Current Advice on Parasite Control:
- Dracunculus insignis
- Dogs or cats with subcutaneous masses or emerging worms should be evaluated to determine if they are infected with Dracunculus.
- Dogs and cats should not be allowed to ingest amphibian paratenic hosts.
- Suspected worms should be preserved in 70-95% ethanol for identification because other worm species can be found in subcutaneous tissues.
Canine and Feline
- Dracunculus insignis is a parasite of numerous wild carnivore species (e.g., raccoons, fisher, mink, muskrat, opossums, fox, coyote, otter) but is occasionally found in dogs. A related species, Dracunculus lutrae, has only been reported in otters.
Overview of Life Cycle
- Dogs, cats and wildlife hosts become infected with D. insignis via ingestion of aquatic copepods infected with L3 larvae. When ingested these larvae migrate to subcutaneous tissues where the mature and mate. After mating, females typically migrate to the legs where they become gravid. Females will emerge and when they come in contact with water will release large numbers of L1 larvae. These larvae are consumed by copepods and develop to the L3 stage in 2-3 weeks.
- The complete life cycle can take 10-14 months.
- An additional possible transmission route is via ingestion of infected paratenic hosts (tadpoles or frogs).
- Adult female worms are detected in the subcutaneous tissues or emerging from skin of dogs and cats. Male worms typically do not migrate to the extremities so are rarely observed. Female worms are large (17-31cm) while males, which are rarely found, are only 3-4 cm long.
- Larvae measure ~500µm x 50µm and have long pointed tails. They may be detected if a female worm is removed and placed in water or in aspirates of masses. Absence of larvae does not rule out Dracunculus because females may not be fertile or gravid.
- Cases in dogs and cats are rarely reported and are generally only detected right before or while the gravid female is emerging or has produced a grossly visible swelling. Adult female D. insignis may produce swelling and inflammation in the subcutaneous tissues and muscle fascia where present. These swellings may or may not be painful. These generally are on the lower portions of the limbs but may occur anywhere on body (e.g., rarely may emerge from face or may be found in eye. Following protrusion of gravid female worms through the skin to expel the larvae, the worm may be observed and there may be focal ulcerations of the skin followed by scarring.
- In a single raccoon case, a streptococcal endocarditis and embolic stroke was believed to be related to a lesions associated with an emerging D. insignis female.
- Diagnosed canine cases of D. insignis are uncommon and only two feline cases have been reported. Canine cases have been reported throughout the eastern half of the United States and the two cat cases were from New York. However, not all females emerge or cause swellings and males do not cause gross lesions so prevalence of infection in dogs and cats is likely higher. One study on ferrets infected with D. insignis found that only about 13% of 153 mature females emerged.
- Infections in various wildlife species have been reported from numerous states in the eastern United States and Ontario, Canada. In some areas, infection prevalence in raccoons may exceed 50% when examined during the time of year (i.e., early-late spring) that females are becoming gravid and/or are emerging.
Host Associations and Transmission Between Hosts
- Dogs and cats become infected with Dracunculus upon ingestion of infested copepods so transmission is tied to aquatic environments.
- Amphibians and possibly other paratenic hosts can harbor infective L3 larvae.
- The parasite is not directly transmitted from an infected wildlife host to a dog or cat.
- A related parasite, the human guinea worm (D. medinensis) infects both humans and dogs in Africa, but there are no known human cases in North America caused by D. insignis.
Prepatent Period and Environmental Factors
- The prepatent period of D. insignis in dogs or cats is not known. However, data from raccoons and ferrets (experimental host) indicate that it takes 10-14 months for female worms to emerge and release L1 larvae.
Site of Infection and Pathogenesis
- Upon infection, larvae of D. insignis migrate out of the gastrointestinal tract and develop in subcutaneous tissues. No clinical signs or lesions are generally observed during this time. A rare case of a developing worm migrating to the spinal cord resulted in hind limb paralysis in a single experimentally infected ferret.
- As female worms develop they migrate subcutaneously to the site where they will emerge (most often a limb). During this migration, some worms may be deep within the muscle fascia which can cause pain and tenderness.
- Surgical removal of the female worms from the mass followed by identification of them using morphologic criteria. Female worms can only be identified to genus so for species identification, worms should be preserved in 75-95% ethanol for molecular identification.
- Male worms may be found in subcutaneous tissues at necropsy.
- L1 larvae may be detected in aspirates of the swelling if females are gravid and if the needle punctures the worm.
Several other parasites may be found in subcutaneous tissues of dogs or cats and must be differentiated from Dracunculus.
- Dirofilaria immitis, heartworm. On rare occasions D. immitis may undergo aberrant migration and be found in subcutaneous tissues. These worms can be distinguished from Dracunculus spp. morphologically or histologically (D. immitis have microfilaria whereas Dracunculus have larvae with long tails).
- Spirometra spp. plerocercoids. These tapeworms are easily distinguished morphologically and histologically from Dracunculus. These plerocercoids are much smaller than Dracunculus females and lack the thick cuticle of Dracunculus. Figure 6 shows a coiled Spirometra in the subcutaneous tissues of a North American otter. Figure 7 shows the same worm removed from the ‘cyst’.
- Acanthocheilonema (=Dipetalonema) reconditum, a filarial worm, resides in the subcutaneous tissues of dogs; however, adults of this species are very small (1.5-2.5cm) and do not cause masses.
- Surgical excision of female worms.
- Various anthelmintic compounds (i.e., diethylcarbamazine, albendazole, ivermectin and, fenbendazole) have been evaluated and none were effective in decreasing worm burdens or killing adult female worms.
Control and Prevention
- Copepods are found in most natural bodies of water. It is currently unknown how commonly dogs and cats acquire infection from ingesting copepods during the act of drinking or if they acquire infections from ingesting amphibian paratenic hosts.
- Dogs and cats should not be allowed to ingest amphibian paratenic hosts.
Public Health Considerations
- D. insignis is not known to infect people; however, experimental exposure of a rhesus macaque (Macaca mulatta) resulted in infection with adult gravid female worms.
- Although infected dogs or cats indicate that the parasite is circulating in an area, these infected animals pose no risk to people or other pets in the household
- A related parasite, Dracunculus medinensis, is the human guinea worm and was once responsible for over 3 million cases annually in numerous countries in Africa and Asia. Beginning in the 1980s, the CDC and The Carter Center have led the efforts to eradicate this parasite using filters to remove copepods from drinking water (https://www.cartercenter.org/health/guinea_worm/index.html). These efforts have decreased the number to only 22 new human cases in four countries during 2015. Unfortunately, in recent years, the number of dog cases in Chad have increased annually.
- Lucio-Forster A, Eberhard ML, Cama VA, Jenks MH, Jones C, Sanders SY, Pongratz JP, Bowman DD. First report of Dracunculus insignis in two naturally infected cats from the northeastern USA. J Feline Med Surg. 2014 Feb;16(2):194-7.
- Beyer TA, Pinckney RD, Cooley AJ. Massive Dracunculus insignis infection in a dog. J Am Vet Med Assoc. 1999 Feb 1;214(3):366-8, 351.