A lousy problem

Head lice are well adapted to living in human hair, but are
almost never found living off of the head.

One of the most fascinating sibling rivalries in entomology has to be that of the human louse, Pediculus humanus.

The version of this species of louse that most of us are likely to encounter is the head louse Pediculus humanus capitus (the reason for the extra Latin name in the scientific name is that the head louse is a subspecies of the human louse).  The head louse, as most elementary school teachers know, lives exclusively on the human head--and seems to especially relish grade-school children.

The other subspecies of the human louse is the body louse, Pediculus humanus humanus. The body louse is mostly physically identical to the head louse, but has a distinct feeding preference for the body rather than the head.  Body lice do not venture to the heads of their human hosts; but leave the head region to their slightly leaner, head-inhabiting cousins.  It is thought that the two subspecies parted ways some 100K years ago, when humans began to don clothing on a regular basis.  Today, the body louse thrives only on people who change their clothing infrequently.  This because body lice rely on clothing as a hiding place between blood meals, and frequent changing of clothes and showering tend to eliminate infestations.  In the U.S. today, the main group of persons who maintain body lice infestations are in homeless communities of large cities.  The two lice subspecies do not interbreed in the wild, although when forced, they will breed in the laboratory under the prying eyes of entomologists.

The body louse, while less familiar to affluent Americans today, has played a huge role in human history. Body lice transmit  human typhus, one of the major deadly human diseases.  When Napoleon's half million man army attacked and was defeated by Russia in 1812, it's said that more French soldiers died from typhus than were killed by Russians.  In 18th century England, "gaol fever" (another name for typhus) killed more prisoners than all the public executioners in the British realm.

Lucky for us that the head louse does not seem to transmit typhus or any other human disease. It's no wonder, then, that entomologists and physicians have long wondered why the difference between the two lice.

Newly published research by University of Illinois entomologists, summarized in the ESA's Entomology Today may provide some insight into this question.  DNA analysis suggests that it all comes down to seemingly minor differences in the immune responses between the subspecies.  According to one of the authors, Barry Pittendrigh, head lice have a stronger immune response than body lice.

“Our experiments suggest that the head louse immune system is fairly effective in fighting off the bacteria that cause trench fever" (another louse-transmitted disease), Pittendrigh said. “However, the body lice don’t seem to have as good an immune response.”

While reasons for the body louse's weaker immune response is speculative at this time, part of the explanation may have something to do with bacterial ecology inside the bug. Many bacteria live inside insects, some good and some bad for the host.  Body lice appear to get extra vitamins from their bacteria, consequently they tend to be a little larger than head lice, according to the story. So for body lice, a weaker immune system may be beneficial.  Unfortunately, the weaker immune system also allows bacteria that cause typhus and trench fever and the like to survive.  Why head lice seem to follow a different survival strategy is still a puzzle.

I'll admit that all of this is pretty far removed from the very practical vocation of pest control, but ultimately these sorts of biological questions have implications for why pests are pests--and sometimes how they can be controlled. Bed bugs, for example, harbor a bacterium called Wolbachia, which a lot of researchers are interested in at the moment. Some think that the Wolbachia bacteria may help explain temperature sensitivities of bed bugs, and might hold a key to new control strategies. Similarly, studies of termite gut micro-flora may someday point to better baits or wood protectants.

So the next time you encounter a head louse, wonder a little bit at the long path it's taken to become one of the most successful human parasites.  And be thankful it has a strong immune system.