6247_Therya

Among the most common animal lifestyles, parasitism stands out, often occurring in groups that also include free-living organisms. Among metazoans, helminths and arthropods include a large number of parasitic species (Goater et al. 2014). Helminths, which constitute a non-natural grouping, comprise organisms belonging to four phyla that are characterized by being worm-like and parasitic: Platyhelminthes (dorsoventrally flattened worms), Rotifera-Acanthocephala (worms with the proboscis armed with hooks), Nematoda (roundworms) and Annelida-Hirudinea (ringed worms). On the other hand, Arthropoda is the phylum that includes the largest number of animals on Earth; they are characterized by the presence of articulated legs, segments fused into tagmas and appendages formed by segments (Brusca et al. 2022).

The systematic study of parasites from both groups in Mexico began asynchronously; the first helminth recorded in the country was the nematode Litomosoides sigmodontis, a parasite of the Hispid Cotton Rat Sigmodon hispidus Say and Ord, 1825 in Jalisco and Michoacán (Ochoterena and Caballero y Caballero 1932). Regarding arthropods, the chewing louse Eutrichophilus mexicanus was reported by Rudow (1866) on the Mexican Hairy Porcupine (Coendou mexicanus (Kerr, 1972)) from an undetermined locality in Mexico. Since then, both lines of research have been addressed by national and international researchers from various institutions, with the most notable being the Instituto de Biología, Universidad Nacional Autónoma de México (UNAM), in Mexico, for both groups (helminths and arthropods), and Facultad de Ciencias, UNAM, and Instituto Politécnico Nacional for arthropods.

The first attempt to quantify the richness of helminths associated with rodents in Mexico was made by García-Prieto et al. (2012); however, these authors considered both wild and synanthropic rodents. In the case of arthropods, Whitaker and Morales-Malacara (2005) gathered published information related to ectosymbionts of wild rodents in a single study. Based on the above, the objective of the present study was to compile, update and list for the first time all nominal species of metazoans that infect and infest wild rodents and analyze their geographic distribution, defining the state of the art and suggesting possible lines of research that will allow us to complete the knowledge of the group.

Materials and methods

Systematic searches were carried out in electronic databases such as BioOne (https://bioone-org), CAB Abstracts (https://www.cabidigitallibrary.org), Clarivate Analytics (https://www.webofscience.com) and Google Scholar (https://scholar.google.com) between 1913 and 2025. In addition, some faunal lists were consulted, such as: Whitaker and Morales-Malacara (2005); García-Prieto et al. (2012); Sánchez-Montes et al. (2013; 2018); Light et al. (2020); Herrera-Mares et al. (2022).

The keywords for the searches were combined in Spanish and English: helminths, flatworms, Platyhelminthes, nematodes, Nematoda, acanthocephalans, Acanthocephala, leeches, Hirudinea, parasitic arthropods, ectoparasites, mites, Acari, fleas, Siphonaptera, ticks, Argasidae, Ixodidae, lice, chiggers, Trombiculidae, Phthiraptera, rodents, Rodentia, Mexico. The classification and nomenclature of parasitic metazoans was updated based on keys and specialized literature. For the nomenclature of rodents we follow The Mammal Diversity Database (2025) V.2.0 (MDD2).

All metazoan records were entered into an Excel spreadsheet, including the following fields for both groups, parasites and hosts: family, genus, and species. The state, bibliographic reference and full citation were included for each record. For the total count of parasitic and host species, only nominal species (identified at the species level) were used. The numbers of metazoan parasites were represented in a choropleth map. The present study is a bibliographic compilation; therefore, it was not required to follow the guidelines of the American Society of Mammalogists or the approval of the Committee for the Care and Use of Animals in Research-Experimentation.

Results

The knowledge of metazoan parasites of rodents in Mexico consists of 3,909 records obtained from all the 32 states; these records correspond to 518 species, from 155 genera and 50 families (Table 1; Figure 1), associated with 204 rodent species classified into 58 genera and eight families. The states with the greatest parasite richness are Estado de México and Oaxaca, with 80 species, associated with 31 and 36 host species, respectively. The most complete record of metazoan parasites was achieved in the states of Jalisco, Nuevo León, and Hidalgo, with seven of the eight groups of metazoans considered in this study associated with rodents. On the other hand, in the state of Aguascalientes, only two species of Ischnocera have been reported. The highest number of studied hosts has been recorded in the states of Chihuahua (38) and Oaxaca (36), whereas in Aguascalientes and Tabasco, only one and two species of rodents have been sampled, respectively. Asymmetric knowledge among metazoan parasitic groups is also evident, with helminths being the most neglected (Table 2).

