Introduction

The vegetation cover of Mexico has been transformed, losing up to 50% of its original coverage, 22% of these areas are now composed of secondary coverage and 27% have been transformed into agricultural and livestock areas. As a consequence of this dramatic shift, mammals in Mexico have become much more endangered (^{López-Ortega, Ballesteros-Barrera, Acosta, & Cervantes-Reza, 2012}; ^{Martínez-Meyer, Sosa-Escalante, & Álvarez, 2014}), along with the helminth parasites these species harbor. Despite the efforts made to expand our knowledge on the biodiversity and natural history of mammals distributed in Mexico (^{Sánchez-Cordero et al., 2014}), only 24% of the species richness has been examined for helminth parasites, and few studies have been conducted in north-central Mexico (e.g., ^{Caspeta-Mandujano, Jiménez, Peralta-Rodríguez, & Guerrero, 2013}; ^{Falcón-Ordaz, Monks, & Pulido-Flores, 2013}; ^{García-Prieto, Falcón-Ordaz, & Guzmán-Cornejo, 2012}; ^{Guzmán-Cornejo, García-Prieto, Acosta-Gutiérrez, Falcón-Ordaz, & León-Paniagua, 2012}; ^{Jiménez-Ruiz, Rosas-Valdez, & Gardner, 2013}; ^{Jiménez-Ruiz, Peralta-Rodríguez, Caspeta-Mandujano, & Ramírez-Díaz, 2014}; ^{Pulido-Flores, Monks, & Falcón-Ordaz, 2013}).

The advances in faunal surveys and inventories of helminth parasites in Mexico are still modest, they have focused only on some host species and are geographically limited (^{Pérez-Ponce de León, García-Prieto, & Mendoza-Garfias, 2011}). As a result, there are undoubtedly many species waiting to be discovered, described and identified for this group of invertebrates. The inventory of the helminth parasite fauna of Rodentia (Bowdich, 1821), which is the most species-rich order in Mexico, comprises 3 phyla: Acanthocephala, Platyhelminthes, and Nematoda, which has the highest percentage of species diversity (e.g., ^{García-Prieto et al., 2012}). The geographical distribution of these parasites include localities in 18 states, with most of them located in Hidalgo, San Luis Potosí, Ciudad de México, and Nuevo León (^{Falcón-Ordaz et al., 2013}; ^{García-Prieto et al., 2012}; ^{García-Prieto, Mendoza-Garfias, & Pérez-Ponce de León, 2014}; ^{García-Prieto, Osorio-Sarabia, & Lamothe-Argumedo, 2014}; ^{Pulido-Flores, Moreno-Flores, & Monks, 2005}; ^{Pulido-Flores et al., 2013}).

Zacatecas is located in north-central Mexico (=Central Mexican Plateau). Its territorial surface encompasses 3.8% of the total area of the country (=75,275 km²), with climate conditions including dry and semi-dry (79% the coverage territory of the state) and temperate sub-humid areas (^{Inegi, 2013}, ²⁰¹⁴). One hundred and fifteen species of mammals have been reported in Zacatecas, of these 66% are rodents (^{López-Ortega et al., 2012}; ^{Sánchez-Cordero et al., 2014}). To the best of our knowledge, there are only 2 published works related to the helminth fauna of 1 species of mammal, the bat Tadarida brasiliensis mexicana Saussure, 1860 (Chiroptera) from Zacatecas (^{Falcón-Ordaz, Guzmán-Cornejo, García-Prieto, & Gardner, 2006}; ^{Guzmán-Cornejo, García-Prieto, Pérez-Ponce de León, & Morales-Malacara, 2003}), but there is no further information on the helminth parasites of rodent wildlife from this state. Loss and changes of vegetation coverage in the state have affected at least half of the territory, which considerably reduces the area available for the establishment of populations of wild mammals (^{López-Ortega et al., 2012}), and their helminth parasites. For this reason, it is important to carry out biodiversity studies in the state for this biological system. Here, a survey of the helminth parasite fauna of rodents from Zacatecas, Mexico is presented for the first time.

