How Plague Spreads: Fleas, Infected Animals and Droplets

The bacterium that causes plague, Yersinia pestis, is carried by fleas and circulates naturally among wild rodents. People usually catch plague from the bite of an infected rodent flea or by handling an infected animal, and can also be infected by sick pets, especially cats.

How people catch it

• Flea bites. The most common route. During a plague epizootic (an outbreak among animals) many rodents die, and their hungry fleas look for other sources of blood. People and animals that visit places where rodents have recently died of plague risk being bitten, and dogs and cats can carry infected fleas into the home. Flea bites can lead to bubonic plague or septicemic plague.
• Contact with infected tissue or fluids. Handling the tissue or body fluids of an infected animal can pass on the bacteria: a hunter skinning a rabbit without gloves, for example. This usually leads to bubonic or septicemic plague.
• Infectious droplets. People and animals with plague pneumonia can cough bacteria-laden droplets into the air, and breathing them in can cause pneumonic plague. This usually takes close, direct contact, and it is the only way plague spreads from person to person. There has been no person-to-person spread in the United States since 1924, but rare cases of pneumonic plague still occur in people exposed to sick cats. Cats are especially susceptible, catching plague by eating infected rodents, and a cat with plague can pass infectious droplets to its owner or vet.

Plague in nature

Plague ecology in the United States. In nature, plague occurs especially in the semi-arid grasslands and scrub woodlands of Arizona, Colorado, New Mexico and Utah, cycling among rock squirrels, ground squirrels, prairie dogs and wood rats. Epizootics there seem more likely in cooler summers after wet winters. Cats usually become very ill and can infect people directly; dogs are less likely to be ill but can bring infected fleas home. CDC.

Y. pestis survives in a cycle between rodents and their fleas. In the United States plague occurs in rural and semi-rural areas of the West, mainly in semi-arid upland forests and grasslands, where many rodent species can be involved. Rock squirrels, wood rats, ground squirrels, prairie dogs, chipmunks, mice, voles and rabbits can all be affected, and wild carnivores can be infected by eating them.

• Enzootic cycle. Scientists think the bacteria circulate at low levels in some rodent populations without killing many of them. These animals and their fleas are the long-term reservoir.
• Epizootic. Now and then other species become infected and an outbreak spreads among animals. People are usually most at risk during or just after one.

In parts of the developing world, plague sometimes occurs in cities heavily infested with rats. The last urban outbreak of rat-borne plague in the United States was in Los Angeles in 1924–1925. More in CDC's plague ecology in the United States (PDF).

Sources

• Centers for Disease Control and Prevention, "How Plague Spreads." https://www.cdc.gov/plague/causes/index.html