Yersinia pestis, the causative organism of the plague, has played an important role in shaping human history. Natural outbreaks devastated entire populations in medieval times, and the organism can still be found today throughout the world, including the United States. The potential use of the bacteria in modern times as an agent of bioterrorism makes understanding this organism a priority. After a brief description of the history of plague pandemics, the authors review the clinical presentations of Yersinia pestis infection and current therapeutic options for this major threat of pandemic outbreaks from the intentional release of this bacterium.

Yersinia pestis, the bacterium that causes plague, has played a significant role in human history and in the history of medicine throughout the ages. The infection caused by Y pestis has been known by many names throughout history, including “plague” and “pestilence.” Similarly, pandemics caused by infection with Y pestis were known by different names, such as the Black Death and the Plague. Three major pandemics have been documented since the Middle Ages. The first was in 541 ad in Egypt. It spread throughout Europe, Asia, and Africa, resulting in estimated population losses of 50% to 60%.¹ The most significant pandemic occurred in medieval Europe and began in 1347. It caused more than 20 million deaths, wiping out at least 30% to 40% of the European population.¹ Shipping and international trade are thought to have facilitated its rapid propagation. The third and last major pandemic occurred in China in 1855 and was responsible for more than 12 million deaths.¹

Fourteenth-century physicians were limited in their medical knowledge and practice. It was believed then that some diseases were caused by an imbalance of bodily humors, and the infection caused by the plague was thought to be no different. In fact, these physicians were partially correct in theorizing that plague was airborne and spread from person to person. However, the possibility that bubonic plague spread from the bite of an infected flea was initially overlooked. Physicians thus recommended that people stay indoors. Bathing was highly discouraged, because it was thought that this would open up the pores to airborne disease. Scores of people fled from the rat-infested cities to the rat-infested farmhouses of the countryside. Bodies piled up and decayed in the larger cities in Europe and were buried in mass graves. In the haste to bury the dead, the mass graves were only superficially covered with soil, allowing the stench of the decaying bodies to spread throughout the surrounding areas.²

Y pestis was not identified as the causative agent for the plague until 1894, when Alexandre Yersin described the bacterium as the causative organism and its association with rats.¹

Geographic Distribution
Endemic plague is found today throughout most of the world, with the exception of Western Europe and Australia. Within the United States, infection with plague is concentrated in the Southwest, with the highest prevalence in Arizona, California, Colorado, and New Mexico (Figure 1). In the year 2000, 6 cases of human plague were reported from 6 US states.³ Although the Centers for Disease Control and Prevention (CDC) estimates that anywhere from 5 to 15 cases of human plague occur in the United States annually,³ the World Health Organization reports that there were only 2 human cases reported in 2001 and 2002 and just 1 case reported in 2003—all of which were bubonic plague.⁴

Epidemiology
Plague is a zoonotic disease primarily affecting rodents; humans do not play a role in the long-term survival of the bacterium. Different fleas (eg, Xenopsylla cheopis) are the vectors of transmission in rodents as well as from rodents to humans. The most common method of transmitting plague to humans is through the bite of an infected flea. After the fleas feed with a blood meal, they are believed to regurgitate bacteria back into uninfected animals (Figure 2).¹ Other methods of transmission to humans include the handling of infected animal carcasses, exposure to aerosols from humans or animals infected with pneumonic plague, and scratches from infected hosts. Most modern cases of endemic or sylvatic plague are contracted from rural wild animals. The most common animal source in the United States is the squirrel (Table 1). Infections usually occur between May and September.¹ Several cases of pneumonic plague have been reported in veterinary workers exposed to infected domestic cats in endemic areas. A sudden and large drop in a host population, such as rodents, may herald an increase in the number of human cases of plague.¹

Clinical Manifestations
The pathogenicity of Y pestis is associated with various molecular mechanisms that allow the bacteria to overcome the defenses of the mammalian host and then overwhelm it with massive growth. In infected mice, significant levels of cytokines only arise just before death.⁵

In the United States, between 1988 and 1994, the prevalence of plague in humans, by clinical manifestations, was: bubonic, 78%; septicemic, 13.2%; pneumonic, 4.4%; meningitic, 1.5%; undetermined, 2.9%.¹ The 3 most common presentations of plague are bubonic, septicemic, and pneumonic (Table 2). Less common presentations include plague meningitis and pharyngitis.

Bubonic plague
The bubonic form is generally thought to be a response to local inoculation from a flea bite or a scratch. Patients develop malaise, fever, headache, and chills within 2 to 6 days of exposure; a swollen and extremely tender lymph node, called a bubo, develops within 24 hours. Buboes typically occur in the groin, axilla, or cervical region, depending on the area of exposure (Figure 3). Accordingly, a flea bite or a scratch on the lower extremity may result in tender and swollen inguinal or femoral lymph nodes. Buboes range in size from 1 to 10 cm in diameter and are generally nonfluctuant; the overlying skin is erythematous and edematous. Occasionally the buboes become fluctuant and suppurate.

