Antiparasitic Drugs For Scabies

This article explores the effectiveness of antiparasitic drugs in managing scabies, a skin infestation resulting from infection by the Sarcoptes scabies mite. Various treatment approaches, including topical solutions like permethrin and oral medications such as ivermectin, will be examined, including a comparison of their mechanisms, application guidelines and potential side effects. We will also look at vulnerable populations and how the scabies mites are so effective in infecting humans.

Introduction 

Scabies is a skin infestation caused by the Sarcoptes scabiei mite, which burrows into the skin, causing extremely itchy and uncomfortable lesions. Every year, over two million people worldwide are infected, particularly in tropical regions.¹ It is important that any cases of scabies are treated as early as possible to prevent the mites from spreading from one person to another and an outbreak from occurring. 

Scabies mainly spread with prolonged skin-to-skin contact between an infected and uninfected person, and so it is important to avoid unnecessary contact while being treated. Quick treatment is also important for the individual, as complications can arise from secondary infections with bacteria and other microbes that exploit lesions on the skin’s barrier and a suppressed immune system. 

There are many treatment options, with permethrin and ivermectin being the gold standard in medicine today. These are relatively safe with simple side effects, and scabies can be easily diagnosed by a GP who can provide these medications. 

Understanding scabies

Overview  

There are two different types of scabies infestations:

• Classical
• Crusted (formerly known as Norwegian scabies)

Crusted scabies is much more likely to be seen in immunocompromised individuals, as it shows a lack of control of the host immune system, meaning the scabies can reproduce much more extensively without being stopped. It is characterised by large, crusted patches of skin on the body as opposed to the smaller pustules associated with classic scabies.²

Mite life cycle

Scabies have a relatively simple life cycle, which lasts around 10-14 days. Female mites mate with males before burying deep into the human skin barrier to lay their eggs. The eggs hatch into larvae, and so on. This helps them survive, as it is harder for treatments to attack mites in this layer of the skin. They also release chemicals, which can stop the immune system from attacking them.⁴ The life cycle takes place entirely in the skin of the affected individual, as the mites cannot survive outside of this. There are no vectors involved to transport the parasite from host to host, which is why they rely on direct skin contact.³ 

Symptoms of scabies infestation 

• Severe itching (pruritus) which tends to get worse at night and can have a severe impact on quality of sleep
• A pimple-like rash, which can secrete pus 
• Centimetre-long burrows across the skin

These symptoms may be harder to visualise on darker skin, but should still be visible due to the raised nature of the pustules. 

It is likely that during the first 4-6 weeks of infection, the individual will have no symptoms and therefore no idea they are infected. This is called the incubation period, during which the body begins to recognise the parasite, and the itchiness is a result of the body’s immune response against the infection.²

Transmission methods and risk factors

Scabies mites are usually transmitted from an infected person to an uninfected by prolonged skin-to-skin contact, including sexual transmission. While it is unlikely to be transmitted through shorter contact, such as hugs, it is recommended that an infected person avoid all contact with others until the infection has been fully cleared. In classic scabies, there is no evidence for transmission through clothing and linens, as the mites cannot survive outside the human host. However, with crusted scabies, mites can survive in shredded skin so more precautions must be taken to ensure the infection has been cleared.³

Transmission is more common in lower and middle-income countries, particularly in overcrowded areas and it is considered endemic in poverty-stricken communities. It has also been established that there is a peak in outbreaks during times of war, likely due to the displacement of individuals into crowded areas, as well as a lack of sanitation as systems break down.²

Children are more likely to be infected, likely due to bed-sharing, lack of symptoms and high reinfestation rates. In the Solomon Islands, 25.7% of children between the ages of 1 and 4 were diagnosed with scabies. In higher-income countries, outbreaks are most common in institutional settings such as care homes or among the homeless. For example, in a majority of the homeless population in Paris, 56.5% of dermatologic consultations were attributed to scabies infection.³

Most complications associated with scabies comes from bacterial infections rather than the mites themselves. The bacteria can enter through abrasions in the skin caused by excessive itching and take advantage of the fact that the mites have been able to suppress the host’s immune system, so they are less likely to be attacked. The most common bacteria involved in complications following scabies infection are Streptococcus pyogenes and Staphylococcus aureus, which can cause severe, life-threatening infections such as sepsis.³

Available treatment options

There are several possible treatment options to manage scabies infestation. Most of these are topical creams, as this is a skin infestation. These include:² 

