In Nigeria, very few reliable studies have been carried out and have reported the helminth fauna, blood parasites, and ectoparasites of small mammals with observed variations in parasite profile and prevalence by location [3,5–8]. For instance, in Ibadan (South-West), Trichostrongylus columbriformis was reportedly the commonest nematode, while Hymenolepis diminuta was the only cestode recovered in Rattus rattus [9]. In Abeokuta (South-West), 64.7% of rodents were infected with one or more of the 7 species of recovered helminths [8]. Rodents trapped in the Niger Delta (South-South) had higher infection rates with nematodes than cestodes and trematodes [5]. Another study in Plateau (North-Central) demonstrates a high prevalence (83%) of blood parasites in rodents [3]. The only study in Ekpoma, and effectively in Edo state, reported 17 helminth parasites in 8 species of small mammals [8]; and currently due to urbanization and climate change, it is believed that some changes in the small mammals’ parasite’s composition, distribution, and rate of infection may have occurred [10]. Additionally, for the first time, information regarding parasites of small mammals trapped in houses located in other major towns in Edo is presented. Intrinsic factors like age and sex of these mammals were also considered as possible determinants of the rate and burden of infection [10]. Furthermore, this study identifies Ekpoma as a potential hotspot for zoonotic parasites.

The vegetation cover in Nigeria is majorly rainforest in the south and savannah in the north. Among the states in Nigeria, Edo has these 2 vegetation types: savannah in the northern part and rainforest in the south. Small mammals in these 2 ecological zones were thus sampled in areas with these vegetation types being extrapolative for Nigeria particularly in locations with similar climatic, sanitary and housing conditions. Edo (6°30’N 6°00’E) is an inland state in southern Nigeria. It has a tropical climate with 2 seasons: wet (April to October) and dry (November to March). The average rainfall is between 150 mm and 250 mm. The 4 major towns of the state are Auchi, Benin, Ekpoma, and Uromi.

Auchi (5°54’N 5°40’E) located in the northern part of the state has a sub-humid tropical climate. The vegetation type is savannah and it is characterized by open grassland and few scattered fire-resistant trees. Topography is relatively undulating sloping from north to south with a sandy soil type. The human population size approximates to 142,819 with a landmass of 358 km². Benin (6°20’N 5°37’E) is the capital city of Edo with human population size of 1,147,188 and land mass of 1,204 km². It has a humid climate located in a rainforest zone. Ekpoma (6°45’N 6º08’E) houses the state university (Ambrose Alli University) and has a population size of above 80,000 with landmass of 62 km². Uromi (6°71’N 6°32’E) has a landmass of 60 km² with population size of less than 65,000.

Before collection of the animals, approval (AAUBREC: 16/008) to trap and sacrifice was obtained from the Ambrose Alli Bioethics Research Committee. Trapping was done for a period of 6 months between April and September 2016. These small mammals were collected using sticky traps baited with pieces of food items, e.g. smoked fish, and positioned within and around human residences. The total number of housing units where small mammals were sampled were 331 across the surveyed areas, Auchi (n=38), Benin (n=110), Ekpoma (n=112), and Uromi (n=71), at night, and the trap was left till the morning of the next day. Most houses in Nigeria have perimeter fences and both are built with cement and mortar; so we ensured that traps placed outdoor were restricted to the fenced areas. Two to 3 sticky traps were positioned indoors and outdoors in each residential building for 2 to 3 days and checked at intervals (between 5 and 6 hr). When catches were made, these mammals were immediately transported to the laboratory for processing.

These small mammals were carefully removed from the sticky traps alive and further immobilized using chloroform. Already dead small mammals on traps were not processed and examined for parasites. Information regarding location (indoor or outdoor) of catches including home and street addresses was noted. Samples were correctly identified. Also, the sex of the small mammals were recorded while the age classes (adults, sub-adults, and juveniles) were equally determined and documented using the indices of the tail length, body length, and body weight [11,12]. After dissection, blood from the heart was collected using needle syringes. Also, capillary tubes were used to collect blood from the retro-orbital plexus. For parasite detection and identification, thick and thin blood smears were respectively made on slides and then stained with Giemsa solution and allowed to dry. Babesia (sometimes stained in circular or tetrad forms) and Plasmodium (sometimes seen as thick dots or shaped in a ring form) in the red blood cells were identified [13,14]. Meanwhile, blood collected in the heparinized capillary tubes was centrifuged, and the buffy coat layer was separated and examined for the detection as well as identification of trypanosomes [15].

