Chapter 13: Infestations and Bites

INTRODUCTION TO CHAPTER

Arthropods have always feasted on humans and humans have tried countless strategies to keep them from doing so. The societal burden of biting and sucking bugs is significant, costing millions of dollars, tremendous discomfort, and immeasurable emotional distress. They bite, we itch, we scratch, and these facts seem destined to endure.

SCABIES

Introduction

Scabies is a common parasitic infection caused by the mite Sarcoptes scabiei var hominis. Transmission is primarily person-to-person by direct contact and although anyone is susceptible, situations that result in more skin-to-skin contact, such as parents with small children, sexual activity, overcrowding, and institutional settings, increase the incidence of infestation. Although the scabies mite has not been shown to transmit any significant pathogens, the intense itching associated with the infestation, the risk of superinfection of excoriated skin, and the fact that up to 300 million people may be affected worldwide annually make scabies a significant public health problem.¹

Pathophysiology

Sarcoptes scabiei is an obligate human parasite that completes its entire 30-day life cycle within the epidermis. The fertilized female weaves through the epidermis and leaves a trail of 60 to 90 eggs and feces (scybala), in her burrow (Figure 13-1). The eggs hatch into larvae that then mature into nymphs and adults. The rash and pruritus of scabies is a result of a hypersensitivity reaction to the mite and its detritus. The incubation period from infestation to pruritus can range from days to months. The first time an individual is infested, it typically takes 2 to 6 weeks to become sensitized and develop symptoms, but in subsequent infestations, the previously hypersensitized individual can begin itching in as little as 1 to 3 days. Some infested individuals never develop hypersensitivity to the mite and never experience symptoms, but can still transmit the infection; these are asymptomatic "carriers."

Clinical Presentation

History

Intense pruritus is the main presenting complaint, although very young children who cannot verbalize itching are often irritable and eat and sleep poorly. Adults often complain that the pruritus is worse at night. Family members and close contacts frequently report similar symptoms.

Physical Examination

The patient presenting with scabies usually has a nondescript, excoriated, papular dermatitis. The most common physical findings are papules, vesicles, pustules, or nodules. These typically occur on the trunk, arms, hands (Figures 13-2 and 13-3), and genitals in adults and may also involve the head, neck, and feet in infants and young children (Figures 13-4 and 13-5). The burrow, a short, ...