Summary
The tapeworms Anoplocephala perfoliata, A. magna and Paranoplocephala mamillana occur in horses around the world. In Canada A. perfoliata is the most common. These parasites have an indirect life cycle requiring a free-living mite intermediate host in which the infective cysticercoids develop following ingestion of eggs from horse faeces. Infection of horses is by ingestion of infected mites. The diagnosis of tapeworm infections in horses has significant problems with test sensitivity because egg production is very sporadic. Examining faecal samples for eggs 24-48 hours after treatment for tapeworms increases this sensitivity. Also, a serological test has been developed that detects antibodies to an A. perfoliata-specific antigen and thereby exposure, but not necessarily current infection, and a coproantigen test has also been shown to be more sensitive than simply flotation. Neither of these immunological tests are routinely available in Canada.
Tapeworms in horses were believed to be relatively non-pathogenic unless present in large numbers or in young, old or otherwise stressed animals. Recent epidemiological investigations have detected a link between infection with A. perfoliata and the occurrence of ileal impaction and spasmodic colic together with a protective effect of treatment for the tapeworms. As a result of this link there is widespread marketing of products containing praziquantel, which has high efficacy against A. perfoliata. A recent Canadian study, however, demonstrated no link between colics and the presence of A. perfoliata. Also, in a recent study of horses naturally infected with A. perfoliata, notable pathological changes have been observed in the mucosa and the submucosal tissues of the small and large intestinal walls adjacent to the ileo-caecal junction, as well as in the local enteric nervous system. The severity of the mucosal and submucosal changes was significantly linked to tapeworm burden.
Taxonomy
Phylum: Platyhelminthes
Class: Cestoidea
Cohort: Cestoidea
Subcohort: Eucestoda
Infracohort: Saccouterina
Order: Cyclophyllidea
Family: Anoplocephalidae
The tapeworms of veterinary importance most closely related to Anoplocephala perfoliata, A.magna and Paranoplocephala mammillana are the two species of Moniezia in ruminants.
Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.
Morphology
Adult Anoplocephala and Paranoplocephala are large, fleshy tapeworms. Anoplocephala perfoliata measures up to approximately 8 cm long by 1 cm wide. The scolex has no rostellum or hooks but four forward-pointing suckers with a backward-pointing lappet behind each. Anoplocephala magna measures up to approximately 80 cm long by 2.5 cm wide, with a similar scoex but no lappets. Paranoplocephala mammillana measures up to approximately 5 cm long by 5 mm wide, the suckers on the scolex are slit-like and point dorsally and ventrally, and there are no lappets.
Eggs of A. perfoliata, A. magna and P. mammillana are irregularly shaped and all have the same basic structure, with the hexacanth larva enclosed in an ice cream cone-like pyriform apparatus. Eggs of A. perfoliata are slightly larger (diameter approximately 65 to 80 µm) than those of the two other species (50 to 60 µm) and are often D-shaped. Eggs of P. mammillana are generally oval. It can be difficult to distinguish the eggs of the three species of tapeworm, but they are all very different from those of other GI parasites of horses.
Host range and geographic distribution
Anoplocephala species occur in horses and donkeys, and P. mammillana in horses, throughout the world. Adult A. perfoliata are found in the ileum and adjacent large intestine, and A. magna and in P. mammillana are usually found in the jejunum and occasionally in the stomach.
Around the world probably the most common and most important species is Anoplocephala perfoliata. A recent post mortem survey of adult horses in Alberta demonstrated a prevalence of approximately 8%, with A. perfoliata burdens in some horses of greater than one thousand. Adult A perfoliata have been identified as a risk factor for some types of colic, and as the cause of significant pathological changes affecting the mucosa and submucosal tissues and the local enteric nervous system adjacent to the ileo-caecal valve in naturally infected horses.