For gastrointestinal parasite investigation, the stomach was dissected and contents emptied; while the feces in the intestines were removed into standing tubes containing sucrose solution on which ova had to float. Cover slips were placed on the standing tubes, and floating helminth ova were allowed to stick to the surface. Parasites were identified using their distinct morphological features [16]. The mean intensity (MI) of ova were estimated and recorded. In addition, the small mammals were decapitated, and the skin removed to expose the skull so as to access portions of the brain by cutting through it. About 1–2 mm sized portion of the cerebrum was separated and squashed between 2 slides and examined for the presence of any brain parasites. Cysts isolated were compared with the features of already described T. gondii cysts from a mouse brain [17]; and features were similar as there was a thin cyst wall containing clusters of bradyzoite cells that are largely shaped in comma form. Carcasses were properly disposed according to Ambrose Alli University animal research ethics guidelines. Gastrointestinal and brain parasites (helminths) were identified under the microscope (AXL [3180976] Labo Germany) using ×40 magnification, while blood parasites were viewed using ×100 magnification.

The Simpson’s index of diversity (SID) formula [18] was applied to the data. Parasite prevalence (number of species of small mammal positive for respective parasite by location/total number of species of small mammal examined for parasitic infection by location ×100) in trapped small mammals were estimated (see additional file). Also, infection rates (number of Mus musculus or/and R. rattus positive for helminth or protozoan parasites by location/total number of M. musculus or/and R. rattus examined for helminth or protozoan parasites by location) for M. musculus and R. rattus with respective parasites are presented in Table 1. Mean differences relating to infection rates and intensity by sex and age for helminth and protozoan parasites were evaluated using student t-test and 1-way ANOVA with GraphPad Prism 5 version 5.01. Results were considered significant at P <0.05.

A total of 502 small mammals were trapped. Overall, catches around house premises included; 3 (0.59%) Apodemus sp., 1 (0.19%) Crocidura sp., 7 (1.3%) Mastomys natalensis, 361 (71.9%) M. musculus, 22 (4.4%) Rattus norvegicus, 101 (20.1%) R. rattus, and 7 (1.4%) Sorex sp. (see additional file). M. musculus, R. rattus, R. norvegicus, and M. natalensis were seen in all locations, while Sorex sp. were trapped in Benin, Ekpoma, and Uromi (see additional file). Apodemus sp. were caught in Benin and Uromi, while Crocidura sp. was trapped in Ekpoma only (see additional file). Species diversity was generally low across locations with Ekpoma higher than others (see additional file). In all, more males were sampled than females and more adults than other age classes.

Cumulatively, there were 12 helminth taxa, namely, Angiostrongylus sp. (larvae), Aspicularis sp. (ova), Capillaria sp. (ova), Gongylonema sp. (ova), Heterakis spumosa (ova), H. diminuta (ova), H. nana (ova), Mastophorus muris (ova), Moniliformis moniliformis (larvae), Nippostrongylus brasiliensis (ova), Strongyloides sp. (ova), Trichosomoides sp. (ova), and Trichuris sp. (ova), identified with at least a helminth taxon recovered in one or more small mammals. The most occurring helminth parasites were N. brasiliensis and Strongyloides sp. with relatively high infection rates (Table 1). Very low mean infection rates were recorded for Capillaria sp. (Auchi), Angiostrongylus sp., H. diminuta, and Trichosomoides sp. in M. musculus from Ekpoma (Table 1). R. rattus (n=1) from Ekpoma was positive for M. moniliformis, while ova of Gongylonema sp. were recovered in R. norvegicus sampled in Benin and Ekpoma (see additional file). In all, the infection rate for R. rattus (0.036±0.009) was higher than M. musculus (0.023±0.006) but mean difference was not significant (t=1.17; P >0.05). Also, Auchi recorded the lowest mean infection rate in relation to other locations but with marginal mean difference (F=0.35; P >0.05) (Table 1). In addition, Strongyloides sp. and H. spumosa were the most prevalent with relatively high mean intensity (MI) (Table 2).

The prevalence of M. musculus and R. rattus with 2 or more helminth taxa against the total number of helminth-positive small mammals per location was: 4 (26.7%) and 1 (33.3%) in Auchi, 18 (46.2%) and 8 (53.3%) in Benin, 13 (31.0%) and 11 (68.8%) in Ekpoma and 10 (55.6%) and 5 (71.4%) in Uromi, respectively. The number of R. norvegicus in which 2 or more helminth parasites were isolated was: Benin (n=3), Ekpoma (n=8), and Uromi (n=7). In Ekpoma, M. natalensis (n=1) and Sorex sp. (n=1) were positive of 2 helminth taxa with similar result for Apodemus sp. (n=1) (Uromi=data not presented).

