**1. Introduction**

*Toxoplasma gondii* (*T. gondii*) is considered as one of the most successful parasites in the world. This success is first illustrated by its worldwide distribution, from arctic to hot desert areas, including isolated islands and in cities [1]. *T. gondii* is also among the most prevalent parasites in the global human population, with around one third of the population being infected [2]. Finally, it is able to infect, or be present in, the highest number of host species: any warm-blooded animal may act as an intermediate host, and oocysts may be transported by invertebrates such as filtrating mussels and oysters [1, 3].

Beyond this ubiquitous distribution lies a fascinating transmission pattern: simply saying that *T. gondii* has a complex life cycle does not encompass all transmission routes and modes that can be used by the parasite to pass from definitive hosts (DHs), where sexual reproduction occurs, to intermediate hosts (IHs). The "classical" complex life cycle uses felids (domestic and wild-living cats) as DHs and their prey as IHs (Figure 1). Felids are infected by eating infected prey and host the sexual multiplication of the parasite. They excrete millions of oocysts that sporulate in the environment. Sporulated oocysts may survive during several years and may disperse through water movements, soil movements and microfauna. Ingesting a single sporulated oocyst may be sufficient to infect an IH and begin the asexual reproduction phase [1]. This classical life cycle thus relies on a preypredator relationship and on environmental contamination, like other parasites, e.g., *Echinococcus multilocularis* [4].

However, beside this classical cycle, *T. gondii* shows specific abilities that allow it to use "complementary" transmission routes (Figure 1). During the phase of asexual multiplication, tachyzoites may disseminate to virtually any organ within the IH, in

© 2012 Gilot-Fromont et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

particular to muscles, brain, placenta, udder and gonads. Asexual forms are then infectious to new hosts, thus direct infection among IH is possible by several routes which epidemiological importance has to be discussed: vertical transmission through the placenta, pseudo-vertical transmission through the milk, and sexual transmission through the sperm [1, 5, 6]. In humans, *T. gondii* may also be transmitted during blood or organ transplant. Finally, the infectivity of asexual forms towards new IHs entails the ability for the parasite to be transmitted among IHs by carnivory. This transmission route is estimated to cause the majority of cases in humans [7], although people may also get contaminated by ingesting oocysts after a contact with contaminated soil, water, vegetables or cat litter. All the possible transmission routes among IH make the parasite able to maintain its life cycle, at least during a few generations, in the absence of DH and without environmental stage [8]. Moreover, at a high dose, oocysts from the environment may also be infectious for DHs [9], thus the parasite may bypass the IH and use a DHs-environment cycle. The infectivity of oocysts towards cats is relatively low thus the importance of this cycle may be questioned [10]. However, taken together, these observations suggest that *T. gondii* may theoretically have two distinct life cycles, one among IHs and the other one between DHs and environment.

The Life Cycle of *Toxoplasma gondii* in the Natural Environment 5

countries, nationwide seroprevalences in women of childbearing age vary from less than 10% to more than 60% [12]. Within countries, a strong variability is also present: in France, the incidence of *T. gondii* varies from 1 to 68 cases per 1000 pregnancies among the 22 metropolitan regions [13]. Finally, spatial heterogeneity is also present at a more local scale, for example among districts within a region, and up to the level of families: within a village, individuals of the same family tended to have identical serological status [14]. Due to this heterogeneous distribution, the burden of toxoplasmosis and the associated socio-economic cost are unevenly distributed. Elucidating the causes of this distribution of *T. gondii* is necessary to improve prevention. However, this proves to be a difficult task, as the variability of parasite prevalence may reflect variations in many aspects of the life cycle. For example, considering the "classical" cat-environment-prey life cycle, the transmission from IH to DH likely depends on the level of predation of DHs on IHs, thus on the presence and densities of DHs and IHs, as well as on the diet of DHs. The survival of oocysts is influenced by temperature, moisture and UV radiation, thus should be determined by the meteorological conditions prevailing in the area, while dispersal depends on soil and water movements, as well as on the accumulation in invertebrates. The complementary routes of infection depend on the presence of omnivorous species (carnivory), the population dynamics of IH populations (vertical transmission) and the social structure (sexual and milk transmission).

