By: Matthew M. Brendal
Deworming & Internal Parasite Prevention
Deworming chemicals are designed to reduce health risks to your horse. ALWAYS consult your veterinarian if you have any concerns. He or she is the subject matter expert. Due to the advent of tube paste wormers, many people have decided to personally give their horses doses of dewormers. I will give some instruction towards the different methods used. It is a simple process that any horseman or horsewoman can accomplish.
INTERNAL PARASITES
Internal parasites can be harmful in many ways. These parasites can potentially rob your horse of nutrition, growth, inflict irritation to the point of mental frustration. and even cause serious health problems. If these parasites are allowed to infest the horse and remain untreated they could cause damage to vital organs such as the intestines, lungs, stomach and liver. Having an effective worming program will help give your horse a better life of quality. It is important as your horse's health manager to have a good parasite control program. Internal parasites can devour vital organs and/or cause colic. CHEMICAL SAFETY
Dewormers and fly repellants are very dangerous chemicals and care should be taken to wash off any residue remaining from any overspray or contact on your hands/other body parts with soap and water. Wash any clothes or gloves that come in contact with these agents. All hazardous chemicals should be stored as per the label in secure storage and be properly disposed when outdated.
Signs of Internal Parasite Infestation
Slower than normal growth rate Dull and rough hair coat
Potbelly (large abdomen but not fat) Tail rubbing
No desire to perform as normal Weak
Emaciated Loss of appetite
Signs of Acute Internal Parasite Infestation
Colic Diarrhea
Coughing Lameness
COMMON INTERNAL PARASITES
Bots (Stomach bots & Gasterophilus)
Physical Description
Mature botflies have an appearance similar to a honeybee. The eggs are light brown specks the size of pin heads in small clusters.
Lifecycle
The female botfly lays her eggs directly to a hair on the horse in areas that will be able to be in contact with the horse's mouth. The areas most commonly targeted are the legs, lips and other spots that allow direct contact with the mouth and nostril during self grooming or grooming another horse. Licking the attached botfly eggs will cause them to hatch and produce the larvae. The hatched larvae will then migrate to the tongue or gums of the horse and burrow in or attach itself for the 3 weeks incubation period. Following the incubation period they free themselves and pass to the lining of the stomach for a duration of approximately 9 months. After the 9-month period the larvae are transported out of the stomach via manure to become an adult botfly. The activity of an adult botfly will last from late spring until the first hard frost.
Problems Caused by Bots
The botfly larvae may cause only minimal damage to the horse, the biggest threat would be intestinal obstruction due to infestation. The larvae may also reduce the efficiency of the stomach to digest effectively.
Signs of Infestation of Bots
If any botfly eggs are observed to attached to your horse you should consider that your horse has ingested some of them. If eggs are observed remove with them as soon as possible with a Bot knife to reduce ingestion.
Intestinal Threadworms (Strongyloides westeri)
Physical Description
As the name implies these worms are long and thin similar to hair. Adults may be less than 2/5 of an inch.
Lifecycle
The horse consumes threadworm larvae via infected forage or penetration of the skin. This worm may remain dormant in an adult mare until the final days of pregnancy and then will be passed via mares milk or penetrate the foals skin. The worm will live in the small intestines of the foal for the complete lifecycle of about 2 weeks.
Problems Caused by Intestinal Threadworms
Although the possible infection period is small the lifecycle is short, which may produce a severe infestation rapidly. Severe infestation leads to dehydration and chronic diarrhea. If larvae enter via the skin they will migrate to the lungs. Upon reaching the lungs they are coughed up and swallowed and travel to the small intestine where they mature. Adult females produce eggs that pass out of the horse by defecation.
Signs of Infestation of Intestinal Threadworms
Loss of appetite, dullness and chronic diarrhea that does not respond to normal treatments. Severe cases develop into respiratory bleeding and inflammation. Rashes may develop at site of skin penetration. Foals should develop a natural immunity to this worm by about 6 months of age.
Hairworms (Trichostrongylus axei)
Physical Description
The adult worm is very small reaching a length of a 1/4 inch with a body like a strand of hair.
Lifecycle
The horse consumes egg-infected grass and the egg hatches allowing the larvae to mature in the stomach of the horse. When the worms mature they produce eggs which are passed through by way of manure.
Problems Caused by Hairworms
These worms target the villi of the gut and in the process damage the capillaries and lymph vessels. This damage will affect nutritional absorption and cause internal bleeding of intestines.
Signs of Infestation of Hairworms
Severely infected horses will produce dark colored foul-smelling diarrhea. The horse may also have anemia. Foals have the highest risk of hairworm infection.
