Plague (Yersinia pestis, Serious zoonosis)

Etiologic Agent

• Bacterium Yersinia pestis

  • Gram negative

  • Nonmotile

  • Non-spore-forming

  • Coccibacillus

  • Heat intolerant

  • Can surive in infected fleas, carcasses or other organic material

It is ZOONOTIC

• droplets through contact with infected animal

• vector-borne

TYPES of PLAGUE

• Black Death

• Bubonic Plague

• Sylvatic Plague

• Urban Plague

Ancient History

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

  • human population lower back then

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

• Last pandemic began in 1894 and lasted until 1930s

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

Early Biowarfare

• During the siege of Kaffa in 1346, the attacking Tatar forces hurled plague-infected corpses over city walls to cause an epidemic

Major Trade Routes

• Humans have moved this disease around

  • rats were a carrier of plague → ships

Recent History

• From 1958 to 1979- nearly 50,000 cases in 30 countries

• From 1971 to 1980- in the Americas there were 7,382 cases

• Same period- 123 cases in the US

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

  • cycles in wild animals or in nature

• Urban plague under control in most industrialized countries due to rodent control

  • rats used to live with people

Geographic Distribution

• Limited to mostly Western U.S

  • nobody knows why

Host Distribution

• Not normally a disease of domesticated animals (cats very susceptible)

• It is a rodent borne (murine) disease

• The primary host are wild (sylvatic) and urban (commensal) rodents (230 species)

• It is transmitted in nature by fleas

• It is zoonotic!

Epidemiology

• Plague is normally a rodent - flea - rodent cycle

• Humans accidentally become involved in the cycle

• Humans may be infected by fleas that have fed on infected rodents

• Humans may be infected by handling infected animals

• Humans can get infected from other animals

Which is NOT a component of One Health?

A. Humans

B. Animals

C. Agent (epidemiologic triad)

D. Environment

Disease Cycle

Disease Cycle Cont.

Comparison

Rats of North America

Plague cycle in a flea

• Infected bloodmeal (flea feeds on rodent carrying plague)

  • 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

• Attempted to refeed, ingested blood mixes w/ bacilli and regurgitated into mammalian host

  • Fun fact: suggested that as many as 11,000 to 24,000 bacilli are regurgitated into the mammalian host

Plague cycle in wild mammalian host

• Flea bite (regurgitation)

• Y. pestis spreads to regional lymph node

  • closest lymph node to wherever that bite occurs

• Multiplies to high numbers → formation of bubo

  • lymph nodes increase in size

• Spreads to bloodstream

Affected species

• Rats

• Humans

• Felines

• Prairie Dogs

• Ground Squirrels

Highly resistant to plague (carnivores)

• Domestic dogs, black bears, badgers, coyotes, raccoons and skunks

Common weather pattterns

• Epizootics more common during cooler summers (favorable to bacteria) following wet winters (more vegetation → favors rodent population) (SW United States)

• Y. pestis prefers lower humidity without excess heat

Black Footed Ferret

• Endangered

• Diet 98% prairie dogs

  • can die from plague

  • left with no food

Distribution of Species

Which species are we least concerned that Plague will limit populations?

A. Black-tailed Prairie Dog (plague more to the west, hotter)

B. White-tailed Prairie Dog

C. Utah Prairie Dog

D. Gunnison’s Prairie Dog

Clinical Forms of Plague (humans and animals)

• Primary septicemic (flea bite, directly into blood)

  • Secondary septicemia possible

    • more common, bubonic 1st, blood

• Bubonic

• Pneumonic (lung)

  • Secondary Pneumonic also possible

Symptoms in Humans

• Incubation period 2-6 days

• Fever, chills, headache, nausea, diarrhea/constipation, shock, rapid pulse, staggering gait, slurred speech, mental confusion, and prostration (looks like flu)

• Septicemic- disease lasts 1-3 days, mortality may be nearly 100% in untreated cases

• Bubonic- painful swelling of lymph nodes, mortality from 15-60% in untreated cases (highest chance to survive)

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

Diagnosis/Treatment

• Clinical- by signs and symptoms (especially if you have history)

• Definitive- by isolation and identification of Yersinia pestis

• Prompt treatment with antibiotics (steptomycin, tetracycline)

Prevention and Control

• Keep humans away from foci of infection

• FIRST Flea control

• SECOND Rodent control (not 1st because fleas will find another host (humans))

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

• Use of sentinel animals such as coyote

  • used to detect plague