Materials and methods

Eighty-six specimens of rodents were collected using live-traps baited with oats from 9 localities within Zacatecas between May 2012 to July 2015 (Table 1; Fig. 1). Four transects were sampled simultaneously for 2 nights at each locality in Spring, Summer and Autumn (15 traps × 4 transects × 2 nights = 120 trap-nights in each locality) (collection data in Table 1). Most specimens were transported live to the laboratory, and euthanized following the standard procedures and techniques of ^{Gannon and Sikes (2007)}, and were examined for helminths by standard methods under a stereoscope. Parasites were placed in 0.85% w/v saline solution, then fixed by sudden immersion in hot (steaming) 70% ethanol, and stored in 70% ethanol to preserve morphological traits for further identification. Nematodes were cleared for morphological study with Ammans's Lactophenol and with a mixture of ethanol-glycerin (2:8). Cestodes were stained with Delafield's Hematoxylin and Mayer's hydrochloric carmine, and whole-mounted in Canada balsam. Taxonomic identification was conducted by comparing morphological traits using taxonomic keys and descriptions from the specialized literature (e.g., ^{Anderson, 2000}; ^{Falcón-Ordaz, Fernández, & García-Prieto, 2010}; ^{García-Prieto, Falcón-Ordaz, Lira-Guerrero, & Mendoza-Garfias, 2008}; ^{Khalil, Jones, & Bray, 1994} ; ^{Kruidenier & Peebles, 1958}; ^{Lichtenfels, 1970}; ^{Quentin, 1970}, ¹⁹⁷³; ^{Read & Millemann, 1953}; ^{Read, 1956}; ^{Schmidt, 1986}; ^{Smith, 1954}; ^{Tiner, 1948}; ^{Yamaguti, 1961}). Parasites identified were counted to obtain the infection parameters according with the definitions suggested by ^{Bush, Lafferty, Lotz, and Shostak (1997)}.

Figure 1 Map of Zacatecas indicating the localities where specimens of some rodents (Cricetidae, Heteromyidae, and Sciuridae) have been examined for helminth parasites (codes in Table 1; gray=Central Mexican Plateau). 

Voucher specimens of host and parasites were deposited in the Colecciones Biológicas, Unidad Académica de Ciencias Biológicas (UACB), Universidad Autónoma de Zacatecas (UAZ), Zacatecas, Zacatecas, Mexico: Colección de Vertebrados (CVZ) (mammals section) (CVZM 0016-0021, 0036-0042, 0054-0057, 0067-0090, 0153-0173); and Colección de Invertebrados no Artrópodos (CINZ) (CINZ 130-147, 175, 186-202), respectively. Additional vouchers were deposited in the Colección Nacional de Helmintos (CNHE), Instituto de Biología, Universidad Nacional Autónoma de México, Mexico City, Mexico (CNHE 9826-9827, 8195, 9996-10004).

Results

Eighty-six rodents were collected and examined for parasites from 7 host species and 9 localities in Zacatecas (44% of all hosts examined were parasitized). Only Chaetodipus sp, Dipodomys merriami atronasus, Neotoma mexicana, Otospermophilus variegatus, and Peromyscus sp. were infected with at least 1 species of helminth. No parasites were found in Reithrodontomys sp., Peromyscus sp. and O. variegatus from La Barranca; Peromyscus sp. from near Tlalticoaloya, El Godillo, and Villa de Cos; and none from Chaetodipus sp. and Mus musculus from Ojocaliente.

