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Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Lassa fever is a viral haemorrhagic fever caused by an arenavirus. The disease is endemic in West African countries, including Guinea. The rodents Mastomys natalensis and Mastomys erythroleucus have been identified as Lassa virus reservoirs in Guinea. In the absence of a vaccine, rodent control and human behavioural changes are the only options to prevent Lassa fever in highly endemic areas. We performed a 4 year intervention based on chemical rodent control, utilizing anticoagulant rodenticides in 3 villages and evaluating the rodent abundance before and after treatment. Three additional villages were investigated as controls. Analyses to assess the effectiveness of the intervention, bait consumption and rodent dynamics were performed. Anthropological investigations accompanied the intervention to integrate local understandings of human-rodent cohabitation and rodent control intervention. Patterns of bait consumption showed a peak at days 5-7 and no consumption at days 28-30. There was no difference between Bromadiolone and Difenacoum bait consumption. The main rodent species found in the houses was M. natalensis. The abundance of M. natalensis, as measured by the trapping success, varied between 3.6 and 16.7% before treatment and decreased significantly to 1-2% after treatment. Individuals in treated villages welcomed the intervention and trapping because mice are generally regarded as a nuisance. Immediate benefits from controlling rodents included protection of food and belongings. Before the intervention, local awareness of Lassa fever was non-existent. Despite their appreciation for the intervention, local individuals noted its limits and the need for complementary actions. Our results demonstrate that chemical treatment provides an effective tool to control local rodent populations and can serve as part of an effective, holistic approach combining rodent trapping, use of local rodenticides, environmental hygiene, house repairs and rodent-proof storage. These actions should be developed in collaboration with local stakeholders and communities.

Norway and black rats (Rattus norvegicus and Rattus rattus) are among the most ubiquitous urban wildlife species and are the source of a number of zoonotic diseases responsible for significant human morbidity and mortality in cities around the world. Rodent ecology is a primary determinant of the dynamics of zoonotic pathogens in rodent populations and the risk of pathogen transmission to people, yet many studies of rat-associated zoonoses do not account for the ecological characteristics of urban rat populations. This hinders the development of an in-depth understanding of the ecology of rat-associated zoonoses, limits comparability among studies, and can lead to erroneous conclusions. We conducted a year-long trapping-removal study to describe the ecological characteristics of urban rat populations in an inner-city neighborhood of Vancouver, Canada. The study focused on factors that might influence the ecology of zoonotic pathogens in these populations and/or our understanding of that ecology. We found that rat population density varied remarkably over short geographical distances, which could explain observed spatial distributions of rat-associated zoonoses and have implications for sampling and data analysis during research and surveillance. Season appeared to influence rat population composition even within the urban environment, which could cause temporal variation in pathogen prevalence. Body mass and bite wounds, which are often used in epidemiologic analyses as simple proxies for age and aggression, were shown to be more complex than previously thought. Finally, we found that factors associated with trapping can determine the size and composition of sampled rat population, and thus influence inferences made about the source population. These findings may help guide future studies of rats and rat-associated zoonoses.

Effective management of rodent pests necessitates efficient population surveillance. Many of the available methods currently used for estimating rodent populations are either costly or time‐intensive. Rodent trapping demands significant resources, while tracking plates (TP) require high technical expertise and weeks to months of dedicated effort to satisfactorily interpret the plates. Here, we propose integrating Machine Learning techniques to evaluate plates with signs of rodent marks and compare their accuracy with that of conventional human‐interpreted plates. We employed the Otsu method to transform plates from RGB color images to grayscale images, highlighting regions of interest. Subsequently, we applied a global threshold to create binary images, assigning values above a globally determined threshold as 1s and others as 0s. The original images were transformed into new versions with 25 small samples, highlighting regions of interest based on the binary images. We used dimensionality reduction methods to identify the fundamental structure of high‐dimensional data and determined the most important patterns of interest on the plates. Among the methods, Principal Component Analysis, Independent Component Analysis, and Legendre Moments methods were used to visualize patterns and conduct exploratory data analysis. The k ‐nearest neighbors, a versatile and intuitive classification method relying on the similarity principle, predicted the feature vector of PCA, ICA, and LM () results. Ultimately, results from PCA and LM compared favorably against the conventional labur‐intensive manual method, thus proffering those in the field of disease ecology a better alternative for conducting timely and cost‐effective rodent surveillance to monitor rodent distribution hotspots during rodent management programs. We propose a novel approach that could significantly enhance the protocols of rodent surveillance programs, particularly in Low‐ and Middle‐Income Countries, where expertise in interpreting TPs may be limited to enhance rodent surveillance evaluation and timely rodent management while contributing to the indirect control of rodent‐borne zoonoses.

