The relationship between internal equine parasites and the risk of colic has been widely known in the cases of several types of worms that infect horses.  In recent years, there have been significant advancements in the understanding of equine tapeworms, the damage they inflict, and their role in equine colic.1  Equine colic is the single most important noninfectious cause of mortality in horses, and parasitism is one of many factors which can lead to colic. Fortunately, this is one of the easiest factors to control through knowledge of a parasite’s life cycle and strategic use of dewormers that can help to control infection.  

The equine tapeworm (Anoplocephala perfoliata) is ubiquitous in most places where horses have pasture access. However, their presence largely depends upon favorable climatic conditions for the orbibatid mite, which is the tapeworm’s intermediate host. In dry and arid states in the US southwest (for example), horses are rarely exposed to tapeworms. 

The overwhelming majority of horses harboring tapeworms tolerate them without any signs of discomfort or development of colic. It is not entirely known why tapeworm-related disease occurs in some horses and not others, but there are several possible reasons. In some cases, a proliferation of the worms may occur (high infection pressure), which could be driven by climatic conditions or overstocked paddocks and pastures. In other cases, a horse with a suppressed immune system may be more susceptible to a heavy tapeworm infection.

Tapeworm Infection in Equines

A. perfoliata live at the junction between the ilieum and the cecum, where the small intestine connects to the large intestine.2 The worms attach to the intestinal wall just inside the cecum. Thus, the resulting colic is related to the ileocecal region. In very rare cases, the intestinal tract can twist and rupture.1 

Tapeworms infect horses of all ages, and horses do not appear to establish immunity to them.2 Clinicians have observed that weanlings and yearlings experiencing their first tapeworm infection may be particularly at risk for developing ileocecal colic. Thus, many owners and managers begin tapeworm treatment right around or shortly after foals are weaned.

Research has shown that fecal egg counts generated with the McMaster technique do not reliably detect tapeworms, and often miss more than 90% of infected horses.1 Fortunately, other methods can detect at least 90% of horses with worm burdens of as little as 20 worms. Veterinarians can also detect antibodies against A. perfoliata in serum or saliva. The presence of tapeworm antibodies means that a horse is either currently infected or has recently been exposed to A. perfoliata in its environment. 

Praziquantel for Horses

Praziquantel is an anticestodal anthelmintic that is FDA-approved for treatment of Dipylidium caninum, Taenia pisiformis, E multilocularis and Echinococcus granulosus in dogs, and for the treatment of D caninum and T taeniaeformis in cats.3 Praziquantel can also be used extra-label to treat trematodes (e.g., Alaria spp, Paragonimus kellicoti) and other cestodes (e.g., Diphyllobothrium spp, Spirometra spp).

“Although praziquantel’s exact mechanism of action against cestodes has not been determined, it may be the result of interacting with phospholipids in parasite integument, causing ion fluxes of sodium, potassium, and calcium. At low concentrations in vitro, the drug appears to impair (ie, paralyze) the worm’s sucker function and stimulates the worm’s motility. At higher concentrations in vitro, praziquantel contracts and paralyzes (irreversibly at very high concentrations) the worm’s strobilla (ie, chain of proglottids). In addition, praziquantel causes irreversible focal vacuolization, with subsequent cestodal disintegration at specific sites of the cestodal integument. The parasite ultimately becomes susceptible to digestion.”3

Oral administration of praziquantel may cause anorexia, gagging or vomiting, lethargy or diarrhea in dogs, with an incidence being less than 5%. In cats given oral praziquantel, adverse effects were less than 2%; occasionally, salivation and diarrhea were reported.

A greater incidence of adverse effects has been reported after use of injectable praziquantel. In dogs, pain at the injection site was the most frequent adverse effect, with vomiting, drowsiness, and/or a staggering gait also reported. Some cats (9.4%) showed clinical signs of diarrhea, weakness, vomiting, salivation, sleepiness, transient anorexia, and/or pain at the injection site.3

Dosing recommendations for praziquantel in equines are as follows (adapted from Plumb’s Veterinary Drugs):

Susceptible parasites (labeled-dose; FDA approved): 1 – 2.5 mg/kg PO using combination products containing praziquantel with ivermectin or moxidectin.

Anoplocephala perfoliata (extra-label): 1.5 – 2 mg/kg PO was 100% effective in treated horses; 1 mg/kg PO was 99.7% effective.

Where to buy Praziquantel

Praziquantel is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. PRAZIQUANTEL 300 MG/ML, ORAL SUSPENSION, 100ML by NexGen Pharmaceuticals is an excellent choice for the treatment of tapeworms and other internal parasites in the horse. 

Praziquantel carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1aaep.org.

2Merck Veterinary Manual.

3Plumb’s Veterinary Drugs.