A small animal veterinarian requested a consultation regarding treatment options for a dog (signalment unknown) with chronic tapeworm infection. The dog did not display any clinical signs, but several praziquantel products failed to resolve the shedding of proglottids. The veterinarian did not see fleas or flea dirt on the physical exam and confirmed that the owner consistently administered flea/tick control. Based on case reports and dosing recommendations from the Companion Animal Parasite Council, nitazoxanide anthelminthic therapy was recommended as a treatment option.^1,2

Overview of Dipylidium caninum

Dipylidium caninum (D. caninum) is the most common tapeworm of dogs and cats in the United States, and this type of cestode infection is typically responsive to praziquantel and epsiprantel treatment.¹ Recently, there have been reports of praziquantel-resistant D. caninum infections in dogs. Isoquinoline drug-resistant D. caninum has not been observed in cats, likely due to differences in the host adapted D. caninum canine genotype infecting dogs versus D. caninum feline genotype infecting cats.³ Occasionally, D. caninum results in abdominal discomfort, vomiting, diarrhea, perianal irritation, and pruritis, in both dogs and cats. Still, most infections do not result in clinical disease. The zoonotic potential of this parasite makes the emergence of resistant strains a public health concern.

All tapeworms have an indirect life cycle. Fleas (Ctenocephalides felis, C. canis, Pulex irritans) and lice (Trichodectes canis) are the intermediate hosts of D. caninum. Ingestion of adult fleas or lice by vertebrate hosts results in cysticercoids colonizing the small intestine, where they develop into adult tapeworms that reproduce and release gravid proglottids into the environment via feces, continuing the life cycle.^1,4

The prepatent period for D. caninum is 2-3 weeks.^1,4 Gravid proglottids are motile segments resembling cucumber seeds or rice grains mainly seen on the fur around the perineum or on the surface of fresh feces. Fecal flotation is an insensitive methodology for detecting egg packets due to their focal distribution in fecal material and inability to float in commonly used flotation solutions.⁵ At the AHDC, resistant D. caninum infections are documented through veterinarian accounts of the continued presence of proglottids after repeated treatments with verified consistent application of appropriate flea control products and the absence of intermediate hosts.

Documented case reports of praziquantel-resistant D. Caninum infections in Dogs

From 2016 to 2018, a group of veterinary investigators examined 5 cases of dogs with D. caninum infections that did not resolve with label doses of praziquantel alone, in combination with epsiprantel, or at varying doses of these two isoquinolines.⁶ The two cases that received two doses of oral nitroscanate (47 mg/kg and 84 mg/kg) given two weeks apart achieved resolution of proglottids.⁶ Another case review documented isoquinoline-resistant D. caninum in an 18-month-old terrier mix that was successfully eliminated using a single dose of nitrosconate.⁷

Treatment Options

Veterinarians should be aware of the emergence of drug-resistant D. caninum in dogs because of its potential public health implications. Treatment options are currently limited. Nitroscanate, as used in the referenced case reports, is not FDA-approved and is currently unavailable in the US. Nitazoxanide is FDA-approved for treating cryptosporidiosis in humans. In veterinary medicine, nitazoxanide is used extra-label to treat resistant D. caninum in dogs at 100 mg/kg (dose suggested by the Companion Animal Parasite Council).¹ Documenting resistance through egg reduction testing is not feasible; therefore, consistent implementation of proper monthly flea control is necessary to rule out reinfection and document true D. caninum resistance.

References:

1. Dipylidium caninum [Internet]. Companion Animal Parasite Council. [cited 2023 Nov 2]. https://capcvet.org/guidelines/dipylidium-caninum/
2. Manoj, R. Current Perspectives Regarding Dipylidium caninum Infection in Companion Animals. Today’s Veterinary Practice: July/August 2023
3. Beugnet F, Labuschagne M, Vos C, Crafford D, Fourie J. 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. 2018;25:31. doi: 10.1051/parasite/2018029. Epub 2018 May 28. PMID: 29806593; PMCID: PMC6013090.
4. Bowman, D. Georgis’ Parasitology for Veterinarians 10th Edition. St. Louis: Elsevier Saunders. 2014.
5. O’Quin, J.Dipylidium caninum: A Potential Zoonosis Easily Missed on Routine Fecal Diagnostics. Clinician’s Brief: March 2023.
6. Chelladurai, J., Kifleyohannes, T., Scott, J., Brewer, M. Praziquantel Resistance in the Zoonotic Cestode Dipylidium caninum. Am. J. Trop. Med. Hyg., 99(5), 2018, pp. 1201-1205.
7. Loftus, J.P., Acevedo, A., Bowman, D.D., Liotta, J.L., Wu, T., Zhu, M. Elimination of probable praziquantel-resistant Dipylidium caninum with nitroscanate in a mixed-breed dog: a case report. Parasit Vectors., 15 (1), 2022, 438.