The eight families of wild rodents examined have at least one species of parasitic metazoan, highlighting that Cricetidae and Heteromyidae are parasitized by six groups of metazoans except for Ischnocera, which only infests Geomyidae and Erethizontidae (Figure 2).

The most widely distributed parasitic metazoan species in the country are the flea Jellisonia breviloba breviloba Traub, 1950 and the chewing louse Geomydoecus (Geomydoecus) welleri Price and Hellenthal, 1981 in 15 states each, followed by the mesostigmate mite Steptolaelaps liomydis in 13 states. The hosts with the greatest parasite richness correspond to species of the Cricetidae family: Peromyscus difficilis (J. A. Allen, 1891) (73 species), Peromyscus maniculatus (J. A. Wagner, 1844) (60), Microtus mexicanus (de Saussure, 1861) (49) and Peromyscus melanotis J. A. Allen and F. M. Chapman, 1897 (47), followed by the heteromyid (Heteromyidae) Heteromys pictus O. Thomas, 1893 (41 species).

To date, the helminthological record for wild rodents in Mexico consists of 70 nominal species, which parasitize 49 host species belonging to five families, among which Cricetidae and Heteromyidae stand out for hosting the largest number of helminth species (38 and 21, respectively). These hosts have been collected in 23 states. Individually, the phylum Nematoda is the most represented (53 species), followed by Platyhelminthes (with 16) and Acanthocephala with one, with no species of Annelida having been recorded to date associated with this group of mammals (Figure 3).

The first helminth described for a Mexican wild rodent is Micropleura sigmodontis Ochoterena and Caballero y Caballero, 1932 (currently Litomosoides sigmodontis (Chandler, 1931)), in the cricetid S. hispidus from Jalisco and Michoacán. The nematode Vexillata vexillata (Hall, 1916) represents the helminth with the widest geographic distribution, having been collected in six states, followed by the nematodes Heteromyoxyuris longejector Quentin, 1973 and Vexillata liomyos Falcón-Ordaz, Gardner and Pérez-Ponce de León, 2001, each in five states. The most extensive helminthological record is harbored by the cricetid P. difficilis, which comprises 13 helminth species.

Mites (Parasitiformes and Acariformes) that are parasitic on rodents in Mexico are represented by 204 species classified in 58 genera from 16 families. These species are widely distributed among rodents throughout Mexico, except for the states of Aguascalientes, Guanajuato, Querétaro, Tabasco, and Tlaxcala. The first records of rodent mites in Mexico correspond to Ornithodoros nicollei (Parasitiformes: Argasidae) in the nests of rats of the genus Hodomys (Brumpt et al. 1939) and Eutrombicula alfreddugesi (Oudemans, 1910) (Acariformes: Trombiculidae) in the ground squirrel Otospermophilus variegatus (Erxleben, 1777) (Islas 1943). The most widely distributed mite species are Steptolaelaps liomydis (Grant, 1947), Androlaelaps fahrenholzi (Berlese, 1911), and E. alfreddugesi, found in rodents from 13, 12, and 10 states, respectively.

These mite species parasitize 132 rodent species from 37 genera and seven families (Cricetidae, Cuniculidae, Dasyproctidae, Erethizontidae, Geomyidae, Heteromyidae, and Sciuridae). The host with the greatest richness is Heteromys pictus (Cricetidae) with 29 associated mite species, followed by Peromyscus difficilis with 19 and H. irroratus with 18. The most studied host, that is, the one with the greatest number of records, is also H. pictus.

Among insects, anoplurans are represented by 40 species belonging to six genera and two families, distributed across 28 states in Mexico. Among the first records of anoplurans in the country are those of Enderleinellus extremus Ferris, 1919 and Enderleinellus suturalis (Osborn, 1891), in squirrels of the genera Sciurus (Chiapas, Oaxaca, Tamaulipas, and Veracruz) and Callospermophilus (Chihuahua), respectively. The most widespread species of sucking lice are Polyplax auricularis Kellogg and Ferris, 1915 and Neohaematopinus sciurinus (Mjöberg, 1910) in eight Mexican states, followed by Fahrenholzia ehrlichi Johnson, 1962 in seven, and Fahrenholzia pinnata Kellogg and Ferris, 1915, and Hoplopleura hirsuta Ferris, 1916 in six states. These lice species are associated with 65 rodent species from 26 genera and three families (Cricetidae, Heteromyidae, and Sciuridae).

