Lecture 24: Insects and Human History – the Plague
What is plague? Plague is a disease of rodents. The term is often misused for outbreaks of other
diseases, such as cholera but it is only the name of the disease caused by the bacterium
. Humans are incidental hosts of plague, which is why the disease is so devastating in
humans. Many rodents, such as chipmunks, are asymptomatic, in other rodents, such as rats, the
disease can be as lethal as it is in humans.
is transmitted from rodent to rodent or
rodent to humans by fleas. The primary rodent source of the disease for humans is the rat.
Vectors to Humans
Two rat species are involved in the plague cycle, the roof rat and the sewer rat. These rats are
peridomestic, that is they live closely with humans, in human habitations and human transformed
habitats, but are not domesticated animals.
The roof or black rat,
is thought to have originated in India. It was probably first
introduced into Europe during the Crusades between 1100 and 1200 AD. During this period
large numbers of people, trade goods and supplies were moved between the Middle East and
Europe. The rats probably first entered Europe through trading ports on the Mediterranean Seas.
This is the rat most likely responsible for the majority of plague epidemics in human history.
Both rat species are omnivores, with dietary requirements similar to those of humans. This
makes them highly successful scavengers in human communities. Each rat species has a
somewhat different biology, so both can be found in the same habitats.
The black rat is subtropical in origin. It is not particularly cold tolerant, so it is only found in
warm sheltered areas in northern climates. This rat is very abundant in Davis and can often be
seen running along telephone lines in summer evenings. This rat is an effective climber and is
often seen in trees, on roofs and in attics. It will feed on pet food left outside, fallen fruit and
nuts, unripe fruit on trees, and similar materials.
The sewer or brown rat,
originated in Russia. It apparently did not enter
Europe until nearly the 1700’s. This rat is very cold tolerant. It tends to be subterranean,
inhabiting burrows along waterways, sewers, basements and similar situations.
A number of flea species are involved in the transmission of plague bacteria from rodent to
rodent or rodent to humans. The most important one though for plague transmission to humans is
the rat flea,
. Second in importance, may be the human flea,
Although there is some question over this flea’s effectiveness as a vector. Human fleas were
certainly abundant in human populations in Europe during major plague epidemics.
Fleas become infected when they feed. When a flea feeds on an infected rat they take in large