What brand of flea medication are you using?
Have you tried any other treatments?
Have you changed his dog food?
Has anything in the environment changed?
What areas of his body are affected?
I can not think of any parasites that could be transfered from cow feces to his skin. It sounds like you have fleas covered.
He could have picked up ringworm from the dirt. This is a fungal infection. The only way to confirm the diagnosis is to have a fungal culture done by your vet. They make anti fungal shampoos and topical lotions containing 2% miconazole. I have found that most often either prescription anti fungal oral medications or prescription lyme sulfur dips are needed to cure it. http://www.veterinarypartner.com/Content.plx?P=A&A=559
I do not know if the flowers and trees are blooming where you are but, if so, he may have inhalant allergies. These are often manifested on the legs. It can be diagnosed with skin testing at the animal dermatologist or blood testing at you regular vet. It is easier, and less expensive, just to treat the symptoms. You can try benadryl at a dose of 1 mg per pound of body weight every 8 hours. Also buy hydrocortisone spray and cover the affected areas twice a day until healed.
More on inhalant allergies http://www.veterinarypartner.com/Content.plx?P=A&A=597
He could also have sarcoptic mange. This is a microscopic parasite that affects the hair follicles and causes intense itching. It is diagnosed by scraping some skin cells from him and examining them on the microscope. It can be treated with oral and topical prescription medications. http://www.veterinarypartner.com/Content.plx?P=A&A=616
I would give him benadryl right away if he is really itchy.