Large Strongyles (Bloodworms or Strongylus vulgaris or Strongylus endentatus or Strongylus equinus)
Physical description
A tubular worm which may reach 2 inches in length when mature.
Lifecycle
The mature worm produces eggs that pass via feces from infected animals. The eggs then develop to the stage of larvae. Larvae are then ingested during consumption of contaminated forage, feed or water. The larvae transit the body seeking the primary blood supply of the intestines or liver. The infestation of the intestines or liver lasts about 3 months, the larvae then migrate to the large intestine to finally mature. The adult worms will lay thousands of eggs a day during the 11-month lifecycle.
Problems Caused by Large Strongyles
The large strongyles are the most serious internal parasitic threat to your horse. They can cause ruptures of blood vessels, restrict proper circulation, cause injury to vital organs and tissues.
Signs of Infestation of Large Strongyles
The infestation of large strongyles may result in colic, diarrhea, fever, loss of appetite and lack of interest in daily activities.
Lungworms (Dictyocaulus arnfieldi)
Physical Description
A long slender worm possibly reaching lengths of about 2 inches.
Lifecycle
The horse consumes lungworm on damp forage. The ingested eggs become larvae and migrate to the circulatory system through the thin wall of the intestinal tract. The mature lungworms will pass to the lungs via the blood stream. Eggs are passed from the mature female through manure. The lifecycle may last up to 4 months.
Problems Caused by Lungworms
If left untreated and severe infestation takes place, a horse can develop bronchitis. In very young horses severe infestation may lead to death. As a horse matures they usually develop a resistance this parasite.
Signs of Infestation of Lungworms
A severe cough, loss of appetite and difficulty breathing are the main signs the horse having bronchial problems due to infestation of this worm
Neck Threadworms (Onchocerca cervicalis)
Physical description
The long thin female worm may reach a length of about 11 inches at maturity. Because these worms remain in a coiled status they may be identified as a bump under the skin.
Lifecycle
These worms rely on a tiny two-winged fly called a biting midge to assist in the infection of the horse. Neck threadworm microfilariae live in the tissue under the horse's skin. When the biting midge bits into the horse's skin, the microfilariae will then leave the horse and attach to the midge's mouth. After about a month in the midge's mouth the microfilariae will develop into infective larvae. During the infective the larvae transfers from the midges as the fly bites the horse.
Problems Caused by Neck Threadworms
The favorite place for this destructive worm to infect is the ligaments. As the name implies it does attack the neck ligaments. In some cause the flexor tendons and suspensory ligaments of the legs will be infected. These worms may cause swelling of infested ligaments or lameness.
Signs of Infestation of Neck Threadworms
Swelling and pain may be caused by infection of mature worms. Bumps under skin on ligaments and tendons may be the sign of dead worms that have hardened.
Pinworms (Oxyuris equi)
Physical Description
Large pinworms reach 3 inches in length and have a thin tubular body. The eggs are yellow in color with a sticky substance to attach to the skin around the anus.
Lifecycle
The eggs are either laid in the large colon and pass with manure or the mature female will transit out of the horse and deposit eggs around the horse's anus. The eggs quickly become larvae and when ingested migrate to the colon. The pinworms will mature to produce eggs in about 5 months.
Problems Caused by Pinworms
These parasites mainly cause irritation that leads to itching. Any age horse can be affected.
Signs of Infestation of Pinworms
The horse will bite or rub at the base of the tail, sometimes causing broken or rubbed off hair. If left untreated this could lead to excessive itching, which may result in open sores and infection.
Roundworms (Ascarids)
Physical Description
Mature worms will be 12-15 inches in length & pencil shaped.
Lifecycle
The cycle begins as the horse consumes forage, feed or water that is contaminated with roundworm eggs. When the eggs hatch into larvae they burrow into the small intestines then use veins to gain access to the liver and heart. The larvae then seek the lungs, where they are then coughed up to be swallowed only to be returned to the small intestines. Upon reaching the small intestine the larvae mature and produce eggs. The duration of the lifecycle lasts approximately 3 months.
Problems Caused by Roundworm
Inflammation and scarring to liver and lung tissue. Disruption of nutritional absorption, intestinal blockage, intestinal ruptures, and colic.
Signs of Infestation of Roundworms
Potbelly, rough haircoat, slow growth, thriftless and nasal discharge with cough. The mature horse usually develops immunity to the roundworm, horses under 3 years of age are primarily infected.
Small Strongyles (Cyathostomes)
Physical Description
The mature parasite may reach a 1-inch in length with a red tubular body.
