Bacteria - Yersinia pestis/Plague (Jan 27 - Feb 3)

Bacterium Yersinia pestis

1. Gram negative

2. Non-motile

3. Non-spore forming

4. Coccibacillus

5. Heat intolerant

6. Can survive in infected fleas, carcasses or other organic mater

Zoonotic and transmission

1. Droplets through contact with infected animal

2. Vector-borne

AKA

: black death, bubonic plague, sylvatic plague, urban plague

Ancient History

1. Many pandemics. First in 542 AD caused 100 million human deaths

2. The 2nd pandemic began in 1346 and lasted 3 centuries. Claimed 25 million victims

3. Last pandemic began in 1894 and lasted until 1930’s

4. Since the last pandemic, foci of infection established in South America, West Africa, South Africa Madagascar, and Southeast Asia

 Recent history

1. From 1958 to 1979 – nearly 50,000 cases in 30 countries

2. From 1971 to 1980 – in the Americas there were 7,382 cases

3. Same period – 123 cases in the US

4. Plague in the US continues to be a problem because of sylvatic plague

5. Urban plague under control in most industrialized countries due to rodent control

6. Plague is limited to western United States (No one knows why tho)

How did the disease move around the globe?

Human move disease through rats (carrier of plague) on ships

Host distribution

1. Not normally a disease of domesticated animals (cats are very susceptible)

2. A rodent borne (murine) disease

3. Primary host

   1. Wild (sylvatic) and urban (commensal) rodents (230 species)

4. Transmitted in nature by fleas

5. Zoonotic!

Plague cycle in a flea

Infected bloodmeal

cleared from some but multiply in stomach of other

2 days later, stomach has clusters of brown specks

w/ Y. pestis

3-9 days later, bacterial masses block ingested blood from reaching stomach

Attempt to re-feed, ingested blood mixes w/bacilli and regurgitated into mammalian host

Epidemiology

1. Plague is normally a rodent - flea - rodent cycle

2. Humans accidentally become involved in the cycle

3. Humans may be infected by fleas that have fed on infected rodents

4. Humans may be infected by handling infected animals

5. Humans can get infected by other humans

6. In terms of urban plague, black rat (radus radus) is the most common spreader

7. Epizootics more common during cooler summers following wet winters (SW United states)

Plague cycle in wild mammalian host

Flea bite

Y. pestis spreads to regional lymph node

Multiplies to high numbers->formation of bubo

Spreads to bloodstream

In prairie dogs and black footed ferrets

Prairie dog is one of the biggest concern of getting plague

Black footed ferrets eat prairie dog and plague kills all prairie dogs

ferrets are susceptible and left without food (sad)

Clinical Forms of plague

1. Primary septicemic - in blood

   1. Secondary septicemic possible 

2. Bubonic - in nymph node

3. Pneumonic - in lung

   1. Secondary pneumonia also possible

Symptoms in humans

1. Incubation period 2-6 days

   1. Fever chills headache (flu-like symptoms)

2. Septicemic - disease lasts 1-3 days, mortality maybe be nearly 100% in untreated cases

3. Bubonic - painful swelling of lymph nodes, mortality from 25-60% in untreated cases

4. Pneumonic - the most dangerous for human-human transmission, mortality almost as high as Septicemic

Diagnosis/Treatment

Clinical - by signs and symptoms

Definitive - by isolation and identification of Yersinia pestis

Prompt treatment with antibiotics (streptomycin, tetracycline) is highly treatable

Prevention and control

1. Keep humans away from foci of infection

   1. First- flea control (this first because without rodent

   2. Second- rodent control

2. Inactivated human vaccines for high risk individuals conferred some protection for less than 6 months. No vaccine currently available in USA

3. Use of sentinel animals such as coyote