Scientific Causes of the Black Death

Lesson 6

The Plague Theory -

• A plague pandemic (infectious disease that spreads through humans across a large region).• Three types of plague – • Bubonic plague• Pneumonic plague• Septicaemic plague

Bubonic Plague

• A disease of rats transmitted to humans by fleas• The fleas bite infected rats and then bite humans, infecting them in turn. The

flea is xenopsylla cheopis, the oriental flea, which feeds on the blood of mice, rats (especially the black rat, rattus rattus) (see page 483 of your text book)• The organism is Yersinia pestis, a rod-shaped bacteria (see page 483 of your

test book)• When the flea bit a human, depositing Yersinia into human tissues, the

bacteria could travel inside white blood cells to nearby lymph nodes where it could multiply, causing the lymph nodes to swell and suppurate (form pus). It could then escape into the bloodstream and spread to other organs of the body, overwhelming and killing its host in 30 — 50 percent of cases.

Why did the fleas bite so many humans?• Efficient transmission of Yersinia pestis

is generally thought to occur only through the bites of fleas whose mid guts become obstructed by replicating Y. pestis several days after feeding on an infected host. • This blockage results in starvation and

aggressive feeding behaviour by fleas that repeatedly attempt to clear their blockage by regurgitation, resulting in thousands of plague bacteria being flushed into the feeding site, infecting the host.

The importance of the Host

• There were two types of rodents involved in the transmission of the disease, who were they and why were they important?• The bubonic plague mechanism was also dependent on two

populations of rodents: one resistant to the disease, which acts as hosts, keeping the disease endemic, and a second that lacks resistance. When the second population dies, the fleas move on to other hosts, including people, thus creating a human epidemic.• If the bacterium reaches the lungs of an infected person, they then

also become a host and can spread the disease

Pneumonic Plague

• Was more deadly. It was caught by breathing in the germs when an infected person coughed or sneezed.• An infection of the respiratory system• Results from inhalation of fine infective

droplets and can be transmitted from human to human without involvement of fleas or animals

Septicaemic Plague

• Very rare form• Spread by infected fleas, but moves directly to the blood stream and

becomes life threatening before there is visible signs of infection on the skin