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The Plague
Rebecca Royten
Concordia University
The first plague epidemic on record was the outbreak among the Philistines in 1320 B.C.
described in the Bible and there have been many pandemics reoccurring throughout time (WHO,
1999). The plague has significantly reduced since then but it has not been eradicated and
continues to be an endemic plague foci in many countries in Africa, the former Soviet Union, the
Americas and Asia (Galimand, 2006). The plague circulates at low rates within the populations
of certain rodents without causing excessive die off. These infected animals and their fleas serve
as long term reservoirs for the bacteria in an enzootic cycle (CDC, 2012b). Humans are
extremely susceptible to plague and may be infected either directly or indirectly, with most
transmissions occurring through the bite of a flea (WHO, 1999). When other animal species
become infected an outbreak can occur among animals, causing an epizootic plague. Humans are
more at risk during or shortly after an epizootic plague (CDC, 2012b). Yersinia pestis’s natural
foci is situated in a broad area in the tropical and sub-tropical latitudes and the warmer parts of
the temperate latitudes around the globe between the parallels 55 degrees north and 40 degrees
south (WHO, 1999). It also naturally occurs in areas where climate conditions are favorable for
high and stable numbers of rodents and fleas, where low annual precipitation and dry seasons
inhibit growth and lead to formations of deserts (WHO, 1999). Disappearance of the disease is
unlikely die to the wide range of pests and fleas (Perry, 1997).
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The plague microbe Yersinia pestis is a non-motile, non-acid fast, non-spore forming,
and Gram negative cocco-bacillus. Yersinia pestis belongs to a group of bacilli with low
resistance to environmental factors (WHO, 1999). Plague is a zoonosis that primarily affects
rodents but several other animal species like cats, rabbits, camels and humans can also be
infected (Galimand, 2006). Plague is an infectious disease caused by the bacteria Yersinia pestis
that circulates in animal reservoirs found on all continents except Australia (WHO, 1999). The
life cycles of rodents and fleas exposes Yersinia pestis to different environmental conditions
which has resulted in traits that facilitate transmission and infection and create mechanisms to
overcome host defenses (Perry, 1997). Yersinia pestis is not only able to parasitize the flea but it
is highly virulent to rodents and humans causing epidemics as a systemic and often fatal disease
(Zhou, 2006). The agent Yersinia pestis penetrates the human organism through skin lesions or
the mucosal membranes of mouth, nose and eyes. When primary plague develops into secondary
pneumonic plague airborne transmission may take place leading to pneumonic plague (WHO,
1999).
People most commonly acquire plague when they are bitten by a flea that is infected with
Yersinia pestis (CDC, 2012a). People can also become infected from direct contact with infected
tissues or fluids when handling an animal that is sick with or has died from plague. People can
also become infected from inhaling repository droplets after close contact with cats and humans
who have pneumonic plague (CDC, 2012a). Plague is divided into three types, bubonic,
septicemic and pneumonic plague. Signs and symptoms vary depending on the type of plague
(Mayo Clinic, 2013). Bubonic plague is usually the result of a flea bite and the bacteria multiply
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in the lymph node closest to where the bacteria entered the body (CDC, 2012a). Symptoms
include sudden onset of fever, headache, chills and weakness (CDC< 2012a). Buboes, swollen
lymph nodes about the size of a chicken egg, develop in the groin, armpit or neck (Mayo Clinic,
2013). Treatment of appropriate antibiotics is highly effective, but if not treated the bacteria can
spread to other parts of the body (CDC, 2012a). Septicemic plague occurs when plague bacteria
multiply in the bloodstream, it can occur as first symptoms of plague or it may develop from
untreated bubonic plague (CDC, 2012a). Symptoms include fever, chills, abdominal pain,
diarrhea, vomiting, bleeding from mouth, nose or rectum, shock and gangrene in extremities
(Mayo Clinic, 2013). Pneumonic plague affects the lungs and is the least common variety of
plague but the most dangerous because it is highly infectious via airborne transmission (Mayo
Clinic, 2013). Symptoms include fever, headache, weakness and rapidly developing pneumonia.
Pneumonic plague may develop from inhaling infectious droplets or from untreated bubonic or
septicemic plague that spreads to the lungs (CDC, 2012a). Pneumonic plague progresses rapidly
and may cause respiratory failure and shock within two days of infection. If antibiotic treatment
isn’t introduced within a day after signs and symptoms occur the infection is likely to be fatal
(Mayo Clinic, 2013).
