Written by: Martin Nielsen, DVM, Ph.D., DEVPC, ACVIM
The equine tapeworm, Anoplocephala perfoliata, is present on most properties where horses have pasture access. Therefore, it's not surprising to find this parasite in a horse. As is the case with all parasite infections, the overwhelming majority of horses harboring tapeworms tolerate them very well without any signs of discomfort or colic. It is just not in the interest of the parasites to cause disease; the horse is their home.
Tapeworms are widespread and common in horses across the world. However, their presence depends on climatic conditions favoring the orbibatid mite, which is the intermediate host. In dry and arid states such as Arizona, Texas, Nevada and parts of California, horses are rarely - if at all - exposed to tapeworms. Rather, tapeworms usually live in areas with lush green pastures.
Why does tapeworm-related disease sometimes occur? There are many possible reasons. Horses might be exposed to an unusually high infection pressure, which basically means an uncommonly large number of tapeworms. This could be driven by climatic conditions or overstocked paddocks and pastures. A horse with a suppressed immune system due to other disease or stressful events is more susceptible to parasite infection and disease.
A. perfoliata tapeworms live at the junction between the ilieum and the cecum, which is where the small intestine connects to the large intestine. They attach to the intestinal wall just inside the cecum. As a result, the disease this parasite usually causes is colic related to the ileocecal region. The horse might experience a simple impaction of the ileum or a more complicated intussusception, in which parts of the ileum telescope into the cecum. In rare cases, the intestinal tract can twist and rupture. While veterinarians can typically treat the simple impactions medically, intussusceptions and twisted intestines definitely require surgery.
Tapeworms infect horses of all ages, and horses do not appear to establish immunity to them. Thus, tapeworm-caused disease can happen at any age. In recent years, clinicians have observed that weanlings and yearlings experiencing their first tapeworm infection might be particularly at risk for developing ileocecal colic. So it is a good rule of thumb to begin tapeworm treatment right around or shortly after weaning.
Researchers have shown that regular fecal egg counts generated with the McMaster technique do not detect tapeworms reliably, missing more than 90% of infected horses. Fortunately, better modifies methods exist. One of these can detect at least 90% of horses with worm burdens of at least 20 worms, which is a moderate to low count. Veterinarians can also detect antibodies against these parasites in either serum or saliva. The presence of tapeworm antibodies means the horse is either currently infected or has recently been exposed to the parasite in the environment. This information can help guide veterinarians toward recommending an appropriate parasite control protocol for the farm.
There are two types of dewormers available for treating equine tapeworms. One is praziquantel, which can be found in several products. It is most often combined with ivermectin or moxidectin, but manufacturers also produce it in a standalone formulation in some countries. Praziquantel has been found to be very effective against A. perfoliata. The other type of dewormer is pyrantel pamoate, which is just as effective as praziquantel when administered in a double dose. A single dose of pyrantel can be expected to reduce tapeworm burdens by 80-85%, while a double dose reduces them by at least 95%.
Equine tapeworms don't appear to be resistant to either of these dewormers, but that does not mean it cannot occur. Researchers have not studied resistance in tapeworms extensively, and it is possible that emerging resistance could go undetected.
There are two other tapeworm species infecting the horse: Anoplocephaloides mamillana and Anoplocephala magna. These are both very rare and have not been associated with disease. Their life cycles are similar to that of A. perfoliata, but they live in the small intestine an not the cecum. Sometimes migrating tapeworm segments (proglottids) of either species can be observed in the feces of healthy horses. This is generally not observed for the other two species. None of the three species can infect other animals or humans. Similarly, horses cannot get tapeworms from dogs, cats or wildlife.
Article provided courtesy of AAEP-Media Partner, The Horse.