Research explores how plague spreads in wild rodent populations

Scientists have long puzzled that Black Death bacteria can either cause stable endemic diseases in rodents or suddenly cause deadly zootic outbreaks that cause massive deaths
in the same rodent population.
A new study concludes that this difference may be related to fleas carrying the virus between
Yersinia pestis.
According to a study published this week in the journal Animal Science, fleas infected early alone are not enough to cause animal epidemics in most wild rodents, but rather favor a more stable endemic state, done by Joseph Hinnebusch of the Rocky Mountain Laboratory at the National Institute of Allergy and Infectious Diseases
, and colleagues.

Plague mainly afflicts rodents, including rats, mice, gerbils, squirrels, marmots, and prairie marmots
.
Bacteria circulate
through these host populations through several flea vectors.
Fleas can spread at different stages after an infectious blood meal; Transmission can occur the next time they eat, a phenomenon known as early transmission
.
This ensues "block-dependent transmission" that forms a bacterial biofilm in the flea's digestive system, blocking the flow of food and allowing blood to flow back to the bite site
after mixing with the biofilm.

In the new study, the researchers empirically assessed for the first time the relative efficiency
of individual flea transmission at different stages.
Swarms of ground squirrel fleas, Oropsylla Montana, are all infected by feeding the blood of mice or rats to the plague genus Yersinia.
The individual spread efficiency of the Omontana researchers then measured the fleas over a four-week period and created models to show how this efficiency translates into the spread of infectious diseases by Yersinia pestis through rodent populations
.

The results show that transmission that relies on obstruction is more effective
than early transmission in terms of probability of transmission, number of bacteria circulated, and ability to drive zoozootic outbreaks.
Models have shown that early transmission can only lead to animal epidemics
in naïve , a very susceptible host population , when the burden of fleas is high.
In addition, low-dose bacteria that are usually transmitted during the early transmission phase may "immunize" many people, promoting endemic animal status
.

"Our model suggests that exposure of most wild rodents to sublethal doses of Yersinia pestis in early stages of transmission can improve rapid zoozootic transmission by reducing the number of susceptible people in the population," the authors said
.
"In many cases, early transmission may be more important
in maintaining endemic animal status than in triggering animal epidemics.
"

。