What is the cause of hair failing in the dogs and cats?

Hair loss in dogs and cats is a result of ticks/ fleas infestation. they stick to the body of the animals and produce saliva which initiate allergic reaction and itching, results in hair loss. there is also genetic component involved.

Ticks and fleas infestion they stick to the body of theanimals and produce saliva 

There are several causes of hair falling in dog and cat among them 

- external parasitic infestation and scabies 

- Zinc deficiency and other minerals like sulfer 

due to the external parasite(ticks, fleas, lice, etc)

sulfur deficiency

zinc deficency

Causes of Hair Loss in Dogs and Cats. ... Hair loss in dogs and cats can appear as areas without hair (bald patches or alopecia) or areas with thinning hair. There may also be changes in the skin with increased pigmentation, redness, swelling, crusting, flaking, or moist oozing and discharge.

deficiency of vitamins

presence of internal parasites

presence of ectoparasites

sensitivity to some meals