Patients with bubonic plague appear acutely ill. Fever and tachycardia are common findings. Buboes contain neutrophils as well as large numbers of Yersinia bacilli, which can be revealed by Gram’s stain. The mortality rate among untreated persons with bubonic plague has been estimated at 40% to 60%.¹ Among recognized and treated cases in the United States, the death rate is 14%.¹

Septicemic plague
Septicemic plague can be primary or secondary. Primary septicemic plague arises with no discernible bubo.⁶ About 12% of patients with bubonic or primary septicemic plague develop secondary pneumonic plague.^1,6 Septicemic plague can lead to disseminated intravascular coagulation, necrosis of small vessels, and purpuric skin lesions.^1,6 Distal necrosis and gangrene (Figure 4) are believed to be responsible for naming the second plague pandemic the Black Death. Abdominal pain may be more prominent in this form of plague than in other forms.¹

Pneumonic plague
Pneumonic plague is the most lethal clinical entity and is highly contagious by droplet dispersion.¹ Human-to-human transmission has occurred by inhalation of aerosolized droplets.¹ If aerosolized as a biological weapon, pneumonic plague can progress rapidly from a febrile, flulike illness to overwhelming pneumonia with the production of bloody sputum. The incubation period characteristically ranges from 1 to 3 days.¹

Pneumonic plague presents with fever, lymph­adenopathy, chest pain, cough, and, occasionally, hemoptysis. Common gastrointestinal complaints that may help limit the differential diagnosis include nausea, vomiting, abdominal pain, and diarrhea. Sputum can be purulent, and a Gram’s stain yields Yersinia bacilli.

Radiographic findings include diffuse pneumonia that may show cavitation (Figure 5). The fatality rate of pneumonic plague is 95% to 100% if treatment is not immediately initiated.⁷

Diagnosis
Plague is clinically difficult to differentiate from other potential aerosolized biological weapons. Recognition is further complicated by the possible simultaneous release of multiple biological weapons. Agents that should be included in the differential diagnosis for flulike symptoms that progress to pneumonia are inhalational anthrax, Q fever, and typhoidal tularemia. Review of these organisms is beyond the scope of this article, but several excellent sources are available.^8-10

Clinical clues that may point to the diagnosis of bubonic plague include travel to endemic areas, a history of contact with an infected species, and the presence of the classic bubo. The rate of clinical deterioration is slower with bubonic plague than with other forms of plague. Early diagnosis of pneumonic plague is difficult but crucial, given the initial protean flulike symptoms and rapid progression to sepsis and death. Early clinical diagnosis is based on epidemiologic factors (eg, geography, mass exposure) as well as the associated clinical syndrome.

It is critical to maintain a high degree of clinical suspicion. The diagnosis of plague must be considered when several patients present with an acute respiratory tract infection, especially if hemoptysis is present. Rapid administration of antibiotics is critical; the physician must not wait for any laboratory confirmation of plague.

Laboratory testing
A diagnosis of plague can normally be confirmed by culture of an infected site, such as a bubo, sputum, blood, or a lymph node. Y pestis bacteria form small colonies and prefer incubation at 30°C. They produced a raised, shiny, but irregular “fried egg” colony morphology, with little or no hemolysis1 (Figure 6). A review of 27 infected cases in New Mexico determined that blood cultures were positive in 24 of 25 cases, whereas bubo aspirate cultures were positive in 10 of 13 cases.¹¹ Wright, Giemsa, or Wayson stain may demonstrate the bipolar or “closed safety pin” appearance of this gram-negative aerobic rod. This appearance can be seen with other gram-negative rods and is, therefore, in and of itself, not diagnostic.

Serology usually requires acute and convalescent titers and is of limited value if rapid diagnosis is needed. However, a single hemagglutination inhibition titer of 1:10 specific to Y pestis F1 antigen is presumptive of the diagnosis of recent infection.¹

New, rapid diagnostic tests
If plague were intentionally aerosolized, traditional methods of diagnosis would be too slow and cumbersome for early detection. For more rapid diagnosis, a direct fluorescent antibody stain (Figure 7) is currently available through many state laboratories or the CDC.

