Wild mammals as hosts of Rickettsia: a molecular evidence-based review

3.1.1. Rodents and insectivores

Wild rodents and insectivores can harbor various Rickettsia species, some of which have zoonotic potential. Understanding the role of rodents in the ecology and transmission of Rickettsia is crucial for assessing potential risks to human health.

Several studies have investigated the presence of rickettsial species in rodents and insectivores in Europe, highlighting their potential role as reservoirs and the involvement of ectoparasites in transmission. A study conducted in Saxony, Germany, showed Rickettsia DNA in 28.6 % of rodents, 3.9 % of fleas, and 41.6 % of ticks, by real-time PCR (Obiegala et al., 2016; 2017). Sequencing identified R. helvetica and R. raoultii in rodents, and R. raoultii, R. monacensis, and R. helvetica in ticks. Notably, this was the first detection of R. raoultii in Myodes glareolus and R. helvetica in Apodemus agrarius in Germany. The high prevalence of R. helvetica in small mammals suggests they may play a role in the transmission of the agent to feeding arthropods. Another study carried out by Galfsky et al. (2019) in Saxony, Germany investigated the prevalence of various tick-borne pathogens, including Rickettsia species, in small mammals and ticks in Saxony, Germany. The prevalence of Rickettsia spp. in rodents was found to be 29.1 %.

The study by Fischer et al. (2018) demonstrated screening of 3939 wild small mammals, including rodents and insectivores, for Rickettsia DNA. Rickettsia helvetica was the most prevalent species, detected in 90.9 % of positive samples. Other species identified included R. felis (7.8 %) and R. raoultii (1.3 %). The highest prevalence of Rickettsia DNA was found in rodents of the genus Apodemus, particularly the yellow-necked mouse (A. flavicollis). Additionally, older and male individuals of certain rodent species exhibited higher rates of Rickettsia DNA presence, and small mammals collected in forested habitats during summer and autumn had higher prevalence rates, suggesting that seasonal and habitat-related factors influence Rickettsia prevalence. The findings highlight the importance of considering ecological factors, such as host species, age, sex, and habitat, in understanding the dynamics of Rickettsia transmission.

In a study conducted by Minichová et al. (2017), the presence of Rickettsia spp. DNA was identified in 0.5 % (3/663) of rodent samples, comprising ear biopsies and/or spleen samples, from rodents (A. flavicollis) captured from Slovakia. The identified Rickettsia spp. were R. helvetica and R. monacensis. Concurrent studies in Slovakia have demonstrated the overall prevalence of Rickettsia spp. in examined small mammals (rodents and insectivores) as 11 %, with R. helvetica the most prevalent species and being confirmed in A. agrarius, A. flavicollis, My. glareolus, and M. arvalis. Furthermore, a Rickettsia species closely related to R. felis was identified in A. flavicollis, while R. slovaca was observed in A. flavicollis (Heglasová et al., 2018). The highest occurrence of rickettsiae-positive small mammals was recorded during the spring and autumn months, possibly related to the seasonal activity of tick vectors. The research by Špitalská et al. (2020) provides insights into the prevalence of Rickettsia species and rickettsial endosymbionts in small mammals and their ectoparasites, with R. helvetica identified with the highest frequency in A. agrarius.

A study conducted in south-western Poland identified the presence of Rickettsia DNA in the tissues of wild rodents, including species such as A. agrarius and My. glareolus. The prevalence of infection was approximately 17.6 %, with some samples specifically detecting the presence of R. helvetica and Rickettsia felis-like organisms (Gajda et al., 2017).

In the northern Appenines, Italy, Rickettsia slovaca was identified by PCR in ear biopsies in 8 Apodemus spp. (n = 37 examined), but not in M. glareolus (n = 9). Rodents were infested by D. marginatus larvae and nymphs. Prevalence of R. slovaca was greater in larvae feeding on PCR-positive Apodemus than in those feeding on negative mice, suggesting that heavily infested mice might play a role as amplifiers of the infection (Martello et al., 2013).