In this chapter we aim to provide a comprehensive overview of factors that are recognized or can be expected to determine *T. gondii* dynamics in animal populations and in the environment, which constitute the reservoir of human infection, *i.e*., a set of epidemiologically connected populations and environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population [15]. Although the risk for people is largely due to the quality and intensity of their contacts with this reservoir, here we only deal with variations of the cycle in the environment and in animals. We summarize which mechanisms are now established and identify areas where data are lacking. We first show that the dynamics of the life cycle varies according to the relative densities of IHs and DHs, in particular along the urban-rural-wild gradient. Then we detail the variations observed in each of these environments at different spatial scales, and the factors that have been found to influence transmission dynamics. We conclude on how the variations described here should affect human exposure and should be

The life cycle of *T. gondii* is dependent on populations of IHs and DHs, and on the level of predation between them. These ecological determinants are themselves dependent on their environment. Because humans exert a major influence on the structure of their environment, the first structuration of these IH-DH communities comes from the urbanization gradient. We first explicit how IHs and DHs populations vary along this gradient before detailing

considered for prevention.

**2. The urban- rural-wild gradient** 

how these variations affect the dynamics of *T. gondii*.

**Figure 1.** Life cycle of *Toxoplasma gondii*: the "classical" life cycle between intermediate hosts (IH, rodents), definitive hosts (DH, felids) and environment (E, soil) is represented with large arrows, while the "complementary" transmission routes (vertical or horizontal transmission among IHs and environment-to-cat transmission) are represented with small arrows.

Moreover, in IHs, the infection of the brain results in several specific clinical manifestations, modifications of host behaviour and life history that influence transmission. As a result of its presence in the brain of IHs, *T. gondii* manipulates host behaviour in two ways, by specifically increasing attractiveness of cat odours to rodent IHs, thus favouring transmission from IH to DH [5, 11], and by increasing the sexual attractiveness of infected males, which favours sexual transmission [6].

These numerous capacities of transmission clearly allow *T. gondii* to be distributed worldwide. However, this does not mean that the risk of toxoplasmosis is identical everywhere. On the contrary, a highly structured pattern of infection can be demonstrated, for example by comparing the level of infection of different human populations. Among countries, nationwide seroprevalences in women of childbearing age vary from less than 10% to more than 60% [12]. Within countries, a strong variability is also present: in France, the incidence of *T. gondii* varies from 1 to 68 cases per 1000 pregnancies among the 22 metropolitan regions [13]. Finally, spatial heterogeneity is also present at a more local scale, for example among districts within a region, and up to the level of families: within a village, individuals of the same family tended to have identical serological status [14]. Due to this heterogeneous distribution, the burden of toxoplasmosis and the associated socio-economic cost are unevenly distributed. Elucidating the causes of this distribution of *T. gondii* is necessary to improve prevention. However, this proves to be a difficult task, as the variability of parasite prevalence may reflect variations in many aspects of the life cycle. For example, considering the "classical" cat-environment-prey life cycle, the transmission from IH to DH likely depends on the level of predation of DHs on IHs, thus on the presence and densities of DHs and IHs, as well as on the diet of DHs. The survival of oocysts is influenced by temperature, moisture and UV radiation, thus should be determined by the meteorological conditions prevailing in the area, while dispersal depends on soil and water movements, as well as on the accumulation in invertebrates. The complementary routes of infection depend on the presence of omnivorous species (carnivory), the population dynamics of IH populations (vertical transmission) and the social structure (sexual and milk transmission).

In this chapter we aim to provide a comprehensive overview of factors that are recognized or can be expected to determine *T. gondii* dynamics in animal populations and in the environment, which constitute the reservoir of human infection, *i.e*., a set of epidemiologically connected populations and environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population [15]. Although the risk for people is largely due to the quality and intensity of their contacts with this reservoir, here we only deal with variations of the cycle in the environment and in animals. We summarize which mechanisms are now established and identify areas where data are lacking. We first show that the dynamics of the life cycle varies according to the relative densities of IHs and DHs, in particular along the urban-rural-wild gradient. Then we detail the variations observed in each of these environments at different spatial scales, and the factors that have been found to influence transmission dynamics. We conclude on how the variations described here should affect human exposure and should be considered for prevention.

#### **2. The urban- rural-wild gradient**

4 Toxoplasmosis – Recent Advances

environment.

particular to muscles, brain, placenta, udder and gonads. Asexual forms are then infectious to new hosts, thus direct infection among IH is possible by several routes which epidemiological importance has to be discussed: vertical transmission through the placenta, pseudo-vertical transmission through the milk, and sexual transmission through the sperm [1, 5, 6]. In humans, *T. gondii* may also be transmitted during blood or organ transplant. Finally, the infectivity of asexual forms towards new IHs entails the ability for the parasite to be transmitted among IHs by carnivory. This transmission route is estimated to cause the majority of cases in humans [7], although people may also get contaminated by ingesting oocysts after a contact with contaminated soil, water, vegetables or cat litter. All the possible transmission routes among IH make the parasite able to maintain its life cycle, at least during a few generations, in the absence of DH and without environmental stage [8]. Moreover, at a high dose, oocysts from the environment may also be infectious for DHs [9], thus the parasite may bypass the IH and use a DHs-environment cycle. The infectivity of oocysts towards cats is relatively low thus the importance of this cycle may be questioned [10]. However, taken together, these observations suggest that *T. gondii* may theoretically have two distinct life cycles, one among IHs and the other one between DHs and

