Infestations and Bites

Description

• ■

  Scabies is a parasitic infestation of the skin caused by the mite Sarcoptes scabiei var. hominis.

• ■

  It is characterized by an intensely pruritic eruption and may be the etiology of “the 7-year itch.”

History

• ■

  Patients with scabies complain of a rash with unremitting itching and an inability to stop scratching, even while being examined.

• ■

  It is uncommon for scabies to present in just one member of a family.

• ■

  Usually, several other members of the family, usually with close contact, especially bed partners, will also have symptoms.

• ■

  Nodular lesions take the longest to heal or completely resolve.

• ■

  Crusted scabies (thousands of mites) may be the source of epidemic scabies and is seen in institutionalized or immunocompromised patients.

• ■

  Persistent itching after adequate treatment is due to a prolonged allergic response to mite feces and body parts.

Skin Findings

• ■

  A linear or curved burrow is the classical lesion of scabies; it is a linear, curved, or S -shaped slightly elevated vesicle or papule up to 1 to 2 mm wide.

• ■

  Burrows and other scabies lesions are most likely to be found in the intertriginous areas, finger webs, wrists, sides of the hands and feet, lateral fingers and toes, and genitalia, including the glans penis, buttocks, and scrotum.

• ■

  The palms, soles, and scalp may be more commonly affected in infants than adults.

• ■

  Scabies may also present with scattered inflamed pustules, linear vesicles, papules, and even larger nodules.

• ■

  Individual lesions may be excoriated, hemorrhagic, and crusted.

• ■

  The scabies rash and associated pruritus usually appear 2 to 6 weeks after exposure.

• ■

  Eczema and impetigo may appear as a secondary consequence.

• ■

  A unique, advanced clinical variant is crusted (Norwegian) scabies. Patients—usually those with dementia, Down syndrome, or immunosuppression—experience thick crusting and eczematous dermatitis, especially on the hands and feet. These lesions contain numerous mites.

Laboratory and Testing

• ■

  Mites, eggs, or feces can be identified in a scabies preparation.

• ■

  A scabies preparation involves applying mineral oil to a burrow, vesicle, or papule to preserve the mite feces. The burrow is scraped with a #15 blade and rubbed on a glass slide, a coverslip is placed on top, and the slide is viewed microscopically.

• ■

  Potassium hydroxide and heat (performed for identification of tinea) will make mites easier to identify but will destroy mite feces.

  
  
  
  
  

• ■

  Mites are not always easily identified, and sometimes body parts, feces, or eggs are more easily visualized for confirmation.

Differential Diagnosis

• ■

  Eczema

• ■

  Xerosis

• ■

  Drug rash

• ■

  Insect bites

• ■

  Impetigo

• ■

  Folliculitis

• ■

  Lymphomatoid papulosis

Treatment

• ■

  Permethrin or lindane is applied to the entire skin surface from the neck down, including under the fingernails and toenails and in the umbilicus.

• ■

  The patient should bathe after about 12 hours. This regimen should be repeated in 1 week. Lindane should be used with caution in infants because of reports of neurotoxicity. Lindane is not available in the United Kingdom.

• ■

  Malathion lotion 0.5 is effective and is used in a similar manner as permethrin.

• ■

  A standard program in developing countries consists of a bath with monosulfiram soap, followed by application of benzyl benzoate lotion to the entire body below the neck. This is washed off in 24 hours and then repeated daily for 3 to 5 consecutive days. It may cause irritant dermatitis, especially in the genital area and on the face. There is no evidence for adverse effects on pregnancy outcome. It is not available in the United States.

• ■

  The head and neck are uncommonly affected, but if lesions are present there and need treatment, care should be taken to avoid the eyes and mouth.

• ■

  All clothes and bedding must be washed in hot water or put in a hot dryer at the time of antiscabetic application.

• ■

  There is no need for fumigation or extermination treatments in the house.

• ■

  A single dose of oral ivermectin (Stromectol 6-mg scored tablet) (200 mcg/kg) is also safe and effective for most patients. Repeating the dose in 1 to 2 weeks is a common practice and may provide a higher cure rate. Combination of ivermectin with topical treatment may also provide a higher cure rate.