In Asia, examined tissues of rodents and insectivores are positive for various Rickettsia species, some of which are zoonotic and can infect humans. A study in Mudanjiang, Heilongjiang Province, detected Candidatus Rickettsia tarasevichiae in 1.29 % of 463 rodents, including Rattus norvegicus and Clethrionomys rufocanus (Yuan et al., 2020). This finding marked the first confirmation of this Rickettsia species in rodents in China. A nationwide survey identified R. raoultii in 2.9 % of 835 wild rodents. This was the first detection of R. raoultii in South Korean rodents and highlights the importance of continuous monitoring to mitigate public health risks (Kim et al., 2024). A comprehensive study in Taiwan documented the capture of 1285 small mammals, comprising 10 rodent species and one shrew species (Kuo et al., 2015). Rattus losea accounted for 54.6 % of rodent captures in a study conducted between 2006 and 2010 in Taiwan. High seropositivity rates were observed for R. conorii or R. rickettsii (94.6 %) and R. typhi (2.3 %). Additionally, DNA of spotted fever group (SFG) and typhus group (TG) Rickettsia species was detected in 41.1 % and 9.4 % of individuals, respectively. Mus caroli, representing 10.6 % of captures, showed seropositivity rates of 48.9 % for R. conorii or R. rickettsii and 5.9 % for R. typhi. SFG and TG Rickettsia DNA were found in 43.2 % and 6.8 % of individuals, respectively. Bandicota indica rodents exhibited seropositivity rates of 88.0 % for R. conorii or R. rickettsii and 0 % for R. typhi. SFG and TG Rickettsia DNA were detected in 41.2 % and 5.9 % of individuals, respectively. The high prevalence of Rickettsia infections in rodents across Taiwan indicates a significant potential for zoonotic transmission. Rodents, especially Rattus species, serve as primary reservoirs for these bacteria, with fleas acting as vectors. Infection with Rickettsia species can lead to murine typhus (R. typhi) and spotted fever group rickettsioses (R. conorii and R. rickettsii). Notably, R. typhi is transmitted by fleas found on rodents and can cause murine (endemic) typhus, characterized by fever, chills, and rash.

A study involving 95 wild rats (R. diardii and R. norvegicus) from Kuala Lumpur and Pulau Pinang in Malaysia detected Rickettsia DNA in 13.7 % of tissue samples (Tay et al., 2014). Sequence analysis revealed 98.6–100 % similarity with R. honei, R. conorii, and R. raoultii.

In the African continent, a study in the Democratic Republic of the Congo found that 13 % of rats (R. rattus and R. norvegicus) were positive for R. typhi, the causative agent of murine typhus (Laudisoit et al., 2014). Fleas (Xenopsylla cheopis) from these rodents also harbored R. typhi in 72 % of pools, highlighting the role of fleas in the transmission of this pathogen. Research across southern Africa and Namibia detected Rickettsia DNA in 15.5 % of 1616 ear-tissue samples from 23 small mammal species (rodents and insectivores) (Essbauer et al., 2018). Four human-pathogenic Rickettsia species were identified: R. conorii, R. massiliae, R. felis, and R. helvetica. Novel species such as Candidatus Rickettsia africaustralis, Candidatus Rickettsia rhabdomydis, and Candidatus Rickettsia muridii were also confirmed. The high prevalence of Rickettsia species in rodents and their fleas across Africa suggests significant public health risks. Rodents and fleas act as reservoirs and vectors, respectively, facilitating the transmission of these pathogens to humans. Infection can lead to diseases such as murine typhus, characterized by fever, chills, and rash. While often mild, severe cases can result in complications affecting organs like the liver, heart, and lungs. The detection of diverse Rickettsia species in rodents and insectivores and their ectoparasites across various African regions underscores the importance of monitoring these populations. Enhanced surveillance and control measures targeting rodents and fleas are essential to mitigate the risk of Rickettsia transmission to humans.

