Chain of Infection for the Plague

Scientific Name: Yersinia pestis

Common Names: ‘the plague’, Bubonic plague, Pneumonic plague, Septicemic plague, the Black Death.

Yersinia pestis is a facultative, gram-negative bacterium. It is adapted to live and multiply in rodent hosts and use insects as vectors. Its ability to produce a biofilm in the rat flea makes the rat flea a particularly effective vector. When Y. pestis enters the body, they are phagocytosed by macrophages that  carry them through the lymphatic vessels to regional lymph nodes where the bubo forms.  Their virulence factors include traits giving them the ability  to resist destruction in macrophages. They also have the ability to free themselves of the macrophage and grow extracellularly when the time is right. Released endotoxin can cause septic shock.

Reservoir: Yersinia pestis is resident in social ground-burrowing rodents. It is found in several native species in southern and central Asia, western North America, and south-eastern Africa and Madagascar.  In the United States it is found in ground squirrels, prairie dogs, and similar native rodents. In Kazakhstan the dominant rodent species is the Great Gerbil. The black rat is capable of carrying Yersinia pestis and is credited with carrying the bacteria over long distances by ship.

Portal of Exit: The most common exit from the reservoir is through the blood meal of a flea that has fed on an infected rodent.

Transmission: The primary means of transmission for bubonic plague is through the bite of a flea that has fed on an infected rodent. The rat flea, Xenopsylla cheopis, is the most effective vector but the fleas of other infected rodents transmit the bacteria well enough. Carnivores, like cats, who prey upon the rodents can also contract Yersinia pestis and transmit it to humans through bites, scratches, respiratory droplets or their fleas.  It can be transmitted human to human by respiratory droplets causing pneumonic plague.  About 2% of modern cases are pneumonic but large pneumonic outbreaks were documented in the early 20th century. In one case from 1997, one patient who developed secondary pneumonic plague transmitted it to 18 people (8 died). In the preantibiotic era, pneumonic plague is nearly 100% fatal. It can also be transmitted by blood-to-blood contact often made through scratches while skinning or butchering an infected animal or potentially while  handling an infected human or corpse. Transmission can also occur through ingestion of an infected animal. Eating infected camels is one of the most common means of contracting gastrointestinal or pharyngeal plague today.

Portal of entry: Y. pestis usually enters the human body through a break in the skin via an flea bite or an abrasion. Less commonly Y. pestis enters the body through inhalation or ingestion.

Vulnerable hosts: All non-immune humans are vulnerable to Y. pestis. Occupational exposure can make some more vulnerable to contract the infection. There is little information available about the vulnerability of the immune suppressed or pregnant women.

References:

Arbaji, A., Kharabsheh S, Al-Azab S, Al-Kayed M, Amr ZS, Abu Baker M & Chu MC. (2005). A 12-case outbreak of pharyngeal plague following the consumption of camel meat, in north-eastern Jordan. Ann. Tropical Medicine and Parasitology, 99 (8), 789-93.

Gage KL, Dennis DT, Orloski KA, Ettestad P, Brown TL, Reynolds PJ, Pape WJ, Fritz CL, Carter LG and Stein JD. (2000). Cases of Cat-Associated Human Plague in the Western US, 1977-1998. Clinical Infectious Disease, 30: 893-900.

Inglesby TV, Dennis DT, Henderson DA, Bartlett JG, Ascher MS, Eitzen E, Fine AD, Friedlander AM, Hauer J,  Koerner JF, Layton M, McDade J,  Osterholm MT,  O’Toole T, G Parker, G. Perl, TM, Russell, PK,  Schoch-Spana,M, & Tonat, K. (2000) Plague as a biological weapon: medical and public health management. Working Group on Civilian Biodefense. JAMA. 283 (17): 2281-90.

Prentice, MB, & Rahalison, L., (2007). Plague. The Lancet, 369: 1196-1207.