• 5-10% sulphur in paraffin (mostly used in Africa and South America)
• 1% lindane (rarely used in Western countries due to concerns of neurotoxicity in hosts)
• 10-25% benzyl benzoate (used widely in Europe and Australia)
• 10% crotamiton 
• 5% tea tree oil used in combination with other treatments

The most common treatments used are 5% topical permethrin and oral ivermectin as they are cheap to make and show relatively good efficacy in clearing infestations

Permethrin 

Topical permethrin is the first line of treatment for scabies infestation and was approved for use by the FDA in 1989.² It is a broad-spectrum antiparasitic, meaning it works against many types of parasites and it has shown relative efficacy in clearing scabies mite infestations.

Application 

Permethrin cream should be applied all over the body and left on for 8 -12 hours. Most people choose to do this overnight, however, there is no issue with doing this through the day as it is quickly absorbed and does not have a strong, obvious smell. It should then be washed off, and it is recommended that the clothes/bed sheets that were used during treatment be washed to remove any remaining cream. 

The application process must be repeated one week after the first treatment, as permethrin can only target living mites and not eggs. This week gives the chance for any remaining eggs to hatch before another round of treatment. 

Common side effects and management

There are few side effects associated with permethrin use, and they are generally restricted to the skin. These include:

• Mild burning
• Swelling 
• Redness where the cream has been applied

The symptoms of scabies may also worsen temporarily after treatment, but this should resolve after 2 - 4 weeks. Ask your doctor for antihistamine tablets to take alongside this treatment if the scabies symptoms are impairing your quality of life, and this should help deal with worsening symptoms.  

Effectiveness and consideration

The lack of ovicidal action (ability to kill mite eggs) shows that Permethrin is relatively limited in its efficacy. Despite the general two-application course of treatment, scabies can be very difficult to treat and may require further applications if itching hasn’t subsided six weeks post-second treatment. There are also some areas where scabies mites are beginning to show resistance to permethrin, likely because they are exposed to it without fully being cleared during the first treatment.⁵  

Ivermectin 

This is the only available oral medication against scabies mites. There is also topical ivermectin, which can be used to treat infestations such as head lice. However, it is much more common to use this orally for scabies. This is also a broad-spectrum antiparasitic, which tends to be used in scabies that hasn’t been cleared by permethrin or crusted scabies. This treatment can be used in combination with permethrin to treat particularly stubborn infections, the necessity of which will be determined by the prescribing doctor.⁵

Dosage recommendations and administration 

The dosage of ivermectin is decided by the prescribing doctor and is dependent on body weight. It is incredibly important to take this medication exactly as instructed by the doctor, as misuse can contribute to microbial resistance or an increased severity of side effects. It is also best to take this medication on an empty stomach with water to increase its efficacy. If a dose is missed, it should be taken as soon as possible unless it is almost time for the next dose, in which case that missed dose should be skipped to prevent double dosing. 

Potential side effects

Common side effects include:

• Stomach pain/digestive issues
• Dry mouth 
• Fatigue and drowsiness
• Muscle pain 

More severe complications include:

• Chest pains 
• Black stools
• Dizziness
• Severe fever

There is an extensive list of possible side effects with some being much rarer than others. It is important to see your doctor if you are concerned about any symptoms after taking ivermectin. 

Mechanisms of antiparasitic drugs

Almost all antiparasitic drugs, including permethrin and ivermectin, work by affecting the nerve and muscle functions of the parasites. Permethrin disrupts a voltage-gated sodium channel on the cell membranes of parasites, which causes imbalances in the amount of sodium in the cells. This then affects the cells’ ability to respond to neurotransmitters from the brain. 

Ivermectin works in a similar way, by disrupting another channel present on parasitic cells, called ligand-gated chloride ion channels.  

These work well against parasites and not our cells as there are major structural differences in these types of channels between vertebrates (e.g. humans) and invertebrates (e.g. scabies mites). 

Summary

Scabies is an infestation of Sarcoptes scabiei mites, which causes itching in infected individuals. 

The condition becomes severe due to secondary infections caused by bacteria invading through damaged skin. 

The most common treatment options are topical permethrin and oral ivermectin, which are both broad-spectrum and function to treat many types of parasites and are somewhat effective against scabies. 

It is hard to treat an infestation as there are no treatments that can target the mites’ eggs, and therefore multiple treatments are needed to clear the infection fully.