Dipylidium caninum

Dipylidium caninum is a tapeworm of the small intestines of domestic dogs and cats generally found in warmer and more humid regions of Canada, such as southern coastal BC.  Like all tapeworms, the parasite has an indirect life cycle, with fleas (and possibly canine chewing lice) serving as intermediate hosts.  Dogs and cats acquire the parasite by ingesting infected fleas or lice containing the cysticercoid infective stage.  Dipylidium caninum is relatively non-pathogenic. Diagnosis is challenging because segments are the most common fecal stage, and neither they nor the large egg packets are likely to be detected on routine fecal flotation.  The short pre-patent period of the parasite (2-3 weeks) means that when segments (resembling cucumber seeds) re-appear in feces shortly after treatment, this can be a result of rapid re-infection from poorly controlled flea infestations, or, as recently described, emerging resistance to cestocides.  Rarely people, especially children, can become infected with D. caninum following ingestion of an infected dog or cat flea.  Treatment for this tapeworm should always be coupled with control of flea intermediate hosts but detection of fleas should not automatically result in prescription of a cestocide, especially as there is little perceptible impact of D. caninum on animal or human health.

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

 

Members of the Family Dilepididae include Dipylidium, Joyeuxiella, and Diplopylidium spp.

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. They have similar structures and life cycles to other cyclophyllid cestodes.

We have recently recognized that dogs and cats have their own host-specific “strains” of Dipylidium caninum, although cross infection can occur.

Adult D. caninum are large (up to 50 cm in length, or more) and have an anterior scolex (attachment or holdfast organ), behind which is the ribbon-shaped, segmented body of the tapeworm. Typically, anterior immature segments are smaller than mature and gravid segments towards the posterior. At the anterior tip of the scolex is a rostellum, which is armed with several circles of hooks. Behind the rostellum are four circular, muscular suckers. Each mature segment of D. caninum contains two sets of reproductive organs, each with a lateral genital pore at about the mid-point of each lateral margin of the segment.

Scolex

Mature segments                                                          Gravid segments

In gravid segments, the eggs of D. caninum are grouped in very distinctive egg packets (egg capsules) throughout the central area of the segment. Each egg packet measures up to 200 µm in diameter and contains up to approximately 30 eggs, each of which is approximately 25 to 50 µm in diameter. In most specimens submitted to a diagnostic laboratory, the genital pores and the egg packets are the only structures visible, even in fixed, stained specimens. Each egg has a fragile shell and contains a typical hexacanth larva with six hooks. Details of the egg packets are not always easily visible in the segments.

Eggs inside of egg packets                                                  Flea IH

Cysticercoids from flea IH

 

Dipylidium caninum occurs commonly in dogs and cats, foxes, and very occasionally in people, especially children, around the world. In Canada, they are present wherever Ctenocephalides spp. fleas are present on dogs and cats, generally in southern and coastal regions with high humidity.

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

Egg packets, or eggs, are ingested by a suitable flea larva (or possibly a chewing louse) although the epidemiological significance of lice is not known. In the arthropod intermediate host (required for completion of the life cycle), the eggs hatch in the GI system. The hexacanth larva released from each egg then penetrates the GI wall and migrates to the body cavity, where it develops into a cysticercoid, which is the metacestode stage for cyclophyllid tapeworms that utilize invertebrate intermediate hosts. The cysticercoid remains in the flea as it develops through its larval stages to the pupa and ultimately to an adult flea. Each cysticercoid contains only a single protoscolex and each protoscolex has the potential to become the scolex of an adult tapeworm.

The life cycle continues when the cysticercoid in the infected flea is eaten by a dog or a cat or other suitable definitive host. Here the infective stage is released from the intermediate host tissues, attaches to the mucosa of the small intestine using the scolex, produces segments and completes its development to an adult. Eggs are produced within 2-3 weeks of infection of the dog or cat; this short pre-patent period has implications for epidemiology and control.

 

The maintenance and spread of D. caninum in dogs and cats is vitally linked to the presence of arthropod intermediate hosts. Treatment and control measures aimed at fleas (and lice) often bring about control of D. caninum. The short pre-patent period combined with rapid re-infection from fleas can lead to perceived treatment failures.

Adult D. caninum in dogs or cats are very rarely associated with clinical signs, which are non-specific. Passage of segments can be disturbing for pet owners, particularly as the segments can be quite lively.

Single gravid segments , or more rarely multiple segments or whole tapeworms, can be detected in pet feces, usually by the owner. The gravid segments of D. caninum look like cucumber seeds. Detection of egg packets on fecal flotation is uncommon, as primarily segments are shed in feces and the egg packets are large and less likely to float. Each egg packet measures up to 200 µm in diameter and contains up to approximately 30 eggs, each of which is approximately 25 to 50 µm in diameter. If you are lucky enough to detect egg packets of D. caninum on flotation, they are quite distinctive, although if their size is not fully appreciated they are sometimes mis-identified as hookworm eggs! If the adult tapeworms/ segments recovered from feces seem dehydrated, immersion in tap water for several minutes will often reveal the diagnostic structures (most notable, the bilateral genital pores). 

There are several products approved in Canada for treating D. caninum in dogs and cats including praziquantel and epsiprantel. Because this tapeworm has a very short pre-patent period (a few weeks), owners may sometimes mistakenly believe that treatment has not been effective, when in fact the dog or cat has been re-infected due to inadequate flea control.  However, resistance to praziquantel in D. caninum has recently been documented.  This suggests that routine cestocide administration to pets in households where fleas have been detected is not a responsible use of anthelmintics.  On detection of fleas, efforts should focus on elimination of fleas and not involve routine prescription of a cestocide.  On detection of D. caninum (which is rarely accomplished by fecal flotation), efforts should focus on elimination of fleas and administration of a cestocide to all pets (dogs and cats) in the household.  

 

Very rarely, children may become infected with Dipylidium caninum by ingesting infected fleas (similar to how pets become infected - people are not exposed directly from pet feces). Clinical signs are usually mild. More common is parental alarm associated with the gravid segments in the child’s feces.

Beugnet et al., (2018 )Analysis of Dipylidium caninum tapeworms from dogs and cats, or their respective fleas Part 2. Distinct canine and feline host association with two different Dipylidium caninum genotypes. Parasite 25:31 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6013090/

Conboy G (2009) Cestodes of dogs and cats in North America. Veterinary Clinics of North America Small Animal Practice 39: 1075-1090.

Challadurai et al. (2018) Praziquantel resistance in the zoonotic cestode Dipylidium caninum.  American Journal of Tropical Medicine and Hygiene 99: 1201-1205.

https://www.ajtmh.org/view/journals/tpmd/99/5/article-p1201.xml

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