Immunization of ferrets against Dirofilaria immitis by means of chemically abbreviated infections.

Ferrets were exposed to two successive inoculations with 30 third-stage larvae of the canine heartworm Dirofilaria immitis, the inoculations being approximately 5 months apart. Each infection was terminated by ivermectin therapy approximately 2 months after inoculation. The ferrets were challenged with 30 larvae 3 weeks after the termination of the second infection, and were necropsied approximately 6 months after challenge. Of the four ferrets that survived this protracted experimentation, two were entirely free of heartworm while the other two had only a single female worm each. In contrast, 14 control ferrets that had not been immunized (four of which had been given ivermectin doses) were all infected at necropsy, yielding a mean of 6.6 worms per ferret.