The helminth inventory of sampled hosts included 9 families and 10 species, as follows: (1) Platyhelminthes; 3 cestodes [Catenotaeniidae gen. sp. Spassky, 1950 (Catenotaeniidae), Hymenolepis sp. Weinland, 1858 (Hymenolepididae) and Raillietina sp. Fuhrmann, 1920 (Davaineidae)], and (2) Nematoda; 6 species [Heteromyoxyuris longejector Hannum, 1934 and Rauschtineria ^{Hugot, 1980} (Oxyuridae), Lamotheoxyuris cf. Ackerti ^{Falcón-Ordaz et al., 2010} (Heteroxynematidae), Pterygodermatites dipodomis ^{Tiner, 1948} (Rictulariidae), Gongylonema sp. Molin, 1857 (Gongylonematidae), Mastophorus dipodomis ^{Read and Millemann, 1953} (Spirocercidae), and Trichuris dipodomis ^{Read, 1956} (Trichuridae)]. No Trematoda were collected in the study.

Prevalence, mean abundance, mean intensity, intensity range, site of infection, hosts and localities are given in Table 2. Nematoda is the group with the highest species richness. Eight of the 10 helminth species inhabit the intestine (Catenotaeniidae gen. sp., Raillietina sp., Hymenolepis sp., H. longejector, L. cf. ackerti, Rauschtineria sp., P. dipodomis, and T. dipodomis), with an intensity ranging from 1 to 22 helminths per infected rodent. D. merriami atronasus presented the highest parasite species richness with 6 helminth species, including 1 cestode (Hymenolepis sp.) and 5 nematodes (Gongylonema sp., H. longejector, P. dipodomis, M. dipodomis, and T. dipodomis) (Table 2). All of the parasite taxa were recovered in the adult stage, with the exception of L. cf. ackerti, which was recovered as a single adult female and 2 larvae (found with the primordium of the vulva). The majority of helminth life cycles sampled here are unknown, however, they could be inferred based on information from the literature based upon closely related species (^{Anderson, 2000}; ^{Falcón-Ordaz et al., 2010}; ^{García-Prieto et al., 2008}; ^{Joyeux & Baer, 1945}; ^{Khalil et al., 1994}; ^{Kruidenier & Peebles, 1958}; ^{Lichtenfels, 1970}; ^{Quentin, 1970}, ¹⁹⁷³; ^{Read & Millemann, 1953}; ^{Read, 1956}; ^{Schmidt, 1986}; ^{Smith, 1954}; ^{Tiner, 1948}; ^{Yamaguti, 1961}) (Table 2).

H. longejector from El Gordillo and Ojocaliente, T. dipodomis from Tlalticoaloya, Rauschtineria sp. from Morelos-Zacatecas Road and Zacatecas City, and L. cf. ackerti from Rancho La Barranca, had the highest prevalence (100%), followed by Hymenolepis sp. (80%) in D. merriami atronasus from Ojocaliente. H. longejector had the highest mean abundance (21), whereas Chaetodipus sp. from El Gordillo had the highest mean intensity (21). The nematode M. dipodomis infected 2 species of rodents (D. merriami atronasus and Chaetodipus sp.) from the same locality (Rancho La Barranca), whereas the nematode T. dipodomis was present in 2 host species (D. merriami atronasus and Chaetodipus sp.) at different localities, Rancho La Barranca and Tlalticoaloya, respectively (Table 2). H. longejector was present in 3 localities, and infected 2 species of rodents (D. merriami atronasus, and Chaetodipus sp.), and P. dipodomis was present in 3 localities also infecting 3 different host (D. merriami atronasus, Chaetodipus and Peromyscus sp.) (Table 2). Finally, Catenotaeniidae gen. sp. and P. dipodomis from San Pedro Dam, were found at the same prevalence, mean abundance, mean intensity, and intensity range, these parameters are equivalent to Hymenolepis sp., and Gongylonema sp. from Rancho La Barranca (Table 2).

Discussion

The class Mammalia is a widespread and charismatic group of animals in Mexico (^{Sánchez-Cordero et al., 2014}). There are very few studies on the diversity of helminths of wild mammals in Mexico, which contrasts with the number of mammalian species present in this country. Moreover, 67% of terrestrial mammals from Zacatecas are rodents (^{López-Ortega et al., 2012}; ^{Sánchez-Cordero et al., 2014}), and have not been examined for helminths (see ^{Falcón-Ordaz et al., 2013}; ^{García-Prieto et al., 2012}; ^{Pulido-Flores et al., 2013}).