**Figure 1.** Life cycle of *Toxoplasma gondii*: the "classical" life cycle between intermediate hosts (IH, rodents), definitive hosts (DH, felids) and environment (E, soil) is represented with large arrows, while

Moreover, in IHs, the infection of the brain results in several specific clinical manifestations, modifications of host behaviour and life history that influence transmission. As a result of its presence in the brain of IHs, *T. gondii* manipulates host behaviour in two ways, by specifically increasing attractiveness of cat odours to rodent IHs, thus favouring transmission from IH to DH [5, 11], and by increasing the sexual attractiveness of infected

These numerous capacities of transmission clearly allow *T. gondii* to be distributed worldwide. However, this does not mean that the risk of toxoplasmosis is identical everywhere. On the contrary, a highly structured pattern of infection can be demonstrated, for example by comparing the level of infection of different human populations. Among

the "complementary" transmission routes (vertical or horizontal transmission among IHs and

environment-to-cat transmission) are represented with small arrows.

males, which favours sexual transmission [6].

The life cycle of *T. gondii* is dependent on populations of IHs and DHs, and on the level of predation between them. These ecological determinants are themselves dependent on their environment. Because humans exert a major influence on the structure of their environment, the first structuration of these IH-DH communities comes from the urbanization gradient. We first explicit how IHs and DHs populations vary along this gradient before detailing how these variations affect the dynamics of *T. gondii*.

#### **2.1. Host densities and predation rates vary along a urbanization gradient**

The Life Cycle of *Toxoplasma gondii* in the Natural Environment 7

**Figure 2.** Variations of human density and anthropogenic food supply, cat density, rodent density and predation along an urban-rural-wild gradient. The magnitude of the bars represents the relative

Because of these variations of three key parameters of *T. gondii* cycle (densities of DHs and IHs, and predation rate), one can hypothesize that the dynamics of *T. gondii* should vary qualitatively and quantitatively along the urban-rural-wild gradient, following the specific features regarding *T. gondii* transmission in each environment. Urban areas, at least in the limited areas where cats live, support the highest densities of DHs. However, in cities, rodent densities are relatively low and predation rate is low due to the availability of anthropogenic food resources. The transmission through predation is not expected to be favoured in this case, but the DH-environment cycle should be maximized. On the contrary, in the wild environment, the level of predation of cats on rodents is maximal, but cat density is low, thus transmission should occur only by predation. Finally, rural areas combine intermediate to high values of IH and DH density, with high predation rates. Thus these may be the most favourable for the transmission of *T. gondii* [40]. This transmission should occur largely through "classical" IH-DH transmission, but transmissions among IHs and

importance of each factor according to the degree of urbanization (modified from [4]).

through a possible DH-environment cycle should also be possible in this case.

**2.2. Variations in** *T. gondii* **dynamics along the urban-rural-wild gradient** 

The hypothesis that the dynamics of *T. gondii* transmission varies along the urban-rural gradient has been tested through a theoretical approach, using an epidemiological model [10]. The aim was to estimate the contributions of the IH-DH and DH-environment cycles in the spread of *T. gondii* according to the predation rate, with stable cat population size. The modelling approach allowed the authors to compare populations differing only by the rate of predation, all else being equal. The model first confirmed that the rural environment (here defined as having predation rates above 21 prey/cat per year [35, 36] is favourable for *T. gondii*, as transmission increases with the predation rate [10]. Seroprevalences predicted for cats ranged from 33.2 to 83.4% in the rural environment *vs*. 6.9 to 33.2% in urban areas. Moreover, in rural-type areas, the contribution of the IH-DH cycle increases with the predation rate, and may reach 70% of the transmission (Figure 3). The DH-environment cycle may theoretically be responsible for more than 50% of the transmission, but only in extremes cases with predation rates lower than 9 prey/cat/yr (Figure 3). It is noteworthy that

