Overview 

Lice types

Lice (singular: louse) are a group of parasites that cannot fly and require a host (human and/or animal) to infect to survive, grow and reproduce as part of their life cycle. Lice suck blood on varying regions of the body, including;

• Scalp and neck - head louse
• On the body - body louse, which may be attached to clothing or around mattress seams opportunistically (as and when the parasite needs nourishment) to take host blood meals 
• Pubic hair - pubic lice attach to host's pubic hair and take blood meals 

How often do lice occur?

In a study of homeless people in a French city, prevalence rates of body lice were recorded as 12.2%, of head lice, 4.5%, and 0.7-61% in another general study and of pubic lice, 3.2%.^1,2

All three lice infestations can occur due to poor hygiene, socioeconomic status and disasters.¹

Lice impacts on public health

Body lice transmit bacteria that cause diseases inside the bite site, including typhus, louse-borne relapsing fever and trench fever. 

All three lice types cause itching, redness, inflammation, pain, swelling, infection due to excessive scratching of the bite by the host, and anaemia (low blood cell count resulting in tiredness, faintness and weakness).³

The general life cycle of lice

1. Adult male and female louse mate, and the mature female lays eggs 
2. The eggs will hatch to become a nymph (a smaller version of the adult louse), which goes from the 1st nymphal stage, 2nd nymphal stage and the final, 3rd nymphal stage by moulting (shedding their outer coat)
3. From the 3rd moult, adult lice will emerge. Step 1 reoccurs, and the life cycle begins again¹

Transmission routes of lice

• Lice can be transmitted by head-to-head contact (head lice), body-to-body skin contact (pubic and body lice)
• Sharing bedding, towels, mattresses, and clothes can cause body and pubic lice transmission 
• Sharing a hair comb/brush and head coverings can spread head lice ^1,3

Introduction to antiparasitic drugs

What are antiparasitic drugs?

Antiparasitic drugs manage and treat the signs and symptoms of parasitic infections caused by parasites, including protozoa (single-celled organisms), parasitic worms (tapeworms, roundworms, and flatworms), and ectoparasites (such as lice).⁴

How do antiparasitic drugs work? 

Antiparasitic drugs work by:

• Inhibiting essential molecules for parasite survival, growth, infectivity, and reproduction
• Helping the host to expel the living parasite via excretion

Categories of antiparasitic drugs 

Broadly, antiparasitic drugs are classified into: 

• Ectoparasiticides (which target parasites of hair and skin) 
• Antihelminthic drugs (which target tapeworms, flatworms, and roundworms)
• Antiprotozoal drugs (which target single-celled eukaryotic parasites such as malarial parasites transmitted by mosquitoes)⁴

Antiparasitic drugs for lice

Permethrin

Permethrin acts on the nervous system (part of the brain) of the adult parasite, targeting and continuously activating the sodium channels (gates on nerve cells which help send nerve impulses) culminating in muscle spasms, whole-body paralysis and subsequent death of the parasite. Sodium ions are responsible for propagating brain nerve impulse transmission. 

Ways to use permethrin for lice

Permethrin is in creams and lotions applied to the skin to treat body and pubic lice, but is not recommended for head lice due to lice resistance to its effects.

Side effects of permethrin

• Itchiness of the skin
• Burning and stinging pain of the skin (upon application of the cream)^4,5,6

Pyrethrins and piperonyl butoxide

Pyrethrins target sodium ion channels (gates on nerve cells which help send nerve impulses), causing whole-body paralysis and subsequent death of the adult parasite. 

Piperonyl butoxide is used with pyrethrins to increase their insecticidal effect by inhibiting parasite metabolism and detoxification of pyrethri, causing high levels of pyrethrin insecticide to remain within the parasite. 

Ways to use pyrethrins and piperonyl butoxide for lice

• Shampoo

Side effects

• Red patches on the skin that are inflamed, bumpy, itchy, and sometimes painful
• Finding it difficult to respire
• Blocked or runny nose^5,6

Malathion

Malathion works by inhibiting (through binding) an important molecule at parasite nerve endings to cause the irreversible paralysis of the parasite by continuously activating nerve cells.

Ways to use malathion for lice

Malathion can be used for head lice. The applied emulsion is allowed to dry and the hair is then washed. For pubic lice, it must be applied around intimate areas. 

