This also relates back to our problems with heartworm treatment.
Doxycycline kills L3 and L4 larval stages of the heartworm, and it decreases embryogenesis (the production of microscopic baby heartworms -- microfilariae). It has intrinsic anti-inflammatory effects. It greatly inhibits Wohlbachia, which is a symbiotic rickettsial organism needed by Dirofilaria immitis (the canine heartworm) to survive.
When you treat with Doxycycline at 10mg/kg/day for 30 days, it will clear microfilariae and it will shrink adult worms. [Doses twice this high have been recommended, but some dogs do not tolerate the dose as well.]
We know that more inflammation is produced in the dog when worms die with Wohlbachia present, than in dogs with worms where the Wohlbachia have been suppressed with Doxycycline.
Clinically the effect is not very great in dogs that have few worms and no microfilariae. [Dogs who have been taking year-round preventive medicine, but develop a positive blood test] In dogs that have not received preventive medicine, who have more worms present, the beneficial effect is more noticeable.
It would be advisable to use Doxycyline in dogs that one suspects to have a significant worm burden and who are not in need of immediate treatment. It is combined with Ivermectin given weekly during the month of Doxycycline treatment, prior to giving Immiticide. This should eliminate microfilariae, shrink the adult worms, and reduce the amount of inflammation associated with the death of the adult worms.
This is not a very satisfying conclusion to my seminar, but this reflects the fact that I don’t have a good answer to my problem. I don’t know the cause of the apparent preventive medicine failures, nor why it is more difficult to eliminate adult worms and microfilariae than it used to be. I'm starting to get used to it, but I've become used to dog-bites, too, and I don't like that either.
I don’t have a strategy to improve the situation, which is why I was eager to share experiences and information with the other veterinarians and the parasitologists. I’m looking forward to seeing what the future holds here.