[Vertebrate Pests] Plague As A Bioweapon

Last month we discussed the disease plague, which despite the perception of many people, still occurs in some areas of the United States and around the world. And because plague typically involves a cycle of bacteria infecting fleas and rats, two common pests associated with cities and towns, some of our clients, the media and health officials occasionally inquire as to whether or not plague, given today’s global awareness on terrorism, could be used by terrorists in some form or fashion as a bioweapon.

And with that in mind, what role, if any, would pest management professionals have in prevention, readiness and response to a plague outbreak or to a bio-attack from plague? Let’s take a look.

PLAGUE AS A BIOWEAPON. Sadly, the word "terrorism" is now an everyday part of our vocabulary. Bioterrorism is defined as "a deliberate release of living organisms (e.g., the plague bacteria, Yersinia pestis) to inflict harm directly or indirectly to a population.

Interestingly, the use of "germs" as bioweapons against humanity is not new. Plague was used as a bioweapon back in the 14th century in a conflict between Muslims and Christians. During WWII, the Japanese dropped "bombs" containing a mixture of grains, paper and plague-infected fleas (Pulex irritans) from airplanes. This operation was relatively successful because most of the fleas survived. About two weeks later, both residents and rats started dying from the plague.

Currently, the Centers for Disease Control and Prevention (CDC) lists Yersina pestis, the causative agent of plague, as a Class A bioweapon. There are several reasons: a) Y. pestis is obtainable on a world-wide basis; b) as a microbe, millions of the bacteria can be maintained in small and easily portable vials or other containers; and c) the bacteria can be distributed in an aerosolized fashion (i.e., released into the air and be distributed via wind and air currents). All of these factors add up to the potential for Y. pestis to cause widespread disease and death.

Of course, ever since Sept. 11 and the subsequent anthrax attack shortly thereafter, plague and other microbial pathogens (e.g., small pox) are now of acute universal interest, and are queued up on the surveillance "radar screen" in many cities. For example, a case of plague that occurred in two people in New York City in 2001 was of interest to homeland security professionals and epidemiologists, and it was no doubt a relief to discover these cases were not the front of an outbreak, but rather non-urban plague transported from wild rodent populations in the Western U.S.

Moreover, to demonstrate how serious the government takes the possibility of plague being used as a weapon, consider that the Department of Justice in 2001 conducted a mock bioterrorism event of releasing Y. pestis in Denver, and a similar event in Chicago in 2003.

How plague might specifically be used as a bioweapon has been studied fairly thoroughly for the past 20 years, by the World Health Organization (WHO), the Centers for Disease Control, the Armed Forces, individual epidemiologists and medical entomologists on a national and global scale. According to these experts, if plague was used as a bioweapon, the most likely form of attack would come in the form of a deliberate release of the bacteria as an aerosol, causing a possible outbreak of the pneumonic phase of plague when people directly inhale the respiratory droplets of the bacteria.

From here it is possible a second-level epidemic could occur in urban rats also affected by the initial release of aerosolized bacteria. Then, should rat populations succumb to the plague (which is referred to as an epizootic outbreak), it is possible for the rodent’s fleas to seek humans as alternate hosts as the rats die. In this way, it is possible for the plague to establish a foothold beyond just the initial "attack."

But, for all of this to occur, several conditions are necessary: 1) a particular city or area must provide the right environmental conditions for the survival of both the bacterial pathogen and a flea vector species that is fairly efficient for transferring the plague (e.g., Oriental rat flea); 2) rat populations must exist in large numbers and in close proximity to people, and 3) enough fleas must be present within a rodent population to support the plague spreading through a rat population on a wide-scale level, (i.e., epizootic) as well presenting a significant threat due to the sheer number of infected fleas that might be present to feed on humans as their alternative hosts.

CITY-LEVEL PROACTIVE PROGRAMS. In locales where these conditions are feasible, surveillance and response programs based on those provided by Homeland Security, the Centers for Disease Control and the World Health Organization are all essential.

Still, inherent in all of this, is the importance of cities everywhere prioritizing prevention via proactive citywide rat management programs. Certainly, this is a critical element in an efficient prevention approach, especially as it relates to the significance of city-wide rat infestations that for past several decades have affected many of our country’s largest cities (e.g., Washington, D.C.; New York; Baltimore; Philadelphia; Chicago; Phoenix; Los Angeles; San Francisco, etc.).

While it is not likely cities can ever eliminate all their rats, cities can (and should) keep rat populations significantly suppressed via well-designed and implemented rat management programs. This in turn, is a critical component towards reducing the threat level of plague, should it ever arrive either via a future terrorist attack, or via a non-terrorist avenue aboard ships, or crossovers from wild rodent populations.

In fact, several government publications over several decades have discussed rat control and control of any associated fleas as essential components of a response to a plague bio-attack to stem off possible secondary human epidemics. In other words, perhaps the most efficient approach would be to prevent city rat populations from ever gaining a stronghold in cities.

Here again, the role of pest management professionals from both within city agencies (e.g., municipal pest control staff), as well as within the private sector (e.g., quality pest control companies) is essential.

It is not an exaggeration to state that rodent control professionals provide a significant part of the front line in plague prevention and the suppression of other rodent borne diseases. City, state and national governments must recognize this, and provide adequate staffing and funding resources. Again, in the famous words of Charles Mayo, "That which can be foreseen can be prevented."

For those interested in obtaining more information on this topic, consult an outstanding paper by Jeff Borchert, entitled Epizoology and Response to the Bioweapon Use of the Plague Organism in Commensal Rodents, published in the proceedings of the 2004 Vertebrate Pest Conference Proceedings (available from the University of California, Davis). His paper also provides an excellent gateway into the literature of bioinvasions, bioterrorism and biosecurity, especially as some of these relate to pest management programs involving rodents, fleas and other urban pests.