) can affect all ages of horses. You can detect pinworms by checking the anal discharge for eggs or by “trapping” the worm with scotch tape.

Signs of pinworms:

• Anal itching
• Restlessness
• Poor eating
• Yellow/ gray discharge from the anus

Treatment

Treat adult pinworms with fenbendazole, oxibendazole, pyrantel pamoate, piperazine or ivermectin. Treat foals for pinworms with fenbendazole.

Threadworm

Threadworms (Strongyloides westeri) usually affect young foals between 10 days and 6 months of age. Fecal flotation can detect threadworms. Threadworms cause swelling and damage to the small intestinal lining. This may impair digestion and uptake of nutrients.

Signs of threadworms:

• Poor growth
• Depression
• Mild diarrhea

Treatment

Foals get threadworms through the mare's milk. Treating the mare with ivermectin within one week of foaling will reduce infection. Treat the foal with oxibendazole or ivermectin if you suspect threadworm.

The key to selecting a dewormer is to know what parasite group each active ingredient targets (Table 1.).

Rare, adverse reactions to moxidectin have lead to its disuse by some veterinarians.

Targeted dosing

This strategy involves testing the total number of parasites in individual animals.

• Standard fecal egg counts should be performed once monthly.
• Tapeworm testing via fecal testing or blood testing (serology) should be done twice yearly.
• All animals that are positive over a certain cut off level should be treated.
• A yearly treatment for botfly larvae (bots) should also be included during the winter.

This program is only appropriate for adult horses and should be considered on a farm with a dedicated manager where good grazing management is in place.

Strategic dosing

This strategy involves treating all pastured animals at regular intervals with an appropriate product.

• The interval between dosing can be determined by the egg reappearance period (ERP) of the medication, which is shorter for young animals. The ERP is the period after medicating an animal with a dewormer until there are significant numbers of parasite eggs present again in the feces.
• Animals are only treated during the spring/summer season when the risk for increased egg loads is highest.

Interval dosing

This strategy is the one most commonly used. It is similar to strategic dosing.

• Animals are treated year round at regular intervals.
• As the duration of parasite kill varies from product to product and even between farms, the interval between doses should be determined by the ERP or by guidelines set by your veterinarian based upon products used.
• Appropriate for farms where there are frequent new additions to the group, at more casually managed (hobby) farms, and in young animals.

Daily deworming

This strategy involves adding a parasite control medication to the horse's daily ration.

• Appropriate for most adult grazing horses.
• Parasites are continuously exposed to a low level of the drug and may become resistant to the medication over time.
• Additional periodic deworming with other products is usually necessary. Consult your veterinarian for current recommendations.

How often should I deworm my horse? 

Fecal egg counts can help you decide whether your horse needs deworming. If your horse is shedding high egg counts you can deworm them for that specific parasite(s) respectively.  Ideally, you’ll find a balance where you control the parasites but don’t over deworm. Over deworming can promote the growth of resistant parasites.

Most farms use a double dose of pyrantel or a praziquantel product once a year for tapeworm control. Farms with a high stocking rates may deworm more often or use daily wormers  for individually fed horses.

When should I deworm mares and foals? 

Deworm mares after foaling to reduce passing Strongyloides to the foal through the milk. Prior to weaning (about 2 months of age), start foals on a deworming program that targets roundworms.

How else can I manage parasites? 

You must use dewormers to manage parasites, but management also plays a key role in parasite control programs. The following management tips can improve parasite control on your farm.

• Isolate and treat new animals on the farm to make sure that highly infected animals don’t shed parasites in the environment and infect other horses. Alternatively, require that new animals be dewormed prior to joining the stable.

• Closely monitor young horses as they are prone to parasitic diseases.

• Avoid feeding off the ground, especially in soiled areas and in box stalls.

• Routinely pick or drag manure in pastures and routinely mow pastures. This will break up manure piles and destroy eggs and parasite larvae through drying, sunlight exposure or freezing.

• If your horse's’ primary forage comes from grazing, make sure animals aren’t overcrowded or the pasture overgrazed. A good recommendation is each 1,000-pound horse needs two acres of pasture.

• Rotational grazing will help reduce parasite exposure by:

  • Spreading out manure

  • Giving manure time to break down

  • Reducing overgrazing

How can I tell if my parasite control program is working?

Sending in fecal samples yearly can help you monitor parasite loads in your horse and the dewormer efficacy. You can also find out if your horse is a heavy egg shedder or if the parasites are resistant.

Monitoring the effectiveness of a parasite preventative program is necessary. You should test fecal egg counts at least once per year.

Testing and monitoring will also help detect unusual parasitic infections. In some situations, certain parasites will not be killed using common strategies and additional medication may be necessary.

As a dedicated horse owner or stable/farm manager, you and your veterinarian can develop a complete and cost-effective parasite control program that is best suited for your farm and your animals. The health, happiness and productivity of your horses will make the effort more than worthwhile.