• ■

  Ivermectin should be used with caution in the treatment of scabies in elderly patients.

• ■

  Topical steroids may be used to control pruritus and inflammation after complete treatment with a scabicide.

• ■

  Persistent nodular lesions can be treated with intralesional steroids.

• ■

  Posttreatment pruritus can occur for weeks or months, even though the mites have been cleared.

  
  
  
  

Pearls

• ■

  Scabies should be considered for any generalized itchy eruption unresponsive to prednisone or any pruritic rash that gets worse with steroid treatment.

  
  

• ■

  Even adequately treated scabies may continue to itch for days to weeks after treatment, and it does not need to be retreated in all cases. Rescraping should be performed if incomplete treatment or reinfection is suspected.

  
  
  
  
  

Description

• ■

  Lice are flattened, wingless insects that infest the hair of the scalp, body, and pubic region.

• ■

  Each variety of insect prefers a specific region of the body.

• ■

  Lice have three pairs of legs located on the anterior part of the body directly behind the head. The legs terminate in sharp claws that are adapted for feeding, permitting the louse to grasp and hold firmly onto hair or clothing.

• ■

  Lice attach to the skin and feed on human blood. They lay eggs or nits on hair shafts.

• ■

  Nits are hard, white oval lice eggs attached to the hair shaft.

• ■

  Rarely, lice can transmit disease, such as epidemic typhus and relapsing fever.

• ■

  Each type of insect is named for the area of the body which it infects:

  • Head lice infestation is caused by Pediculus humanus var. capitis.

  • Body lice infestation is caused by P. humanus var. corporis.

  • Pubic lice infestation is caused by Pthirus pubis.

History

• ■

  Head lice infestation is typically diagnosed by a schoolteacher, school nurse, or camp nurse.

• ■

  Girls are affected more often than boys.

• ■

  Fomite transmission by hats, brushes, or earphones is common.

• ■

  Infestation may cause mild itching at the nape of the neck or no symptoms at all.

• ■

  Posterior cervical adenopathy is occasionally noted.

• ■

  Infestation is rare in African American patients.

• ■

  Infestation of the eyelashes is more common in children.

Skin Findings

• ■

  Nits are small white eggs firmly cemented to the hair shafts.

• ■

  Nits are sometimes easier to see than lice.

  
  

• ■

  Head lice are 3 to 4 mm in length. They can be seen on the hair shafts and scalp with careful observation.

• ■

  Diagnosis is usually not difficult but may require repeated examinations.

• ■

  Head lice have an elongated body, similar to a body louse, though smaller.

• ■

  Honey-colored crusting or secondary impetigo and adenopathy occur if the papules become infected.

• ■

  Infestation may induce blepharitis with lid pruritus, scaling, crusting, and purulent discharge.

• ■

  Nits are fluorescent, so a Wood's light examination can be used to screen children.

Differential Diagnosis

• ■

  Seborrheic dermatitis

• ■

  Impetigo

• ■

  Insect bites

Treatment

Description

• ■

  Myiasis is an infestation of the body tissues of animals or humans by the larval stage of nonbiting flies.

• ■

  Dermatobia hominis causes human botfly infestation.

• ■

  This species of fly is native to Central and South America.

• ■

  The female fly uses the underbelly of mosquitoes to transmit its eggs.

History

• ■

  The human botfly larvae infestation is seen in travelers returning from Central America and South America.

• ■

  Dermatobia females use mosquitoes as vectors to transmit their eggs by capturing mosquitoes and depositing eggs on their bodies before releasing them.

• ■

  Fly larvae are deposited on the skin incidentally by the mosquitoes and penetrate to the subcutaneous tissue, sometimes in the mosquito bite area, where they mature into maggots.

• ■

  The larva is alive in the subcutaneous tissue and appears as an erythematous papule or nodule that can be mistaken as an inflamed cyst or furuncle.

• ■

  Within the papule is a 1- to 2-mm central punctum consisting of the breathing tube of the larva.

• ■

  Many patients report discomfort and a moving sensation within the skin.

• ■

  At maturation, the larva exits the body and drops to the ground and matures into an adult fly.

Skin Findings

• ■

  The tender red nodule is about 2 to 10 mm in diameter.