Trypanosoma cruzi, the causative agent of Chagas disease, has been detected in mammalian hosts occupying densely populated urban environments. This suggests that the risk of transmission to humans is higher than prevailing estimates, which largely reflect conditions in rural and peri-urban areas. Understanding the risks posed by T. cruzi thus requires further study of transmission pathways in part because triatomines - the primary vectors for T. cruzi - appear to be uncommon or absent in urban landscapes. Here we test the hypothesis that vertical transmission contributes to the prevalence of infection and diversity of T. cruzi in urban reservoirs. We assessed whether embryos of T. cruzi-positive parous female rodents also exhibit evidence of infection. A diagnostic PCR assay detected T. cruzi in 15 out of 66 (22.7%) embryos from Norway rats, black rats, and house mice captured in New Orleans (LA, USA). Genotyping PCR identified the presence of TcI and non-TcI discrete typing units (DTUs) in individual infected embryos, providing evidence of mixed infection. Next-generation sequencing provided additional evidence of mixed infection in individual embryos. Our findings provide additional evidence that vertical transmission can occur in natural populations of reservoir species and demonstrates for the first time that multiple DTUs can transmit from mother to offspring. Our study also demonstrates that vertical transmission can contribute to the prevalence of infection and diversity of T. cruzi in multiple reservoir species occupying urban landscapes where vectors appear to be rare or absent, providing a new baseline for understanding transmission pathways and eco-epidemiological cycling of T.cruzi.

Although leptospirosis is traditionally considered a disease of rural, agricultural and flooded environments, Leptospira spp. are found in a range of habitats and infect numerous host species, with rodents among the most significant reservoirs and vectors. To explore the local ecology of Leptospira spp. in a city experiencing rapid urbanization, we assessed Leptospira prevalence in rodents from three locations in Malaysian Borneo with differing levels of anthropogenic influence: 1) high but stable influence (urban); 2) moderate yet increasing (developing); and 3) low (rural). A total of 116 urban, 122 developing and 78 rural rodents were sampled, with the majority of individuals assigned to either the Rattus rattus lineage R3 (n = 165) or Sundamys muelleri (n = 100). Leptospira spp. DNA was detected in 31.6% of all rodents, with more urban rodents positive (44.8%), than developing (32.0%) or rural rodents (28.1%), and these differences were statistically significant. The majority of positive samples were identified by sequence comparison to belong to known human pathogens L. interrogans (n = 57) and L. borgpetersenii (n = 38). Statistical analyses revealed that both Leptospira species occurred more commonly at sites with higher anthropogenic influence, particularly those with a combination of commercial and residential activity, while L. interrogans infection was also associated with low forest cover, and L. borgpetersenii was more likely to be identified at sites without natural bodies of water. This study suggests that some features associated with urbanization may promote the circulation of Leptospira spp., resulting in a potential public health risk in cities that may be substantially underestimated.

Schistosomiasis is a neglected tropical disease of public health significance. In view of its elimination as a public health problem by 2030, adopting a One Health approach is necessary, considering its multidimensional nature. Animal reservoirs, in particular, pose a significant threat to schistosomiasis control in Africa and beyond. In this study, we conducted a spatio-temporal survey of Schistosoma infections in small mammal communities and intermediate snail hosts in the vicinity of Lake Guiers in northern Senegal. Sampling campaigns were undertaken four times between April 2021 and August 2022 around eight villages. A total of 534 small mammals of four species, primarily Hubert's multimammate mice Mastomys huberti, were captured. Out of 498 individuals examined, only 18 rodents (17 M. huberti and 1 Arvicanthis niloticus) were infected with schistosomes. The infection rates in M. huberti varied over time (prevalence range: 2.4% to 9.3%, intensity range: 4 to 132), and space (prevalence range: 3.1% to 40%, intensity range: 2 to 110) and were higher in adult hosts captured during or just after the rainy season, a time when older individuals dominate in rodent populations. Using a multi-locus molecular approach (cox1 and ITS) on Schistosoma larvae (cercariae and miracidia) and adult worms, we identified Schistosoma mansoni as the most widespread species. We also detected Schistosoma bovis and Schistosoma haematobium in M. huberti from one locality (Temeye). Although no Schistosoma hybrids were found, the discovery of a male S. mansoni and a female S. bovis pair raises concerns about potential hybridization patterns that could occur in rodents. Finally, three snail species were found infected (25 Biomphalaria pfeifferi, 3 Bulinus truncatus and 1 Bulinus senegalensis) including with S. mansoni, S. bovis, S. haematobium and S. haematobium x S. bovis hybrids. Our findings highlight the spatial-temporal variations of Schistosoma infections in rodents and emphasize the need for fine-scale monitoring over time and space for effective One Health measures and ensuring the sustainability of schistosomiasis control efforts.Author summary: Schistosomiasis is a neglected tropical disease of major human and veterinary health concern. To ensure its sustainable control, the role of animal reservoirs that could maintain and spread the disease needs a better understanding. We investigated the spatio-temporal variations of Schistosoma infections in small mammal communities around Lake Guiers, northern Senegal. Small mammal species diversity was relatively low at transmission sites. Most infected rodents were found during or just after the rainy season, a time when older individuals dominate in rodent populations. From a spatial perspective, transmission was highly focalized and infection parameters (prevalence, intensity) highly variable, making it challenging to establish clear patterns. Schistosoma mansoni, responsible for human intestinal schistosomiasis, was the most widespread species found. However, sporadic infections with species of human (S. haematobium) and animal significance (S. bovis), including a case of mixed infections (S. mansoni x S. bovis) in rodents raises concerns about hybridization events that may occur in these hosts. Our findings confirm the role of rodents as reservoir or spill-over hosts for Schistosoma in northern Senegal and suggest that fine scale monitoring of animals at transmission sites could help to ensure the sustainability of schistosomiasis control.