Side effects

• Hypersensitivity
• Skin reactions
• Swollen eye or body area^6,7

Ivermectin

Ivermectin over-activates tiny gates on nerve cells in the parasite body to cause paralysis (inability to move, therefore unable to complete essential activities such as feeding and reproduction), leading to death of the parasite.

Ways to use ivermectin

• As a lotion

Side effects

• Skin rash, swelling, hives^6,8

Spinosad

Spinosad works by inhibiting (through binding) an important molecule at parasite nerve endings to cause the irreversible paralysis of the parasite by continuously activating nerve cells.

It is highly selective and broad, with limited to no unwanted effects on humans and other beneficial insects such as parasitic wasps, midges, bees and spiders.

Ways to use spinosad

• Used as lotion/cream for head and body lice

Side effects

• Irritation with redness at the site of application^6,9

Future antiparasitic drugs and other remedies

New antiparasitic drugs

A newly approved drug called abametapir is a single-application, topically applied lotion which treats head lice in individuals over 6 months of age. 

Abametapir inhibits vital enzymes (proteins) in the outer shell casing of the parasite, which are important for head louse egg development and hatching, therefore hindering the life cycle of the parasite. It also kills adult lice.¹⁰ 

Natural remedies

Oils

Essential plant oils may have a repellent and insecticidal effect on lice. These oils, amongst many include:⁶

• Tea tree 
• Ylang ylang 
• Cinnamon 
• Eucalyptus 

Hot air device treatment

In a study, an FDA-approved device called AirAllé, provided a half-hour exposure of hot air to head lice to cause loss of moisture and thus death. The death of 100% of lice eggs and 80% of hatched lice eggs was caused by hot air treatment.⁶

Diagnosis and treatment of head lice 

Using a nit comb

Nit combs are utilised to find and remove head lice.

Nit combing involves:

1. Wash hair with shampoo and conditioner and comb
2. Use a nit comb to comb starting from the roots of the hair down to the ends of the hair. use a tissue to remove accumulated head lice on the nit comb 
3. Wash out the hair conditioner 
4. Repeat step 2 to ensure all head lice are removed 

Many head lice treatments provide a solution containing ingredients such as dimethicone to apply over the head and scalp and a nit comb to remove head lice. 

They then advise the user to wash their hair with shampoo afterwards and repeat this process weekly after no adult lice and their eggs are detected.¹

Antiparasitic drug resistance 

Drug resistance methods 

Resistance to antiparasitic drugs can be achieved by:

• Decreasing uptake or increasing outtake of the drug inside the parasite
• Stopping the drug from being broken down and activated
• Inactivating the active form of the drug 
• Creating new copies of and increasing expression of drug resistance mediating genes

Repeated inadequate application of singular drugs can speed up parasite resistance to the drug. 

Ways to reduce antiparasitic drug resistance

• Use a combination therapy of multiple drugs with multiple targets in the parasite 
• Creating new drugs with alternative modes of action
• Controlling vectors that transmit parasitic infection 
• Observation and tracking of drug resistance patterns to determine treatment trajectory, prevention, and alternatives¹¹

FAQs

How do I check myself for lice?

Use a nit comb and visual inspection by eye to find dead nits, eggs and live lice after having applied anti-nit treatment or shampoo and conditioner.

What antiparasitic is for head lice?

Pyrethrin, ivermectin, permethrin and other anti-head lice treatments containing dimethicone (a desiccant that causes loss of moisture from and subsequent death of the parasite) are available over-the-counter. Visiting a doctor is advised to determine the best course of treatment.

Summary

Lice cause morbidity by skin inflammation, itching and bacteria-transmitted fevers. The life cycle of lice goes from an egg produced from male-female louse copulation hatching to a nymph, then developing into a mature adult. 

Antiparasitic drugs are categorised into anti-protozoal, antihelminthic and ectoparasiticide types. 

Ectoparasiticides include pyrethrin, permethrin and piperonyl butoxide, malathion, ivermectin and the newly approved Spinosad. 

Emerging resistance to drugs means more selective and broad-spectrum combination therapy from different drug classes would be effective in treating lice infestations. The efficacy and potency of alternative remedies against parasitic infection need further investigation.