Taenia species

The cestode genus Taenia includes manyspecies that cycle between predator-prey assemblages around the world.  Adult cestodes live in the small intestine of dogs, cats, and free-ranging carnivores around the world.  Examples include T. hydatigena and T. pisiformis in dogs and T. taeniaeformis in cats.  All species have indirect life cycles that require a mammalian intermediate host, which becomes infected by consumption of immediately infective, environmentally resistant eggs passed in the feces of the carnivore definitive host.  Each species of Taenia has a characteristic larval stage (metacestode) that develops in the tissues or organs of the  intermediate host.  In Canada, the most common form is the cysticercus.   Carnivores become infected when they consume the metacestode stage inside an intermediate host (through predation or scavenging).  Adult Taenia are usually non-pathogenic, but some larval stages in the intermediate hosts can be associated with significant pathology.  Diagnosis relies on detection of taeniid eggs on fecal flotation (or preferably sedimentation), or segments passed in feces.  Since methods based on recovery of eggs from feces have poor sensitivity, and taeniid eggs cannot be identified to genus level based on morphology alone, PCR based methods are often preferable for diagnosis in dogs, allowing differentiation of Taenia from Echinococcus spp.  There are many products labeled for Taenia spp. in dogs and cats, usually based on praziquantel. Control relies on preventing dogs and cats from accessing carcasses of infected wildlife or domestic livestock serving as intermediate hosts.   None of the species of Taenia infecting dogs in Canada are zoonotic.

 

Phylum: Platyhelminthes
Class: Cestoda
Order: Cyclophyllida
Family: Taeniidae 

The Order Cyclophyllida includes several families that contain virtually all tapeworms of domestic animals and birds. Adults of most genera within the family Taeniidae are parasites of carnivores and omnivores, and have many morphological and biological similarities. Other tapeworms of veterinary importance - Diphyllobothrium (Dibothriocephalus) and Spirometra are classified in another Order, the Pseudophyllida. 

All cyclophyllid tapeworms have basically similar structures and life cycles, although the details differ. Similarities among the genera within a cyclophyllid family are greater than those between families. 

Taeniid cestodes include many different species of Taenia and several species of Echinococcus.  These species circulate in fairly specific host assemblages and produce morphologically indistinguishable eggs.

Note: Our understanding of the taxonomy of parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on Deplazes et al. eds. Parasitology in Veterinary Medicine, Wageningen Academic Publishers, 2016.

Adults of the species of Taenia that infect dogs or cats are large (up to 100 cm in length, or more) and have an anterior scolex (attachment or holdfast organ), behind which is the ribbon-shaped body composed of segments. Typically, anterior immature segments are smaller than the mature or gravid segments towards the posterior. At the anterior tip of the scolex is a rostellum, which is armed with two circles of hooks. Behind the rostellum are four circular, muscular suckers. Each mature segment of Taenia species contains a single set of reproductive organs, with a lateral genital pore at about the mid-point of the segment. In mature segments, details of the reproductive structures can be seen in fixed and stained, but not fresh, specimens. In gravid segments, however, the laterally-branched uterus (with the structure of a “Christmas tree”) is usually visible, even in fresh specimens. 

Mature segments (stained)                       Gravid segment (stained)

Gravid segments (unstained)                        Taenia spp. gravid segments found in the environment

Eggs of Taenia species are round to oval, measure approximately 30 to 35 µm in diameter and have a thick, radially striated shell. Each egg contains a hexacanth larva with six hooks, not all of which are visible in every egg. The eggs of the various species of Taenia cannot be distinguished microscopically from each other, or from the eggs of Echinococcus species.

 

Within the genus Taenia, some species infect primarily dogs and other canids, and others primarily cats and other felids, around the world. Taenia tapeworms are very common in Canada, especially in pets that spend time out of doors and thus have access to the intermediate hosts. Species commonly found in dogs include T. pisiformis, T. hydatigena, T. krabbei, T. ovis, and T. serialis. The most common species in cats in Canada is T. taeniaeformis.  Taenia spp. use a parasite species-specific range of herbivore intermediate hosts, including rabbits, rodents, cervids, and domestic livestock.

Adult Taenia species live in the small intestine of the definitive hosts and, like all cestodes of veterinary importance, are hermaphrodites. Gravid segments, which are full of eggs, drop off and pass intact in the feces or disintegrate in the GI lumen, releasing the eggs, which are also passed in the feces. 

If an egg is ingested by a suitable mammalian intermediate host (required for completion of the life cycle) the egg hatches in the stomach. The hexacanth larva that is released then invades the intestinal wall – using its six hooks - and migrates in the bloodstream to various tissues, where it develops into the metacestode, or larval stage. Each species of Taenia has a specific metacestode stage; for most species in Canada, this is the cysticercus, which serves as the infective stage for the canid definitive host (T. pisiformis, abdominal organs of rabbits and rodents; T. hydatigena, abdominal organs of domestic and wild ruminants; T. ovis, muscle in sheep and goats; T. krabbei, muscle of cervids; T. polyacantha, abdominal organs of rabbits and rodents; T. crassiceps, connective tissue and body cavity of rodents). Each cysticercus contains only a single protoscolex. Each protoscolex has the potential to become the scolex of an adult tapeworm.