An investigational rapid dipstick test for bubonic and pneumonic plague appears promising. This test uses colloidal gold conjugated to an anti–F1 antibody. Testing against cultured strains has demonstrated a 100% sensitivity and specificity.¹² When tested in the field in Madagascar, the country with the highest endemic rate of plague, this test was more sensitive than standard bacteriologic methods and enzyme-linked immunosorbent assay (ELISA). Using a combination of bacterial methods and ELISA as the reference standard, the positive and negative predictive values of this rapid dipstick test were 90.6% and 86.7%, respectively.¹²

Bioterrorism
The first known use of Y pestis as a bioweapon dates back to the 14th century. Townspeople who were under attack were known to throw plague-ridden corpses over protective walls at invading troops.² During World War II, the secret branch of the Japanese army known as Unit 731 reportedly dropped plague-infested fleas over populated areas of China, causing plague outbreaks. Both the United States and the government of the former Soviet Union were known to have researched and developed methods of aerosolizing plague. The US bioweapons program was terminated in 1969.

The clinical presentation of plague in a bioweapons attack would be dramatically different from the more common bubonic form of infection with Y pestis. It would present in its pneumonic form because of the release of aerosolized Y pestis. Symptoms would typically begin within 3 days but may take as long as 6 days to develop. Masses of people would start to pre­sent with signs and symptoms resembling pneumonia or influenza. One key feature may be hemoptysis, although this should not be considered a definitive diagnostic finding.

In 2000, the US Congress directed the Department of Justice to undertake a study called TOPOFF to “assess the nation’s crisis and consequence management capacity under extraordinarily stressful conditions.”¹³ The study simulated a simultaneous chemical weapons, radiological event, and bioweapons attack on 3 American cities. On May 17, 2000, there was a simulated covert release of Y pestis at the Denver Performing Arts Center. The conclusions of the study were clear: the nation was not ready for even a localized bioweapons attack. The following areas were deemed in need of significant improvement¹⁴:

• Prioritization and distribution of scarce resources
• Initial outbreak containment
• Communication and decision-making
• Management of civil unrest.

Therapy
Pharmacotherapy
Many therapies are suitable for the treatment and prophylaxis of plague. Recognizing the potential for this infection and starting antibiotic therapy quickly are crucial to successful outcome. Pneumonic plague will be rapidly fatal if not treated within 18 to 24 hours.

Parenteral administration of antibiotics is the route of choice for a limited outbreak. Streptomycin sulfate has historically been the preferred agent.⁶ It is given by intramuscular injection in twice-daily doses of 1 g for adults and 15 mg/kg for children. The difficulty with the use of streptomycin lies in its toxicity and lack of widespread availability in the United States.

Gentamicin sulfate has similar efficacy, is widely available, and has fewer side effects. Gentamicin is given in daily doses of 5 mg/kg intramuscularly or intravenously (IV) once daily or a 2-mg/kg loading dose, followed by 1.7 mg/kg intramuscularly or IV 3 times daily in adults, based on ideal body weight and normal renal function.⁶ The efficacy of once-daily dosing has not been established in children, and a dose of 2.5 mg/kg intramuscularly or IV 3 times daily is recommended. Neonates up to 1 week old and premature infants should receive 2.5 mg/kg IV twice daily.⁶

The oral agents tetracycline (Sumycin) and doxycycline (eg, Adoxa, Doryx, Vibramycin) are indicated for the treatment and prophylaxis of plague. Oral therapy is useful if parenteral administration is not possible, or if hospital resources are overwhelmed. The dose of doxycycline for adults and for children weighing 45 kg or more is 100 mg twice daily.⁶ Children who weigh less than 45 kg are given 2.2 mg/kg IV twice daily (maximum 100 mg twice daily, weight based). Tetracyclines should be used with caution in children younger than age 8 because these drugs can cause tooth discoloration and growth retardation.

A recent randomized, controlled trial comparing gentamicin and doxycycline for treatment of plague in Tanzania showed that these agents are effective for the treatment of plague in adult and pediatric patients.¹⁵ Few side effects were noted.

The oral fluoroquinolones ciprofloxacin (Cipro), levo­floxacin (Levaquin), and ofloxacin (Floxin) may be used off-label for treatment or prophylaxis.⁶ The dose of ciprofloxacin is 500 mg twice daily for adults and 15 mg/kg IV twice daily for children (maximum 400 mg IV twice daily for children). As with the tetracyclines, use of quinolones in children must be weighed against the potential risk of arthropathy and tendon damage. Ciprofloxacin in children should only be used in the case of dire emergency. Antimicrobial prophylaxis should be given for 7 days; if disease is confirmed, treatment should be continued for a minimum of 10 days.

Isolation
Patients suspected of or diagnosed with pneumonic plague should be placed in respiratory droplet isolation for 2 to 4 days while receiving antibiotic therapy until clinical improvement is noted. Those who have come in close contact with infected individuals should receive antibiotic prophylaxis. In mass casualty situations, persons with fever or cough should be treated for presumed plague pneumonia.

