Chrysanthemums, Pyrethrin, and Getting Rid of Head Lice
An Annoying Problem
Head lice have lived on human scalps since ancient times. They are a common and very annoying parasite. Although they don't cause disease, they can be extremely irritating. The flowers of some chrysanthemum species contain an insecticidal material known as pyrethrum. One of the active ingredients in this material is a chemical called pyrethrin. The chemical is useful because it kills insects but is relatively safe for humans and pets compared to other insecticides. It's a common ingredient in lice treatments.
Unfortunately, although pyrethrin has been very effective at getting rid of head lice in the past, the insects are becoming resistant to the chemical. Pyrethrin is still used to treat lice problems, but sometimes other control methods are needed. There are potential problems or difficulties with these methods, however. We really need a new treatment that is effective, convenient, and safe.
Although the word "pyrethrin" is often used in the singular, multiple varieties of the chemical exist. This is why the word is sometimes used in its plural form.
Chrysanthemums, or "mums", belong to the family Asteraceae and the genus Chrysanthemum. Their family is also known as the Compositae. This name reflects the fact that their flower is technically a composite made of smaller flowers, which are sometimes known as florets. There are two types of florets. The disk florets are located in the centre of the flower and don't have petals. The ray florets surround the disk florets and each contain one petal.
Mums are beautiful and popular plants. Their flowers have lovely colours and patterns and take many different forms. Some chrysanthemum flowers are flat and resemble daisies. Others have multiple layers of ray florets that are upturned and hide the disk florets. Some flowers are almost globular and look like buttons or pom poms. In some species the ray florets are narrow and tubular, making the flower look like a spider.
The genus is the first word in the scientific name of an organism. Some biologists think that the plants that produce pyrethrin should be placed in the genus Tanacetum instead of the genus Chrysanthemum.
The chrysanthemum that is generally used for pyrethrin production is Chrysanthemum cinerariifolium. Its common name is the Dalmatian chrysanthemum. Some people may not realize that this plant is a chrysanthemum because its flower looks like a white daisy and its leaves are finely divided like those of some ferns.
The Persian chrysanthemum or painted daisy (Chrysanthemum coccineum) is also used as a source of pyrethrin. It contains a much lower content of the chemical than its relative, however. Its flower resembles a daisy and is white, pink, or red in colour.
Chrysanthemums whose flowers look like daisies and produce an insecticide are sometimes known as pyrethrums because they were once classified in the genus Pyrethrum. The raw insecticidal material produced by the plant is also known as pyrethrum. The material contains the chemical named pyrethrin.
Both dried and powdered chrysanthemum flowers and an extract obtained from them are used to kill insects. The pyrethrin is located in the seed cases of the flowers. In some countries pyrethrums are an important cash crop due to their ability to make a relatively safe insecticide.
Pyrethrin and Its Uses
Pyrethrin is a neurotoxin for insects. It interferes with the normal functioning of an insect's nervous system, paralyzingly the animal and often killing It. The chemical kills other insects besides lice. It's appreciated for its safety for mammals (when used in recommended amounts) and the fact that it's broken down by light and air. In general, pyrethrin doesn't persist in the environment after use.
Pyrethrin is often mixed with piperonyl butoxide in insecticides. The latter chemical acts as a synergist. A synergist doesn't produce a benefit by itself but makes another substance more effective. Enzymes in a louse's body normally break down pyrethrin, which may allow the insect to recover from a treatment. Piperonyl butoxide causes the chemical to stay intact for long enough to produce a permanent effect.
Pyrethrin is often used as a lice treatment for pets as well as humans. It has a very low toxicity in humans and other mammals and at recommended concentrations usually creates no problems. Some people experience an allergic skin reaction when they are exposed to the chemical, though. Inhaling a high concentration of pyrethrin can cause breathing difficulties and nausea. In test animals, very high concentrations have produced convulsions and paralysis.
