On Lice: Two Parasites Are More Alike Than We Thought
April 10, 2012

On Lice: Two Parasites Are More Alike Than We Thought

Genetic evidence suggests head lice and body lice may be the same species. This finding is of particular importance because body lice have been known to transmit deadly bacterial diseases, while head lice do not.

This study can be found in the journal Insect Molecular Biology.

The debate about whether these two forms of lice are the same species or not has had scientists disagreeing for years. The head louse (or Pediculus humanus capitis) is a common enemy in elementary schools and summer camps, laying its eggs in the hair of its host. Once there, the louse will bite deep into the scalp to feed on blood many times a day.

Head lice are unable to fly or jump and can only move from one host to another by crawling. Head lice are often transmitted through head-to-head contact or by sharing hats, scarves, or other clothing which lice have laid their eggs in.

The body louse (also known as the Pediculus humanus humanus) is usually larger than its cranial cousin. A more dangerous parasite, this louse lays its eggs on clothing and in bedding and feeds on larger amounts of blood. This louse can also transmit diseases such as relapsing fever, trench fever, and typhus. Body lice can only be transmitted via direct contact. While dangerous, the only actual infestation known in the United States is within the homeless and others who do not have access to regular baths and clean clothes.

When found present on the same host, previous research found these two lice will not encroach on one another´s territory. Likewise, these two cousins do not breed with one another in the wild, though previous research has shown they will reproduce under very specific laboratory conditions.

While head lice can appear regardless of human hygiene, body lice tend to arrive when hygiene is at its poorest, such wartime or the depression.

This new study analyzed the genetic sequences of both types of lice during each life stage.

University of Illinois professor of entomology Barry Pittendrigh led to study to determine the differences between these two lice. According to a press release, Pittendrigh came into the study with some questions: “Do they have the same number of genes? Do those genes look very similar or are they very different? What we found is that these two organisms are extremely similar in terms of their protein-coding genes.”

In order to capture a wide amount of gene variety, Pittendrigh and his team exposed these lice to different environmental conditions.

“My colleagues at the University of Massachusetts, led by veterinary and animal sciences professor John Clark, collected lice at every developmental stage, exposed them to every pesticide they could get their hands on, multiple bacterial challenges, several physical challenges – cold, heat – to get the lice to express as many genes as possible,” Pittendrigh said.

Through each of these tests, there were very few differences detected in the number of sequences detected. There were so few, in fact, the researchers are now led to believe the two lice might have been confused with one another.

Says Pittendrigh, “The differences in their sequences were so minor that if we didn´t know they were separate groups, we would have considered them the same species.”

This discovery could lead other researchers to discover why body lice are able to transmit diseases while head lice are not.


Image Caption: The head louse, left, and body louse, right, differ in habits, habitat and in their ability to transmit disease, but a new genetic analysis indicates they are likely the same species. Credit: Centers for Disease Control and Prevention