Stomach bots are larvae of the botfly (Gasterophilus) and not actually internal worms. Female botflies lay their eggs by attaching them to the hairs of the horse. Different species lay their eggs on different parts of the horse’s body. Gasterophilus nasalis lay their eggs between the jawbones. Gasterophilus hemorrhoidalis lay their eggs on the short hairs of the lip. Gasterophilus intestinalis lay their eggs on the forelimb and shoulder. The eggs on the legs are stimulated to hatch by the lip action and warm saliva as the horse licks its leg. Those seen around the nostrils and lips hatch in one to one and a half weeks spontaneously. Larvae attach and burrow into the tongue and gums of the mouth and incubate there for three weeks. After incubation, they are swallowed and attach to the lining of the stomach. Bots spend approximately nine months attached to the stomach lining before passing out with the manure. These larvae pupate into adult flies. The life cycle depends on the parasite larvae overwintering in the stomach, then passing out in the manure in spring and subsequently developing into adult flies. The adult flies are active from late spring to the killing frost in the late fall. Botflies have no vestigial mouth parts and never feed. They live their adult life cycle on stored energy. They have yellow stripes on their body and resemble a sweat bee. Botfly larvae probably cause minimal damage to the stomach, but can cause damage to the lining of the stomach and outflow obstruction.
Control
Various types of chemicals called antiparasitics or anthelmintics have been developed to eliminate parasites. These chemicals work in a number of ways. Some paralyze the parasite, thus allowing the host to expel them. Other chemicals prevent nutrient utilization or limit reproductive capabilities in the parasites, thus killing them or stopping the life cycle. A large number of commercial anthelmintic or antiparasitic compounds are currently on the market to remove internal parasites from horses. These antiparasitics are separated into six major classes. The more common classes are avermictins/milbimycins, benzimidazoles, and pyrimides. These anthelmintics are available in different physical forms (paste, feed additives, gel, drench) and are sold under several trade names. Antiparasitics are effective by all routes given, if an appropriate dose is administered based on the horse’s weight and the entire dose gets into the horse. Knowledge of anthelmintics is important because these chemicals vary in their ability to remove specific parasites. For example, a compound may be effective at controlling strongyles and ascarids, but not bots or tapeworms, where another chemical is effective in controlling ascarids, strongyles, and tapeworms, but not bots. In addition, some anthelmintics are not safe for certain classes or ages of horses.
A rotational treatment protocol is often uesd to control bots. Rotational treatment protocol is the switching between different classes of anthelmintics to avoid resistance to an anthelmintic. There are several deworming strategies used in equine parasite control, and all have advantages and disadvantages. Some of the common strategies are:
- Interval rotational treatment (rotating drugs 4 to 6 times a year)
- Annual rotation (using a different drug each year)
- Daily (continuous) treatment (also administer a botacide at least twice a year)
- No rotation (using the same drug 4 to 6 times a year and the same one every year)
- Targeted treatment (target specific parasites)
- Strategic treatments (administer drugs at specific times of the year)
Factors such as climate, humidity, season, rainfall, stocking rate, age of the horse, and financial resources of the owner all affect which strategy is chosen. It is critical to consult a veterinarian in establishing an effective parasite control program. In most circumstances, a horse will need to be dewormed 4 to 6 times a year starting at about four to eight weeks of age. Some anthelmintics are toxic to young foals, and the labels and package inserts should be read carefully. Typically, parasite control programs are most effective if treatments are administered at the times when environmental conditions are favorable for hatching of eggs or development of larvae, which is the time when transmission of infection is likely to occur. An essential component to an effective parasite control program is to check the efficacy of that program by evaluating fecal samples for parasite eggs on an annual basis.
Management Practices
- Deworm all foals at four to eight weeks of age.
- Repeat every 30 to 60 days, depending on the circumstances of the environment.
- Regular rotation of pastures.
- Small pastures from one to 10 acres can be divided into smaller areas so horses can be rotated. This will help lower the worm burden as well as give forage a chance to recover.
- Rotate cattle, sheep, or goats behind the horse(s), if possible. These species will consume the infective larvae of the horses’ parasites. These larvae will be inactivated.
Prevention
Clean stalls on a regular basis and compost manure or spread thinly over pasture not being grazed by horses. Stalled horses become reinfested from larvae crawling up the walls and being licked off by the horses. Mowing and harrowing pastures to break up fecal piles during the hottest and driest season of the year will decrease numbers of infective larvae. Feed horses grain and hay from some type of rack or trough. This includes pastured horses. Yearly fecal examination by a veterinarian will evaluate how well the program is working. Avoid overstocking a pasture as this will increase the risk of exposure to infective larvae or eggs. Remove bot eggs quickly and regularly from the horse's hair coat to prevent ingestion. Alternate anthelmintic classes to decrease possible parasite resistance to an anthelmintic class, and administer anthelmintics to all horses at the same time when they are kept together. Always read and follow the label instructions of an anthelmintic when administering.
