Current Perspectives Regarding Dipylidiasis Infection in Companion Animals

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Dipylidiasis is a tapeworm infection caused by the cestode Dipylidium caninum (commonly known as the flea tapeworm, double-pored tapeworm, or cucumber seed tapeworm), which is found worldwide.^1,2 Key factors that influence the probability of infection for dogs and cats include the geographic region and the availability of fleas or lice as intermediate hosts. Although the reported prevalence of this infection varies from 4% to 60% in dogs and 1.8% to 52.7% in cats,³ the actual prevalence may vary as the data generated by fecal flotation alone almost certainly underestimate the actual number. A survey of euthanized cats from shelters in Oklahoma revealed that 40 of 116 cats were positive for D caninum worms, although no infections had been diagnosed by using centrifugal sugar flotation.⁴ This finding is a reminder to not rely on a single diagnostic method for the presumptive diagnosis of dipylidiasis. Recent molecular evidence suggests the existence of separate canine and feline genotypes for this parasite.⁵

D CANINUM LIFE CYCLE

As the common name “flea tapeworm” indicates, D caninum parasites have an indirect life cycle that requires fleas and lice as intermediate hosts and vertebrates as definitive hosts. Grossly visible mature proglottids are found in the perianal region, the pet’s and human’s bedding, or the fecal material itself. The eggs released from the crawling proglottids are taken up by larval fleas or nymphal lice, where oncospheres (tapeworm embryos) hatch and develop to infective cysticercoids. The insects retain the cysticercoids during metamorphosis, and the pet gets infected while nipping at the adult insects during grooming. Children get infected by accidentally ingesting fleas or lice while playing with pets or directly from the environment (FIGURE 1).^1-3 The final transformation of larval cysticercoids into segment-shedding tapeworms takes only 2 to 3 weeks, at which time they attach to the small intestinal mucosa of the definitive hosts via a scolex and 4 suckers. Adult tapeworms measure 15 to 70 cm long and have a retractable rostellum armed with 30 to 150 small hooks.

Figure 1. Life cycle of the Dipylidium caninum tapeworm. Courtesy Ranju Ravindran Santhakumari Manoj; created with biorender.com.

DIPYLIDIASIS DIAGNOSIS

D caninum infection does not lead to ill health in companion animals or humans. Occasional symptoms are mild abdominal pain, vomiting, and diarrhea. Dragging the perianal region along bedding or walls to relieve the anal pruritus caused by the crawling proglottids cannot be considered a characteristic clinical sign as that behavior also accompanies anal gland impaction. The most striking feature of infection and the most significant concern for clients is the unsightly emergence of motile proglottids from the anus. They can be found in the perianal region, in the feces, on diapers, and occasionally on floor coverings and furniture.

Proglottids, shaped like cucumber seeds and having double genital organs, can be seen with a hand lens and are often mistaken for maggots or fly larvae (FIGURE 2).^2,3 Preliminary diagnosis is based mainly on identification of proglottids. If they are dried and distorted, crushing the segments between 2 glass slides and observing them under a microscope may help reveal the characteristic egg packets (FIGURE 3), thus differentiating the segment from that of Taenia species tapeworms, which contain individual eggs. The individual eggs of both species contain a hexacanth embryo.

Figure 2. Cucumber seed–shaped gravid Dipylidium caninum proglottids recovered from the feces of an infected animal.

Figure 3. Dipylidium caninum egg packet seen in crushed proglottids, containing numerous oncospheres (average 25–30 eggs) with an individual egg size of 35–60 µm. Scale bar indicates 50 μm.

Identification of egg packets on fecal flotation is less sensitive due to the lack of uniform distribution of proglottids in the fecal material and inconsistent floating of tapeworm eggs; therefore, other diagnostic methods (e.g., molecular diagnosis by polymerase chain reaction [PCR],⁶ detection of antigen produced by mature proglottids) should be combined with fecal flotation for accurate diagnosis.³

DIPYLIDIASIS TREATMENT AND CONTROL

Treatment and control of D caninum infection must be executed together. Anthelmintic therapy is not beneficial unless the reservoirs of infection (fleas and lice) are also brought under control.¹ Hence, administration of anthelmintics should be combined with use of U.S. Food and Drug Administration (FDA)- or Environmental Protection Agency (EPA)-approved insecticides.^1,2 The primary approved drug of choice for dipylidiasis in dogs and cats is praziquantel at 5 mg/kg PO or SC.^1-3 A closely related isoquinoline, epsiprantel, can also be administered at 5.5 mg/kg PO for dogs and 2.75 mg/kg PO for cats. However, refractoriness to praziquantel and epsiprantel was noted for D caninum in 5 dogs from Colorado, Iowa, Michigan, and Minnesota after multiple increased doses of praziquantel.⁷ Some of the infections were cleared by using an off-label drug, nitroscanate, or a compounded combination of pyrantel, praziquantel, and oxantel.⁷ Off-label use of nitazoxanide (100 mg/kg) may also be helpful for drug-resistant cases.³ However, practitioners should carefully rule out the possibility of reinfection before declaring praziquantel resistance. In addition to on-host insecticide use, the animal’s bedding and resting places should be cleaned and sprayed with insecticide to eliminate the immature flea and lice stages.