Life cycle - indirect
The adult tapeworms live in the intestine, the exact locations(s) depending on species. Eggs pass in the feces. The eggs are immediately infective for the free-living oribatid mite intermediate host. Following ingestion by the mite, the hexacanth larva develops into a cysticercoid, which is infective for the horse. Infection of horses by ingestion of infected mites.
Life Cycle: Anoplocephala and Paranoplocephala
Epidemiology
Because of the requirement for a free-living arthropod intermediate host, significant transmission of tapeworms among horses requires access to pasture, and in Canada most transmission is likely to occur during the summers.
Pathology and clinical signs
Many horses infected with Anoplocephala and/or Paranoplocephala show no adverse effects. Sometimes heavy burdens may cause vague clinical signs, especially in foals and young horses, or in animals with other health problems. Very rarely, the tapeworms may be associated with intestinal perforation probably caused by the activities of the scolex.
Anoplocephala perfoliata has been identified epidemiologically as a risk factor for some forms of colic, especially ileal impaction and, to a lesser extent, spasmodic colic. Ironically, a recent study has demonstrated a possible epidemiological link between mild colic and the treatment of horses with combined ivermectin and praziquantel (the latter product specifically for tapeworms). Also, in horses naturally infected with A. perfoliata, notable pathological changes have been observed in all layers of the intestinal wall, and most recently in the local enteric nervous system, around the ileo-caecal junction. The severity of mucosal and submucosal pathology has been significantly linked to tapeworm burdens.
Diagnosis
Adult tapeworms may be seen in the feces, particularly in the fall, and typical eggs recovered by faecal flotation. The sensitivity for egg detection is, however, poor because egg output is often sporadic. A serological test that detects antibodies to an antigen specific for A. perfoliata has been developed, and is commercially available in the UK and the US, but not in Canada. This test is to some extent quantitative and provides an estimate of parasite burdens, but antibody levels can remain high in the absence of adult tapeworms from the intestinal lumen. A coproantigen test (not commercially available in Canada) has been shown to be more sensitive than simple flotation. Recent work in Canada has demonstrated that in some horses a faecal flotation done 24-48 hours following treatment with an anthelmintic effective against A. perfoliata (for example, praziquantel) significantly increases test sensitivity compared to a flotation prior to treatment.
Treatment and control
Historically, the only effective treatment for tapeworms in horses was the extralabel use of pyrantel pamoate at two or three times its normal therapeutic dose. Recently, several products containing praziquantel have been developed specifically for tapeworms in horses. Praziquantel is a very effective treatment for these parasites. The products are EQUIMAX and EQVALAN GOLD (both praziqunatel with ivermectin) and QUEST PLUS (praziquantel with moxidectin).
Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.
Appropriate use of the new products containing praziquantel can be helpful in minimizing tapeworm transmission among horses, but other control measures are of limited use in horses that graze during the summer, when they have access to the free-living mite intermediate hosts.
Public health significance
Neither Anoplocephala species nor P. mammillana are known to be zoonotic
References
Reinemeyer CR (2012) Anthelmintic resistance in non-strongylid parasites of horses. Veterinary Parasitology 185: 9-15.
Elsener J et al. (2011) Does examination of fecal samples 24 hours after cestocide treatment increase the sensitivity of Anoplocephala spp. detection in naturally infected horses? Canadian Veterinary Journal 52: 158-161.
Pavone S et al. (2011) Pathological changes caused by Anoplocephala perfoliata in the mucosa/submucosa and in the enteric nervous system of equine ileo-caecal junction. Veterinary Parasitology 176: 43-52.
Skotarek SL et al. (2010) Evaluation of diagnostic techniques for Anoplocephala perfoliata in horses from Alberta, Canada. Veterinary Parasitology 172: 249-255.
Reinemeyer CR et al. (2009) Parasitism and colic. Veterinary Clinics of North America Equine Practice 25: 233-245.
Trotz-Williams L et al., (2008) Occurrence of Anoplocephala perfoliata infection in horses in Ontario, Canada, and associations with colic and management practices. Veterinary Parasitology 153: 73-84.