Objective: Wild rodents act as important hosts and reservoirs for both zoonotic and non-zoonotic pathogens, playing a key role in maintaining and transmitting infectious agents in nature. Their presence can lead to contamination of food and water sources, affecting both humans and animals. Methods: This study examined 138 dead rodents from six species (Microtus socialis, Rattus norvegicus, Mus musculus, Meriones libycus, Apodemus witherbyi, and Ellobius lutescens) collected from three regions in Iran. Fecal samples were analyzed for Eimeria spp. and Cryptosporidium spp. using potassium dichromate cultivation and sugar flotation for Eimeria, and modified Ziehl-Neelsen staining for Cryptosporidium. Results: The infection rate for Eimeria spp. was 5.79%, and the identified species included E. falciformis, E. papillata, E. miyairii, E. musculoidei, and E. hungaryensis. For Cryptosporidium spp., a 4.34% infection rate was observed. While Eimeria infections were limited to three rodent species, Cryptosporidium was detected in all six. Conclusion: This study presents the first morphological identification of Eimeria species in rodents in Iran, with findings consistent with host-parasite relationships reported globally. Additionally, the widespread presence of Cryptosporidium spp. in multiple rodent species emphasizes the epidemiological importance of these animals as potential reservoirs of zoonotic pathogens. These results contribute to a better understanding of protozoan diversity and distribution in rodent populations of Iran. Keywords: Rodent, Emeria, Cryptosporidium, Iran Amac: Yabani kemirgenler, hem zoonotik hem de zoonotik olmayan patojenler icin onemli konakci ve rezervuarlar olup, dogada enfeksiyoz etkenlerin devamliliginin saglanmasi ve yayilmasinda kilit rol oynamaktadir. Bu canlilarin varligi, hem insan hem de hayvanlar icin gida ve su kaynaklarinin kontaminasyonuna neden olabilmektedir. Yontemler: Bu calismada, Iran'in uc farkli bolgesinden toplanan alti ture ait (Microtus socialis, Rattus norvegicus, Mus musculus, Meriones libycus, Apodemus witherbyi ve Ellobius lutescens) toplam 138 olu kemirgen incelenmistir. Diski ornekleri, Eimeria spp. icin potasyum dikromat inkubasyonu ve seker flotasyon yontemi, Cryptosporidium spp. icin ise modifiye Ziehl-Neelsen boyama yontemi kullanilarak analiz edilmistir. Bulgular: Eimeria spp. enfeksiyon orani %5,79 olarak belirlenmis; tanimlanan turler arasinda E. falciformis, E. papillata, E. miyairii, E. musculoidei ve E. hungaryensis yer almistir. Cryptosporidium spp. icin saptanan enfeksiyon orani ise %4,34'tur. Eimeria enfeksiyonlari yalnizca uc kemirgen turunde gorulurken, Cryptosporidium alti turun tamaminda tespit edilmistir. Sonuc: Bu calisma, Iran'daki kemirgenlerde Eimeria turlerinin ilk morfolojik tanimlamasini sunmakta olup, elde edilen bulgular dunya genelinde bildirilen konak-parazit iliskileri ile uyumludur. Ayrica, Cryptosporidium spp.'nin bircok kemirgen turunde yaygin olarak bulunmasi, bu hayvanlarin zoonotik patojenler icin potansiyel rezervuarlar olarak epidemiyolojik onemini vurgulamaktadir. Elde edilen veriler, Iran'daki kemirgen populasyonlarinda protozoon cesitliligi ve dagiliminin daha iyi anlasilmasina katki saglamaktadir. Anahtar Kelimeler: Kemirgen, Eimeria, Cryptosporidium, Iran

We report serological surveillance for exposure to SARS-CoV-2 in 1,237 wild rodents and other small mammals across Europe. All samples were negative with the possible exception of one. Given the ongoing circulation of this virus in humans and potential host jumps, we suggest such surveillance be continued.