Parasite control in horses has become simpler and more convenient in recent years. No longer is it necessary to subject your horses to passing a naso-gastric tube to administer various anthelmintics (dewormers). Oral paste anthelmintics administered at the correct dose and fully swallowed are just as effective as tube deworming.
With the release of broad-spectrum ivermectins (Zimectrin, Eqvalan, Rotectin I and Equimectrin), we seemed to be winning the battle against parasites.
Two parasites have recently been scrutinized more closely, however. Small strongyles (bloodworms) are being examined because of their association with winter cyanthostomiasis. Tapeworms have been implicated with certain types of colic. Both of these parasites have unique characteristics and anthelmintic requirements.
When I graduated from vet school 15 years ago, the small strongyle was not thought to be a major problem. All of our attention was focused on anthelmintic programs against the large strongyle (S. vulgaris), the primary cause of colic.
THE LARVAL STAGES of this nasty worm migrate through the arteries of the intestines and cause major problems. All veterinarians welcomed the release of ivermectin because of its effectiveness in killing migrating S. vulgaris larvae.
There are more than 40 species of small strongyles, with 10 to 15 most common. These smaller first cousins to S. vulgaris are notorious for developing resistance to certain anthelmintics. Winter cyanthostomiasis was first reported in 1913, but only recently has it become a concern with veterinarians. Cyanthostome is the official term for the small strongyle. Winter cyanthostomiasis is a clinical parasitic disease that usually occurs in the winter.
This syndrome is most common in young horses, but any horse is susceptible. Horses will show unexplained weight loss, sudden diarrhea, swelling under the belly, poor coats and, sometimes, colic. A typical history includes a herd with questionable parasite control. However, even well-managed horses can have problems. Sometimes symptoms develop after routine deworming in late fall or early winter.
The actual cause of the syndrome is a larval (immature) stage of the small strongyle that forms a cyst in the wall of the large intestines. In its normal life cycle, this parasite spends some time in the mucosal layers of the large intestine.
For unknown reasons, these larva sometimes stop developing and stay in the intestinal wall, causing it to become inflamed and thickened. If enough of these cysts develop, rough hair coat, loose stools and mild weight loss result.
It's thought that a warm, wet fall will increase the number of infective small strongyles and, therefore, increase the overall worm burden. It is also theorized that a severe winter following a warm, wet fall will trigger arrested cyst development.
Some veterinarians report that problems with infected larva can vary with the local weather patterns. Dr. Christine A. Uhlinger of North Carolina has found that the most dangerous time of exposure to parasite larva in northern locations is in the spring and fall. However, in southern locales, pastures can be located with larva during mild winter weather.
The classic syndrome of cyanthostomiasis occurs in winter or early spring, but your regions may experience small strongyles at other times of year.
A MORE SEVERE reaction and clinical signs develop when these sleeping cystic larva come out. When these larva emerge, or "bloom", all at once, the horse suffers with sudden diarrhea, severe weight loss and colic.
Although there's no clinically known reason why the larva bloom again, one theory is that when the resident adult small stongyles are removed with conventional anthelmintics, the junior larva "jump-start" their life cycle again.
Frequently, a horse may become ill after an anthelmintic treatment. Previously we assumed this was a reaction to the dead parasites. It is possible that some other post-anthelmintic signs are actually blooming small strongyle larva hatching out or their protective cysts. The protective cysts prevent anthelmintics from reaching the larvae.
The anthelmintic ivermectins (Zimectric, Rotectin I, Eqvalan, and Equimectrin) that are effective against migrating large stongyles.
Many veterinarians are now mending the larvacidal (larva-killing) dose of fenbendazole (Panacur or Safeguard) for treatment of horses. This product has been approved for control of fourth-stage larva of Strongylus vulgaris.
To achieve larvacidal activity, a double dose (two 25 - gm tubes of paste) of fenbendazole is given daily for five consecutive days. This treatment is usually performed in late fall or winter or with any new animals upon arrival to the farm. Consult with your veterinarian on the use and timing of this larvacidal program.
Good preventive husbandry is helpful in preventing ingestion of small strongyles. To decrease the number of small strongyles, avoid spreading manure in pastures. Use the correct dose of paste wormer and make sure the drug is fully swallowed. Maintaining a rigid rotational anthelmintic program will also help.
The strongyle larvae cannot survive the heat and dryness of the summer. Breaking up manure clumps at this time can help. The best approach, but the most time-consuming, is picking the manure up in the pasture at least once a week - an ideal project for a couch-potato spouse or child!
Another family of worms that once weren't thought to be as much of a problem, but now are, are tapes. Tapeworms are creamy white and can grow up to 86 cm long. These segmented parasites are attracted to the junction of the various types of colic. Tapeworms at this location will cause inflammation and swelling. For these reasons, they must be considered in any effective anthelmintic program.
How does a tapeworm get into your horse? The intermediate host for tapeworms is a mite. This mite is commonly found on older pasture with thick undergrowth. The mite ingests the tapeworm eggs, and the horse ingests the mite on vegetation.
The prevalence of tapeworms varies by region. In one study of southern Ontario, 14 percent of horses were found with tapes. It has also been suggested that tapeworm infestations are more common with horses grazing in low, swampy areas.
Like the encysted small strongyle, tapeworms are now affected by ivermectin wormers. The most effective drug against tapeworms is pyrantel pamoate (Strongid P or Rotectin II).
The normal dose of pyrantel pamoate will remove up to 70 percent of tapes, but many veterinarians recommend a double dose at least once or twice a year to minimize the incidence of tapes.
Because this is an "off-the label" use, you are strongly encouraged to consult your veterinarian before using double dose of pyrantel for tapeworms.