CASE SCENARIO

Signalment and History

An 11-year-old neutered male American foxhound dog with a history of regular deworming and vaccination was presented to a pet clinic after the client noticed gravid proglottids in the feces. The first proglottid was seen in the feces during February 2021, after which the client noticed gravid proglottids quite often and collected them in a tube. The dog had been adopted in 2020 from a hunt club, where he had hunted coyotes and foxes. According to the client, the dog had been receiving a monthly chewable heartworm preventive (2.3 mg milbemycin oxime with 22.8 mg praziquantel) and always wore flea and tick collars containing imidacloprid and flumethrin.

Earlier routine fecal examinations were positive for Ancylostoma caninum, Trichuris species, and Toxocara canis, for which the dog received 1.5 tablets of oral praziquantel/pyrantel pamoate/febantel (Drontal Plus; Elanco, elanco.com). The treatment was repeated every 3 weeks (total 3 times), which cleared all parasites except A caninum, which was cleared by fenbendazole oral suspension (50 mg/kg). The client reported no other clinical signs.

Physical Examination

Examination did not detect fleas or “flea dirt” on the patient’s body. The tapeworm segments and a fresh fecal sample collected by the client were sent to the reference laboratory for further parasite identification.

Diagnostic Test Results

No eggs were detected in centrifugal fecal flotation using zinc sulfate (1.18 specific gravity) or sugar (1.33 specific gravity) solutions. The tapeworm segments had an elongated shape, resembling cucumber seeds. Because the segments were dried, they were crushed between 2 glass slides and mounted with 70% alcohol. The crushed proglottids revealed characteristic D caninum egg packets,⁸ 120 to 200 µm, containing numerous oncospheres (average 25 to 30 eggs), with an individual egg size of 35 to 60 µm (FIGURE 3).

Treatment Plan

The patient received a single oral dose of 1.5 tablets of Drontal Plus whenever proglottids were seen in the feces. No improvement has been noted to date, and the patient was still passing proglottids at regular intervals at the most recent follow-up visit.

Discussion

This patient is infected with a parasite suspected to be resistant to praziquantel. D caninum worms that are refractory to treatment after repeated praziquantel therapy at ≥5 mg/kg body weight and adequate flea control measures are considered to be drug resistant. For the patient presented here, the treating veterinarian was not willing to use unapproved drugs. Because the chances of reinfection were ruled out by ensuring continuous use of insecticide collars containing imidacloprid and flumethrin, this case represents suspected praziquantel drug resistance as reported in the previously mentioned study.⁷

According to the literature, the next options for infection refractory to praziquantel are nitroscanate or nitazoxanide. Nitroscanate was introduced in 1973 and has been shown to possess 98% to 99.8% activity against D caninum in dogs at a single dose of 50 to 56 mg/kg PO^9,10; however, this drug is currently not available in the United States.

The next alternative is off-label use of nitazoxanide at 100 mg/kg,³ which is available in the United States albeit expensive. A cost-effective way to buy this drug is through GoodRx (goodrx.com) or Canada Drugs Online (canadadrugsonline.com). This drug is used to treat Cryptosporidium infection, and the Companion Animal Parasite Council indicates its use as follows for Cryptosporidium species infection: 100 mg q12h for 5 days in animals 24 to 47 months of age and 200 mg q12h for 5 days in animals 4 to 11 years of age. However, the efficacy of nitazoxanide in dogs and cats is not known and there is no published report on an approved dosage for treating canine tapeworm. According to personal communication from Dr. Manigandan Lejeune (Cornell University), a veterinarian from Florida successfully used this drug to treat praziquantel-resistant D caninum. However, another veterinarian who used the drug at the above dosage to treat D caninum observed protracted vomiting after only 2 doses; the vomiting was controlled by using antiemetics, and tapeworm segment shedding stopped right after the 2 doses. According to another personal communication with Drs. John Loftus (Cornell University) and Kelli Ferris (North Carolina State University), a single dose did not effectively control the infection in suspected resistant cases, but a second dose was effective.

More work to identify resistant strains of D caninum tapeworms is needed. Case studies of successful treatment of praziquantel-resistant D caninum infection in the United States have been reported.^7,10 Recent diagnostic advances, such as capsule endoscopy, can be used to confirm praziquantel resistance by monitoring presence or absence of adult parasites in the small intestine before and after treatment. The same method coupled with pre- and post-treatment fecal examination and gross monitoring of the feces for proglottid presence or absence may be used to evaluate anthelmintic effectiveness.¹¹

Summary

D caninum cestodes commonly infect dogs and cats and occasionally humans. Infections are typically asymptomatic and not life-threatening. Definitive hosts contract dipylidiasis by ingesting adult fleas or lice containing the cysticercoid stages of this parasite. Humans are prone to infection through interactions with their pets. Obtaining an accurate diagnosis includes gross monitoring of the proglottids, fecal flotation, PCR, and detection of antigen from mature proglottids. Treatment and control must be executed together; hence, simultaneous administration of anthelmintics and insecticides to target the intermediate hosts are needed for effective parasite control. If the parasite is refractory to praziquantel and epsiprantel, nitroscanate or nitazoxanide may be effective. Although dipylidiasis is not life-threatening, the zoonotic significance of dipylidiasis warrants timely diagnosis and treatment.