Intestinal parasites or worms can cause varied symptoms such as colic, diarrhea, weight loss and poor performance. Severe infestations can be life threatening. There is no set worming program that suits all horses and situations, as it will depend on the number and ages of horses present, pasture management and geographic location. The following describes the main parasites and gives a guide to controlling them. Appropriate management for your situation should always be discussed with your veterinarian.
Types of intestinal parasites
The most important worms, in terms of health risk are:
• Small strongyles (cyathostomes/small redworms)
• Large strongyles (redworms)
• Roundworms (ascarids)
• Tapeworms
Small strongyles/ cyathostomes/ small redworms
These are the most common equine parasite of which there are over 50 different species. As a consequence to infestation, horses can develop diarrhea, colic and weight loss. Light infestations are often inapparent, but after treatment an infected horses general health and performance often improve. Eggs are ingested from the pasture and as part of their development, then the larval stages undergo a period of arrested development in the walls of the large intestine. When the encysted stage emerges, usually in early spring, they cause damage to the gut wall. Treatment for the encysted worms should be done in late fall or early spring using moxidectin or a 5 day course of double dose fenbendazole.
Large strongyles/large redworms
Immature stages of these worms migrate through blood vessels that supply the intestines, and can result in anemia, lethargy, weight loss and even potentially fatal colic. Luckily these worms are controlled by all classes of dewormers and a regular worming program will keep worm populations down.
Roundworms
Young horses (foals-yearlings) are particularly susceptible to the roundworm Parascaris equorum as they lack age acquired immunity. The adult worms live in the small intestine and can cause poor growth and condition, coughing and impaction colics. Colic is most likely in foals over 3 months of age that are heavily parasitized when wormed for the first time. They are best controlled with fenbendazole or pyrantel every month until the foal is a year old.
Tapeworms
The adult tapeworm Anoplocephala perfoliata live in the cecum where the small and large intestines connect and can cause weight loss and colic. A worming product that will control tapeworms (one containing praziquantel) should be used once per year.
Others
Bots, the larvae of which live in the stomach, and pinworms which cause intense itchiness around the anus can both be controlled with ivermectin or moxidectin, neither of these parasites cause major health problems. Threadworms can occasionally be a problem in young foals causing diarrhea. Lungworms can cause chronic coughing.
Worm control
The best management for your situation should be discussed with your veterinarian.
Fecal egg counts
In order to develop effective worming programs and monitor their effectiveness fecal eggs counts should be performed by your veterinarian. Fresh manure samples are analysed microscopically looking for worm eggs, allowing us to determine which parasites are present and how severe the infestation is. Egg counts are expressed in eggs per gram (epg) of faeces. Counts of less than 200epg indicate a light parasite load and counts over 500 epg indicate a heavy parasite load. Negative fecal egg counts don’t necessarily mean a horse has no parasites as some parasites produce eggs intermittently. Performing fecal egg counts is a cost effective way to determine what dewormers should be used and the efficacy of the deworming program.
Rotating products
This is a controversial issue! If worming products of the same class are used repeatedly parasites may become resistant and the product becomes less effective or ineffective. However if products are rotated too quickly parasites could become resistant to multiple products. Please discuss this issue with your veterinarian to decide the best system for your situation.
Treatment interval
The necessary interval between each deworming will depend on your circumstances – your geographical location, pasture management, fecal egg counts and type of antheltmintic drug product.
Daily dewormers
Using a daily dewormer is an alternative worming strategy where a small amount of dewormer is fed every day in an attempt to prevent new infections from larvae picked up during grazing. This will not resolve existing infections and will not kill tapeworm, encysted small redworms or bots, therefore should not be relied upon as the only method of parasite control.
Adult horses
At the start of the parasite season in the spring, it is advisable to have a fecal sample analysed to determine what type of worming your horse requires.
If your horse has a high worm egg count (>500 epg), this generally means the individual animal is susceptible to worms and should be wormed with a product which will kill encysted larvae (5 days double dose fenbendazole or moxidectin). Moxidectin should not be used on horses in poor body condition. Ideally, the fecal counts should be rechecked post treatment, or the horse should be wormed regularly throughout the worm season with ivermectin, pyrantel or fenbendazole, according to the product directions.
If your horse has a moderate fecal egg count it should be wormed using your choice of product. These horses generally only require one treatment, but ideally a fecal sample should be analysed for worm eggs approximately 8 weeks post treatment, depending on the product used.
If your horse has a low fecal egg count (<200 epg), it likely does not require to be wormed, but should be rechecked in approximately 8 weeks. In the late fall, your horse should be dewormed to control encysted cyathostomes, tapeworms and bots. We recommend using Quest Plus which contains moxidectin plus praziquantel. This product should not be used on horses less than 4 months old or if they are in poor body condition.
Foals
Foals can be infected with parasistes on day one, and these worms take approximately a month to develop. Foals should therefore be dewormed when they are a month old with fenbendazole or pyrantel and this should be repeated monthly. Please consult your veterinarian for further advice.
Pregnant mares
Pregnant mares should be dewormed as normal adult horses, however should not be dewormed within 6 weeks of their foaling due date.
Pasture management
• Remove feces, at least twice per week
• Prevent overgrazing – the ideal stocking density is 1-2 horses/acre of grazing, in order to reduce pasture contamination.
• Harrow pastures – but only when the temperature is > 80°F, so the eggs will be killed.
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