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Intenstinal Threadworm

(Strongyloides stercoralis)

Strongyloides spp. infect dogs, cats and humans. Dogs become infected when they ingest infective larvae through mammary milk or when these larvae actively penetrate into the dogs’ skin.

Parasite: Strongyloides stercoralis (syn. Strongyloides canis)
Common name: Intestinal threadworm
Host: Dogs, humans ± cats
Pre-patent period: 6-10 days; autoinfection possible
Location of adults: Small intestine
Distribution: Worldwide
Transmission route: percutaneous, trans-mammary and auto-infection
Zoonotic: Yes

Distribution

Worldwide.

Clinical signs

Most dogs are asymptomatic, developing a strong immunity to infection and stop shedding larvae within the first 8-12 weeks of life. In young pups, mild and self-limiting watery or mucus diarrhoea may result. In heavy infections, wasting and signs of bronchopneumonia due to migrating of auto-infective larvae may be present. Pododermatitis may result from percutaneous penetration of larvae.

Diagnosis

The Baermann technique (SOP 3) is the test of choice for larval isolation and identification. Strongyle eggs possess a first stage larvae (Fig 1), which may be isolated on standard faecal flotation (S.G. 1.20). (SOP 1) The first-stage larvae can be recognized via their prominent genital primordium (Fig 2) and must be differentiated from larvae of lungworms (Fig 3) and hookworms. Diagnosis of Strongyloides spp. infection is complicated by the fact that larvae may be very low in number or absent from the faeces, even in symptomatic cases. In these cases, faeces can be tested multiple times (3 times over the course 5 to 7 days).

Figure 1. Strongyloides eggs containing first stage larvae on faecal flotation. (Image credit: The University of Melbourne parasitology image library.)
Figure 2. Larva of Strongyloides spp. containing a prominent genital primordium (arrow). (Image credit: The University of Melbourne parasitology image library.)

Treatment

Off-label use of ivermectin at 200 µg/ kg, as a single oral dose and fenbendazole 50 mg/kg once daily for 5 consecutive days is effective at removing adult worms. Re-test faeces twice at 2- and 4-weeks following treatment and monthly thereafter, for a total period of 6 months.  Re-treatment may be necessary in some cases.

Figure 3. First stage larva of canine lungworm containing a ‘kink’ in the tail. (Image credit: Dr Rebecca Traub.)

Control

In Strongyloides-endemic areas, consider testing dogs prior to initiating any immunosuppressive therapy, particularly corticosteroids. Latent intestinal infections can be reactivated when the host is immunocompromised (e.g. iatrogenic, neoplasia) to produce auto-infective larvae which can cause life-threatening disseminate infection. Infected dogs should be isolated from other animals. For further control options, refer to the General Considerations and Recommendations section.

Public health considerations

In humans, clinical signs of S. stercoralis infection may range from being asymptomatic to causing gastrointestinal disorders (e.g. abdominal pain, diarrhoea) and cough. Percutaneous penetration of infective larvae may also cause larva currens. In immunocompromised people, auto-infection may result in hyper-infection syndrome, disseminate strongyloidiasis and bacteraemia, which may prove fatal.