Cysticerci from IH                                              Cyticercus section

The life cycle continues when the metacestode in the infected intermediate host is eaten by a dog or a cat or another suitable definitive host. Here the infective larval stage is released from the intermediate host tissues, protoscolices evert and attach to the mucosa of the small intestine, forming  the scolex of the adult cestode, which buds off segments (proglottids) to form the  strobila of the adult cestode. The pre-patent period varies for different species but is roughly 6-9 weeks.

Taenia taeniaeformis has an atypical metacestode stage called a strobilocercus, which is found in the liver of rodents. For T. multiceps (not in Canada) and T. serialis, the larval stage is a coenurus, which develops from a single egg and which contains several protoscolices attached to the wall of the thin walled cyst. In these two species, therefore, the parasite can increase its numbers in the intermediate hosts (T. serialis, connective tissue of rabbits; T. multiceps, central nervous system and connective tissue of wild and domestic ungulates).  While dogs and cats are generally definitive hosts for Taenia spp., rarely dogs, cats, and people can serve as aberrant intermediate hosts for Taenia spp., sometimes with serious consequences (i.e. coenurus of T. serialis in the brain of cats). 

Coenurus from IH                                         Coenurus section

Important for the maintenance and spread of Taenia species tapeworms of dogs and cats are the ability of individual adult parasites to produce large numbers of eggs, the potentially long survival of the immediately infective eggs in a range of environments (months to years under cool, moist conditions), and the reservoir of infection in the wildlife intermediate hosts.  Unlike many other parasites, infection is not always more common in young dogs and cats - older animals with access to intermediate hosts are commonly infected, often with multiple species of taeniid tapeworms.  In a recent national study of shelter dogs and cats, prevalence of taeniid eggs (potentially Taenia or Echinococcus spp.) based on fecal flotation (which is notoriously insensitive) was 2% in dogs and 4% in cats nationally, with prevalence in the prairie provinces higher (3% of dogs and 6% of cats).  Prevalence may be higher in western Canada as more dogs and cats are free ranging and/or have access to carcasses of livestock and wildlife.    

Adult tapeworms in dogs and cats are not usually a clinical problem unless present in very large numbers in young animals or in animals with other health problems. Occasionally the larval stages of Taenia species that have infected domestic animals or wildlife as intermediate hosts are associated with clinical signs, particularly if the central nervous system is involved. In addition, sometimes in sheep flocks in western Canada the larval stages of T. ovis are so numerous that the carcasses are condemned for aesthetic reasons (T. ovis is not zoonotic). In general, however, the larval stages of Taenia species occurring in Canada in dogs and cats are not considered a significant clinical problem in their intermediate hosts.

Whole tapeworms, or single or multiple gravid segments, can often be detected in feces.. If the adult tapeworms or segments recovered seem dehydrated, immersion in tap water for several minutes will often reveal the diagnostic structures.   Eggs are dense and sporadically shed, requiring centrifugation-flotation with a high specific gravity solution or, ideally, sedimentation.  Fecal flotation has a low sensitivity ( <30%) for diagnosis of adult taeniid cestode infection in carnivore definitive hosts. 

The gravid segments and eggs of the Taenia that infect dogs and cats cannot be identified to the species level macroscopically; this requires microscopic examination of the hooks on the rostellum or, most recently, commercially available coproPCR methods which can detect non-patent infections and differentiate Taenia from Echinococcus spp., important because of the zoonotic potential of the latter.

The goal of treatment is to reduce environmental contamination with eggs and eliminate passage of adult cestodes and segments in feces, which many owners find distasteful.  In addition, detection of Taenia spp. tapeworm infections in a dog or cat indicate that the animal is high risk for consumption of wildlife and organs or raw meat, placing them at risk of exposure for many species of parasites, including, most concerningly, Echinococcus spp.  Therefore, such animals should be treated regularly (monthly in regions endemic for E. multilocularis) with an adult cestocide.   There are several products approved in Canada for treating Taenia species tapeworms in dogs and cats, including fenbendazole, nitroscanate, and praziquantel based products.    Prevention requires preventing access of dogs and cats to intermediate hosts, and preventing dogs and cats from fecally contaminating the environment, especially livestock pasture and feed for T. ovis in dogs.  Offal and raw meat should be thoroughly cooked or frozen solid prior to feeding to companion animals.

None of the species of Taenia in Canada that utilize carnivores or omnivores, including dogs and cats, as definitive hosts are known to be zoonotic.  Elsewhere in the world, people can become infected with adult Taenia spp. by consumption of cysticerci in raw beef (T. saginatta) or pork (T. solium).  More concerningly, Taenia solium can also establish in people as intermediate hosts, causing neural cysticercosis, a serious and even fatal condition. 

Kolapo T et al. (2021) Copro-polymerase chain reaction has higher sensitivity compared to centrifugal fecal flotation in the diagnosis of taeniid cestodes, especially Echinococcus spp, in canids.  Veterinary Parasitology 292 https://doi.org/10.1016/j.vetpar.2021.109400

 Villeneuve, L. Polley, E.J. Jenkins, J.M. Schurer, J. Gilleard, S. Kutz, G. Conboy, D. Benoit, W. Seewald, F. Gagné. 2015. Parasite prevalence in fecal samples from dogs and cats across the Canadian provinces. Parasites and Vectors 8:281 doi:10.1186/s13071-015-0870-x

https://research-groups.usask.ca/cpep/parasites/cestodes-supplementary.php