Taking advantage of a high adaptability and following human migrations, the domestic cat *Felis catus* has colonized a wide variety of habitats, ranging from urban areas to nonanthropized islands, through, agricultural areas, arid or semi-arid areas, villages or cities, from polar to equatorial climatic regions [16]. However, due to the behavioural plasticity of this species, population density and structure vary, depending on the abundance and distribution of food resources and shelters [16, 17]. In particular, cat populations are structured differently along an urban-rural-non-anthropized ("wild") gradient (Figure 2). The highest densities of cats are found in urban populations of stray cats locally more than 1000 cats/ km² [18, 19]. At these high densities, cats form large multimale–multifemale social groups and share their territory, as well as available resources [16]. Most resources are provided directly or not, by people (feeders, garbage) [19]. In rural areas, population density is moderate (100-300 cats/ km²) [20, 21, 22]. Most cats have an owner who provides food and shelter but cats are generally free to roam [23]. An important part of cats diet result from predation: 15 to 90% depending on cat lifestyle [24, 16]. In rural areas, the spatial distribution of cats is based on human settlements: the social groups are based on a house or farm that provides most of the feeding and nesting resources. Around a feeding point, cats may form groups of up to 20 individuals, often constituted by related females and their kittens [16]. In fact, in such areas, a gradient can be observed between pet-owned cats mostly fed by the owner, to farm cats and feral cats mostly living on predation. Finally, feral cats occupying non-anthropized areas (sub-Antarctic, arid or forested areas), survive exclusively through predation, live at low density (1 to 10 cats/ km²), in large and nonoverlapping home ranges [25, 26].

Rodent densities also vary along the urban-rural-wild gradient (Figure 2). However, comparisons are not straightforward since many species are concerned and most of them are not present in all environments. The available estimates suggest that some species may live at very high densities in agricultural landscapes: for example, common voles *Microtus arvalis* and water voles *Arvicola terrestris* may reach 100 000 individuals/km² [27]. In contrast, in urban areas, the density of wood mice *Apodemus sylvaticus* was estimated to lie around 2 000 - 8 000 mice/km² [28, 29].

The third parameter that varies along the urban-rural-wild gradient is the rate of predation of rodents by cats, *i.e*., how many rodents does a cat ingest per unit of time. This parameter is crucial for the transmission of *T. gondii* from IH to DH: combined with the prevalence in prey, it determines the risk for a cat to get infected. The importance of the predation rate is illustrated by the finding that cats with frequent outdoor access show higher predation rates [16] and higher prevalences than cats not allowed to roam [30, 31, 32, 33, 34]. The predation rate depends on the availability of rodents, *i.e*., on the density of rodents relative to cats, and on the availability of other food resources provided by people. The predation rate is lowest in urban populations, ranging from 10 to 27 prey/cat/year [35, 36, 37]. For suburban and rural sites, estimated values for predation rates range from 21 to 436 prey/cat/year [10, 24, 38, 39]. Finally, the predation rate should be highest in non-anthropized areas, where cat exclusively live on predation.

The Life Cycle of *Toxoplasma gondii* in the Natural Environment 7

overlapping home ranges [25, 26].


exclusively live on predation.

**2.1. Host densities and predation rates vary along a urbanization gradient** 

Taking advantage of a high adaptability and following human migrations, the domestic cat *Felis catus* has colonized a wide variety of habitats, ranging from urban areas to nonanthropized islands, through, agricultural areas, arid or semi-arid areas, villages or cities, from polar to equatorial climatic regions [16]. However, due to the behavioural plasticity of this species, population density and structure vary, depending on the abundance and distribution of food resources and shelters [16, 17]. In particular, cat populations are structured differently along an urban-rural-non-anthropized ("wild") gradient (Figure 2). The highest densities of cats are found in urban populations of stray cats locally more than 1000 cats/ km² [18, 19]. At these high densities, cats form large multimale–multifemale social groups and share their territory, as well as available resources [16]. Most resources are provided directly or not, by people (feeders, garbage) [19]. In rural areas, population density is moderate (100-300 cats/ km²) [20, 21, 22]. Most cats have an owner who provides food and shelter but cats are generally free to roam [23]. An important part of cats diet result from predation: 15 to 90% depending on cat lifestyle [24, 16]. In rural areas, the spatial distribution of cats is based on human settlements: the social groups are based on a house or farm that provides most of the feeding and nesting resources. Around a feeding point, cats may form groups of up to 20 individuals, often constituted by related females and their kittens [16]. In fact, in such areas, a gradient can be observed between pet-owned cats mostly fed by the owner, to farm cats and feral cats mostly living on predation. Finally, feral cats occupying non-anthropized areas (sub-Antarctic, arid or forested areas), survive exclusively through predation, live at low density (1 to 10 cats/ km²), in large and non-

