Strongyloides stercoralis is a soil-transmitted helminth that is increasingly reported in dogs across Europe. While most infections are subclinical, hyperinfection syndrome, marked by excessive larval proliferation and systemic spread, can occur in immunocompromised hosts. An 11-year-old male Boston Terrier dog, originating from southern France with a history of chronic diarrhoea and recent splenectomy was presented with severe regenerative anaemia, thrombocytopenia, and a degenerative left shift. Haemotropic Mycoplasma haemocanis infection was diagnosed, and doxycycline therapy was initiated. During hospitalisation, the dog developed dyspnoea, pneumonia, and progressive pleural effusion containing motile nematode larvae. Strongyloides stercoralis infection was diagnosed by morphological identification of larvae in pleural effusion (third-stage larvae) and faeces (first- and third-stage larvae) and confirmed by PCR targeting mitochondrial cox1 and nuclear 18S rDNA hypervariable regions (HVR) I and IV), followed by sequencing, which revealed the haplotypes HP16 (cox1), VI (HVR-I) and A (HVR-IV). The HVR-IV haplotype A is found in both dogs and humans, characterizing the zoonotic population of S. stercoralis. Respiratory signs worsened despite combined fenbendazole and escalating ivermectin treatment, requiring euthanasia. Post-mortem examination revealed chronic granulomatous pneumonia, a pulmonary carcinomatous nodule, lymphoplasmacytic enterocolitis, and glomerulonephritis. This case represents the first report of S. stercoralis hyperinfection in an adult splenectomised dog. It highlights the importance of including strongyloidiasis in differential diagnoses of dogs with respiratory and gastrointestinal signs, particularly in animals with compromised immunity or relevant travel history.
