ABSTRACTIn the Tours area of the Val‐de Loire, haemograms carried out in 40 dogs with canine parvovirus disease showed that in 30 cases the viral infection was complicated by canine haemobartonellosis. To the picture, typical of the former infection, were added very severe anaemia, fever, locomotor difficulties, arthralgia, adenopathy, respiratory difficulties (dyspnoea, polypnoea and anoxia) and circulatory disorders (tachycardia and capillary fragility). The blood picture was markedly abnormal, anisocytosis, poikilocytosis and anisochromia, being found together with alterations in the blood counts (neutrophils 85 to 90% high; leucocytes, 400 to 500 mm3; erythrocytes, 0·6 to 1·5 × 106mm3).Symptomatic treatment and serotherapy (equine hyperimmune serum against feline panleucopenia) were given for the parvovirus infection. Of the antimicrobial agents, only chloramphenicol, a spiramycin metronidazole association and sulphamethoxypyridazine gave good results.A high dose of intramuscular chlorpromazine (2 to 4 mg/kg b.w.) repeated 48 h later, followed by an 8‐day course of oral treatment at a moderate dosage (1 mg/kg b.w. daily) produced complete clinical cure and disappearance ofHaemobartonella canisfrom the peripheral blood.It is considered that these results support the view that an intercurrent infection may upset the balance existing between the host and parasite in carriers ofH. canisand produce a virulent infection (see Fig. 1).