In this paper, we present new locality records for 10 taxa of helminths that are recorded in Zacatecas for the first time, and 2 species are reported for the first time in Mexico (Rauschtineria sp. and M. dipodomis). Nine species of helminths were found that result in 21 new host records, 5 taxa were identified to species, 4 to genus, 1 only to family, and 5 taxa have indirect life cycles (Table 2). Of these helminthes, 30% were cestodes (3 taxa) and the rest were nematodes (7). Nematodes showed the highest species richness, a pattern previously observed for mammal hosts in North America (e.g., ^{Decker, Duszynski, & Patrick, 2001}; ^{García-Prieto, Mendoza-Garfias et al., 2014}; ^{García-Prieto, Osorio-Sarabia et al., 2014}; ^{King & Babero, 1974}; ^{Munger & Slichter, 1995}; ^{Read & Millemann, 1953}) (Table 2).

Studies on the life cycles of the helminths of mammals are relatively few (^{Georgiev, Bray, & Littlewood, 2006}; ^{Morand, Bouamer, & Hugot, 2006}). We present information of congener species or species of the same family or order and extrapolate them to the present infections only when they are constant at the relevant taxonomic level. This includes an arthropod as the intermediate host for tapeworms and the direct pattern of transmission for most pinworms (i.e., ^{Anderson, 2000}; ^{Joyeux & Baer, 1945}; ^{Lichtenfels, 1970}; ^{Schmidt, 1986}; ^{Yamaguti, 1961}). This allow us to understand the relationships among a variety of host-related and environmental-related factors in semi-arid and arid environmental (^{Bienek & Klikoff, 1974}; ^{Reichman, 1975}).

The cestodes observed here include 3 taxa, Catenotaeniidae gen. sp., Hymenolepis sp. and Railletina sp. Regarding their life cycles, it is known that members of Catenotaeniidae (whose host range is restricted to rodents) (Table 2), use mites (Siphonaptera) as intermediate hosts (^{Joyeux & Baer, 1945}), while hymenolepidids use beetles (Coleoptera) (^{Cunningham & Olson, 2010}). Therefore, it is assumed that arthropods are included in the diet of the rodents studied. However, the cestodes have a relative low prevalence (Table 2), suggesting that arthropods represent a low proportion of the diet reported in the literature (^{Decker et al., 2001}).

For identification to species level, more specimens of cestodes will be required, allowing us to confirm if these specimens correspond to other species recorded in the country. For example, Catenotaeniids are represented in Mexico by 1 species from a Central Mexican locality (^{Carmona-Huerta, 1994}). Hymenolepids include 3 species, Hymenolepis diminuta Rudolphi, 1819, Hymenolepis horrida Linstow, 1910 and Hymenolepis sp. from 3 different hosts in the Transmexican Volcanic Belt (^{Carmona-Huerta, 1994}; ^{García-Prieto et al., 2012}) and hosts of Raillietina sp. from Mexico include 7 rodent species from the Mexican Plateau and Mexican Transvolcanic Belt (^{García-Prieto et al., 2012}; ^{Underwood, Owen, & Engstrom, 1986}). The distribution previously reported for cestodes and the typical occurrence of these in rodents, is supported by the data found in the current study. It should be noted that hymenolepid cestodes are typically parasites of heteromyids from North America (^{Gardner, 1985}; ^{Gardner & Schmidt, 1987}).

The nematode species and localities reported herein represent 12 new hosts and 25 new locality records (Table 2). Specifically, within these host mammals Gongylonema sp. and Rauschtineria sp., are recorded for the first time in D. merriami atronasus and O. variegatus, respectively for Mexico. It is noteworthy that N. mexicana represents a new host record for L. cf. ackerti, a species recorded exclusively from the genus Neotoma, Neotoma nelsoni Goldman, 1905 in North America (^{Falcón-Ordaz et al., 2010}). M. dipodomis is recorded for the first time in Mexico. Identification to species level was possible with the exception of Gongylonema sp., whose individual were all females, as male specimens are required for proper identification.