Rickettsia rickettsii is the main etiological agent of severe spotted fever, named with different names across north, central and south America (e.g. Rocky Mountain spotted fever, Brazilian spotted fever), and different tick species are responsible of its transmission to humans. Studies in Latin America show that ticks are rarely infected by R. rickettsii in nature, possibly due to the pathogenic effect of the rickettsia for ticks. Thus, transovarial transmission is not sufficient to maintain it in nature, and amplifier vertebrate hosts are needed to infected new tick lines through horizontal transmission (Labruna, 2009). In Brazil, Amblyomma sculptum and A. aureolatum ticks are the main vectors of R. rickettsii to humans and xenodiagnostic experiments have demonstrated that capybaras (Hydrochoerus hydrochaeris) and opossums (Didelphis aurita) act as amplifier host (Horta et al., 2009; Souza et al., 2009; Ramírez-Hernández et al., 2020a, Ramírez-Hernandez et al., 2020b). In other studies in South America, a wide variety of rodent species were examined, but Rickettsia spp. was found in a very low percentage and only in a few rodent species in Mexico, including R. parkeri identified in Mus musculus (Arroyo-Ramírez et al., 2023; López-Pérez et al., 2022a, López-Pérez et al., 2022b, Table 4).

3.1.2. Bats

Molecular studies have demonstrated the presence of Rickettsia sp. in European bats. In a study conducted in Hungary, 13 out of 221 (5.9 %) bat faecal samples were found to contain rickettsia DNA. These included sequences of a rickettsial insect endosymbiont, a novel Rickettsia genotype, and R. helvetica (Hornok et al., 2018a, Hornok et al., 2018b). Furthermore, a study conducted in Romania revealed a high prevalence of R. monacensis in two species of insectivorous bats, Nyctalus noctula and Pipistrellus pipistrellus (Matei et al., 2021). The study utilised PCR detection techniques, confirming the presence of Rickettsia DNA in 14.6 % of the bat samples. Furthermore, the presence of Rickettsia spp. DNA has been confirmed in bats in Africa. A study conducted across South Africa and Swaziland found diverse Rickettsia spp. in both insectivorous and frugivorous bats, as well as their blood-feeding ectoparasites. Subsequent analysis of the partial gltA sequence data has enabled the identification of bat-associated Rickettsia spp. that are closely related to R. conorii (Dietrich et al., 2016).

3.1.3. Game animals/ungulates

Only a few studies report the occurrence of Rickettsia DNA in tissues of wild living ungulates. Much more work concerns the ticks collected from these animals. In Europe, among game animals, cervids were particularly investigated since they are major hosts for I. ricinus, the most important tick vector in this region. Rickettsia spp. was reported in Dama dama and Capreolus capreolus, with a prevalence between 1.2 % and 3.3 % in Hungary and the Netherlands, respectively (Hornok et al., 2018a, Hornok et al., 2018b; Wijburg et al., 2022). The higher level of R. helvetica infections (6.3 %) was in the blood of Hungarian mouflons (Hornok et al., 2018a, Hornok et al., 2018b).

The interesting results were recorded in Kenya, where in topi (Damaliscus lunatus jimela), Rickettsia DNA was detected in 30 % of blood isolates, whereas in other ungulates the prevalence was an order of magnitude lower, or no DNA of the tested bacterium was detected (Ndeereh et al., 2017).

Wild boars are also suspected as having a role in rickettsial cycle. In Europe, R. slovaca was found in a wild boar skin biopsy in Tuscany, where SENLAT cases occur (Selmi et al., 2009). However, whether wild boars are able to develop a sufficient rickettsemia to infect feeding ticks or simply act as hosts for infected vectors remains unclear. A recent study showed that wild boars do not act as competent amplifying hosts of R. rickettsii for A. sculptum ticks in Brazil. Experimental infection of four wild boars through R. rickettsii – infected A. sculptum did not led to detectable rickettsemia (Neves er al., 2025). A moderate seroconversion was detected, as already shown in other serological studies (Sousa et al., 2024). However, wild boars are deemed important for to the dissemination of infected A. sculptum, especially in areas where competent hosts for R. rickettsii are present (Neves et al., 2023).

