Demodex is a tiny parasitic mite that lives on mammals, near the hair follicles and inside them. There are over 60 species of these mites, and two species are known to live on humans. These are Demodex folliculorum that make their home in the hair follicles and Demodex brevis that live in the sebaceous glands near the hair follicles. They are sometimes referred to as eyelash mites. These parasites are very common in humans, and they seldom cause problems. In some cases, they can cause skin diseases in humans. Humans can have them near their nose or on their eyelashes or eyebrows, and sometimes elsewhere on the body.
The mites are microscopic, and are about 0.3mm to 0.4 mm long. They have a semi-transparent body made of two parts. The front body segment has eight legs, and the entire body has a cover of scales. These are used to anchor to the hair follicle. Mites do not like light, so they usually come out only at night to walk on the skin where they look for oils, hormones, and skin cells to eat. During the day, they stay in the hair follicle and eat. Mites reproduce on the skin or the entrance to the hair follicle, and one female is capable of laying up to 25 eggs at a time inside one hair follicle.
About two-thirds of elderly people have mites on their bodies because they produce more sebum than younger people. Around one-half of adults have them, and approximately one-third of all children do as well. They can be transferred from person to person, but it is not usually possible to get them from other species. Some scientists believe that Demodex mites may be an underlying cause of thinning hair, but this has not been proven to be either true or untrue at this point. The mites are usually not noticed by the person they infest unless a skin disease results from them.
Illness or stress can cause the immune system to not function properly, and if this happens, the amount of mites on the body can increase by great numbers. Also, those with cancer or HIV/AIDS are more prone to getting skin diseases from the mites because their bodies cannot produce the antibodies needed to fight against them. They may develop demodicosis or mite bite with resulting itching and inflammation. Another disease caused by the mites is blepharitis, which is an inflammation of the eyelids. In a person with a normal immune system, the body produces antibodies that keep the amount of mites on their body under control. It is sometimes believed that the mites are necessary for humans to make the antigen that helps the body make protective antibodies.
The condition in dogs that is called Demodex mange is caused by one of three mites that live on the skin of dogs. All puppies have these mites transferred to them from their mother during the first weeks of their lives. This type of mange is not contagious like sarcoptic mange. Demodex mites live inside the hair follicles of dogs, so it is difficult to reach them with any type of chemical that could kill them. Dogs can also develop skin diseases from an excessive amount of mites on their bodies.
Humans who have an excessive amount of Demodex need to be treated with special products. These can be homeopathic, or a dermatologist can treat the resulting diseases, but not all doctors are in agreement that Demodex causes any problems for humans. There is still a lot of controversy surrounding these mites and the role that they play in certain human disorders.