Juliana, the only black ectoparasites seen on dogs are fleas and lice. When you use the adjective "large", lice are more likely. Please see images of lice here: https://www.google.com/search?q=dog+lice+images&rlz=1C1CHBF_enUS720US720&oq=dog+lice+images&aqs=chrome..69i57.2951j0j7&sourceid=chrome&ie=UTF-8 Here's how they're addressed:
1. Affected and all in-contact same-species animals should be treated.
2. Matted hairs should be clipped away.
3. Traditional therapy is to topically treat your dog's entire body with over the counter 2% lime sulfur, pyrethroids, carbaryl, or organophosphate shampoo, powder, spray, or dip twice 2 weeks apart. Just about any flea insecticide kills lice.
4. Alternative treatments include the following:
Injectable ivermectin given by Millie's vet at 0.2 mg/kg orally or subcutaneously 2 weeks apart.
Selamectin (Revolution) spot-on (as per label), topically twice 2 weeks apart. Treatment administered every 2 weeks at least four times may be more effective. This is a prescription drug available through your vet.
Injectable doramectin given at 0.2-0.4 mg/kg orally every week for 3-4 weeks. This is a prescription drug available through your vet.
0.25% fipronil pump spray (over the counter Frontline, e.g.) 6ml/kg, topically, twice, 2 weeks apart.
10% fipronil spot-on (over the counter Frontline, PetArmor, Fiproguard, e.g.) topically, twice 2 weeks apart.
5. Bedding, grooming tools, and environment should be cleaned at least once. A professional fumigator is suggested in light of the length of time you've struggled with this problem.
6. Prophylactic use of insecticidal flea collars may protect exposed animals from infestation, but avoidance of infected animals is ideal.
The prognosis is good. Lice are highly contagious from dog to dog and from cat to cat, but they aren't considered contagious from dogs or cats to humans.
If they truly are fleas, however, please note that many populations of fleas have developed resistance to the fipronil in Frontline. Here's how you can address a flea infestation:
Our dermatologists tell us to provide one of the newer prescription products available from Millie's vet even if fleas aren’t seen. Over the counter products containing imidocloprid (Advantage, e.g.) or fipronil (Frontline, e.g.) may be ineffective because many populations of fleas have developed resistance to those chemicals. Consider products containing a different class of insecticide such as Bravecto, NexGard, Simparica, Comfortis, and Vectra. Dogs can be such effective groomers so as to eliminate all evidence of flea infestation. Dogs who remain primarily indoors can contract fleas because we walk them in on us and flea eggs and larva can remain viable in your home for months. As the weather warms or you turn on heaters at this time of year, egg hatches are common. If the area between the edge of Millie's rib cage and tail (the “saddle” area) is particularly excoriated, a flea saliva allergy should be the most important differential diagnosis. In severe cases, an anti-allergenic prescription glucocorticoid (steroid) such as prednisone will work wonders for dogs allergic to the saliva of the flea. If you have other pets they may have fleas too but may not be allergic to the flea’s saliva. Be sure to treat your premises with an over the counter area treatment spray that contains an insect growth regulator (IGR) such as Siphotrol Area Treatment Spray containing the IGR methoprene. The IGRs don't allow flea eggs and larvae to develop into adult fleas and so the life cycle of the flea is broken.