Health risk and exposure assessment

Public Health Significance of Urban Pests 159

5.3. Health risk and exposure assessment

Plague, caused by the Gram-negative bacterium Yersinia pestis, is the most significant zoo- nosis that involves fleas. The fleas transfer the bacterial infection among their rodent hosts and to people Gage Kosoy, 2005. A plague infection is characterized by fever, chills, headache and malaise that lead to extreme physical weakness; the symptoms vary, depending on the form of plague. The principal clinical forms include bubonic plague, septicaemic plague and pneumonic plague. Untreated bubonic plague has a case-fatality rate of about 50–60, but current therapies, including treatment with an appropriate course of antibiotics, markedly reduce fatality from it. Untreated primary septicaemic plague and pneumonic plague are invariably fatal. Septicaemic plague and pneumonic plague also respond to early diagnosis and treatment; however, patients who do not receive appropriate therapy for primary pneumonic plague within 18 hours after the onset of symptoms are unlikely to survive. Historically, plague has resulted in considerable human mortality, producing significant population declines along with sociological changes. In the Middle Ages, bubonic plague the so-called Black Death killed between a quarter and a third of Europe’s population within just a few decades Gage Kosoy, 2005. During the last pandemic, between 1896 and 1911, more than seven million people died of plague in India. During the last half of the 20 th century, plague outbreaks were reported in Africa, Asia, and North and South America. Nowadays, the number of annual cases is about 2000–3000 worldwide with more than 90 of them in Africa, with a case fatality rate of about 7 WHO, 2004. The infection obviously remains entrenched in sylvatic rural rodent–flea ecosystems throughout the world, and international travel and transportation make reintroduction and re-emergence likely. Plague rarely occurs in the European Region, but it does occur regularly in North America, with chronic zoonotic mainte- nance of the plague pathogen in the south-western United States provoking ongoing concern. Plague in non-commensal rodents and lagomorphs is dealt with in greater depth in Chapter 13 of this report. The oriental rat flea Xenopsylla cheopis and Xenopsylla brasiliensis are the most impor- tant vectors of plague bacilli, from rat to rat and from rat to human. In the laboratory, other flea species, such as the northern rat flea Nosopsyllus fasciatus, cat and dog fleas Ctenocephalides felis and Ctenocephalides canis, respectively, and the human flea Pulex irritans , have been shown to be capable of transmitting the plague organism, so they may play some role in disease maintenance Traub, 1983. In the American Southwest, ani- mals such as ground squirrels Spermophilusspp. and chipmunks Tamias spp. are reser- voirs of sylvatic plague plague in wild rodents, and rodent fleas play an important role in its maintenance and transmission Davis et al., 2002. The risk of people becoming infected is greatest in urban–rural interfaces, parks and recreation areas, because these are areas where people are most likely to encounter infected hosts and their fleas. The other flea-borne disease of significance is murine typhus, an endemic zoonosis cau- sed by an obligate intracellular bacterium, Rickettsia typhi Azad, 1990. Murine typhus is Fleas 158 flea faeces in their environment. Being negatively phototactic moving away from light sources and positively geotropic burrowing, flea larvae avoid sunlight and actively move deep into carpet or under organic debris. Larvae typically require 5–11 days to com- plete their three instars, but the larval developmental period may be extended up to three weeks, depending on food availability and climatic conditions Silverman, Rust Reierson, 1981. Because flea larvae are highly susceptible to desiccation, the larval environment is defi- ned by relative humidities over 50; larvae maintained in soil with low moisture fail to develop Silverman, Rust Reierson, 1981. Due to their susceptibility to heat and desic- cation, flea larvae cannot survive outdoors in areas exposed to the sun. Flea hosts prefer shaded areas, so flea eggs are more likely to be deposited in shade, with the resulting flea larvae developing where the ground is shaded and moist. Similarly, flea larvae are pro- tected under the carpet canopy in indoor habitats, where air movement is minimized and humidity is highest. The third instar larva secretes silk and spins a cocoon within which it pupates. Because the silk is sticky, debris from the environment adheres to it, camouflaging the cocoon as a lint ball or dirt clod. The cocoon is ovoid and about 3 mm long. Inside its cocoon, the larva molts to a pupa and the pupa then molts to an adult. The length of residence within the cocoon varies. Under conducive conditions around 27°C and 80 relative humidity, the adult flea may emerge five days after the cocoon is formed, while under adverse environmental conditions or absence of a host the adult fleas may not emerge for many months perhaps over a year. Typically, fleas emerge within two weeks following cocoon formation. After the adult develops inside the cocoon, a stimulus is required to cause the flea to emerge. Such stimuli as movement, heat and carbon dioxide signal the flea that a potential host is nearby, triggering emergence from the cocoon. The pre-emerged adult flea inside the cocoon is more resistant to desiccation than are eggs and larvae Rust Dryden, 1997. Using its rudimentary eyes, the newly emerged flea orients itself towards a potential host by cueing in on movement. Extending its tarsal hooks, the flea leaps towards the host. If it lands on the host, the hooks permit it to cling. Once an adult cat flea acquires a host, it typically remains on that animal for the duration of its life, with host grooming being the most common mortality factor. Unlike cat fleas, rodent fleas spend the majority of their lives in their host’s nest. Because most rodents are nocturnal, rodent fleas tend to feed in the daytime, after their rodent hosts have returned to the nest and are resting. So rodent flea eggs are found in the host nest, as are most of the developmental life stages. Public Health Significance of Urban Pests 161 In North America, plague has been reported primarily in the West, ranging from south- western Canada to Mexico. Human cases average fewer than 10 a year in North America, a rate that has remained relatively constant since plague was introduced into the Americas in 1899 WHO, 2004. Worldwide, the number of murine typhus cases is low, but because symptoms are non- specific and frequently misdiagnosed, infection is presumed to be much more common than reported Gratz, 2004. In Europe, murine typhus has been reported from Bosnia and H erzegovina, Croatia, the Czech Republic, France, Greece, Italy, Montenegro, Portugal, the Russian Federation, Serbia, Slovakia, Slovenia and Spain, and it is consi- dered likely to be present in most other countries, as well Gratz, 2004.

5.5. Economic burden of flea infestations