Y pestis is very sensitive to heat and sunlight and does not survive very long outside the host.⁶ There is minimal risk of environmental transmission 1 hour after the release of aerosolized Y pestis.⁶

Vaccines
No commercially available vaccine is effective against pneumonic plague. A live attenuated vaccine has been in use since 1908, but its safety is questionable, and it has caused deaths in mouse models.¹⁶ A formalin-inactivated vaccine that showed questionable efficacy was discontinued in 1999.

Subunit vaccines in development include recombinant F1 antigen and V antigen; the 2 antigens combined have offered better protection in mice than either antigen alone.⁷ Data from a phase 1 clinical trial have shown a human immune response to the recombinant F1 and V antigens plague vaccine against plague,¹⁷ and other trials in humans are ongoing.

Conclusion
The plague has been a part of human history since at least the sixth-century ad. It has altered the course of human history, and had cultural, political, and economic implications. Plague has been responsible for millions of deaths in the past and remains a threat despite advances in medicine and in public health. Bioterrorism using Y pestis as a weapon of mass destruction has renewed the specter of plague as an ever-present threat. Adequate preparation as well as skill in recognizing and treating patients infected with plague are essential in preventing a fourth pandemic.

Self-assessment test
1. What is the most common method of acquiring Y pestis infection in humans?
A. Inhalation of infected droplets
B. Infected rodent bite
C. Infected flea bite
D. Handling of infected animal carcasses

2. Which of these findings is most characteristic of Y pestis infection?
A. Rash
B. Bradycardia
C. Hypotension
D. Lymphadenopathy

3. Which of these symptom pairs is most suggestive of bubonic plague?
A. Fever with cough
B. Prominent abdominal pain
C. Fever and headache
D. Nausea and vomiting

4. Which of the following diagnostic modalities would be most beneficial in the event of a plague outbreak?
A. Gram’s stain
B. Culture
C. Serology
D. Fluorescent antibody stain

5. All these dosages are appropriate for the treatment of an adult with plague, except:
A. Streptomycin, 1 g intramuscularly, twice daily
B. Gentamicin, 2.5 mg/kg IV, 3 times daily
C. Oral doxycycline, 100 mg, twice daily
D. Oral ciprofloxacin, 500 mg, twice daily

References
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2. Cantor NF. In the Wake of the Plague: The Black Death and the World It Made. New York, NY: Free Press; 2001.

3. Centers for Disease Control and Prevention. CDC Plague Home Page [online]. 2005. Available at www.cdc.gov/ncidod/dvbid/plague/.

4. World Health Organization. Human plague in 2002 and 2003. Wkly Epidemiol Rec. 2004;79:301-306.

5. Cornelis GR. Molecular and cell biology aspects of plague. Proc Natl Acad Sci U S A. 2000;97: 8778-8783.

6. Inglesby TV, Dennis DT, Henderson DA, et al, for the Working Group on Civilian Biodefense. Plague as a biological weapon: medical and public health management. JAMA. 2000;283:2281-2290.

7. World Health Organization. Initiative for Vaccine Research. Zoonotic Infections: Plague. Available at www.who.int/vaccine_research/diseases/zoonotic/en/index4.html.

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9. Maurin M, Raoult D. Q fever. Clin Microbiol Rev. 1999;12:518-553.

10. Dennis DT, Inglesby TV, Henderson DA, et al, for the Working Group on Civilian Biodefense. Tularemia as a biological weapon: medical and public health management. JAMA. 2001;285:2763-2773.

11. Crook LD, Tempest B. Plague. A clinical review of 27 cases. Arch Intern Med. 1992;152:1253-1256.

12. Chanteau S, Rahalison L, Ralafiarisoa L, et al. Development and testing of a rapid diagnostic test for bubonic and pneumonic plague. Lancet. 2003;361:211-216.

13. Justice Department, Federal Emergency Management Agency to conduct domestic counterterrorism exercises [press release]. April 24, 2000. Available at www.usdoj.gov/opa/pr/2000/April/230ag.htm.

14. Inglesby TV. Observations from the Top Off exercise. Public Health Rep. 2001;116(suppl 2):64-68.

15. Mwengee W, Butler T, Mgema S, et al. Treatment of plague with gentamicin or doxycycline in a randomized clinical trial in Tanzania. Clin Infect Dis. 2006;42:614-621.

16. Titball RW, Williamson ED. Vaccination against bubonic and pneumonic plague. Vaccine. 2001;19: 4175-4184.

17. Williamson ED, Flick-Smith HC, LeButt C, et al. Human immune response to a plague vaccine comprising recombinant F1 and V antigens. Infect Immun. 2005;73: 3598-3608.