The Head Louse
A head louse has the scientific name Pediculus humanus capitis. Lice are parasitic insects that feed on human blood. They have no wings and can't fly or jump. Instead, they crawl from place to place. Like other insects, they have three pairs of legs. Each leg ends in a claw, which resembles a hook. The claws enable a louse to cling to a hair. The females are slightly bigger than the males.
Head lice pierce our skin with their mouth parts and inject saliva containing an anticoagulent to enourage blood flow. They feed on small amounts of blood several times a day. Though the blood loss doesn't seem to affect us, the insect's saliva often triggers an allergic reaction. An itch is a major part of this reaction. People differ in their sensitivity to the saliva, however. For some people, the itch is intense and develops quickly. For others, it doesn't appear until several weeks after the arrival of the first louse and is milder. Some people don't experience an itch from a lice infestation.
The body louse is a close relative of the head louse but belongs to a different subspecies. Its scientific name is Pediculus humanus humanus. According to the Institute of Food and Agricultural Services at the University of Florida, head lice are more common in children and body lice are more common in adults.
The Secret Life of Head Lice
Life Cycle of a Head Louse
A nit is the egg of a head louse. Nits are very small and are white or yellow in colour. They can be mistaken for dandruff. The female louse deposits the nits at the base of the hair shafts next to a person's scalp, cementing them to the hair with a glue. The nits hatch in about a week.
A tiny nymph emerges from a nit. The nymph looks like an adult head louse but is much smaller. The empty nit stays in place, so if someone is exploring the hair of someone with head lice, the nits that they find may be full or empty.
The nymph molts to form the slightly larger second nymphal stage. This in turn molts to form the larger third nymphal stage. The third nymph molts to form the adult house louse. The time period from nit hatching to the appearance of the adult is generally around nine to twelve days.
How Do Head Lice Spread?
Lice are spread by head-to-head contact or by the transfer of an item that has been in contact with an infested person's head. Example of these items include hats, scarves, combs, hairbrushes, pillows, and headphones. The items shouldn't be shared in case someone has a lice infestation. As the Mayo Clinic quote mentions below, though, there is one exception to this rule.
Head lice can affect people of any age but tend to infest children that form close and interactive groups, such as those formed in day cares and classrooms. A relatively new method of spreading lice has arisen due to the popularity of taking selfies. Children and teenagers often bring their heads together to take group selfies, allowing lice to crawl from one head to another.
A worry about head lice transmission is not considered a good reason to avoid sharing protective headgear for sports and bicycling when sharing is necessary.— Mayo Clinic
Possible Symptoms of an Infestation
Possible symptoms of a head louse infestation include the following:
- Itching is the most common symptom.
- The affected person may also experience a crawling or ticking sensation.
- When someone examines the scalp of a person with head lice they may see red spots.
- White nit cases may be visible. These don't necessarily indicate an active infestation, however. They may be remants of a previous one.
- Some people don't experience any symptoms from an infestation.
The Mayo Clinic recommends that someone is checked by a doctor if a head louse invasion is suspected in order to avoid unnecessary treatment. Dandruff, scabs, dirt, and residue from shampoos and other hair products may be mistaken for lice or nits. Hair casts may also be mistaken for the insects. A hair cast is material dislodged from the surface of a hair.
Becoming infected by head lice doesn't mean that a person has poor hygiene. Anybody can get lice.
Pyrethrin is frequently found in head lice shampoos and treatments. It's been a mainstay treatment for getting rid of lice for a long time. Permethrin is a synthetic version of pyrethrin and is also a common ingredient in lice treatments. Pyrethrin and permethrin are known as pyrethroids. Pyrethroids kill adult lice but not nits. Other insecticidal chemicals are available for treating an infestation. These may not be as safe for humans, however.
