Strongyloidiasis

Synopsis

CAPC Recommends

• Strongyloidiasis should be considered for any dog presenting with respiratory and/or gastrointestinal signs, especially in neonates and immunocompromised patients.
• Diagnosis is most often based on observing the characteristic first-stage larvae in feces.
• Treatment is not always effective and may need to be repeated. The most commonly used anthelmintics are fenbendazole and ivermectin (see Treatment guidelines).

Species

Canine

• Strongyloides stercoralis is the causative agent of strongyloidiasis in canines and humans.
• Infections in domestic cats have also been reported.

Overview of the Life Cycle

• Strongyloides stercoralis alternates between free-living and parasitic generations.
• Only adult filariform females are parasitic.
• Adult filariform females reside in the small intestine of the host and produce eggs by mitotic parthenogenesis.
• Eggs develop quickly within the host and first-stage larvae are passed in feces.
• In the environment, male first-stage larvae will develop into free-living adults. Female larvae molt to either infective third-stage filariform larvae or to free-living adults.
• The free-living adult males and females reproduce sexually and produce infective filariform larvae.
• Dogs become infected when filariform third-stage larvae penetrate the skin or oral mucosa. Transmammary transmission and, in rare instances, autoinfection have also been described.
• The migration route of larvae in the host is complex and not fully understood. Larvae reach the small intestines within 4-5 days and mature to adults.

Stages

• First-stage larvae: Passed in feces and are characterized by a rhabditiform esophagus, prominent genital primordium, and straight tail. Larvae are 150 to 390 µm long.
• Third-stage filariform larvae: The infective stage to both dogs and humans. Third-stage larvae are 490 to 630 µm long and are identified by their long (filariform) esophagus and notched tail.
• Adult: Parasitic and free-living adults can be easily distinguished by the morphology of their esophagus and body length. Parasitic females are 2 to 3 mm long, slender, and have an elongated (filariform) esophagus compared to their free-living counterparts.

Disease

• Dogs infected with S. stercoralis display a range of clinical signs, although many are asymptomatic.
• Clinical signs vary depending upon the severity of the disease. Mild cases are often associated with a recurrent cough, diarrhea, and lethargy.
• In more severe cases, dogs often present with a mixture of severe respiratory and gastrointestinal disease. Such signs include coughing, dyspnea, tachypnea, severe watery or mucoid diarrhea, vomiting, dehydration, anorexia, weight loss, and lethargy.
• Laboratory findings may include leukocytosis characterized by neutrophilia and eosinophilia, hypoalbuminemia, and anemia.

Prevalence

• Strongyloides stercoralis is found infecting dogs and humans worldwide.
• The estimated global prevalence of canine strongyloidiasis is 6%.
• The prevalence of infected dogs in the United States is quite low with several studies reporting a prevalence ranging from 0.0 to 1.4%.
• Higher prevalence is often reported for stray and shelter animals.

Host Associations – Transmission Between Hosts

• Dogs primarily become infected when filariform third-stage larvae penetrate the skin or oral mucosa.
• Transmammary transmission may occur if a pregnant or nursing dog is infected during late gestation or lactation.
• Autoinfection, where first-stage larvae molt to third-stage larvae and eventually to adults without leaving the host, have also been described. This is favored in neonates or in dogs that are immunosuppressed which may also lead to hyperinfection and disseminated infections.
• Infected dogs can serve as a source of environmental contamination for other dogs and humans.

Prepatent Period - Environmental Factors

• The prepatent period of S. stercoralis from the time of infection to when first-stage larvae are detected in the feces of canines ranges from 5 - 21 days.

Site of Infection and Pathogenesis

• Adult female S. stercoralis are present in the mucosal epithelium of the small intestine and sometimes the large intestines. Infections may produce an enteritis with watery or mucoid diarrhea.
• Villous blunting along with cellular infiltration of the lamina propria by lymphocytes, plasma cells, and eosinophils may be observed in the small intestines. Similar lesions may also be present in the lamina propria and submucosa of the large intestines.
• Infective third-stage filariform larvae penetrate the skin and mucosa which may result in local inflammation and irritation.
• Larval migration through the body may result in various lesions depending upon the location of the migration. Lesions are more severe in cases of disseminated strongyloidiasis and hyperinfection.
• Larval migration through the lungs may result in overt pulmonary disease as a result of bronchopneumonia, edema, hemorrhage, fibrinous exudate, and areas of focal necrosis.

Diagnosis

• Diagnosis is made by demonstrating first-stage larvae with characteristic rhabditiform esophagus, prominent genital primordium, and straight tail. This can be accomplished by the Baermann technique using a fresh fecal sample. Larvae, and sometimes larvated eggs, can also be recovered by fecal flotation using zinc sulfate, however morphologic characteristics of the larvae may be distorted if not analyzed promptly.
• The use of other flotation solutions is not recommended as the morphological characteristics of the larvae will be distorted. Larvated eggs can be recovered by zinc sulfate or Sheather’s sugar, if present.

