The development of symptoms, parasitic infection and immunity in human scabies

Abstract

1. Scabies is usually transmitted by intimate personal contact.2. The young, newly fertilized adult female Sarcoptes is the stage usually responsible for transmission.3. Patients with a high parasite rate (over 100 adult female mites) are much more likely to spread scabies than those with few parasites.4. The distribution of the parasites in clinical scabies gives no clue to the original sites of infection.5. When a volunteer is infected for the first time he gives no reaction for about a month. During this period he may be quite unaware that the parasites are burrowing in his cuticle.6. After a month the patient becomes sensitized, itching and other symptoms develop.7. In a first infection the mites increase in numbers far less rapidly than is theoretically possible. A parasite rate of about 25 may be reached in 50 days and of up to 500 in 100 days; after this the number of mites decreases rapidly.8. The second time an individual has scabies he itches at the sites of infection within 24 hr. His mite population seldom rises to a fraction of the height reached in his first infection.9. Reinfection of cured cases is much more difficult than infection for the first time.10. Sarcoptes causes antibody function in man. This causes a partial immunity.11. The mechanism of immunity is due to three reactions: (a) scratching by the host which removes the parasites mechanically; (b) oedema renders the cuticle unsuitable for colonization and causes the mites to vacate their burrows; (c) scratching produces sepsis which is fatal to Sarcoptes.12. The partial immunity obtained may account for fluctuations in the incidence of the disease.