Grazing horses are more likely to have tapeworm problems than stabled horses.

We asked Dr. Craig Reinemeyer, an esteemed expert in internal parasites, if he would help us determine how strong a threat tapeworms are to our horses and if there are any special measures we should take in controlling them.

We seem to be hearing more about tapeworms. Are they more of a threat now or just being recognized more?

The increased interest in equine tapeworms is mostly due to the combined influence of two factors. The first was formal approval by the FDA in the last few years of equine dewormers with label claims for efficacy against tapeworms, otherwise known as Anoplocephala perfoliata. The pharmaceutical sponsors escorted the launch of these new animal health products with marketing campaigns, so the drug manufacturers have created some buzz to educate horse owners and to help sell more products. The second factor is publication of research findings that described improved diagnostic methods for detecting tapeworm infection, and documented tapeworms as serious pathogens of the horse.

Are tapeworms any more, or less, dangerous to the horse than other types of intestinal parasites?
Parasites differ from viruses and bacteria in that they can’t amplify their numbers within the host. Rather, they must cycle through the environment with each new generation. So, parasitic disease is consequently a numbers game. More worms cause greater damage.

What types of problems do tapeworms cause for horses?
Epidemiologic research has demonstrated that tapeworm infection is associated with an increased incidence of three syndromes: spasmodic colic, ileocecal intussusception, and ileal impaction. These are potentially serious problems that may have to be managed surgically.

A recently published study from the Netherlands found evidence of tapeworm infestation was associated with a risk of colic in general, of all types. How can tapeworms influence the health or function of the digestive tract even in areas where they aren’t attached?

Most A. perfoliata are attached around the ileocecal valve, the point where the small intestine empties into the large intestine, and they cause intense inflammation and scarring of local tissues. This may interfere mechanically with the passage of ingesta from the ileum into the cecum. A less obvious explanation is that tapeworms in the cecum are somehow able to modulate motility in distant portions of the alimentary tract through altered nerve conduction, production of active chemicals, etc.

How many different types of tapeworms are there? Are any kinds more harmful than others?
In the United States, there are three species of tapeworms that exclusively infect horses. Anoplocephala perfoliata is far and away the most common, comprising 98% or more of all infections. Anoplocephala magna is next in prevalence, being found in only 1 to 2% of all horses infected with tapeworms. Paranoplocephala mamillana is the smallest of the three and is found only rarely.

How do horses get tapeworms?
The tapeworm eggs that pass in the manure of a horse are ingested by certain types of mites (Oribatids) that live in the soil and in the thatch layer of forage. Within a mite, the tapeworm egg develops into a stage (cysticercoid) that is infective to horses. Horses acquire tapeworms by inadvertently swallowing infective oribatid mites while they graze.

Are tapeworms only a problem for horses at pasture?
Confined horses with no exposure to pasture and that are fed stored hay or complete feeds have virtually no risk of tapeworm infection. Tapeworms are acquired almost exclusively by grazing, but there is some evidence that oribatid mites may invade hay bales that are stored outside in direct contact with the ground.

Are any age groups more at risk?
Epidemiologic studies in the United Kingdom and in the United States have determined that horses less than 2 years of age and older than 15 years are more likely to be exposed to tapeworm infection. This probably has less to do with age per se, and more to do with the average amount of pasture exposure enjoyed by the various age groups.

Are there any areas of the country where tapeworms are more or less of a problem than others?
A survey of antibodies to tapeworms from a representative population of horses across the continental U.S. found that exposure to infection was more common in horses from the upper Midwest, and least common in the arid southwest. Overall results of the study indicated that 54% of all the horses sampled had been exposed to tapeworms.

Is there any way for owners to minimize the risk of their horse picking up tapeworms?
The simplest method is to use dewormers that are effective against Anoplocephala. Ultimately, killing adult worms should decrease pasture contamination with eggs, and thereby reduce exposure of grazing horses. At the present time, we don’t know enough about tapeworm infections to recommend a seasonal or annual schedule for specific deworming.

How long can a tapeworm live inside a horse?
I’m not sure whether the longevity of an individual Anoplocephala has ever been determined in a horse. In other animals, however, certain tapeworms can easily outlive their hosts. Given the seasonal nature of most pasture-based worm infections, it’s likely that tapeworms survive within the horse for weeks to months, rather than for days or years.

Can tapeworms be diagnosed from fecal examinations?
Tapeworm eggs can be observed by centrifugal fecal flotation procedures, using a variety of standard reagents such as sucrose or zinc sulfate. But, the overall sensitivity of this method is extremely low.

What is the best treatment?
Two currently marketed drugs have label claims for efficacy against tapeworms in horses: praziquantel, which is generally sold in combination with ivermectin or with moxidectin and pyrantel pamoate paste.

Are herbal dewormers or diatomaceous earth effective?
No.

If a horse is known or suspected to have a large number of tapeworms, can there be complications of deworming? Can any special protocol be used to minimize this risk?
No.

Can tapeworms develop resistance to dewormers?
Resistance to dewormers is certainly possible from a biological standpoint. Although anthelmintic-resistant tapeworms have not yet been demonstrated, effective cestocides have been available in the U.S. since 2003. Because two different drugs (praziquantel and pyrantel pamaote) are currently recognized as effective cestocides, it would be a good management practice to rotate these compounds to help minimize the potential development of resistance.

We know it isn’t FDA-approved for this, but is it true that a month-long treatment with pyrantel tartrate (Strongid C and similar daily dewormers) can eliminate tapeworms?
There is some preliminary evidence that supports the cestocidal activity of daily pyrantel tartrate. Because this is an extra-label application, however, the use of daily dewormers for tapeworm control can’t be recommended.

How often should horses be dewormed for tapeworms? When?
No scientific studies have been conducted to support an optimal program for tapeworm control. The most prevalent recommendation is to deworm twice annually, in the spring and again in autumn, and there is some solid epidemiologic rationale behind each of those applications.

Is it best to just assume they’re there and deworm accordingly, or should testing be done first?
Current testing techniques are so insensitive that a negative result may not reveal anything about the infection status. The serum antibody test is a good indicator of exposure for a herd but can’t identify which animals in the group should be dewormed today.