Rodent densities also vary along the urban-rural-wild gradient (Figure 2). However, comparisons are not straightforward since many species are concerned and most of them are not present in all environments. The available estimates suggest that some species may live at very high densities in agricultural landscapes: for example, common voles *Microtus arvalis* and water voles *Arvicola terrestris* may reach 100 000 individuals/km² [27]. In contrast, in urban areas, the density of wood mice *Apodemus sylvaticus* was estimated to lie around 2 000

The third parameter that varies along the urban-rural-wild gradient is the rate of predation of rodents by cats, *i.e*., how many rodents does a cat ingest per unit of time. This parameter is crucial for the transmission of *T. gondii* from IH to DH: combined with the prevalence in prey, it determines the risk for a cat to get infected. The importance of the predation rate is illustrated by the finding that cats with frequent outdoor access show higher predation rates [16] and higher prevalences than cats not allowed to roam [30, 31, 32, 33, 34]. The predation rate depends on the availability of rodents, *i.e*., on the density of rodents relative to cats, and on the availability of other food resources provided by people. The predation rate is lowest in urban populations, ranging from 10 to 27 prey/cat/year [35, 36, 37]. For suburban and rural sites, estimated values for predation rates range from 21 to 436 prey/cat/year [10, 24, 38, 39]. Finally, the predation rate should be highest in non-anthropized areas, where cat

**Figure 2.** Variations of human density and anthropogenic food supply, cat density, rodent density and predation along an urban-rural-wild gradient. The magnitude of the bars represents the relative importance of each factor according to the degree of urbanization (modified from [4]).

Because of these variations of three key parameters of *T. gondii* cycle (densities of DHs and IHs, and predation rate), one can hypothesize that the dynamics of *T. gondii* should vary qualitatively and quantitatively along the urban-rural-wild gradient, following the specific features regarding *T. gondii* transmission in each environment. Urban areas, at least in the limited areas where cats live, support the highest densities of DHs. However, in cities, rodent densities are relatively low and predation rate is low due to the availability of anthropogenic food resources. The transmission through predation is not expected to be favoured in this case, but the DH-environment cycle should be maximized. On the contrary, in the wild environment, the level of predation of cats on rodents is maximal, but cat density is low, thus transmission should occur only by predation. Finally, rural areas combine intermediate to high values of IH and DH density, with high predation rates. Thus these may be the most favourable for the transmission of *T. gondii* [40]. This transmission should occur largely through "classical" IH-DH transmission, but transmissions among IHs and through a possible DH-environment cycle should also be possible in this case.

#### **2.2. Variations in** *T. gondii* **dynamics along the urban-rural-wild gradient**

The hypothesis that the dynamics of *T. gondii* transmission varies along the urban-rural gradient has been tested through a theoretical approach, using an epidemiological model [10]. The aim was to estimate the contributions of the IH-DH and DH-environment cycles in the spread of *T. gondii* according to the predation rate, with stable cat population size. The modelling approach allowed the authors to compare populations differing only by the rate of predation, all else being equal. The model first confirmed that the rural environment (here defined as having predation rates above 21 prey/cat per year [35, 36] is favourable for *T. gondii*, as transmission increases with the predation rate [10]. Seroprevalences predicted for cats ranged from 33.2 to 83.4% in the rural environment *vs*. 6.9 to 33.2% in urban areas. Moreover, in rural-type areas, the contribution of the IH-DH cycle increases with the predation rate, and may reach 70% of the transmission (Figure 3). The DH-environment cycle may theoretically be responsible for more than 50% of the transmission, but only in extremes cases with predation rates lower than 9 prey/cat/yr (Figure 3). It is noteworthy that

the predicted prey seroprevalences, from 2.4 to 5 % along the gradient, was always low compared to the magnitude of cat seroprevalences.

The Life Cycle of *Toxoplasma gondii* in the Natural Environment 9

Aimargues, Saint-Just Chaleyssin, Barisey (Rural)

Lyon Croix-Rousse (Urban)

on oocyst shedding, Dabritz *et al.* [51, 52] estimated that 0.04 infections could occur per cat-

Incidences estimates may be combined to local cat densities, in order determine the number of infection that could occur each year in a given site. In urban sites, even if incidence is low, very high densities of cats lead to expect a high number of infections: 165 infections per km² per year could occur in the dense population studied by Afonso *et al.* [41, 50]. In rural sites where cats live in density varying between 120 and 200 cats per km², [32] estimated that 31 to 72 infections per km² per year could occur. In Kerguelen, where incidence is high but density is only 1-3 cats/km2, the number of new cases per year would be only around 1/km2/year. Based on the assumption that primary infected cats shed between 1 and 50 millions of potentially infectious oocysts in the environment, oocyst burden may be estimated in each case, as was proposed for rural populations [32]. The results of the cited studies are summarized in Table 1, to give a range of possible estimates for oocyst burden. This may be compared to the estimate from a recent study on owned cats living in coastal California: [51] estimated that the annual burden of oocysts in the environment ranged