Two life cycle groups of nematodes are recognized. Gongylonema sp., M. dipodomis and P. dipodomis have indirect life cycle (heteroxenic parasites species), where the individuals of the 3 genera require an arthropod as an intermediate host (Coleoptera, Lepidoptera, Orthoptera or Siphonaptera) to complete their life cycles (^{Dyer & Olsen, 1967}; ^{King & Babero, 1974}; ^{Ransom & Hall, 1915}). The other 4 taxa have direct life cycles (monoxenic parasite species) with no intermediate hosts (^{Anderson, 2000}).

The nematode Rauschtineria sp. is known to parasitize terrestrial Sciuridae by habiting the intestinal caecum (^{Hugot, 1980}; ^{Hugot, Feliu, Douangboupha, & Ribas, 2013}), and may develop through a direct life cycle as other oxyurids (^{Anderson, 2000}; ^{Gibbons, 2010}; ^{Hugot et al., 2013}), and this species has not been previously reported in Mexico. There are other known Oxyuridae parasites of squirrels in México, namely Syphatineria sp., a parasite of aerial Sicuridae, Sciurus aureogaster Cuvier, 1829 from México City, and Citellina abdita (^{Caballero-y Caballero, 1937}), parasitizing the terrestrial Sciuridae, O. variegatus (=reported originally as Spermophilus variegatus) from Guanajuato and Hidalgo (^{García-Prieto et al., 2012}). This is the first record of the species and a new host record for the genus in Mexico.

No trematodes were found in this study; however, 6 species of trematode parasites in rodents from Mexico have previously been reported (^{García-Prieto et al., 2012}). One species, Caballerolecythus ibunami^{Lamothe-Argumedo, Falcón-Ordaz, García-Prieto, and Fernández-Fernández (2005)}, has been previously reported in other heteromyid host (Liomys irroratus) from the Transmexican Volcanic Belt (^{Falcón-Ordaz, Acosta-Gutiérrez, Fernández, & Lira-Guerrero, 2012}; ^{Lamothe-Argumedo et al., 2005}). For localities where trematode species were reported from semiarid zones with similar conditions as the localities from Zacatecas, however, the prevalence of trematodes was low. A possible explanation for this phenomenon is that arid ecosystems generate a lower exposure rate of arthropods (second intermediate host) and to snails (first intermediate host). Conversely, species of Brachylaima spp. are parasites of heteromyids (i.e. Liomys pictus) and Crisetids (i.e. Peromyscus guatemalensis and P. difficilis), from tropical zones in Mexico (^{Caballero-Deloya, 1970}; ^{Ubelaker & Dailey, 1966}), where habitats are optimal for the survival of snails and exposure to secondary intermediate hosts should be more frequent (^{Yamaguti, 1971}, ¹⁹⁷⁵).

Rodent diet studies associated with semi-arid and arid regions which has been scarcely documented (e.g., ^{Bienek & Klikoff, 1974}; ^{Reichman, 1975}). ^{Decker et al. (2001)} suggested that rodents in dry environments consume variable quantities of arthropods (15.5% of diet) depending upon availability in the ecosystem, while green vegetation made up only 6.1% of the diet (see ^{Reichman, 1975}). In contrast, in higher moisture environments, a granivorous diet is preferred. About half of the diversity we sampled is represented by helminths with indirect life cycles (Table 2), which parasitize host species that are primarily considered as “granivores” (^{González-Salazar, Martínez-Meyer, & López-Santiago, 2014}), which suggests that the diet of these hosts is variable (excluding N. mexicana). Even though indirect life cycles are predominant in the species reported here, the nematode H. longejector (direct life cycle) is the most abundant species and is present in all of the seasons analyzed (Flores-Rodríguez & Martínez-Salazar, Pers. Obs.).

