Vet was incompetent,
Ivermectin injections work 100%, but are too cheap (100bht) for most vets to use.
I'd suggest using a different vet, one that puts animal health before profits.
Ivermectin stays in the blood and poisons all insects that suck blood (seen or unseen).
2-3 injections protects the animal from recurrence for up to a year.