From Alzheimer’s to Zebrafish: Eclectic Science and Regulatory Stories 144
The second stanza of Robert Burns’ poem, “To a Louse,” describes a bug crawling on a
woman’s bonnet and in her hair as follows:
“Ye ugly, creepin, blastin wonner,
Detested. Shunned by saunt an’ sinner,
How daur ye set your fit upon her,
Sae fine a lady!
Gae somewhere else and seek your dinner,
On some poor body.”
Back in Robert Burns’ time (1759–96), personal hygiene was not as big a deal as it is now,
and the bug he describes may not have been a head louse. A louse is only the size of a
sesame seed. Chances are, however, that the woman he described was infested. Although
elegance of manner and dress were cultivated in that period, cleanliness was not.
Unfortunately, head lice are still most common. In fact, Pediculosis capitus has been a
companion of the human species since antiquity. In the US, head lice infestation is more
prevalent among children five to 11 years of age. It is endemic worldwide and affects
persons of all ages and socioeconomic backgrounds.1
More important is that while head lice may not cause fatal disease, they do cause
great distress to the patient. The head louse effectively infests only the human head, and
thus is distinct from body or pubic lice. It feeds by sucking blood, simultaneously inject-
ing saliva, which sometimes causes itching of the scalp and, in neglected cases, secondary
infection and general malaise.2,3
Body lice (Pediculus humanus) are another matter. They are responsible for epidemic
typhus, an acute, severe, febrile disease. The puncture wound from the louse’s bite trans-
fers the causative agent, Rickettsia prowazekii,* through the skin by scratching. Dried
louse feces may also transfer the causative agent to the mucous membranes of the victim’s
eyes or mouth.
Typhus remained endemic in the whole of Europe from the 17th to the early 20th cen-
tury.4 Approximately 20 million cases of true typhus occurred in European Russia alone
between 1917 and 1921, causing 2.5–3 million deaths.
Because lice are species specific, they are important for another reason unrelated to
disease transmission. Biologists have used them to peer back 130 million years in time,
illuminating the catastrophic period during which dinosaurs perished and birds and
animals rose.5 Lice have also been genetically studied, and the analysis suggests that two
distinct species of early humans had close physical contact after a long period of isolation.6
The history of lice, in essence, parallels our own, and lice can be considered evolutionary
partners in tracking human origins.
Lineage
As mentioned above, lice are specialists, feeding on a single species to whose fur, hair or
feathers their claws are adapted. The adaptation is so precise that when a louse’s host species
evolves into a new one, the louse will diversify into different species too. This information
led to a reconstruction of a family tree of lice. This tree stretches so far back in time that the
host of the first louse would have been a dinosaur, probably one that was an ancestor of
birds.7 Researchers used the recent discovery of two fossil lice, one 44 million years old and
the other about 100 million years old, to calibrate a molecular clock for louse evolution.
Evolutionary biologists have compared the mitochondrial DNA from lice, primarily
Pediculus humanus, to existing data on human evolution. They analyzed six louse species,
including two from humans, three from other primates and one from a rodent. According
to the parasites’ DNA, lice specific to chimpanzees appeared 5.6 million years ago, sug-
gesting that the ancestors of chimps and humans diverged at about this time.
The second stanza of Robert Burns’ poem, “To a Louse,” describes a bug crawling on a
woman’s bonnet and in her hair as follows:
“Ye ugly, creepin, blastin wonner,
Detested. Shunned by saunt an’ sinner,
How daur ye set your fit upon her,
Sae fine a lady!
Gae somewhere else and seek your dinner,
On some poor body.”
Back in Robert Burns’ time (1759–96), personal hygiene was not as big a deal as it is now,
and the bug he describes may not have been a head louse. A louse is only the size of a
sesame seed. Chances are, however, that the woman he described was infested. Although
elegance of manner and dress were cultivated in that period, cleanliness was not.
Unfortunately, head lice are still most common. In fact, Pediculosis capitus has been a
companion of the human species since antiquity. In the US, head lice infestation is more
prevalent among children five to 11 years of age. It is endemic worldwide and affects
persons of all ages and socioeconomic backgrounds.1
More important is that while head lice may not cause fatal disease, they do cause
great distress to the patient. The head louse effectively infests only the human head, and
thus is distinct from body or pubic lice. It feeds by sucking blood, simultaneously inject-
ing saliva, which sometimes causes itching of the scalp and, in neglected cases, secondary
infection and general malaise.2,3
Body lice (Pediculus humanus) are another matter. They are responsible for epidemic
typhus, an acute, severe, febrile disease. The puncture wound from the louse’s bite trans-
fers the causative agent, Rickettsia prowazekii,* through the skin by scratching. Dried
louse feces may also transfer the causative agent to the mucous membranes of the victim’s
eyes or mouth.
Typhus remained endemic in the whole of Europe from the 17th to the early 20th cen-
tury.4 Approximately 20 million cases of true typhus occurred in European Russia alone
between 1917 and 1921, causing 2.5–3 million deaths.
Because lice are species specific, they are important for another reason unrelated to
disease transmission. Biologists have used them to peer back 130 million years in time,
illuminating the catastrophic period during which dinosaurs perished and birds and
animals rose.5 Lice have also been genetically studied, and the analysis suggests that two
distinct species of early humans had close physical contact after a long period of isolation.6
The history of lice, in essence, parallels our own, and lice can be considered evolutionary
partners in tracking human origins.
Lineage
As mentioned above, lice are specialists, feeding on a single species to whose fur, hair or
feathers their claws are adapted. The adaptation is so precise that when a louse’s host species
evolves into a new one, the louse will diversify into different species too. This information
led to a reconstruction of a family tree of lice. This tree stretches so far back in time that the
host of the first louse would have been a dinosaur, probably one that was an ancestor of
birds.7 Researchers used the recent discovery of two fossil lice, one 44 million years old and
the other about 100 million years old, to calibrate a molecular clock for louse evolution.
Evolutionary biologists have compared the mitochondrial DNA from lice, primarily
Pediculus humanus, to existing data on human evolution. They analyzed six louse species,
including two from humans, three from other primates and one from a rodent. According
to the parasites’ DNA, lice specific to chimpanzees appeared 5.6 million years ago, sug-
gesting that the ancestors of chimps and humans diverged at about this time.