The urban-rural-wild gradient is thus a key determinant of the *T. gondii* dynamics. The general level of transmission varies along this gradient, rural areas being particularly favourable for *T. gondii* transmission. Moreover, the relative importance of different transmission routes is not equivalent along this gradient. In particular, the DH-environment cycle may become significant at very low levels of predation rate, especially in urban areas. These variations are expected to influence the risk for other target species, and especially for people, to get infected. In particular, generally speaking, the level of soil contamination is expected to be highest in the areas where urban feral cats are concentrated, and lowest in the wild environment. However, with each environment, spatial and temporal heterogeneities

are present. They will be detailed in the following paragraphs.

(Non-anthropized)

Seroprevalence in cats (%) 36.2 – 55.0 47.4 – 55.1 18.6

of new infections/cat/year) 0.28 – 0.65 0.26 – 0.39 0.17

infections/km2 0.66 – 1.3 31 - 72 165

deposited /year/m2) 17 - 33 775 - 1800 4125

environments. The table summarizes the studies of one population in a non-anthropized island (2 study sites) [46], three rural populations [32] and one urban population [41]. Oocyst burdens are estimated

**Table 1.** Estimated levels of contamination by oocysts in five populations located in different

Population Kerguelen

considering that an infected cat produces 25 millions oocysts.

year in cats recruited through local veterinarians in coastal cities in California (USA).

between 94 and 4671 oocysts/m².

Incidence in cats (number

Number of new cat

Oocyst burden (number

The cat serological prevalences predicted by [10] agree with values observed along the urban rural gradient: when natural populations (as opposed to heterogeneous samples constituted from veterinary clinics or facilities) are considered, seroprevalences are clearly lower in urban (between 15 % to 26% [41, 33, 34, 42, 43]) than in rural areas (48% to 87.3% [44, 30, 45, 32, 42]). They also reach high values in non–anthropized areas: 51% in Kerguelen island [46]. In rodents, prevalence is generally low (0 – 10% [47, 46]), which renders comparisons difficult. High seroprevalences have been occasionally reported in brown rats (70% in Italy [48]) and in house mice (59% in rural and sub-urban areas in England [49]). However, these limited data do not permit to draw a clear pattern among environments in rodents. Interestingly, the usually low rodent seroprevalences are in accordance with predictions of the model [10]. The model also suggested that cat seroprevalence is less dependent on prey seroprevalence than on predation rates and prey availability. Thus obtaining accurate estimate of these two last parameters should be more important to understand *T. gondii* epidemiology than estimating rodent seroprevalence.

**Figure 3.** *Contributions of the DH-Environment and IH-DH cycles in the basic reproductive rate R0 of T. gondii according to the predation rate of IHs by DHs. Predation rates below 27 prey/cat per year represent urban areas, values above 21 prey/cat per year represent rural areas. Modified with permission from* [10].

The last way to compare environments would be to compare the levels of soil contamination among environments. However, estimating the level of environmental contamination requires information on the number of new infections in cats (incidence) through longitudinal studies. Based on serological follow-up of cats, incidence was estimated to 0.26- 0.39 infections/cat per year in three rural populations located in France [32]. Incidence was also estimated in one urban site (0.17 infections/cat/year [50]) and in one population living in a non-anthropized environment [46], using the age-seroprevalence relationship. Using data on oocyst shedding, Dabritz *et al.* [51, 52] estimated that 0.04 infections could occur per catyear in cats recruited through local veterinarians in coastal cities in California (USA).

8 Toxoplasmosis – Recent Advances

compared to the magnitude of cat seroprevalences.

the predicted prey seroprevalences, from 2.4 to 5 % along the gradient, was always low

The cat serological prevalences predicted by [10] agree with values observed along the urban rural gradient: when natural populations (as opposed to heterogeneous samples constituted from veterinary clinics or facilities) are considered, seroprevalences are clearly lower in urban (between 15 % to 26% [41, 33, 34, 42, 43]) than in rural areas (48% to 87.3% [44, 30, 45, 32, 42]). They also reach high values in non–anthropized areas: 51% in Kerguelen island [46]. In rodents, prevalence is generally low (0 – 10% [47, 46]), which renders comparisons difficult. High seroprevalences have been occasionally reported in brown rats (70% in Italy [48]) and in house mice (59% in rural and sub-urban areas in England [49]). However, these limited data do not permit to draw a clear pattern among environments in rodents. Interestingly, the usually low rodent seroprevalences are in accordance with predictions of the model [10]. The model also suggested that cat seroprevalence is less dependent on prey seroprevalence than on predation rates and prey availability. Thus obtaining accurate estimate of these two last parameters should be more important to

understand *T. gondii* epidemiology than estimating rodent seroprevalence.

