Another popular theory is that infection of hair follicles by the Demodex folliculorum parasite is correlated with pattern baldness. By infiltrating the sebaceous gland of hair follicles the parasite causes an immune response and inflammation of surrounding tissue. Through long term invasion the parasite exhausts the hair bulb and shifts the hair cycle from anagen to telogen. This theory is promoted by certain companies that just happen to have a range of products that kill off Demodex folliculorum! At least one of the companies has conducted research that allegedly showed 88% of 240 men with pattern baldness had Demodex folliculorum infection in their hair follicles. There are several problems with this theory.

First, Demodex folliculorum infection is very common. Up to 75% of the population, men, women, and children, have Demodex folliculorum infection. However, there is a clear bias towards men have pattern hair loss even though children and women are equally susceptible to Demodex folliculorum infection. If Demodex folliculorum infection was causing inflammation that pushed hair follicles into telogen then one would expect to see some children with pattern baldness and that women would be equally affected with hair loss. Indeed, women have a stronger immune system and so one might expect more women than men to have pattern baldness, but this is not the case.

Second, research has shown that there are excessive numbers of the Demodex folliculorum parasite in eyelash follicles. (Fulk et al. Optom Vis Sci 1996 Dec;73(12):742-745). However, people with pattern baldness do not lose their eyelash hairs.

Third, it would be impossible for hair follicle transplants to work. Given thousands of transplant procedures are done every year and 88% of men have Demodex folliculorum infection, then most of the men with hair transplants must also have Demodex folliculorum infection. In a hair transplant, follicles are moved from the back of the head to the top of the same individual. If the individual is infected with Demodex folliculorum one would expect that the transplanted follicles would either already be infected, or become infected in their new location. However, transplanted follicles grow in every individual who has had the procedure done and pattern baldness does not redevelop.

Fourth, there is evidence to show that pattern baldness is associated with hair follicle inflammation for some people. However, studies have been done to look at the effects of topical anti-inflammatory solutions on pattern baldness. While the anti-inflammatory solutions did reduce hair follicle inflammation they did not promote hair follicle regrowth.

Fifth, there is a widely available treatment to Demodex folliculorum infection, pilocarpine gel. However, using this gel on the skin does not promote any hair growth.

The more likely explanation for Demodex folliculorum infection in men with pattern baldness is that the sebaceous glands of alopecia affected hair follicles become larger and more active, producing oils at a faster rate, under the influence of dihydrotestosterone (DHT). The oils combine with dead cells from the hair follicle to make sebum. The sebum is a rich source of nutrients and this is the food that Demodex folliculorum eats. The oil food supply increases in most hair follicles affected by pattern baldness so these hair follicles can accommodate a greater number of Demodex folliculorum parasites. Rather than the parasites causing pattern alopecia, they simply take advantage of the increased food produced. The parasite infection is a consequence of pattern baldness and not a cause of the hair loss.