• ■

  Lesions are typically found on the scalp, face, or upper arms or chest.

• ■

  The larval breathing tube is mobile and can be seen opening and closing within the skin about once every minute.

• ■

  An inflamed cyst-like structure enlarges over days to weeks and is known as a warble .

• ■

  Serous or seropurulent material can be discharged from the opening.

• ■

  Unless this nodule is inspected closely, or botfly infestation is suspected, it may be misdiagnosed.

• ■

  Travel to an endemic area can aid in suspicion of the diagnosis.

Laboratory

• ■

  After extraction, the botfly larva appears as a juicy, white, pouch-like worm, with circular black spicules and with a breathing tube at one end. It is about 10 to 15 mm.

• ■

  It is important to remove or extract the entire larva because infection can occur with incomplete removal.

• ■

  There is only one larva per lesion.

Differential Diagnosis

• ■

  Inflamed or ruptured epidermal cyst

• ■

  Furuncle or abscess

• ■

  Bite reaction

Treatment

• ■

  The human botfly larva needs oxygen, so it can be forced to the surface with occlusion by the application of petroleum jelly (Vaseline) or bacitracin. In some situations, bacon fat, tape, or nail polish can be applied over the opening to deplete oxygen.

• ■

  The larva is smothered and enters the greasy trap while coming up for air. Larvae can be removed with forceps, often within 30 minutes to 3 hours after application.

• ■

  Another technique is to inject lidocaine below the larva. The pressure may force it out of the orifice.

• ■

  On occasion the opening needs to be enlarged with a #11 blade in a cross or cruciate pattern, which allows easy extraction of the larvae.

Pearls

• ■

  The life cycle of the fly is unique, in that the female glues her eggs to the body of a mosquito or tick, which unknowingly deposits the eggs or larvae while biting or feeding.

• ■

  Most cases of botfly myiasis are suspected when recent travel to Central or South America is noted.

  
  

• ■

  Another less common type of myiasis is tungiasis, caused by the red-brown sand flea Tunga penetrans.

• ■

  Tungiasis is seen on the soles, toe webs, and ankles of travelers returning from Africa or Central or South America.

Description

• ■

  Honeybees are the most common source of insect stings and can cause severe allergic reactions.

• ■

  The stinger of the honeybee separates from the bee's abdomen while stinging and remains embedded in the vertebrate's tissue.

• ■

  The stingers of other bees and wasps do not detach. The detached stinger is a useful diagnostic feature for distinguishing stings of honeybees from those of other bees and wasps.

History

• ■

  The initial sharp or painful sting lasts for a few minutes and is followed by moderate burning. Symptoms resolve in a few days.

• ■

  Most reactions in children are mild.

• ■

  Children with deeper dermal reactions still have a benign course and are unlikely to have recurrent reactions.

• ■

  Severe reactions are more common in adults.

• ■

  Localized or systemic allergic reaction may develop.

• ■

  Patients sensitized by prior stings may develop large, local reactions, with edematous swelling forming hours after the sting and resolving in a few days.

• ■

  Edema is more prominent with head and neck stings.

• ■

  A toxic systemic reaction may develop hours after the sting.

• ■

  Vomiting, diarrhea, headache, fever, muscle spasm, and loss of consciousness can occur.

• ■

  Allergic anaphylactic reactions involve itching, hives, shortness of breath, wheezing, nausea, and abdominal cramps. They occur within minutes to an hour after the sting.

• ■

  Most fatal bee and wasp stings occur in a hypersensitive person older than 40 years who has received a single sting on the head or neck. Deaths are caused by respiratory dysfunction or anaphylaxis.

• ■

  Delayed-onset allergic symptoms (up to 1 week after the sting) range from anaphylaxis to serum sickness.

• ■

  Multiple stings can cause death in nonallergic people.

• ■

  The median lethal dose of bee venom is estimated to be 500 to 1500 stings.

Skin Findings

• ■

  A hive or raised pink wheal with a central pinpoint red punctum appears minutes after the sting and lasts for about 20 minutes.

• ■

  Angioedema may occur, which is a localized reaction that appears thick, hard, and edematous over an area as large as 10 to 50 cm.