Lifecycle
The horse via natural forage consumes infective larvae. These then larvae pass to the intestines and burrow in until mature. They will then migrate to the large intestine to feed and lay eggs. The eggs are then passed through manure to begin the lifecycle.
Problems Caused by Small Strongyles
These worms may contribute to diseases in the large intestine by penetrating the intestinal walls.
Signs of Infestation of Small Strongyles
Most horses will not exhibit outward signs of infestation from this worm unless they are severe. Severe infestation can lead to weight loss, diarrhea or colic.
Stomach Worms (Draschia megastoma or Habronema muscae)
Physical Description
A small white worm that may reach about 1 inch in length.
Lifecycle
The stomach worm larvae are ingested by housefly or stable fly maggots developing in manure. The ingested worm larvae continue maturing inside until the maggots mature into flies. The mature flies then deposit the stomach worm larvae in open wounds, mouth area, eyes or other naturally moist sites on the horse. The larvae are then orally ingested by the horse or contaminate the wounds. The larvae then reside in the stomach or remain to infect the wound.
Problems Caused by Small Stomach Worms
The stomach worm can cause summer sores (non-healing wounds), gastritis or tumors like growths and even swelling of the mucous lining in the eye.
Signs of Infestation of Small Stomach Worms
Non-healing sores, eye swelling and stomach problems.
Tapeworms (Anoplocephala perfoliata)
Physical Description
When mature these worms are about anywhere from 3 to 30 inches in length and 1/2 inch wide. The body is white and flat with a tapered head.
Lifecycle
The lifecycle begins when tapeworm eggs begin to develop inside the oribatid mite. Horses consume the infected mites, which live in the grass, as they eat. The ingested tapeworms will then mature in about 6 weeks.
Problems Caused by Tapeworms
Severe infestation may cause intestinal irritation, inflammation, bleeding or ulcers. If the tapeworms accumulate and completely block the junction of the cecum, small intestine and large intestine the horse may die. It is very important that your deworming program includes chemicals for tapeworms. It has been estimated that the national average of tapeworm infected horses is over 50%. The only FDA approved equine dewormer chemical formulated specifically to rid the horse of tapeworms is Praziquantel. A praziquantel dosage of 1 mg/kg or greater is reported to be 100% effective. Effective dose normally given twice a year, Spring and late Fall. Zimecterin Gold and Equimax are the only FDA approved equine dewormers formulated specifically to rid the horse of tapeworms.
Signs of Infestation of Tapeworms
Signs may be digestive disturbances, poor growth, unthriftiness or colic.
INTERNAL PARASITE CONTROL PROGRAM
This program may have many variables to consider before deciding on a course of action. Some variables to consider are geographic region, season, climate, density of equine population that your horse comes in contact with, age and others items that could lead to a direct contact of a parasitic environment.
The type of worming product and the frequency are also important aspects to an effective deworming program. All chemical dewormers are not equal. The chemicals used to stop parasite infestations are called anthelmintics. Different classes of anthelmintics may have different affects to rid the horse of parasites. Depending on the class they could cause the parasites the inability to absorb nutrients, cause them to be paralyzed or stop reproductive capabilities. The only way to effectively judge your deworming program is determining fecal egg count.
This test takes a fecal ball and determines eggs per gram (epg) by using a microscope and visually counting the eggs. This should be done yearly or based on the veterinarian's recommendations. Below is listed the type of chemical dewormers and there effectiveness against common internal parasites. They will be rated on 10 scale, with 10 being the best and 0 having no effect.