3.1.4. Primates

Studies on natural infection of Rickettsia in wild living primates are rare, but interesting. In Africa, in South Luangwa National Park, Zambia, spleen samples derived from 88 free-ranging non-human primates, comprising yellow baboons and vervet monkeys, were studied. Rickettsia DNA was detected in almost 40 % of isolates (Nakayima et al., 2014). Sequencing of selected positive samples revealed 99 % identity with R. africae, a known causative agent of African tick-bite fever. The other study from Central Africa analyzed over 1000 fecal samples from wild chimpanzees, gorillas, and bonobos. Using real-time PCR targeting the gltA gene, Rickettsia DNA was detected in 11 % of samples, with gorillas showing the highest positivity rate at 17.2 %. The sequences matched R. felis, a flea-borne pathogen known to infect humans (Keita et al., 2013). It is worth considering whether it is possible that the DNA was ingested or encountered through environmental contact (e.g., grooming behavior or eating contaminated food). The study conducted in Asia, in Malaysia, examined blood samples from 50 cynomolgus monkeys captured in residential areas. The Rickettsia DNA was confirmed in 24 % of blood samples, indicating that these monkeys were naturally infected.

These finding highlights the potential role of primates in the transmission cycles of Rickettsia species, which may have additional significance in the case of monkeys living near humans (e.g., urban or peridomestic macaques) that could be part of a zoonotic cycle of pathogens.

Research provided in Australia and South America has shown that marsupials do not play a role in the circulation and maintenance of Rickettsia spp. in the environment (de Sousa et al., 2018; de Oliveira et al., 2020; Gofton et al., 2021, Table 3, Table 4).

3.1.5. Carnivores

There is a broad consensus that wild and domestic carnivores constitute a substantial reservoir for tick-borne zoonotic agents transmitted to humans. The dynamics of tick-borne pathogen transmission are contingent upon the presence of diverse species of vertebrates in proximity, and, in such instances, the convergence of ecological niches among species engenders opportunities for parasites to expand their geographic distribution, abundance, and host range. Consequently, numerous newly discovered arthropod-borne pathogens that originated in wildlife have emerged or reemerged (Otranto et al., 2015; André, 2018).

The extant literature on the subject is inconclusive and limited in scope, with a paucity of studies examining the role of the red fox in the epidemiology of rickettsiosis. However, a recent study by Liu et al. (2021) has identified the presence of R. raoultii in the organs of three red foxes, Candidatus Rickettsia barbariae in the organs of a red fox, R. sibirica in the liver of a red fox, and R. raoultii in two tick species (Ixodes canisuga and D. marginatus) from foxes in China. Conversely, no foxes were found to be positive for Rickettsia spp. in Italy (Torina et al., 2013). In the same study, the flea pool that contained the two Ctenocephalides felis specimens also tested positive for R. felis.

Furthermore, the presence of R. raoultii DNA has been identified in feeding I. ricinus ticks removed from wolves in northwestern Spain (Del Cerro et al., 2022) and in Dermacentor reticulatus ticks removed from a migrating golden jackal (C. aureus) in Denmark (Klitgaard et al., 2017). Conversely, a study by Wymazał et al. (2024) revealed the absence of rickettsial DNA in the blood and tissue (muscle) samples of wolves from an expanding population in Eastern Europe (Poland). A similar finding was reported for the North American canid, the red wolf (Canis rufus), which was found to be devoid of Rickettsia DNA in its blood samples (Tyrrell et al., 2020).

On the other hand, a variety of rickettsiae are in circulation among wild canids in South America. Between the years 2005 and 2012, a survey was conducted on maned wolves (C. brachyurus) residing within the Serra da Canastra National Park (SCNP), a Cerrado preservation area located in the south-eastern region of Brazil (Arrais et al., 2021). Despite the absence of PCR-positive blood samples for Rickettsia, serological reactions were identified through the indirect immunofluorescence assay, which tested the serum samples from these wolves against the antigens of R. parkeri, R. rickettsii, R. amblyommatis, R. rhipicephali, and R. bellii. Of the wolves that were tested, 95 % (74/78) exhibited a positive reaction to at least one Rickettsia species, with R. parkeri producing the highest endpoint titers. Furthermore, some maned wolves that were recaptured during the study demonstrated a seroconversion to R. parkeri. Furthermore, molecular analyses conducted on ticks extracted from the aforementioned subjects revealed the presence of Candidatus Rickettsia andeanae and R. parkeri in adult A. tigrinum ticks (Arrais et al., 2021).

