Horse, Itch and Worms

My 7 year old gelding sometimes has a terrible itch on his top line mid way between his withers and the top of his head and also scratches the head of his tail. He is an Andalusian and usually has much mane and tail but this seems to be disappearing as he continues to rub. I have ruled out sweet itch and lice or something like that. Do you think this could be some allergy and is there any herb to help with this type of condition. Also, do you suggest any particular herbs or concoctions for keeping internal parasites at bay instead of using the regular de-wormers which are full of potentially harmful ingredients to the animal?

Whenever one looks at skin conditions in animals, a number of questions spring to mind. Have you changed any of the products you have been using externally on your horse or on his saddlecloths? Did the itchiness begin at the same time you started with a new shampoo or fly repellent? Has anything changed about his stall or diet? Has anything else about his digestion changed? If any of these statements hold true then removing the cause of the problem would be the first place to start.
Is the area inflamed and/or has the skin been scratched open, thus increasing the chances of a secondary bacterial infection? Each skin condition has a specific look and set of symptoms which in turn will help differentiate the factors contributing to the condition. Your best bet would be to have a qualified herbal practitioner who works with horses actually see the areas that have been affected so as to better reccommend which herbs would be appropriate for internal use.
In the meantime, you could make a strong tea of Calendula flowers (Calendula officinalis) and Chickweed herb (Stellaria media). Strain. Let cool and dip a clean cotton cloth into the tea. Bathe the affected areas two to three times a day to give your horse some relief as well as help to mend the places where underlying tissue may have been exposed.
As for your second question, various treatments have been used for each type of worm, depending on which is affecting your horse. I would suggest you look at Juliette de Bairacli Levy's book, "The Complete Herbal Handbook for Farm and Stable". She lists several recommendations there.