Treatment

• There are currently no approved anthelmintics for strongyloidiasis in dogs, and off-label treatments show reduced efficacy against migrating larvae.
• Repeated courses of anthelmintics are often required to eliminate infections. The decision to retreat should be based on follow-up fecal examinations and clinical signs.
• Follow-up fecal examinations should be performed weekly for two to three weeks following treatment.
• The following medications and regimens have been used successfully to treat uncomplicated infections.
  • Fenbendazole 50 mg/kg once daily for 3-5 days has been used successfully to treat uncomplicated strongyloidiasis. The addition of imidacloprid/moxidectin spot-on has also shown some efficacy.
  • Ivermectin 200 mg/kg once daily for 1-2 days can also be used to treat uncomplicated infections.
• The treatment of disseminated strongyloidiasis and hyperinfection is more complex. The following treatment regimens have been suggested.
  • Fenbendazole 50 mg/kg once daily for 7-14 days.
  • Ivermectin 200 mg/kg once every four days for 3 or 4 doses.
  • The combination of fenbendazole 50 mg/kg twice daily for 14 days and ivermectin 200 mg/kg once daily every four days for two doses has been used to treat S. stercoralis hyperinfection in a dog successfully.
  • Patients being treated for disseminated strongyloidiasis should be closely monitored, especially for respiratory distress. The die-off of migrating worms throughout the body can produce a severe inflammatory reaction.

Control and Prevention

• Prompt removal of feces from the yard will help prevent the contamination of the environment with large numbers of S. stercoralis larvae.
• Strict enforcement of leash laws and requiring owners to promptly remove their pets’ feces can limit environmental contamination of public areas with S. stercoralis.
• Appropriate sanitation and management practices in kennels and boarding facilities is helpful in preventing infections.

Public Health Considerations

• Stronglyoides stercoralis is considered a zoonotic parasite capable of infecting humans.
• Clinical signs in humans are similar to those reported in dogs.
• Studies have revealed that dogs carry two distinct lineages of S. stercoralis. One appears to be specific to canines whereas the other is similar to human isolates.
• Due to the potential risk of transmission, prompt removal of feces from the environment is key to limiting environmental contamination.
• To prevent larval penetration, individuals should avoid direct skin contact with soil where dogs may have access to defecate.

Selected References

• Eslahi AV, Hashemipour S, Olfatifar M, Houshmand E, Hajialilo E, Mahmoudi R, Badri M, Ketzis JK. 2022. Global prevalence and epidemiology of Strongyloides stercoralis in dogs: a systematic review and meta-analysis. Parasit Vectors 15.
• Graham JA, Sato M, Moore AR, McGrew AK, Ballweber LR, Byas AD, Dowers KL. 2019. Disseminated Strongyloides stercoralis infection in a dog following long-term treatment with budesonide. JAVMA 254, 974-978.
• Grove DI, Heenan PJ, Northern C. 1983. Persistent and disseminated infections with Strongyloides stercoralis in immunosuppressed dogs. Int J Parasitol 13, 483-490.
• Grove DI, Lumsden J, Northern C.1988. Efficacy of albendazole against Strongyloides ratti and S. stercoralis in vitro, in mice, and in normal and immunosuppressed dogs. J Antimicrob Chemother 21, 75-84.
• Itoh N, Kanai N, Hori Y, Nakao R, Hoshi F, Higuchi S. 2009. Fenbendazole treatment of dogs with naturally acquired Strongyloides stercoralis infection. Vet Rec 164, 559-560.
• Jaleta TG, Zhou S, Bemm FM, Schär F, Khieu V, Muth S, Odermatt P, Lok JB, Streit A. 2017. Different but overlapping populations of Strongyloides stercoralis in dogs and humans – Dogs as a possible source for zoonotic strongyloidiasis. PLoS Negl Trop Dis 11.
• Lee E, Seo K, Song K. 2021. Treatment of Strongyloides stercoralis hyperinfection in a dog. JVC 38, 210-212.
• Mansfield LS, Schad GA. 1992. Ivermectin treatment of naturally acquired and experimentally induced Strongyloides stercoralis infections in dogs. JAVMA 201, 726-730.
• Nolan TJ. 2001. Canine Strongyloidiasis. In Companion and Exotic Animal Parasitology (ed. Bowman DD), International Veterinary Information Service (www.ivis.org), Ithaca, New York, USA.
• Schad GA, Hellman ME, Muncey DW. 1984. Strongyloides stercoralis: Hyperinfection in immunosuppressed dogs. Exp Parasitol 57, 287-296.
• Shoop WL, Michael BF, Eary CH, Haines HW. 2002. Transmammary transmission of Strongyloides stercoralis in dogs. J Parasitol 88, 536-539.