Antiparasitic agents are drugs used to treat infestations of parasites in the body. This includes skin conditions, such as infestations with scabies, head lice, pubic lice and “creeping eruption.” Parasites are organisms that survive by living on and feeding from a host. These medications work by either paralyzing parasites or by killing them outright.

Antiparasitic agents are usually well-tolerated by patients and cause few side effects. Adverse reactions that do occur tend to be minor, such as skin irritation. Most people can use these drugs safely and effectively regardless of any other medical conditions they might have.

Antiparasitic agents are available in the form of pills, creams, lotions and shampoos. Some of these drugs are available in over-the-counter formulations, whereas others can only be obtained with a physician’s prescription.

Antiparasitic agents are medications used to treat conditions that occur when parasites infest the body. Parasites survive by living on and feeding off a host. There are many different types of antiparasitic agents, including several that are used to treat skin conditions, such as scabies, head lice, pubic lice and “creeping eruption.”

These medications work by either paralyzing parasites or by killing them outright. They are available in the form of pills, creams, lotions and shampoos. Some of these drugs are available in over-the-counter formulations, whereas others can only be obtained through a physician’s prescription.

Many of these drugs require patients to cover their entire bodies with a lotion or cream, starting with the scalp (as long as it is not already inflamed) and continuing from the neck to the toes, while excluding the face. Patients should pay particular attention to ensuring that the medicine covers skin folds, creases and the areas between the fingers and toes. The medication must also be applied under the fingernails. Repeat applications may be necessary if new lesions occur after the treatment has been performed.

Patients should be aware that the itchiness associated with scabies, lice and creeping eruption generally continues for anywhere from a couple of days to as long as several weeks after antiparasitic medication is first used. This is normal and does not indicate that the treatment is unsuccessful.

There are many different types of antiparasitic agents. Those that are most commonly used to treat conditions that affect the skin include (over-the-counter drugs are marked “OTC”):

Antiparasitic agents are prescribed to treat a variety of skin-related conditions, including:

• Lice. Head lice (pediculus capitis) are bloodsucking, wingless insects approximately the size of a sesame seed. They are the most common form of lice infestation in the United States, and they are shaped like a small capsule that resembles dandruff. Head lice are spread through personal contact and by shared combs, brushes, hats and other personal items. Pubic lice (Pthirus pubis) are usually transmitted by intimate or sexual contact. Pubic lice bear a resemblance to the common crab, with infestations sometimes referred to as a case of “crabs.” They are most often located around the genitals, but may spread to other areas (e.g., abdomen, armpits). Itching (pruritus) is the most common symptom of lice infestation. Lice treatments are typically repeated after 7 to 10 days to kill lice that hatch after the initial treatment.
  
  
• Scabies. An infectious skin disease that results from a mite infestation. Scabies occurs throughout the world and is spread by personal contact and shared use of clothing and bedding. The mites burrow into the skin and cause an itchy rash. They then deposit eggs under the skin, which appear as a burrow that resembles a pencil mark. Eggs mature in 21 days. Scabies can occur with small blisters, and is commonly found on the genitals.
  
  
• “Creeping eruption.” An often serpiginous (winding) skin rash that results when a person becomes infected with dog or cat hookworm (larva migrans) through contact with the animals’ feces, and the larvae begin to “creep” beneath the skin. Symptoms usually appear on the feet, buttocks or back. Creeping eruption tends to occur in warm climates when people are exposed to the larvae found in sands and soils. In the United States, it is most common in the Southeastern states.

Most antiparasitic agents can be used by patients regardless of other medical conditions the patient may have. However, patients likely will be urged not to use these medications if they have a prior history of reaction to the drugs or if their skin is raw or inflamed.

Thiabendazole (used to treat “creeping eruption”) should not be used in patients with kidney or liver disorders, severe malnutrition or anemia. Lindane (used to treat scabies) should not be used in premature infants, pregnant women, patients with a history of seizures or patients with Norwegian scabies, a condition in which patients are infected with hundreds or thousands of scabies that cause severe skin manifestations. In addition, people with severely inflamed skin should not apply benzyl benzoate or crotamiton to the skin to treat scabies.  

Individuals with asthma should avoid taking ivermectin to treat scabies and head lice because the drug can worsen asthma. In addition, permethrin should not be applied to an inflamed scalp for the treatment of scabies or lice.

Antiparasitic agents are usually well-tolerated by patients who use them. Side effects that do occur tend to be minor, such as skin irritation. Patients who experience significant irritation should wash off the medication with soap and water and discontinue treatment.

Thiabendazole – the drug used to treat “creeping eruption” – may cause nausea, vomiting or diarrhea when it is prescribed in oral formulation. Other symptoms associated with thiabendazole include impaired mental alertness, low blood pressure (hypotension), skin rash, itchiness (pruritus), loss of appetite (anorexia) and unusual smelling urine (e.g., asparagus-like). Hallucinations, irritability, drowsiness, numbness or tingling in the hands or feet, dryness in the eyes or mouth and ringing or buzzing in the ears may also occur.

The scabies medication lindane has been associated with episodes in which the central nervous system is affected, causing seizures. Though rare, such incidences have resulted in some expert recommendations that lindane no longer be used to treat scabies.

Other side effects sometimes associated with various other antiparasitic agents include dizziness, redness, burning or swelling of the skin, skin rash, numbness, diarrhea, nausea, pruritus, worsened asthma, rapid heart rate (tachycardia), fever and headache.

Individuals experiencing severe or persistent side effects should contact their physician. Patients should contact their physician immediately if any of the following symptoms occur:

• Muscle and joint aches
• Blurred vision
• Chills
• Seizures
• Dark urine
• Pain or burning during urination
• Fever
• Lower back pain
• Weakness
• Unusual tiredness
• Jaundice (yellowing of the eyes and skin)
• Changes in the skin (e.g., blistering, peeling, loosening)

There are no significant drug interactions associated with most antiparasitic agents. However, use of thiabendazole may inhibit the body’s ability to metabolize aminophylline or may elevate the levels of theophylline found in the body.  Both drugs are used to treat breathing problems related to asthma, chronic bronchitis, emphysema and other lung diseases.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to antiparasitic agents:

1. Are antiparasitic agents safe for me to take?
   
   
2. How and when should I take this drug?
   
   
3. What side effects may I develop?
   
   
4. For what side effects should I contact you?
   
   
5. How long will it be before the drug takes effect?
   
   
6. How many times should I use this antiparasitic agent?
   
   
7. Are there medications, nutritional supplements or herbal medications I should avoid while taking this drug?
   
   
8. How will I know if the drug is working?
   
   
9. What are other possible treatments if the drug does not work?