Cutaneous larva migrans is a skin condition in which the larvae of some hookworm species migrate within the skin to produce an intensely pruritic, serpiginous or linear rash. This is known as a creeping eruption.
The etiologic agents are the filariform larvae of hookworms, which have dogs or cats as their definitive hosts. Ancylostoma braziliense is the most common species to cause this rash in humans.
The lifecycle starts with the adult hookworms in the intestines of dogs and cats. These lay their eggs in the intestines, which are passed with the feces into the soil. The soil is often the sandy soil of beaches or the soil under houses. The contaminated soil contains hookworm eggs, which hatch and pass through a couple of molts to form filariform larvae.
The filariform larvae are able to penetrate through skin, either broken or intact, with the help of their protease enzymes. They then start migrating through the upper dermis, but are limited by the basement membrane. Since they cannot reach the intestines and mature, they eventually die and the lifecycle ends. This happens, because humans are accidental hosts.
The life cycle is as follows:
- Eggs are passed in the feces of the dog or cat and are deposited in the soil.
- Under the right conditions, namely, warmth, moisture and shade, the larvae hatch within 1 – 2 days.
- The larvae that are hatched are rhabditiform larvae and continue to grow for 5 – 10 days.
- Within this period they undergo two molts and become infective filariform or third-stage larvae. These are capable of surviving outside the body of the host for up to 4 weeks if the environment is not too harsh.
- Once the bare skin of an animal host comes into contact with the filariform larva, it penetrates the skin to enter the blood vessels. It can enter through intact skin, or through hair follicles, or tiny cracks, because of their proteases.
- The larvae are carried to the pulmonary vessels through the heart, and bore through the lung alveoli to enter the airways.
- They ascend the airways to reach the pharynx where they are swallowed to reach the stomach and eventually the small intestine.
- The small intestine is their home till they mature into the adult form. At this point they attach to the wall of the gut and lay more eggs, to repeat the cycle.
Some larvae persist in other tissues and may be passed on to young animals by vertical transmission.
In the case of human infection with filariform larvae, the larvae cannot penetrate the basement membrane of the skin in most cases. This includes the species A. braziliense, A. caninum and Uncinaria stenocephala. Thus their migration is unsuccessful, being limited to the superficial layers of the skin, and they finally die. A few larvae, usually of A. caninum, may end up in deeper tissues.