Chemical- Ivermectin Moxidectin Fenbendazole
Bots- 10 9 0
Pinworms- 10 10 9
Roundworms- 10 10 8
Large strongyles 10 10 9
Small strongyles 10 10 9
Chemical- Oxibendazole Pyrantel Pamoate Pyrantel Tartrate
Bots- 0 0 Prevents
Pinworms- 9 5 infective
Roundworms- 8 9 larvae from
Large strongyles 9 7 entering
Small strongyles 9 9 tissues
Chemical Class and Active Ingredients (Average Effectiveness)
Benzimidazoles Macrocyclic Lactones
Fenbendazole (4 weeks) Ivermectin (7 weeks)
Oxibendazole (4 weeks) Moxidectin (12 Weeks)
Pyrantel Pamoate Pyrantel Tartrate
Pyrantel Pamoate (4 weeks) Pyrantel Tartrate (4 weeks)
Brand Names and Chemical Class
Ivercare = Ivermectin Zimectrin = Ivermectin
Quest = Moxidectin Strongid P = Pyrantel Pamoate
Safe-Guard = Fenbendazole Panacur = Fenbendazole
Continuex = Pyrantel Tartrate Anthelcide EQ = Oxibendazole
Zimectrin Gold = Ivermectin & Praziquantel
Example of Deworming Rotation Based on Chemical Class
Horses More Than 1 Year Old
1 Jan - Ivermectin
15 Feb - Ivermectin
1 Apr - Ivermectin
15 May - Pyrantel Pamoate
15 Jun - Moxidectin
15 Sep - Ivermectin
1 Nov - Ivermectin & Praziquantel
15 Jan - Benzimidazole
Horses Less Than 12 Months Old
1 month old - Pyrantel Pamoate
2 month old - Ivermectin
3 month old - Pyrantel Pamoate
4 month old - Ivermectin
5 month old - Pyrantel Pamoate
6 month old - Ivermectin & Praziquantel
7 month old - Pyrantel Pamoate
8 month old - Ivermectin
9 month old - Pyrantel Pamoate
10 month old - Ivermectin
11 month old - Pyrantel Pamoate
12 month old - Ivermectin & Praziquantel
Pregnant Mares (60 days or more) & Minis
1 Jan - Pyrantel Pamoate
15 Feb - Ivermectin & Praziquantel
15 May - Pyrantel Pamoate
1 Jul - Ivermectin
15 Aug - Pyrantel Pamoate
1 Oct - Ivermectin & Praziquantel
1 Dec - Fenbendazole
Large Pasture & Minimal Manure Contamination
1 Jan - Pyrantel Pamoate
1 Apr - Ivermectin
1 Jul - Pyrantel Pamoate
1 Oct - Ivermectin & Praziquantel (After first frost)
The most commonly recommended dewormer program is an interval type. This would mean for example that every 8 weeks the horse is given a chemical dewormer, usually a broad-spectrum type to rid the horse of any and all internal parasites.
There are also seasonal dewormer programs, these programs anticipate the parasites most infectious stage of the lifecycle and are administered accordingly. These programs traditionally center on the first killing frost and peak parasite reproduction times such as June or July. This program may also require some monitoring of fecal egg counts to ensure effectiveness. The daily program requires you to give a dewormer on a daily basis.
Not all dewormers are the same, neither are the parasites. Some parasites may develop a resistance to certain chemical dewormers. The internal parasite that will most likely become resistant is the small stronglyes. Dewormers are usually given by the horse's body weight. Because of weight tape variables and wanting to ensure an adequate dosage I like add 10% to the horse's weight and make dosage adjustments that I call the "waste of paste" factor.
The foundation of your program should be prevention of parasite infection. Chemical dewormers may have an impact on the digestive health of the horse. Probiotics may be added to the horse's diet the day after the dewormer is administered. I would continue probiotics as a supplement for week. I would also be cautious of "natural dewormers", they may not be effective. If you suspect that your deworming program is not effective collect a manure sample from the horse in question. You can do this by obtaining a ping pong ball size sample, putting it in a airtight container, properly labeling with name, address, telephone number, name of horse and date collected. Unless you go immediately to the veterinarian after obtaining the sample, it should be refrigerated until taken to the clinic.
Here is a List of Recommendations to Reduce Internal Parasites
Having a clean and well kept area that the horse(s) is confined to is paramount.
Many internal parasites use manure in some form or another in their lifecycle. Help break up that part of the lifecycle by cleaning the areas the horse leaves large quantities of manure or if the horse is restricted to small areas of confinement, such as stalls and paddocks.
The cleaning of stalls or small paddocks should be done daily.
If the horse is kept in a pasture you may want to clean piles that are very big and harrow that pasture monthly to help break up and scatter the dried manure.
Practice composting of manure and bedding. Use bedding the horse will not have a desire to eat, such as pine shavings. When bedding such as straw is used in stalls, the horse may eat it along with infected manure.
If at all possible rotate pastures. Avoid over stocking pastures, feed horses from a manger or troughs to reduce mouth contact with the ground, helping to reduce parasite ingestion.
Quarantine new horses away from other horses upon arrival and treat with dewormer. Wait 3-4 days before allowing them direct contact with other horses.
All horses at the same location should be dewormed at the same time
Deworm foals the first time at 2 months of age for roundworms
The most effective time to break the lifecycle is deworming 2 weeks after first the frost.
Machines such as Mosquito Magnets lure and destroy mosquitoes as well as other similar insect pests that may use internal parasites in their lifecycle.
There are also powders and liquids that can be applied to the ground and plants which will kill insects. As with any chemicals read and follow the label so as you don't harm any animals or children.
