Quarantine: Pneumonic Plague Outbreak in China
Pneumonic plague has killed three during a recent outbreak which began at the end of July in Ziketan, North West China. Pneumonic plague is extremely rare, but is the most deadly type of plague. Since the start of the outbreak, nine additional human infections with Yersinia pestis, the bacteria which causes plague, have been confirmed in this Tibetan Area of Qinghai Province. The threat of disease spread was considered serious enough that the Chinese authorities took aggressive measures to keep 10,000 inhabitants of Qinghai province under quarantine. Media reports suggest that these restrictions have now been lifted.
Let’s be clear that this outbreak represents neither a disease pandemic nor an epidemic, so perhaps it may not ‘warrant’ top headline status. Annually, between 1,000 and 3,000 human Y. pestis infections are reported to the World Health Organisation (WHO) 1. News of the current outbreak has largely been relegated to short paragraphs in world news sections for any number of reasons. Limited access to information about this outbreak in this remote Chinese province may provide some explanation for the lack of media attention, as may the limited number of cases, the relative frequency of similar small plague outbreaks and the existing public health media focus on the swine flu pandemic.
Y. pestis has adapted to exist in rodents with various degrees of symptomatic illness; in some rodent hosts the bacteria can circulate without any ill-effect. As such, animals including rats, mice, ground squirrels, chipmunks and marmots can act as a reservoir for the bacteria across the globe. Transmission between these animals and to human hosts generally occurs via the bite of Y. pestis infected male and female fleas. The primary flea vector is the oriental rat flea (Xenopsylla cheopis). Transmission can also occur by handling or skinning infected animals, or by inhaling infected droplets from the cough of a pneumonic plague infected individual 1 2. Media reports suggest that the first fatal human case in the Qinghai outbreak may have resulted from handling a dead Y. pestis infected dog which had eaten an infected marmot.
There are three types of disease caused by infection with the Y. pestis bacterium: bubonic, septicaemic and pneumonic plague. Bubonic and septicaemic plagues can have high mortality rates (up to 60%) without treatment; pneumonic plague has 100% mortality in the absence of treatment 3. Bubonic plague is characterized by high fever and infection of the lymph nodes; here bacteria multiply to result in characteristic enlarged, tender and necrotic glands. Septicaemic plague is rarer than bubonic plague, and occurs when the bacteria circulate in the blood rather than localizing in the lymph nodes. Bubonic and septicemic plagues can only be transmitted after direct contact with infected fluid from the lymph nodes (bubonic) or infected blood (septicaemic), so person-to-person transmission generally does not occur. Pneumonic plague is characterized by infiltration of bacteria to the lung, followed by pneumonia with coughing and dispersal of respiratory droplets in the air. Person-to-person transmission can occur via airborne bacteria after close physical contact with pneumonic plague sufferers 3-5.
Upon hearing of the Chinese plague outbreak, I was reminded of my medieval history lessons at school. I wondered about the relationship between the current outbreak and the ‘black death’ plague of the middle ages. Modern day plague outbreaks do not even begin to approach the scope of those of the 14th century ‘black death’, which is widely considered to have resulted from a bubonic plague pandemic caused by infection with Y. pestis. The ‘black death’ affected people in both urban and rural areas without discrimination. Populations were decimated to the extent that 25 million deaths are estimated to have occurred, wiping out a third of Europe’s population. If indeed Y. pestis was the primary pathogen, the 14th century pandemic most likely included sufferers with all 3 types of plague. It has also been suggested that The Justinian plague of 600AD may have resulted from Y. pestis infection 4.
In recent years, African countries have accounted for around 90% of plague cases reported to the WHO, but outbreaks have occurred across the globe. One recent plague outbreak occurred in Northwestern Uganda in 2006. Here 127 plague cases were confirmed, 102 patients had documented symptoms; 90 had bubonic, and 12 had pneumonic plague. There were 11 pneumonic plague deaths during this outbreak, but overall mortality rates were much lower at 22%. During this Ugandan outbreak there were reports of dead black rats (Rattus rattus). Half of the recovered dead rats were infected with Y. pestis, live rats caught within infected villages had an average of 2 fleas. The flea bite transmits the bacteria from the rodent to the human host, so multiple fleas on infected rodents increase the risk of transmission 6. Small numbers (2-17) of plague cases are reported annually in the USA 7 and recent outbreaks of plague have also occurred in Algeria (2002), India (2002) and the Democratic Republic of Congo (DRC) 8. The ongoing humanitarian crisis in DRC has meant that ‘all efforts at controlling plague have stopped’; since 2001, around 1000 suspected plague cases have been reported annually from DRC 9.
Y. pestis is considered a high risk pathogen and a potential biological weapon. The bacteria can be transmitted via aerosol with 100% mortality rates in humans in the absence of treatment (e.g. pneumonic plague). In addition, as symptoms of pneumonic plague may not be evident for up to 6 days after infection with Y. pestis, this could possibly allow dispersal of the pathogen to a large population before antibiotic intervention occurs 5. However, the bacteria remain infectious as airborne pathogens for less than one hour and can only spread within a few metres from the pneumonic plague sufferer; these factors may limit the potential for Y. pestis transmission to large populations 3-5.
While all this plague outbreak talk may seem quite frightening, outbreaks of plague are extremely rare, even in the least developed countries of the world. Where outbreaks do occur, disease containment is usually swift and certain. Plague deaths are rare and antibiotics can effectively treat the disease, especially when it’s identified early. Biomedical research is also underway globally to further study the genetic and virulence characteristics of Y. pestis, as well as to develop fast acting vaccines against the bacteria. For your own sanity please don’t have nightmares about modern day plague scenarios! Despite existing reservoirs for the Y. pestis bacteria in some wild rodent populations, the risk of plague epidemics or pandemics is miniscule.
1. CDC. Plague Fact Sheet. 2005; http://www.cdc.gov/ncidod/dvbid/plague/resources/plagueFactSheet.pdf
2. CDC. Protect yourself from plague. http://www.cdc.gov/ncidod/dvbid/plague/resources/plaguebrochure.pdf
3. Butler T. Plague into the 21st century. Clin Infect Dis 2009; 49 (5) 736-42 http://www.journals.uchicago.edu/doi/pdf/10.1086/604718
4. Crawford D. Deadly Companions: How Microbes Shaped Our History. 2007; Chapter 4 85-106 http://www.amazon.co.uk/gp/reader/0192807196/ref=sib_dp_pt#reader-link
5. CDC. Frequently asked questions about the plague. 2005; http://www.bt.cdc.gov/agent/plague/faq.asp
6. CDC. Bubonic and pneumonic plague – Uganda, 2006. MMWR Morb Mortal Wkly Rep 2009; 58 (28) 778-81 http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5828a3.htm
7. CDC. Notifiable Diseases/Deaths in Selected Cities Weekly Information. MMWR Morb Mortal Wkly Rep 2009; 58 (29) 808-819 http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5829md.htm
8. WHO. Global Alert and Response (GAR): Plague. http://www.who.int/csr/don/archive/disease/plague/en/index.html
9. CDC. Plague: Enhancing country readiness. 2009; http://www.who.int/csr/disease/plague/readiness2005_1_11/en/index.html
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