[Columnist] Vertebrate Pests - Bobby Corrigan

Forgotten plague?

Recently, a friend of mine called and asked me the following question: "We read in the paper today that our neighborhood is battling some rat infestations. Some were seen behind the mall, and one was spotted not too far from the elementary school. I read on the Internet and I remember from high school that rats can transmit plague. If these rats get out of control, is there any chance of a plague outbreak in our city? Or has plague now been eradicated?"

If this question were posed to you tomorrow from one of your clients, how would you reply? It’s no secret that Norway and/or roof rats have and can serve as reservoirs for the plague organism, and that these rats are well established throughout most of the United States.

But can rats trigger a plague outbreak in our cities or in your town? Perhaps even a timelier question: Could terrorists introduce plague as a bio-weapon to America using rats or fleas or via some other means?

This month let’s take a closer look at plague and its current status in the United States. Next month, we will examine why plague is currently an organism of significant interest to epidemiologists and homeland security officials relative to its use by terrorists as a bio-weapon.

BACKGROUND. Among the history of vector-borne diseases, plague on a global scale is still to this day the most significant flea-born disease of all diseases associated with commensal rodents. Plague has impacted entire societies, the outcomes of wars and global politics. A staggering statistic to keep in mind is that plague is responsible for the loss of more human lives than of all wars combined throughout history.

But because plague epidemics have not occurred in the United States in nearly a century some people believe it has been completely eradicated in this country. But this is not so.

Globally, there are 1,000 to 3,000 cases of plague every year. Plague remains endemic in regions of Africa, Asia, and North and South America. From 1983 to 1997, there were 28,570 cases with 2,331 deaths in 24 countries reported to the World Health Organization (WHO). In 1997, the total number of cases reported by 14 countries to the WHO was 5,419, of which 274 were fatal. In the U.S. however, there were four cases and only one death in 1997. From 1970 to 1990, 284 cases of sylvatic plague occurred in the United States.

PLAGUE 101. Plague acts as an acute disease of the circulatory and respiratory systems, caused by the pathogenic bacterium, Yersinia pestis. This bacterium is transmitted to rats and several other rodents by fleas.

Medical entomologists estimate that there are at least 125 species of fleas capable of transmitting plague, including the widespread human flea (Pulex irritans) and the northern rat flea (Nosopsyllus fasciatus). The northern rat flea is a common flea of domestic rats especially in the temperate and northern regions of the world. Nevertheless, relative to a human transmission threat, it is the Oriental rat flea (Xenopsylla cheopis) that is the principle vector of the plague organism.

People contract plague when fleas containing the bacterium bite them. Fleas obtain the plague bacteria after taking a blood meal from an infected host (e.g., rats, ground squirrels, prairie dogs, woodrats, chipmunks, etc.). Plague kills the rodents it infects. When the rodent dies, the fleas leave the host and seek another suitable host. For the most part, humans are considered "accidental hosts."

In humans, plague manifests itself in three clinical forms: (1) bubonic, (2) septicemic and (3) pneumonic. In general, plague is characterized by the sudden onset of fever and chills, followed by the development of swollen and painful lymph nodes (buboes) in the armpits, groin and other areas two to six days following exposure. Although there is no effective vaccine for plague, it is readily treated and cured using antibiotics, providing a timely diagnosis is made. Without treatment, mortality rates range upwards to 95 percent depending on the type of plague and the severity.

From an epidemiological aspect, two types of plague occur: urban (cities and towns) and sylvatic (wild/wooded areas) plague. Urban plague is usually transmitted from commensal rodents (e.g., roof and Norway rats) by the bite of the Oriental rat flea and less often by other flea species. Urban plague cases are rare in the U.S., although isolated cases have occurred in Norway rats near urban areas in the West and Southwest. The rats in these cases likely became infected from the fleas of wild rodents, in areas where the plague bacteria are established among the wild mammal populations. Because of urban sprawl, new sub-developments are now spreading into what were previously wild areas. Thus, there is increasing concern as to the potential for sylvatic plague to be transferred to domestic rodents and become urban plague.

Away from urban areas, sylvatic plague is transmitted to people from ground squirrels, chipmunks, prairie dog populations, wood-rats, deer mice, California ground squirrels, rabbits and hares. The fleas of these mammals may attack humans (usually hunters) when the animals are handled or prepared as skins and so forth. For the 284 human cases of sylvatic plague reported between 1970 and 1990, about 50 percent of these cases were reported in New Mexico, with the remaining cases occurring among Arizona, California and Colorado.

Plague is also recorded regularly in cats in the U.S., mainly as a result of their contact with infected rodents and other small mammals when cats prowl for food. In New Mexico, which had the most reported cases of human plague, 119 cases of plague were reported in cats between 1977 and 1988.

WHAT PREVENTS OUTBREAKS? But why doesn’t plague occur everywhere? Rats are well established in most of our cities. So why don’t we hear of plague in New York, Miami, St. Louis or other large metropolises?

Plague cannot be ruled out for any area of the United States. And epidemiologists are on the constant alert for the emergence of plague. In 2002 for example, a couple succumbed to plague in New York City, and this caught the attention of health officials. However, the origin of this plague was traced back to New Mexico and not New York rats.

The calming news, however, is that several factors are at play that help minimize the chances of urban plague from occurring in the major cities of the country.

First, it is believed that the first introduction of plague occurred via ship-borne rats arriving in San Francisco in 1899. From here, the plague organism eventually spread to 14 western states (some scientists, however, believe the plague existed in the U.S. prior to 1899). This plague continues to persist in the native rodents in these western states and areas.

Second, key environmental factors may determine whether or not plague can gain a significant foothold in a particular area. For example, although many species of fleas are technically capable of serving as vectors (animals that can carry and transmit pathogens), the primary flea vector which can most readily also adapt to feeding on humans and other animals is the Oriental rat flea.

This flea may occur in many areas, but its preferred environments are dry habitats and tropical and semi-tropical zones. And thus, it is in these types of habitats and zones that have served as the foci (areas where a pathogen exists in greatest concentrations or continued cycles) for the plague cycles.

Where environmental factors tend to keep the number of flea species in a given area low, plague is subsequently suppressed or even totally absent. The more temperate regions of the world or country subject to cold winters, wet springs and falls, high humidity and so forth are not as conducive to the spread of the Oriental rat flea and the other plague-efficient flea vectors.

Third, effective rodent pest management programs play a major role in helping prevent plague from gaining a foothold in cites and modern sanitation and antibiotics, facilitate any sporadic cases to be treated.

And finally, the inspection and fumigation of ships and attention to rat control in harbors helps prevent the introduction of plague-infected animals from overseas.

All this is not to say that there is no need for plague vigilance — for all cities and areas of the U.S. and around the world. When it comes to epidemiological systems (the dynamic relationships between pathogens, vectors, reservoirs, environmental parameters, etc.), things can change — especially given some time. Climates may shift just slightly enough to change these systems; or perhaps some subtle change in a previously minor vector now facilitates the vector playing a more important role.

It is also somewhat problematic that plague is rare around cities outside of the current foci regions, because physicians are not likely to be cognizant of the potential arrival of plague. In this regard, pest management professionals play an important role. Should any rat die-offs occur in a neighborhood, unassociated with any extermination campaigns, such die offs can be thought of as potential "canaries in the coal mine". Vigilant pest professionals should always report this and ay other pest population anomalies to local health authorities.

This is just another way in which pest management professionals serve a critical role in helping to maintain the health of our communities everywhere.