The protozoan parasites detected were; blood parasites (Babesia sp., Trypanosoma lewisi, and Plasmodium sp.), intestinal parasites (oocysts of Eimeria sp. and Isospora sp.), and brain parasites (cysts of Toxoplasma gondii). Of significance was the presence of Plasmodium sp. in more than a half of the number of rodents examined and the detection of Babesia sp. in all surveyed sites (Tables 1, 2). T. lewisi was seen only in Ekpoma (university hostel) and Benin (Sapele road) foci (Table 1). Differences in the rates of helminth infections for M. musculus (0.149±0.040) and R. rattus (0.142±0.040) including across locations were not significant (P >0.05) (Table 1).

Data analysis indicated null effect of sex on helminth infection rate in M. musculus (male=0.023±0.007; female=0.015±0.010; t=0.67, P =0.50), but marked difference for R. rattus (male=0.204±0.008; female=0.061±0.019; t=2.45, P=0.04). Generally, adults had higher infection rates than sub-adults and juveniles, but mean differences were not significant for M. musculus (adults=0.025±0.012; sub-adults=0.016±0.005; juveniles= 0.0127±0.004; F=0.54, P =0.58) and R. rattus (adults =0.047±0.016; sub-adults=0.035±0.017; juveniles=0.018±0.011; F=1.02, P =0.37).

The parasite load for 6 helminths (Aspicularis sp., H. spumosa, M. muris, Trichuris sp., Strongyloides sp., and N. brasiliensis) having ≥7 positive samples in M. musculus and R. rattus was pooled to estimate the parasite MI. The differences in the MI relating to sex for M. musculus (male=427.3±195.7; female= 244.3±136.5; t=0.76; P =0.45) and R. rattus (male= 91.6±36.6; female=70.8±42.1) were not significant (t=0.37, P =0.71). With regards to parasite load and age of M. musculus, adults had higher MI (544.4±212.4) than sub-adults (238.9±88.5) and juveniles (65.8±35.6) (F=9.27; P =0.009). For R. rattus, sub-adults (95.8±449.2) had higher MI than adults (42.2±23.2) and juveniles (9.2±9.2) (F=8.088; P =0.017). Overall, the MI for M. musculus (386.5±177.2) was higher than R. rattus (50.5±9.1) (t=2.62, P =0.043).

Considering mean infection rate with protozoan parasites, sexes were of null effect for M. musculus (male=0.023±0.006; female=0.036±0.009; t=0.75, P=0.314) and R. rattus (male= 0.172±0.041; female=0.087±0.041; t=0.82, P =0.426). Similarly, there was no significant difference across age classes for M. musculus (adults=0.164±0.092; sub-adults=0.1873±0.126; juveniles=0.183±0.147; F=0.0093, P =0.99); while for R. rattus, sub-adults had higher infection rate than other age classes (adults=0.072±0.051; sub-adults=0.154±0.067; juveniles= 0.082±0.052) with marginal mean difference (F=0.62, P =0.54).

Small mammals especially rodents are seen in most homes in Nigeria and are known to facilitate the deterioration of household items. So, the importance of delineating the spectrum of rodents in a given location as well as determining its rate of occurrence cannot be overstated as they would guide on the appropriate trapping methods for optimum results. Data presented here have generally shown that small mammals in sampled locations around and within residential buildings are primarily M. musculus followed by R. rattus and R. norvegicus.

Overall, across study locations, a total of 12 helminth taxa were recovered in the sampled small mammals. In Auchi, helminth diversity was relatively low. The results of parasites diversity in Auchi in comparison to Benin, Ekpoma, and Uromi could attest to the variation in ecological conditions and better sanitary conditions in Auchi (personal observations). In contrast, for Ekpoma, we observed that waste disposal is relatively a big challenge as the town is littered with piles of household waste. More so, due to increasing population size, sanitary conditions have worsened with dilapidated roads and poor drainage systems. However, it is important to add that previous report from Ekpoma indicated greater number of helminth parasites in small mammals; and this is largely attributable to survey efforts covering wider sampling areas [7].

Aside from external conditions, some intrinsic factors like sex and age of small mammals have been advanced to influence the occurrence of infection with helminths [7,19]. We only analyzed data for M. musculus and R. rattus because of a relatively large sample size. Results for M. musculus showed null effect of sex and age on infection rate but higher helminth-parasite load for adults than other age groups. However, for R. rattus, the rate of helminth infections in male was higher than female; and also, sub-adults had greater MI than other age classes. This suggests that the sub-adults are more active in foraging for food materials and thus more exposed to helminth infections whilst still immunologically naive [7]. Furthermore, males are considered to have weaker immune responses than females and hence less resistant to helminth parasites [20].