A comparable pattern was identified in the case of the foxes examined. A total of 78 C. thous foxes from the Pantanal region of Brazil were sampled between 2013 and 2015 and subjected to serological and molecular testing. The results demonstrated that 75.6 % (59/78) of the animals exhibited seroreactivity to at least one Rickettsia species (R. rickettsii, R. parkeri, R. amblyommatis) in all subjects. Additionally, one crab-eating fox was also PCR-positive for SFG Rickettsia. Of the 1582 ticks removed from the tested foxes, predominantly of the Amblyomma spp. variety, 23.5 % were found to be PCR positive for SFG Rickettsia. Furthermore, 30 samples yielded evidence of Candidatus Rickettsia andeanae (de Sousa et al., 2018). Additionally, the Pampas fox (Chrysocyon brachyurus) and South American grey fox (Lycalopex gymnocercus) from the Brazilian Pampa were examined. It was found that 62.5 % (20/32) of the animals were seropositive for R. parkeri. Furthermore, 7.5 % (22/292) of the ticks collected from the aforementioned foxes were found to contain R. parkeri DNA. However, the pathogenicity of R. parkeri for domestic and wild canids remains unknown (Dall’Agnol et al., 2018). Additionally, DNA of rickettsiae was found in two desert kit foxes in California (Shultz et al., 2023).

In the study by Santoro et al. (2017), the authors presented evidence of Rickettsia pathogen infection in the Eurasian otter (Lutra lutra) from the southern Italian population for the first time. Despite the limited number of carcasses examined, the researchers employed molecular methods to demonstrate that Eurasian otters can be naturally infected by Rickettsia sp. A molecular test was employed to detect Rickettsia-specific for R. conorii and R. rickettsii, yet the test was unable to distinguish between the two species.

It has been hypothesized that coyotes residing in the USA may be involved in the dissemination of R. rickettsii. A study was conducted on coyotes from Arizona, USA, and the results revealed the presence of R. rickettsii antigen. Furthermore, polymerase chain reaction (PCR) analyses of skin samples from the same animals demonstrated evidence of Rickettsia spp. in 2.9 % (4/138) of samples, despite the inability to identify the specific rickettsial species (Green et al., 2020). Recently, R. rickettsii has been identified in the blood of a coyote from the US–Mexico transboundary region, confirming the susceptibility of this wild canid to the infection (López-Pérez et al., 2022a, López-Pérez et al., 2022b).

Research has demonstrated that wild hyenas, encompassing both spotted and brown varieties, have also been found to carry Rickettsia species. A study conducted in Namibia and Tanzania identified R. raoultii DNA in hyena (Parahyaena brunnea and Crocuta crocuta) blood samples (Krücken et al., 2021).

Furthermore, Szekeres et al. (2019) reported the detection of Rickettsia DNA in the muscle tissues of the European lesser weasel (Mustela nivalis) and R. helvetica in the stone marten (Martes foina). Additionally, procyonids have been shown to play a significant role in the epidemiology of rickettsial agents. For instance, serological evidence of exposure to R. rickettsii, R. montana, R. parkeri, and R. bellii (369-C strain) has been documented in raccoons in the USA (Castellaw et al., 2011; Rainwater et al., 2017; Tufts et al., 2020). Furthermore, molecular identification of other Rickettsia species has been documented in skin fragments from introduced raccoons in Germany and Poland (Hildebrand et al., 2022). These included R. monacensis, R. raoultii, Candidatus Rickettsia kotlanii-like, R. helvetica, and Rickettsia sp. endosymbionts, which were also identified in spleen tissue. Samples from feral raccoons introduced into Japan yielded the following Rickettsia species: R. japonica, R. felis, R. heliongjiangensis/R. japonica, R. amblyommi, R. helvetica, and Rickettsia sp. strain Hj126 (Sashika et al., 2010; Baba et al., 2012). In Brazil, Rickettsia amblyommatis and R. bellii were identified in Amblyomma coelebs and A. ovale, respectively, obtained from coatis in Iguaçu National Park (Paraná state) (Magalhães-Matos et al., 2021).