Scientists say that although pyrethroids were once very successful at killing lice, they have lost some of their effectiveness due to the development of resistance in the insects. Animals in a species vary genetically. Some lice may possess a gene or combination of genes that makes them resistant to pyrethroids. When these lice reproduce, they pass some of their genes to their offspring, enabling some of the offspring to resist the insecticide, too. Gradually, as the insecticide kills the lice that are susceptible to damage by pyrethroids, a population of resistant insects becomes dominant.
While different sources seem to agree that the effectiveness of pyrethroids has decreased recently, they disagree about the extent of this decrease. The current effectiveness may depend on the identity of the pyrethroid and on the genetic makeup of the head lice in a community.
Behaviour of Head Lice
Safety When Using Lice Shampoos
A lice shampoo, lotion, or cream containing pyrethrin should be kept out of a person's eyes. The chemical may irritate both the eyes and the scalp. It's often considered to be the safest insecticide to use, however, and is usually available over-the-counter (that is, without a prescription). The instructions on the packet regarding the amount of product that should be used, the treatment method, and the frequency of application should be read carefully.
Some lice treatment products contain a different insecticide and are generally available only by prescription. The safety of each of these products should be investigated. It's advisable to discuss a product's safety with a doctor or a pharmacist.
Products that kill lice are often referred to as pediculicides.
Preventing Another Infestation
Head lice don't live for long once they are removed from the human body. Adults require a meal of human blood within two days of their last meal. Nymphs which have just left the egg require a blood meal within twenty-four hours. Eggs require the heat of our body in order to hatch.
The CDC (Centers for Disease Control and Prevention) says that all clothing, bedding, and towels used by an affected person in the two-day period before treatment starts should be washed in hot water and dried by hot air. If the items can't be washed, they should be dry cleaned or sealed in a plastic bag for two weeks. Combs and hairbrushes should be washed in very hot water. Floors and furniture such as sofas should be vacuumed, since we shed hairs on a daily basis. These hairs may have lice attached to them.
How to Use a Lice or Nit Comb
Several natural creams, oils, and liquids are said to kill head lice. Health experts say that there is little evidence that these treatments work. Lice don't suffocate or drown easily. The experts say that the wet-combing method of removing head lice is effective, however,
The Wet-Combing Treatment for Head Lice
One non-chemical treatment for head lice that can be effective is called the wet-combing method. In this treatment a large amount of conditioner is applied to the hair. A fine toothed metal lice comb is then run though the hair to pick up lice and nits. A lice comb has a special design. It's not simply an everyday comb with fine teeth. Lice combs are available at drug stores.
The process of wet combing can be boring for the person being treated, especially if that person is a child, so it's good if they have something to entertain them while they're sitting down. Good lighting is needed so that the person doing the combing can see the hair and scalp clearly. The process involves carefully combing a section of hair and then pinning it out of the way so that another section can be combed. The comb needs to be wiped with a tissue before cleaning a new section of hair. The tissues must be carefully disposed of after use.
The wet combing method needs to be repeated every three to four days for around two weeks in order to remove newly hatched lice. It's more time consuming and requires more effort than using a chemical method to control lice, but it has important advantages. It avoids the application of an insecticide to the scalp. It can also overcome the problem of diminishing effects of pyrethroid treatment due to lice resistance.
Lice have been part of human lives for thousands of years. It looks like we'll be fighting them for many years to come. Hopefully new and improved treatments for head lice will appear soon.
- Information about head and body lice from the University of Florida
- Head lice facts from the Mayo Clinic
- Facts about treating head lice from the Centers for Disease Control and Prevention
- A head lice report from the American Association of Pediatrics
- Bond, C.; Buhl, K.; Stone, D. 2014. Pyrethrins General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/pyrethrins.html.
- A report about the reduced effectiveness of pyrethroid treatment for head lice from the CBC (Canadian Broadcasting Corporation)
- A webpage from the University of Nebraska-Lincoln that gives complete instructions for wet combing in addition to a description of other management techniques for head lice
Questions & Answers
© 2014 Linda Crampton