Changing patterns in man’s ecosystems caused by man have led among other things to a spreading of arthropod-related zoonoses in impoverished as well as in industrial countries. In Third World countries it is caused by a lower hygiene standard and the lack of hosts and reservoirs, and in industrial countries it is because of increasing cohabitation with domestic and exotic pets and the spread of synanthropic animals in urbanized areas (Rosiky 1978; WHO/WSAVA 1981; WHO 1988).

Moreover, urbanization has acted as a selective factor which has led progressively to a drastic decline in biological diversity (Brown and Roughgarden 1989), while a restricted number of animal species has been demonstrated to be able to share the urban habitat with man, these species being dependent on human activities to maintain themselves (Genchi 1992).

Efficacious vector of many pathogens

For the flea, its biology makes it a very efficacious vector of many pathogens (Genchi 1992). In towns the cat flea is mainly synanthropic and maintains its life cycle indoors feeding on pets (Genchi 1992). However, it must be emphasized that only about 5% of the flea population lives and feeds on the animals, the remaining 95% (eggs, larvae and pupae) are spread around indoor habitat. In buffered microclimate situations such as buildings in towns, cat flea populations grow throughout the year, showing unexpected large peaks caused by the sudden breakdown of nymphal diapause of a very large number of individuals when habitations are abandoned by domestic animals and their owners for longer or shorter times (Genchi 1992).

The reasons that the cat flea is such an extremely successful and ubiquitous parasite are the wide range of possible hosts and its status as a rather permanent parasite (Dryden 1993; Grant 1996). Altogether the cat flea has been found on more than 50 hosts throughout the world (Hopkins and Rothschild 1953; Williams 1986).

Since all the life cycle stages of C. felis are susceptible to desiccation, only those eggs that fall into protected microhabitats hatch larvae that will ultimately develop into adults (Dryden 1989a). As stated by Byron (1987), suitable breeding sites are not widespread in homes but confined to specific sites. Areas that may be suitable for flea development in the house are (Dryden 1989a):

• pet’s bedding
• thick shag carpet
• carpeted or dirt floor basements

Potentially favorable developmental sites outside occur where there is moist soil and shade:

• dog houses
• flower beds
• reas under bushes
• damp crawl spaces
• gardens
• any places where the flea-infested animal might rest during the heat of the day

Areas in the house such as wood or tile floors and well-traveled hallways are less likely to support development. Likewise, open areas of the lawn that are exposed to prolonged sunlight offer poor growth conditions (Dryden 1989a). Summarizing, significantly more fleas are found in rooms where pets spend most of their time resting (Osbrink et al. 1986).

Opportunities of flea exchange are created by host movement and interaction (Marshall 1981). In this context the ranges of pets as well as wild life have to be considered. Cats can have home ranges in urban areas of <1 ha o r up to 270 ha rurally, depending on cat density and the availability and distribution of food (Liberg and Sandell 1988). Urban dogs may have home ranges of 1.5-2.6 ha (Beck 1973). Wild animals are mobile, increasingly abundant in urban areas and often serve as alternative hosts for the cat flea (Bossard et al. 1998).

Seasonal fluctuations

The abundance of adult cat fleas fluctuates with seasonal changes. The warm months of spring and summer give rise to the highest numbers, whereas few are found during the cold months of late fall and winter (Metzger and Rust 1997). Infestations of cat fleas consistently recur during the warm months of the year (Osbrink and Rust 1985a). No life stage of the cat flea can survive extended periods of subfreezing temperatures, and no reports of a diapausing stage exist (Silverman and Rust 1983). Furthermore cat flea populations are rarely detected on domestic hosts during winter months, but reinfestation of unknown origin are nevertheless common in spring and summer (Metzger and Rust 1997). Therefore two hypotheses have been proposed as possible overwintering strategies of cat fleas (Metzger and Rust 1997):

1. Feral mammals, whose territories extend into urban areas, harbor cat fleas all year and represent the source of reinfestation for domestic animals (Dryden and Rust 1994).
2. An unknown percentage of preemerged adults remain inside the cocoons for extended periods and emerge when conditions are favorable for immature development.

The survival and maintenance mechanisms of C. felis, important for epidemiological considerations can be summarized as follows (Rust and Dryden 1997):

1. The presence of adults on domestic and feral cats and dogs.
2. The presence of adults on urbanized small wild mammals (such as raccoons and opossums).
3. A delayed development of immature stages in freeze-protected underground dens of wildlife.
4. A delayed development of pupae and emergence of adults in the in-home environment.