Dracunculosis in a German shepherd dog

A 9-year-old, spayed female, German shepherd presented for evaluation of a mass on her right hind foot and for ventral abdominal pruritis that had developed after swimming in swampy water. Examination revealed a papular dermatitis involving the ventral abdominal area and a 2-cm subcutaneous mass on the dorsal metatarsal aspect of the right hind foot. Microscopic examination of a fine needle biopsy of the mass revealed moderate pyogranulomatous inflammation and a large number of coiled nematode larvae. No eosinophils were observed. The larvae were approximately 35 μm wide and 500 μm long, including a 70- to 100-μm long, whip-like tail.

A 2-cm subcutaneous pseudocyst containing a 17-cm long nematode was excised from the dog's foot and the incision was closed using a routine 2-layer skin closure. A diagnosis of dracunculosis was made, based on the morphology of the larvae and length of the adult worm (Figure 1).

Figure 1. Photograph of an adult Dracunculus insignis nematode taken after excision of the pseudocyst from the right hind foot of the German shepherd.

Dracunculus insignis is a parasite of dogs and wild carnivores in North America; it is related to the guinea worm D. medinensis seen in humans and animals in Asia and Africa. It is common in raccoons in south western Ontario. Hosts become affected after ingesting a crus tacean from the genus Cyclops in contaminated water. The larvae develop into adults over an 8- to 12-month period. Adults develop within a pseudocyst in the subcutaneous tissue of the abdomen and limbs. The host may develop itchiness, urticaria, and mild pyrexia. The nodules eventually develop a fistula, through which the larvae escape. The female worm is stimulated to release larvae when the host enters cold water. The lesions are often painful and pruritic.

Cytologic examination will often reveal neutrophils, eosinophils, macrophages, and larvae (Figure 2).

Figure 2. Dracunculus insignis larvae from a dog (original by D.S. Lindsay, http://www.ksu.edu/parasitology/625tutorials/Nematodes11.html).

Treatment options include removing the worm by winding it up on a stick (the origin of the medical symbol) or by excising the nodule. Medical treatment with thiabendazole, metronidazole, niridazole, or diethyl carbamazine may be effective. We elected to try ivermectin at a dose of 200 μg/kg bodyweight after excising the mass and worm. The dog has done well since the surgery and treatment, and lesions have not recurred.