Before antibiotics mortality among those infected with plague was 66%, but with the
discovery of antibiotics mortality was reduced to 11% (CDC, 2012a). It is hard to assess
mortality rates of plague in developing countries because few cases are reliably diagnosed and
reported to the World Health Organization (WHO), but the WHO organization cites mortality
rates of 8-10% (CDC, 2012a). Worldwide between 1,000 to 2,000 cases each year are reported to
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the WHO (CDC, 2012a). Effective treatment methods enable almost all plague patients to be
cured if diagnosed in time. The use of these measures has led to a sharp reduction in the
distribution of plague worldwide (WHO, 1999). Treatment is a grouping of powerful antibiotics
like gentamicin, doxycycline, ciprofloxacin and levofloxacin (Mayo Clinic, 2013). Treatment is
effective but complications of plague may include gangrene leading to amputation. Most people
who receive prompt antibiotic treatment survive but untreated plague has high fatality rates
(Mayo Clinic, 2013). Since the early 1990’s an increase of human plague has occurred especially
in Africa (WHO, 2004). Three geographical areas have seen an increase of outbreaks; India,
Indonesia and Algeria (WHO, 2004). Because of these increases and reappearance of morbidity
associated with the plague it has been categorized as a reemerging disease (Galimand, 2006).
Another change in morbidity is the isolation in Madagascar of two multi-drug resistant strains of
Yersinia pestis, one is resistant to all of the anti-microbial agents recommend for treatment of the
plague (Galimand, 2006).
Plague has occurred in people of all ages from infants up to age 96, but 50% of these
cases fall in the age group of 12-45 (CDC, 2012a). Camping, hunting and hiking are all activities
with an increased risk of coming into contact with plague infected animals (Mayo Clinic, 2013).
These activities are more common in age groups of 12-45. Environmental attributes have more
influence on distribution of plague cases than any other factor. Plague foci change in response to
shifts in climate, landscapes and rodent populations (WHO, 1999). Plague foci is high in areas
with low precipitation and dry seasons that inhibit growth, like deserts (WHO, 1999). In the U.S.
reported plague cases are highest in the southwest where these conditions exist (CDC, 2013).
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Plague cases occur most commonly in rural and semi-rural areas due to overcrowding,
poor sanitation and a high rat population (CDC, 2012a). Areas of low socioeconomic factors
have high density of rat infestations and diverse habitats (CDC, 2012b). This allows the plague
bacteria Yersinia pestis to cycle freely between rats and their fleas with an increased chance of
human contacts. Plague epidemics have occurred in Africa, Asia and South America but since
1990 most cases have occurred in Africa (CDC, 2013). Almost all cases in the last 20 years have
been in small towns or villages over urban areas (CDC, 2013).
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Veterinarians and their assistants have a higher occupational risk because of their
increased contact with domestic cats that may be infected with the plague. The other
occupational factor are people who work outdoors in areas where plague infected animals reside
(Mayo Clinic, 2013). Cases can occur any time throughout the year but most cases are acquired
from the late spring to early fall (CDC, 2012a).
The decrease in the incidence of plague today is due primarily to the improvement of
living standards and health services (WHO, 1999). Control of rat populations, keeping pets free
from fleas, insect repellants and improved living conditions in rural areas have contributed to the
decreased distribution of plague cases (Mayo Clinic, 2013). The gaps of knowledge include not
having a cure even though there is effective treatment. Treatment needs to be administered
within two days in order to be effective (CDC, 2012a). Another gap is that no vaccine has been
developed. No vaccine is available as of now but there are vaccines in development (CDC,
2012a). Also, not all plague foci have been found and identified and the time extended without
plague cases does not mean the plague has disappeared from that region (WHO, 1999). Though
there is no cure, no vaccine and all sources of transmission have not been identified we do have
effective treatments, better living conditions and better health care making plague outbreaks
preventable. If there are any areas of epidemiological research it would be to find all the plague
foci and transmissions and to develop a vaccine. Also it would be beneficial if diagnosing and
reporting became more consistent in order to have a clear idea of distribution.
References
Centers for Disease Control. (2012a). Plague. Retrieved from
http://www.cdc.gov/plague/faq/index.html
Centers for Disease Control. (2012b). Plague ecology and transmission. Retrieved from
http://www.cdc.gov/plague/transmission/index.html
Centers for Disease Control. (2013). Maps and statistics. Retrieved from
http://www.cdc.gov/plague/maps/index.html
Galimand, M., Carniel, E., Courvalin, P. (Oct 2006). Resistance of yersinia pestis to
antimicrobial agents. Antimicrobial agents and chemotherapy. 50(10):3233-3236.
doi:10.1128/AAC.00306-06.
Mayo Clinic. (2013). Disease and conditions plague. Retrieved from
http://www.mayoclinic.org/diseases-conditions/plague/basics/risk-factors/co-20021610?
Perry, R., Fetherston, J. (Jan 1997). Yersinia pestis- etiological agent of plague. Clinical
Microbiology Review. 10(1):35-66.
World Health Organization. (1999). Plague manual: epidemiology, distribution, surveillance and
Control. Retrieved from http://www.who.int/csr/resources/publications/plague/WHO_
_CDS_CSR_EDC_99_2EN/en/
World Health Organization. (2004). Global alert and response impact of plague. Retrieved from
http://www.who.int/csr/disease/plague/impact/en/
Zhou, D., Han, Y., Yang R. (Jan 2006). Molecular and physiological insights into plague
transmission, virulence and etiology. Microbes Infection. 8:1:273-84.