Parasites can be indicative of food-web structure and environmental conditions (^{Decker et al., 2001}; ^{Marcogliese & Cone, 1997}), and based upon the presence of helminth parasites with both types of life cycles, we suggest that the localities analyzed still represent an optimal ecological environment for them. Unfortunately, not much is known about the life cycles of these particular species since most of the information is known at the supra-specific level; such is the case of the cestodes, Catenotaeniidae gen sp., Hymenolepis sp., and the nematodes P. dipodomis, Gongylonema sp. and M. dipodomis. All of these species are thought to require arthropods as an intermediate host to complete their life cycles (^{Cunningham & Olson, 2010}; ^{King & Babero, 1974}; ^{Luong & Hudson, 2012}; ^{Lyons, 1978}). Systematic studies of the hosts that characterize the diet would help to elucidate the life cycles of their parasites, and may help to answer the question as to why helminths with indirect life cycles are prevalent in these rodents from arid and semi-arid environments (^{Bienek & Klikoff, 1974}; ^{Decker et al., 2001}; ^{Garner, Richardson, & Felts, 1976}; ^{Reichman, 1975}).

As for the geographical distribution of our results, no apparent pattern is observed in terms of the closeness of the locations and parasites. For example, sites 2, 3 and 8, have 4 species of nematodes in common (T. dipodomis, H. longejector, P. dipodomis and M. dipodomis), however even though localities 2 and 8 are geographically close, locality 3 is located away far away from them. Furthermore, the presence of parasite species appears to be more related to the rodent family they occur within; we observed that there is host specificity by parasites for the families Heteromydae and Cricetidae. This assertion is supported by the fact although species of both families were found in locations 2, 3, 7, 8 and 9, no parasite species are shared by members of both families. Furthermore, only one locality has a shared parasite (T. dipodomis) between Peromyscus sp. (Cricetidae) and D. merriami atronasus (Heteromydae). Species richness could be underestimated based on the sampling effort in our study (Tables 1 and 2); therefore, a greater sampling effort is required at specific localities (e.g. Morelos-Zacatecas Road, San Pedro Dam, Ciudad Cuauhtémoc or Zacatecas city). Although the number of hosts collected was asymmetric, we found fauna typical of D. merriami from North America (i.e., H. longejector, P. dipodomis, Gongylonema sp. and M. dipodomis) (^{Decker et al., 2001}; ^{King & Babero, 1974}; ^{Voge, 1956}). The helminth fauna found in D. merriami of Zacatecas is similar to other localities of the Mexican Plateau (Durango, San Luis Potosí and Guanajuato, Falcón-Ordaz, Pers. Obs.). The presence of these species of nematodes in D. merriami atronasus provides evidence that the environments where the host is similar throughout their distribution. Records for helminth parasites and their geographical distribution was not enough to demonstrate a geographical pattern in Mexico, but nevertheless, Nearctic affinities within helminth fauna reported here in the Central Mexican Plateau is evident.

The geographic distribution and helminth species richness was heterogeneous in this study. Most of the sampling effort during these years was focused on Rancho La Barranca primarily due to the accessibility of the site (Fig. 1, Table 2). This site is next to anthropogenic activities which includes livestock and pastureland, but the landscape is relatively conserved, as well as the other collecting sites. All sites sampled presented almost 1 species of parasite for each (Tables 1 and 2), which may reflect a relatively healthy ecosystem.

It is necessary to increase our knowledge of the dietary preferences of rodents associated with arid and semi-arid desert environments, as well as information on the life cycles of parasitic helminths as an indicator of the food-web structure and trophic interactions. Likewise, monitoring the populations of these host-parasite systems associated with global climate change data and the correlation with anthropogenic landscape changes, could contribute to conservation in arid and semi-arid environments within Mexico in order to provide information for conservation. This is the first study in this region for these host-parasite systems and the beginning of a systematic inventory of parasites of wild mammals. This allows us to address future questions related to the evolution and biogeography of the parasite-host systems in Zacatecas.