Scientists Identify Genes Critical to Transmission of Bubonic Plague
Throughout human history, no other infectious disease has inflicted more havoc, or aroused more fear, than the plague. The Black Death, as it came to be known during the 14th century pandemic, originates with a bacterium called Yersinia pestis, transmitted from rodent to rodent by fleas and to humans usually incidentally.
Centuries after the peak of its destruction, scientists at the National Institute of Allergy and Infectious Diseases (NIAID) Rocky Mountain Laboratories (RML) in Hamilton, Mont., have identified a critical genetic link to its transmission. As reported in the July 19, 1996, issue of Science, they found that three genes in Y. pestis change it from a harmless, long-term inhabitant in the flea midgut to one that amasses in its foregut. As a result of this obstruction, the flea begins to starve, leading to a blood-feeding frenzy during which it regurgitates the mass of bacteria and thereby efficiently transmits the plague.
Although scientists have known that plague transmission depends on the plug of bacteria developing in the flea's foregut, until now, they had little understanding about the molecular and genetic mechanisms by which this colonization occurs. "This particular system is probably particular to plague and fleas, and not a strategy used by other arthropod-borne microbes," says medical microbiologist B. Joseph Hinnebusch, Ph.D., primary author of the paper and staff fellow in RML's Laboratory of Microbial Structure and Function (LMSF). Senior scientist and medical entomologist in LMSF, Tom G. Schwan, Ph.D., is a co-author on the paper.
Their experiments, which began about four years ago, focused on three hemin storage (hms) genes, which together comprise less than 1 percent of the total genome of Y. pestis. Hemin, abundant in red blood cells, is the iron-containing part of the hemoglobin molecule that binds oxygen.
To investigate the role of these genes in its flea host, Dr. Hinnebusch and his co-workers conducted a series of experiments during which they gave Oriental rat fleas (Xenopsylla cheopis), the main vector of plague, blood meals containing normal Y. pestis or a mutant form missing the hms genes. Co-author Robert D. Perry, Ph.D., of the University of Kentucky in Lexington, who had previously cloned and sequenced the hms genes, provided both forms of the bacterium.
After four weeks of observation, the RML scientists found that only those fleas infected with the normal bacteria developed the foregut blockage, and this was accompanied by a high rate of mortality. These results indicated that the hms genes are required for Y. pestis to cause the foregut blockage.
In further experiments, they discovered that the failure of the mutant Y. pestis to block fleas could not be attributed to rapid elimination of this form of the bacteria, because the same percentage of fleas infected with either normal or mutant Y. pestis strains were heavily infected after four weeks.
To better visualize where the bacteria locate and develop in the flea gut, they next infected X. cheopis fleas with either of the two forms of Y. pestis modified to fluoresce green. Each week for eight weeks they used a high-powered dissecting microscope to examine the digestive tracts of five to 10 male and female fleas. A striking difference in the infections caused by the two different bacteria became apparent after the first week: the mutant bacteria remained in the midgut while the normal bacteria had migrated to the foregut in many fleas. Eventually, the foregut of these fleas became packed with bacteria. Small clusters of the mutant bacteria were sometimes seen on the spines of the foregut but didn't stay there long. The researchers speculate that the mutant bacteria may fail to colonize the foregut because, as they write in their paper, "being less cohesive, they are disrupted and flushed back into the midgut during feeding."
The RML investigators are now examining other genes in Y. pestis that may be related to its ability to transmit infection. For example, scientists have observed experimentally that the blockage that develops in the flea foregut breaks down at temperatures above 80 to 85 degrees Fahrenheit. The RML investigators are trying to determine why this occurs and if such temperature changes might suppress the products of hms or other genes.
Background Information on Human Plague
There are three clinical forms of human plague: bubonic, septicemic and pneumonic. The classic form, bubonic plague, usually is transmitted from the bite of an infected flea, but also may result from direct contact through a break in the skin with plague-infected tissues or body fluids of infected animals or people. Characteristic symptoms include fever, headache, malaise and one or more very painful swollen lymph nodes. Septicemic plague is a direct invasion of the blood stream without lymph node involvement, and may develop secondarily to bubonic plague. Pneumonic plague also may be primary or secondary. Primary pneumonic plague, the most severe form of the disease, results from inhalation of infective droplets expelled from another person or an animal; overcrowding facilitates transmission. Pneumonic plague often leads to death.
Once the plague organism is introduced into a human, a progressive infection generally results unless specific antibiotic therapy, usually starting with streptomycin, is given. Plague patients are generally isolated until they have received at least 48 hours of antibiotic therapy.
Epidemics of plague in humans usually involve house rats and their fleas. Other rodent species such as rock squirrels, ground squirrels, prairie dogs and rabbits may also serve as sources of human infection. Domestic cats and dogs that bring infected fleas into the home or are otherwise exposed to infected fleas or rodents may also facilitate transmission to humans.
In the sixth century, the first well-documented plague pandemic felled more than 100 million people during a 50-year period. Again in the 14th century the plague cut a broad swath in Europe, killing one-fourth of the entire population, or 25 million people. Gradually, improved sanitation and housing reduced the impact of the disease in Europe. But until the end of the 19th century, smaller epidemics continued to ravage other areas around the world.
Currently, some 2,000 cases of plague worldwide are reported to the World Health Organization annually. From 1984-1993, an average of 12 plague cases have occurred each year in the United States. The last U.S. epidemic to include human-to-human transmission occurred in Los Angeles in 1924-5. Since then, most U.S. human plague cases have been acquired from wild rodents or their fleas. Outbreaks still arise occasionally in scattered small towns and villages or agricultural areas of Africa, Asia or South America, usually as a result of association with domestic rats.
According to information from the Centers for Disease Control and Prevention, "Plague will probably continue to exist in its many localized geographic areas around the world and plague outbreaks in wild rodent hosts will continue to occur. Attempts to eliminate wild rodent plague are impractical and futile." Nevertheless, to reduce the threat of infection to humans in high-risk situations -- for example, researchers working with the plague bacterium, military personnel deployed in endemic areas and people in close contact with infected people or animals -- public health authorities support the use of insecticides and other methods of environmental management, and the use of prophylactic drugs, quarantine and vaccines, where warranted.
NIAID is a component of the National Institutes of Health (NIH). NIAID conducts and supports research to prevent, diagnose and treat illnesses such as AIDS and other sexually transmitted diseases, tuberculosis, asthma and allergies. NIH is an agency of the U.S. Public Health Service, U.S. Department of Health and Human Services.
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