**Figure 3.** *Contributions of the DH-Environment and IH-DH cycles in the basic reproductive rate R0 of T. gondii according to the predation rate of IHs by DHs. Predation rates below 27 prey/cat per year represent urban areas,* 

The last way to compare environments would be to compare the levels of soil contamination among environments. However, estimating the level of environmental contamination requires information on the number of new infections in cats (incidence) through longitudinal studies. Based on serological follow-up of cats, incidence was estimated to 0.26- 0.39 infections/cat per year in three rural populations located in France [32]. Incidence was also estimated in one urban site (0.17 infections/cat/year [50]) and in one population living in a non-anthropized environment [46], using the age-seroprevalence relationship. Using data

*values above 21 prey/cat per year represent rural areas. Modified with permission from* [10].

Incidences estimates may be combined to local cat densities, in order determine the number of infection that could occur each year in a given site. In urban sites, even if incidence is low, very high densities of cats lead to expect a high number of infections: 165 infections per km² per year could occur in the dense population studied by Afonso *et al.* [41, 50]. In rural sites where cats live in density varying between 120 and 200 cats per km², [32] estimated that 31 to 72 infections per km² per year could occur. In Kerguelen, where incidence is high but density is only 1-3 cats/km2, the number of new cases per year would be only around 1/km2/year. Based on the assumption that primary infected cats shed between 1 and 50 millions of potentially infectious oocysts in the environment, oocyst burden may be estimated in each case, as was proposed for rural populations [32]. The results of the cited studies are summarized in Table 1, to give a range of possible estimates for oocyst burden. This may be compared to the estimate from a recent study on owned cats living in coastal California: [51] estimated that the annual burden of oocysts in the environment ranged between 94 and 4671 oocysts/m².

The urban-rural-wild gradient is thus a key determinant of the *T. gondii* dynamics. The general level of transmission varies along this gradient, rural areas being particularly favourable for *T. gondii* transmission. Moreover, the relative importance of different transmission routes is not equivalent along this gradient. In particular, the DH-environment cycle may become significant at very low levels of predation rate, especially in urban areas. These variations are expected to influence the risk for other target species, and especially for people, to get infected. In particular, generally speaking, the level of soil contamination is expected to be highest in the areas where urban feral cats are concentrated, and lowest in the wild environment. However, with each environment, spatial and temporal heterogeneities are present. They will be detailed in the following paragraphs.


**Table 1.** Estimated levels of contamination by oocysts in five populations located in different environments. The table summarizes the studies of one population in a non-anthropized island (2 study sites) [46], three rural populations [32] and one urban population [41]. Oocyst burdens are estimated considering that an infected cat produces 25 millions oocysts.

#### **3. Urban toxoplasmosis**

Urban landscapes are characterized by highly fragmented natural or semi-natural habitats resulting in a mosaic of patches varying in size and quality. Dispersal abilities of animal species between patches are generally affected by roads or by distance to the nearest patches [53, 54]. Many animal species are thus restricted to parks, artificial forest fragments or recreation areas [55]. This results in local extinctions, increased local population density, or social disturbance. Natural populations of domestic cats are present in urban areas in various sites including hospital gardens [56], parks [57, 58], cemeteries or squares [59], taking advantage of the abundance of shelters, food wastes linked to human activity or food provided by cat lovers [59, 17, 54]. Densities regularly exceed 250 cats per km² [16], and can reach up to 2000 cats per km² like in urban parks in Italy [58].

The Life Cycle of *Toxoplasma gondii* in the Natural Environment 11

area, only a few humans are likely to be directly exposed. These persons include children playing in sand pits, persons feeding the cats, gardeners, maintenance workers in these sites and also dog owners who allow pets to roam in these sites and become indirectly exposed

Overall, toxoplasmosis in urban areas should be characterized by heterogeneous dynamics, with usually low levels of prevalence in cats, but locally high levels of soil contamination,

Rural areas, and in particular agricultural landscapes, are suitable for *T. gondii* transmission, due to the high densities of both DHs and IHs [72], and to the high level of predation. However, this does not mean that rural areas are evenly infected. Spatial and temporal variations have been detected at several scales. We first present the temporal dynamics to

A temporal variability in the dynamics of *T. gondii* life cycle has been detected, both at the year-to-year level and between seasons. It is first important to notice that temporal variability is uneasy to study using serological data, because of the lifelong persistence of antibodies. In long-lived species, temporal variations in the rate of appearance of new cases (incidence) may be masked by the persistence of antibodies. The easiest ways to study temporal dynamics of *T. gondii* should be to consider short-lived species, species where antibody response does not persist lifelong, individual serological follow-up, or to consider

indicators of acute infection, *i.e*., type M immunoglobulins or oocyst excretion in cats.

