A 28-year-old man comes to the office due to a rash on the lower extremity. The patient noticed a pruritic papule on the dorsum of his right foot just prior to returning from a south Florida beach 2 days ago. He walked barefoot on the beach but does not recall any trauma. Since his return home, the rash and itching have progressively worsened. The patient has no other medical problems and takes no medications. Physical examination findings are shown below.
Show Explanatory Sources
Which of the following is the most likely cause of this patient's current condition?
This patient, who developed a serpiginous, pruritic rash shortly after walking barefoot on a beach, likely has hookworm infection. Hookworms spread when egg-containing feces are deposited into shady, warm, moist sand or soil. Eggs hatch into larvae and are transmitted when they come into direct contact with human skin (eg, when walking barefoot).
Dermal penetration is usually marked by a pruritic maculopapular lesion ("ground itch") at the site of larval entry (eg, foot, toe webs). Spread of larvae through the adjacent dermal tissue can lead to the formation of migrating, reddish-brown, serpiginous tracks. Deeper spread may occur, depending on the species of infecting hookworm, as follows:
Human hookworms (eg, Necator americanus, A duodenale) penetrate the basement membrane and spread from the dermis to the bloodstream. They rupture into the alveoli and are coughed up and swallowed into the small intestine. There, larvae mature into adult hookworms that feed on blood from the duodenal mucosa (often causing iron deficiency anemia) and shed up to 10,000 eggs/day, thereby spreading the infection to others.
Cat and dog hookworms (eg, Ancylostoma braziliense, A caninum) cause a dermal eruption in humans but are unable to penetrate the cutaneous basement membrane and spread to deeper tissues. Therefore, larvae do not mature into adult hookworms or spread from human-to-human (humans are incidental hosts).
(Choice A) A brown recluse spider bite usually appears as a red plaque or papule with central pallor; it may develop a dark central eschar that ulcerates due to necrosis. Pain is often severe.
(Choice C) Mycobacterium marinum lesions usually form a few weeks after exposure to contaminated fresh/salt water (eg, swimming pool, fish tank). Lesions begin as solitary papules or nodules that eventually ulcerate and scar. Serpiginous tracts would be uncommon.
(Choice D) Sporothrix schenckii, a dimorphic fungus, is usually transmitted when contaminated organic material (eg, moss, soil) is inoculated into the skin or subcutaneous tissue. A papule forms at the inoculation site and usually ulcerates; additional lesions form along the proximal route of lymphatic drainage.
(Choice E) Staphylococcus aureus is the most common cause of purulent cellulitis. Lesions tend to develop over days and are characterized by painful areas of fluctuance with surrounding erythema.
(Choice F) Vibrio vulnificus is a free-living bacterium that grows in brackish water and marine environments. It can contaminate wounds and cause mild cellulitis. More severe manifestations (eg, myositis, necrotizing fasciitis, sepsis) can develop in patients with certain underlying comorbidities (eg, liver disease, hemochromatosis).
Educational objective:
Hookworm infections are transmitted via direct contact between human skin and contaminated soil/sand (eg, walking barefoot). Dermal penetration is often characterized by an intensely pruritic papule that may form serpiginous tracks due to the subcutaneous migration of hookworm larvae.