[Efficacy of ivermectin in preventing Dirofilaria repens infestation in dogs naturally exposed to contagion].

The ability of ivermectin to control canine dirofilariosis in naturally infected dogs was tested. Two studies were carried out in the coast line of Grosseto district (Italy), where D. repens infection is endemic. A sample of 384 dogs from the area was examined as background controls and we found D. repens in 21.1%, D. immitis in 7.8% and both species in 6.5%. In the first study we used 72 privately-owned dogs, of varying sex, breed and age; all animals were negative to Knott's modified test for microfilariae and to CITE test for the adult antigens. All the dogs were treated monthly per os with tablets of ivermectin (> or = 6 mcg/kg) seven times from May 1991. All dogs were found negative for microfilariae 6-8 months after the last treatment. The second experiment was carried out on 16 Beagles aged 5.5-7 months (8 control dogs and 8 treated dogs). The dogs lived in a semi-open kennel in the selected area. We treated 8 dogs--8 times at monthly intervals, starting in June--with ivermectin tablets per os (> or = 6 mcg/kg). The monthly examination for circulating microfilariae (Knott's modified test) showed D. repens in two control dogs 7 months after the beginning of the experiment, one month later in 6 control and in another control animal also positive for D. immitis between the 10th and 11th month. Treated dogs were always negative. At the necropsy of all the dogs, about 11 months after the beginning of the experiment, we found adults or pre-adults of filariae in all the control dogs. Four animals had a double infection, 3 with D. repens and one with D. immitis. We never found adults or pre-adults of filariae in treated dogs.