Public Health Significance of Urban Pests
441
bats, and bites from these animals are the most common sources of human infection. Rodents and lagomorphs also occasionally contract rabies from these sources and poten-
tially could pass these infections to people handling these animals, although the risk is thought to be very low Cappucci, Emmons Sampson, 1972; Fleming Caslick, 1978;
Roher et al., 1981; Dowda DiSalvo, 1984; Fishbein et al., 1986; Moro et al., 1991; Childs et al., 1997; Zaikovskaia et al., 2005.
13.4.2. Major rodent- and lagomorph-related bacterial and rickettsial agents
The following subsections discuss the major rodent-related bacterial and rickettsial agents that are believed to cause illness in people in Europe or North America.
13.4.2.1. Francisella tularensis
Tularaemia occurs endemically in much of Europe and North America Petersen Schriefer, 2005. The causative organism, F. tularensis, is a Gram-negative bacterium that
occurs in a large variety of wild rodent species, as well as in rabbits and hares Hopla, 1974; Morner, 1992; Hopla Hopla, 1994; Ellis et al., 2002. People are likely to become infec-
ted through handling infected animals especially rabbits and hares, ingesting contami- nated water or food, or being bitten by an infectious arthropod vector, most often a tick,
although biting flies and mosquitoes also have been implicated as vectors in the western United States and Europe, respectively. In the United States, lagomorphs particularly
Sylvilagus
spp. are commonly infected and, along with their ticks, are particularly impor- tant in the ecology of type A tularaemia strains F. tularensis tularensis, which are often
vector-borne. This form of the tularaemia bacterium, certain strains of which cause the most severe cases of tularaemia, occurs only in North America and is most likely to be vec-
tor-borne. In addition to their association with lagomorphs, tularaemia cases in people in North America have also been frequently associated with outbreaks in muskrats,
Canadian beavers and voles Microtusspp.. Tularaemia strains found in these animals are typically type B F. tularensis holarctica, which are often waterborne, cause less severe dis-
ease than many type-A strains and are found throughout much of North America, nor- thern Asia and Europe. Francisella tularensishas also been recognized in ground squirrels
and prairie dogs in North America Jellison, 1974; Peterson et al., 2004. The species most commonly found infected in urban settings and surrounding areas in Europe are rabbits
and voles, including water voles and common voles Microtus spp.. In North America, tularaemia infections also have occurred in primates that live in zoos, and the most likely
source of infection in one of these incidents was thought to be Richardson’s ground squir- rels Spermophilus richardsoni that lived within the zoo Nayar, Crawshaw Neufeld,
1979.
Outbreaks of tularaemia in people have been reported frequently from certain European nations and regions, including Spain in 1997 and 1998 585 and 19 cases, respectively and
Sweden in 2000 270 cases Anda et al., 2001; Bossi et al., 2004. The 1997 and 1998 Spanish outbreaks involved the handling of infected hares and crayfish, respectively
Anda et al., 2001. The Swedish outbreaks have been linked to contact with infected hares or voles, or the bites of infectious mosquitoes. In the United States, a total of 1368
cases were reported between 1990 and 2000, although most public health officials believe
Non-commensal rodents and lagomorphs
440
13.4.1.3. California group viruses primarily La Crosse virus
La Crosse virus causes encephalitis and aseptic meningitis, particularly in children, in the Midwestern and mid-Atlantic regions of the United States Beaty, 2001. The eastern
treehole mosquito Aedes triseriatus transmits this virus. Unlike many arthropod-borne viruses arboviruses, La Crosse and other California serogroup viruses can be transmit-
ted transovarially and transstadially in their mosquito vector. Eastern chipmunks and gray squirrels are the principal vertebrate hosts and serve as sources for infecting mos-
quitoes. Another California serogroup virus, Tahyna virus, occurs primarily in central and eastern Europe, where it occasionally causes an acute influenza-like disease that
occurs primarily in children, although most infections are believed to be unapparent Labuda, 2001. The primary vectors are the mosquito Ae. vexans, although other Aedes
spp. might play a role in transmission, and the virus purportedly can overwinter in Culex and Culiseta mosquitoes. The European rabbit and European or brown hare are sus-
ceptible to infection, produce high viraemias and can serve as sources for infecting mos- quitoes with Tahyna virus.
13.4.1.4. CTF virus
This virus causes fever, chills, headache and myalgia, but is rarely fatal. It is fairly com- mon in some mountainous regions of western North America, where it is transmitted by
the Rocky Mountain wood tick. Rodents, some larger mammals and certain birds are important hosts for this virus and can serve as sources of infection for feeding ticks,
although ticks also can transmit CT F virus transovarially, as well as transstadially Callisher, 2001. People walking in areas where adult Rocky Mountain wood ticks quest
are at risk of becoming infected with this virus. Immature stages of this tick do not feed on people, but rather infest rodents and rabbits. Although most cases are acquired while
camping or hiking, Rocky Mountain wood ticks and important rodent hosts can occur near towns in some mountainous areas.
13.4.1.5. Cowpox virus
It is generally accepted that the reservoir hosts of cowpox virus are wild rodents, although direct evidence for this is lacking throughout most of the geographic range of the virus.
Chantrey and colleagues 1999 demonstrated that the main hosts in Great Britain are bank voles, wood mice and short-tailed field voles. They also suggested that wood mice
may not be able to maintain infection alone, thus explaining the absence of cowpox from Ireland, where voles are generally absent. Infection in wild rodents varies seasonally, and
this variation probably underlies the marked variable seasonal incidence of infection in accidental hosts, such as people and domestic cats. Although rarely reported, a recent case
of cowpox in a 16-year-old boy was attributed to handling a rat Honlinger et al., 2005. An increase in human cases from various sources can be expected because people are no
longer routinely immunized with Vaccinia virus to protect against smallpox, a procedure that also provides cross-protection against cowpox infection.
13.4.1.6. Rabies virus
Rabies virus causes an acute viral encephalopathy that is virtually always fatal, unless peo- ple exposed to it are promptly given appropriate treatment Beran, 1994. In nature, the
most common hosts of the various strains of rabies virus are dogs, wild carnivores and
Public Health Significance of Urban Pests
443
The most important and severe rickettsial disease in the United States is RMSF, which is caused by R. rickettsii Dumler, 1994; Sexton, 2001. This species of Rickettsia is trans-
mitted primarily by the American dog tick D. variabilis in the eastern United States and by the Rocky Mountain wood tick in the central and northern Rocky Mountains of the
United States and south-western Canada. Non-commensal rodents act as both tick hosts and sources of infection for the immature stages of the above two tick species.
Serological evidence of human infection with A. phagocytophilum, the agent that causes HGA, has been identified in several European countries Strle, 2004. Non-commensal
rodents act as reservoir hosts and sources for infecting ticks with A. phagocytophilum Parola Raoult, 2001; Strle, 2004; Parola, Davoust Raoult, 2005. The primary vector
of this agent is the castor bean tick, which feeds heavily on non-commensal rodents, inclu- ding wood mice, yellow-necked mice and bank voles Panchola et al., 1995. The primary
vector of HGA in the eastern United States is the deer tick, which feeds on rodents, birds and reptiles Piesman 2002. In the Pacific states, the primary vector is the western black-
legged tick, which feeds during its immature stages on non-commensal rodents, birds and lizards Kramer et al., 1999; Piesman 2002 Additional information on tick-borne
rickettsiae can be found in Chapter 10, on ticks and TBDs.
13.4.2.5. Yersiniae
