This month:

Tapeworms are relatively benign in comparison with other equine parasites. We know far less about tapeworms, their life cycles, and their ultimate impact on equine health, because there has been less research over the years; the research emphasis has instead been on more harmful parasites such as strongyles. However, tapeworms can result in a dull-coated, underweight horse with frequent colics or digestive problems, one that is labeled as 'failing to thrive.'
 
Here's what we do know about tapeworms: Horses get them from consuming oribatid mites that have eaten tapeworm eggs and are carrying tapeworm larvae. Oribatids are an essential link in the recycling of organic wastes and enriching of soil, and thousands of species of these mites live on each continent, from Africa to Antarctica.
 
Grazing horses consume mites that have crawled up blades of grass. Once ingested, the larvae emerge from the mites and then pass through the horse's digestive system, latching onto the intestinal walls with the sharp teeth in their mouths. As the tapeworms receive nutrition from the horse, they grow in a flat, segmented pattern, with each segment capable of reproducing by itself. Eggs are produced inside the segments, and the segments at the end of the tapeworm periodically break off, typically dissolving in the horse's large intestine. The eggs then pass outside the horse with the feces.
 
Once excreted, the fertile eggs are eaten by the oribatid mites as part of the natural breakdown of organic matter; the larvae hatch from the eggs inside the mites, and the mites are once again eaten by grazing horses, completing the sequence and perpetuating the cycle.
 
There are three types of tapeworms that we typically find in horses. The smallest are the Paranoplocephala mamillana, which are about a half-inch long. The most common, Anoplocephala perfoliata, are usually about an inch in length. And the largest by far are the Anoplocephala magna, which can range from eight to 25 cm in length.
 
Tapeworms aren't typically found in foals that are still nursing, and many horses seem to manage a tapeworm population throughout their lives without issue, since it's been shown through post-mortem examinations that 50-60% of horses have a tapeworm infestation.
 
While grazing horses are frequently the most susceptible since the oribatid mites are a normal part of the outdoor environment, stalled horses fed recently-chopped grass or hay that's been resting on the ground can also be at risk of consuming mites and tapeworm larvae.
 
If you have a horse that's failing to thrive and that seems to have frequent digestive issues or episodes of colic, it might be time to check with your veterinarian to determine if a course of dewormer is in order. Caution should be exercised when using broad-spectrum dewormers with pregnant mares, foals, and breeding stallions, since these deworming medications might not have been tested on these specific horse populations.
 
As with all parasites, prevention is the key. Understanding what is known about tapeworms and their lifecycle, and taking steps to eliminate infestation through proper sanitation and environment management techniques, will go a long way in both prevention and eradication.