The helminth community profiled in M. musculus and R. rattus broadly places Strongyloides sp. and H. spumosa as the most occurring with the highest helminth-parasite burden. Strongyloides sp. have unusual life cycle and the relatively high prevalence may be partly due to suitable ecological conditions for the development of larvae into infective larvae stage [21].

In Ekpoma, 4 potentially zoonotic helminth parasites (Angiostrongylus sp., H. diminuta, M. moniliformis, and Gongylonema sp.) were isolated. The larvae of Angiostrongylus sp. were recovered in the brain tissue of M. musculus. Generally, snails are the primary intermediate hosts but in particular, only Achatina fulica, an edible land snail has been identified in Edo to harbour Angiostrongylus sp. [22]. Humans may become infected through ingestion of larvae and could develop eosinophilic meningitis [23]. M. musculus, R. norvegicus and Sorex sp. were positive for H. diminuta. The presence of positive rodents with H. diminuta is a major risk factor to human health [24]. M. moniliformis was another intestinal parasite seen in a rodent (R. rattus); and its presence could potentially be of public health importance [25]. In addition, Gongylonema sp. was isolated in R. norvegicus from Ekpoma and Benin. Human infection with Gongylonema sp. occurs mostly through intake of contaminated water with symptom of discomfort in the mouth, lips, and in the soft palate area [26]. Capillaria sp. ova were recovered in the intestine of M. musculus in Auchi. Capillaria hepatica is known to potentially cause human infections [27].

Oocysts of Eimeria and Isospora in the intestinal feces were differentiated by the presence or absence of micropyle cap on its respective surfaces. The prevalence and MI of Eimeria and Isospora in rodents in all study locations indicates their heavy presence in the environment. This clearly suggests that the animals in these locations may be heavily burdened with Isospora and Eimeria parasites. Infection with Isospora could lead to transient diarrhoea, colic, weight loss and fever in dogs, cats, pigs, and man; while for Eimeria, diarrhoea is the major clinical manifestation in goats, sheep, cattle, and birds. Also, the cyst of T. gondii was seen only in the brain sample of rodents (M. musculus [n=1] and R. ratttus [n=1]) caught in Ekpoma, suggesting that this area could be potentially endemic for the parasite. Pregnant women and immune-compromised persons like HIV positive persons are most susceptible because they could suffer complications arising from T. gondii infection [28].

The findings of Babesia and Plasmodim in rodents in the 4 study locations suggested active transmission of these parasites by ticks and mosquitoes, respectively. The prevalence of Babesia among humans is not known in these areas. Since there is regular man-dog contact, there is the possibility of human infection with Babesia albeit very low. Because diagnosis of blood samples for Babesia is not a routine procedure, transfusion of infected blood to others is likely. More than half of the trapped rodents were positive of Plasmodium and to a certain degree could be a reflection of the incidence level of human malaria. The pathology of infection with Plasmodium berghei can be comparable to infection with P. falciparum [29].

Our result shows that of the 12 rodents (M. musculus and R. rattus) caught in a particular location in Ekpoma (university hostel), nine were positive of T. lewisi. Similarly, in Benin (Sapele road), a positive sample was recorded. To our knowledge this is the first report of the presence of trypanosomes in Edo state. In November 2015 in Sapele, a human case of trypanosome-positive blood was detected in this area (personal communication). Therefore, atypical human trypanosomiasis is/are likely in these foci [15] and thus should be further investigated.

Clearly, we have identified Ekpoma as a potential hotspot for the presence of zoonotic parasites. By extension, environmental sanitary conditions with similar ecological factors could berth similar results in a number of places in Nigeria and possibly in some developing tropical countries across the globe. So, Ekpoma may just be a case in point to further advocate for improved personal hygiene and strengthen environmental sanitation system. However, the number of helminth parasites isolated across locations may not be exhaustive as only limited tissues/organs were examined. Likewise, in future, the application of molecular tools to further discriminate some of these parasites may be highly useful. From the results on trypanosome-positive rodents, we urge the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) and relevant bodies to screen human population in places where trypanosomes were isolated; and also carry out comprehensive tsetse surveillance across Edo state to properly delineate tsetse belt. As much as possible, houses should be constructed in a way they are small-mammal proof; and that continued efforts to rid homes of small mammals be highly encouraged.