Toxocara spp

From Dog
Egg of T. canis under light microscopy, showing the characteristic pitted-edge surface[1]
Larval stage of T. canis

Toxocara spp are the most common zoonotic[2] parasitic small-intestinal nematode of dogs and cats worldwide[3].

This species transmits the nematophagous fungal symbionts, Pochonia chlamydosporia[4] and Paecilomyces lilacinus[5] and control of these fungi affects survival and reproductive performance of the nematode in vitro[6].

Co-infections with other intestinal parasites such as Anyclostoma caninum and Toxascaris leonina is common[7].

Infections in humans is via direct transmission from dogs without the involvement of vectors or intermediate hosts[8]. Eggs are present in dog hair, which are easily ingested by humans following close contact, grooming, petting, etc[9]. In humans, Toxocara infections cause allergies, asthma, fetal mutagenesis, ocular toxocariasis and neurotoxocariasis[10].

The life cycle involves ingestion of eggs in soil or encysted larvae in prey or a paratenic host[11]. Eggs are proteolytically activated in the dog stomach and infective larvae (L3) migrate into the mesenteric and portal veins, reach the liver, and then pass to the lungs. Here they may disseminate via the circulation to other organs such as the eye, brain, kidneys, where they encyst as arrested infective larvae, or penetrate the bronchioles, trachea, and pharynx and are swallowed and develop into adults in the small intestine[12].

During the last trimester of pregnancy, recrudescence of encysted larvae may occur, resulting in transplacental transmission to unborn pups and shedding of larvae in milk[13].

Species which are pathogenic to dogs include:

  • Toxocara canis
  • Toxocara cati

Heavy prenatal infections can cause severe abdominal discomfort, anemia and death in young puppies. Infected puppies often present at 4 - 6 weeks of age with pot-bellied, anemic and poor weight gain despite good appetite. In adult and young dogs, other symptoms, related to visceral migration include endophthalmitis, meningitis and pneumonia.

Diagnosis is usually based on the presence of eggs in feces, but larvae can be visualized from necropsy samples. Coprophagy, which is not unusual in dogs, may amplify the clinical relevance of coprological studies[14]. ELISA and PCR assays are also available to help differentiate from other visceral larvae[15].

A differential diagnosis of causes of coughing would include the lungworm Aelurostrongylus abstrusus and Angiostrongylus vasorum. With perinatal mortality in puppies, canine herpesvirus would be a consideration.

Treatment is relatively effective with milbemycin oxime, ivermectin[16] and older benzimidazole preparations.

References

  1. Buckelew
  2. Souza RF et al (2011) Prevalence and risk factors of human infection by Toxocara canis in Salvador, State of Bahia, Brazil. Rev Soc Bras Med Trop 44(4):516-519
  3. Beiromvand M et al (2012) Prevalence of zoonotic intestinal parasites in domestic and stray dogs in a rural area of Iran. Prev Vet Med Oct 5
  4. Araujo JM et al' (2012) Survival of Pochonia chlamydosporia in the gastrointestinal tract of experimentally treated dogs. Res Vet Sci 93(2):803-806
  5. Carvalho RO et al (2010) Ovicidal activity of Pochonia chlamydosporia and Paecilomyces lilacinus on Toxocara canis eggs. Vet Parasitol 169(1-2):123-127
  6. Frassy LN et al (2010) Destruction of Toxocara canis eggs by the nematophagous fungus Pochonia chlamydosporia. Rev Soc Bras Med Trop 43(1):102-104
  7. Dado D et al (2012) Detection of zoonotic intestinal parasites in public parks of Spain. Potential epidemiological role of microsporidia. Zoonoses Public Health 59(1):23-28
  8. Deplazes P et al (2011) Role of pet dogs and cats in the transmission of helminthic zoonoses in Europe, with a focus on echinococcosis and toxocarosis. Vet Parasitol 182(1):41-53
  9. El-Tras WFet al (2011) Risk of Toxocara canis eggs in stray and domestic dog hair in Egypt. Vet Parasitol 178(3-4):319-323
  10. Stensvold CR et al (2011) Toxocariasis. Ugeskr Laeger 173(3):186-189
  11. Choi D et al (2012) Transmission of Toxocara canis via ingestion of raw cow liver: a cross-sectional study in healthy adults. Korean J Parasitol 50(1):23-27
  12. Bowman, DD (2009) Georgis' parasitology for veterinarians. Elsevier Saunders, Missouri. pp:203-205
  13. Fahrion AS et al (2008) Patent Toxocara canis infections in previously exposed and in helminth-free dogs after infection with low numbers of embryonated eggs. Vet Parasitol 152(1-2):108-115
  14. Fahrion AS et al (2011) Toxocara eggs shed by dogs and cats and their molecular and morphometric species-specific identification: is the finding of T. cati eggs shed by dogs of epidemiological relevance? Vet Parasitol 177(1-2):186-189
  15. Jefferies R et al (2011) Improved detection of canine Angiostrongylus vasorum infection using real-time PCR and indirect ELISA. Parasitol Res 109(6):1577-1583
  16. Nolan TJ & Lok JB (2012) Macrocyclic lactones in the treatment and control of parasitism in small companion animals. Curr Pharm Biotechnol 13(6):1078-1094