Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that both your dog and your grandmother's dog, after being exposed to your dog, have symptoms of intense itchiness near their lower back and tailhead, and sores on the belly. Of course this makes a contagious parasite more likely. The most common reason for these symptoms is flea allergy dermatitis
. Flea allergy dermatitis is horrible. In allergic animals it only takes one bite a month for them to itch like crazy. If you don't see any fleas you might not have a huge population and if they are very allergic they may be removing any the evidence with their excess grooming. Ideally you need to apply topical flea control every 3 weeks during in an allergic animal. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II, Advantix, or Frontline Plus every 3 weeks for at least 4 months and then all year round monthly. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem. It may help to treat areas where the animal spend a lot of time with an area treatment like Siphotrol Plus to get rid of developing eggs
and larvae. If they seem to have more dander then usual and the dander seems to be especially bad along their back then we need to worry about a microscopic parasite called Cheyletiella or "walking
dander". This is a highly contagious skin surface mite that stimulates skin cell production and causes extreme itchiness. Your veterinarian can take a sample of the dander and look at it under the microscope to look for mites. If they are present all mammal pets in the home should be treated and a thorough clean-up of the environment should be done to get rid of the mites, and their eggs. The most effective treatments for dogs and cats are topical Selamectin (Revolution) or oral Ivermectin which are prescription products. In light infestations we can use a flea shampoo followed up with flea control topicals containing Fipronil (like Frontline Plus) every 3 weeks. We must be sure to also clean up the environment, including kennels, floors and bedding as this mite can survive in the environment and reinfestation occurs if a thorough cleaning isn't done. You may need to use flea area treatment products like Siphotrol Plus II premise spray. Your veterinarian can also test them for another microscopic mite called Sarcoptes which can cause very itchy skin anywhere, but usually especially on the ears
, bottom of the chest, elbows and hocks. Treatment is with Revolution (selamectin) or ivermectin.Your veterinarian can also culture their lesions for a contagious fungal infection called ringworm
. I would treat all mammal pets in the home just in case she has passed it on to them. This mite doesn't survive long in the environment so simply washing bedding in very hot water should be enough to treat the environment.Because they are so itchy and you haven't seen any flea you may want to take them for testing to your veterinarian now. In the meantime to control itchy skin symptoms in dogs or cats you can try:1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic
to cats and dogs) at 1mg to 2mg per pound of body weight every 8 hours. That's a half of 25mg tablet every 8 hours for a pet that is 8 to 15 pounds. OR 2) Chlorpheniramine at 2mg to 4mg once or twice daily. OR 3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat or dog given orally every 24 hours. (Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats and dogs cannot tolerate decongestants.)Combined with high doses of omega-3 fatty acids antihistamines work to relieve the itchiness. I like 3V caps or Derm Caps. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give 20mg of EPA per pound of body weight per day. For example an 80 pound animal could take 1600mg of EPA per day. Together antihistamines and omega-3 fatty acids work synergistically. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use. Best of luck with your pets please let me know if you have any further questions.