Due to the difficulty to organize long-term surveys, year-to-year variations have been found in a few populations only: in roe deer *Capreolus capreolus* in Spain [73] and in Sweden [74], in red deer *Cervus elaphus* in Scotland [75], as well as in Canadian seals [76]. Tizard *et al.* [77] performed the largest survey to our knowledge, with nearly 12,000 persons studied over 14 years. This survey revealed inter-annual 6-year cycles and showed that year-to-year variations follow rainfall levels with a correlation coefficient as strong as 0.71. Accordingly, a longitudinal survey of rural populations of domestic cats in France showed important interannual variations in incidence among years, related to variations in the level of rainfall [32]. In an urban site, seroprevalence in cats was highest during years with a hot and moist weather or with a moderate and less moist weather [41]. The same trend was observed during a long-term follow-up of two populations of roe deer, with maximal seroprevalence

The first explanation that has been proposed for the correlation between meteorological conditions and *T. gondii* dynamics involves the survival of oocysts. The free stage of *T. gondii* is subject to hard environmental conditions: in the terrestrial environment, its survival in soil depends on temperature and moisture. Oocyst survival is maximal (> 200 days) for

identify mechanisms of heterogeneity that may also explain spatial variations.

through contact with dogs [71, 67, 68].

**4.1. Temporal dynamics** 

under cold/dry, or cool/moist years [78].

which may favour the environment-DH cycle.

**4. Heterogeneity in the rural environment** 

In urban areas, rodent densities are heterogeneous and generally strongly related to vegetation cover, predation pressure [29], and/or on how the presence of rodents is controlled by trapping or poisoning. For example, the density of field mouse *Apodemus sp*. can range from no individual in areas occupied by dense populations of predators, to 20,000 individuals per km² in isolated patches [28, 29]. Communities of small mammals can persist at high density in small habitat patches sparsely settled by predators [28]. It is therefore unlikely that cat and IHs populations coexist at high densities in the same habitat patches. In addition, urban cats are attracted by food provided by humans, easily accessible all over the year, and that requires no effort of predation. The presence of such a resource can reduce the motivation to hunt in cats [16, 59]. Observations made on urban cats in hunting activity are thus rare in such areas [41]. The altered predator-prey dynamics limits IH-HD *T. gondii* transmission, however toxoplasmosis does occur in urban hosts. Most surveys conducted in stray cats show low prevalences ranging from 5 to 20% [60, 61, 62, 41]. However, high values have occasionally been found: 35.4% in Sao Paolo [62], 51.9 Barcelona [63], 70.2 in Ghent, Belgium [64]. These cases may correspond to areas where cats have access to predation.

The high local densities of cats also entail a high local level of environmental contamination by *T. gondii* oocysts. Beside density, in such areas, cats often use the same place to defecate where they burry or expose their faeces as scent marks [16, 65], and a single location may be used by several cats when cat density is high [66]. Moreover, this behaviour is expected to favour the direct contamination of cats by oocysts while defecating, since oocyst load in defecating areas is extremely high, and cats are exposed through scratching the soil, before cleaning their paws and fur. These defecation sites spread over cat territory cumulate a high concentration of oocysts in areas closed to humans. A study of a cat population living in the Croix-Rousse hospital (Lyon, France) showed that defecation sites were the areas most often found to be positive for *T. gondii* DNA, and may be viewed as hot spots of environmental risk to humans [50]. Similarly, in Poland and in China, contaminated soil samples have been found in public parks and sand pits [67, 68]. Contact with soil, and particularly gardening and consumption of raw vegetables have been demonstrated to be significant risk factors for toxoplasmosis in humans [69, 7, 70]. Contact with defecation sites is thus expected to result in a high risk of infection, but, because contaminated sites represent a low proportion of the area, only a few humans are likely to be directly exposed. These persons include children playing in sand pits, persons feeding the cats, gardeners, maintenance workers in these sites and also dog owners who allow pets to roam in these sites and become indirectly exposed through contact with dogs [71, 67, 68].

Overall, toxoplasmosis in urban areas should be characterized by heterogeneous dynamics, with usually low levels of prevalence in cats, but locally high levels of soil contamination, which may favour the environment-DH cycle.
