Controlling and treating parasites in your horse
Controlling parasites in your horse involves more than deworming on a regular basis. The most important thing you can do is to reduce the number of parasites and eggs in the horse’s environment.
When you do treat for parasites, use a method that is effective with the fewest number of treatments. The treatment program should be broad spectrum to control many different types of parasites.
Preventative medication is a very important component of parasite control. But there is no single program that works for all situations.
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
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 (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:
- Loss of appetite
- Weight loss
- Death, if not treated
With chronic infections, random repeated colic is a major sign of disease.
Use ivermectin and moxidectin to treat against larval stages. And use oxibendazole, fenbendazole, or pyrantel pamoate to treat against adult worms.
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:
- Weight loss
- Severely underweight, if not treated
Use ivermectin, oxibendazole, pyrantel pamoate, or piperazine to treat against adult worms. You can use ivermectin, moxidectin, or pyrantel tartrate to treat against larvae.
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. Fecal tests can detect tapeworms. Horses with tapeworms have no visible signs of parasitism. But tapeworms may cause:
- Slow growth
- Loss of body condition
Use twice the normal dose of pyrantel pamoate to treat against tapeworm.
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.
Treat against stomach bots with ivermectin or moxidectin.
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
- Poor eating
- Yellow/ gray discharge from the anus
Treat adult pinworms with fenbendazole, oxibendazole, pyrantel pamoate, piperazine or ivermectin. Treat foals for pinworms with fenbendazole.
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
- Mild diarrhea
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.
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.
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.
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.
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 long do these medications last?
The best way to determine how long the medication is working is to have your horse's manure tested for parasite eggs. Specific products give a rough guideline for the estimated length of time the medication is good for. Most are effective for from 4 to 6 weeks.
Check with your veterinarian and on product labels for details about the medication you are using. Deworming your horse every few months will not be enough in most cases.
Is there anything I can do besides give dewormers?
Dewormers are necessary, however, management is an essential aspect of a parasite control program and one that is often overlooked. Managing new arrivals, the existing herd, the pastures and paddocks are all important components. Here are some tips:
- Isolate and treat new animals on the premises to ensure that highly infected animals do not have a chance to shed parasites on the environment and infect other horses. Alternatively, require that new animals be dewormed prior to joining the stable.
- Monitor young horses closely as they are particularly susceptible to parasitic diseases.
- If your horse's’ primary forage comes from grazing, make sure animals are not 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 the manure time to break down and help reduce overgrazing.
How can I tell if my parasite control program is working?
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.
Reviewed in 2018