The host with the greatest richness is Sciurus aureogaster F. Cuvier in É. Geoffroy Saint-Hilaire and F. Cuvier, 1829 with four associated species (Enderleinellus deppei Kim, 1966, E. extremus, Enderleinellus mexicanus Werneck, 1948 and N. sciurinus); the most studied host is Heteromys irroratus J. E. Gray, 1868 with records in nine states of Mexico.

Regarding chewing lice, the national record includes 67 species and 21 subspecies (Ischnocera), belonging to three genera and one family (Trichodectidae); the oldest record for this group is Eutrichophilus mexicanus (Rudow 1866) associated with C. mexicanus. The most widely distributed species in the country is G. (Geomydoecus) welleri in 15 states, followed by Thomomydoecus (Thomomydoecus) zacatecae (Price and Hellenthal, 1980) in seven.

Chewing lice are associated with 21 rodent species from eight genera and two families. The host with the greatest number of associated species is Megascapheus umbrinus (J. Richardson, 1829) (=Thomomys umbrinus, which included several subspecies), with 22 species and three subspecies. It is also the most studied host in 18 states of Mexico.

Finally, Siphonaptera (fleas) is represented by 136 species belonging to 49 genera and nine families, distributed in 27 states. The first record of flea species (Pleochaetis mundus Jordan and Rothschild, 1922) in wild rodents of Mexico was made in two species of rodents: Reithrodontomys megalotis (S. F. Baird, 1857) and P. difficilis. The most widely distributed species of fleas are J. breviloba breviloba and Plusetis mathesoni in 15 and 12 states of Mexico respectively, followed by Jellisonia weismani Eads, 1951 and Plusaetis sibynus (Jordan, 1925), in 11 states. Flea species are associated with 101 species of rodents from 32 genera and four families (Sciuridae, Cricetidae, Heteromyidae and Geomyidae). The host with the greatest richness is P. maniculatus with 45 flea species, while the most studied is P. difficilis with 166 records.

Discussion

This work represents the first approach towards a comprehensive understanding of the richness of metazoan parasites (helminths and arthropods) in wild rodents in Mexico. The results of this study reveal a significant asymmetry in knowledge between the two groups. An important explanation for these differences can be found in the greater adaptive radiation shown by arthropods compared to any other animal group, since we are dealing with the most diverse group on the planet (Brusca et al. 2022). Another possible reason for this is the different start dates for the study of each group, with the study of helminths in rodents beginning in 1932 (Ochoterena and Caballero y Caballero 1932) and that of arthropods in 1866 (Rudow 1866).

However, this pattern may also be explained by other factors, such as the requirement to euthanize hosts to obtain helminths, in contrast to arthropod collection, which does not require euthanasia and thus facilitates their capture in larger numbers. Additionally, there is a bias toward the study of helminths infecting synanthropic rodents (Panti-May et al. 2018), as well as challenges related to the morphological complexity of some parasite groups, particularly trichostrongylid nematodes, which frequently infect these hosts. Furthermore, some helminth groups, such as Acanthocephala, exhibit relatively low richness, reducing the likelihood of their detection in wild rodents. Supporting this, our electronic literature search conducted to compile the information analyzed in this study yielded eight records of acanthocephalans, of which only one was associated with wild rodents (Lynggaard et al. 2021).

From a biological point of view, the differences in intra-group (metazoan groups) and inter-group (helminths vs. arthropods) richness could be attributed to the intrinsic nature of each one, that is, its particular richness and abundance, as well as the geographical distribution and host specificity they exhibit, their ability to evade the immune response, and the characteristic overdispersed distribution exhibited by the parasites.

Additionally, the process for the study of arthropods is relatively easier, so it requires less time. The processing of helminths applies particular techniques for each group, such as heat fixation, staining of specimens, clearing and mounting in Canada balsam (turpentine) or, in the case of nematodes, cross-sections of the body and clearing with Amman lactophenol (Guzmán-Cornejo et al. 2012).

Another issue we identified is that some records of ectoparasitic arthropods were attributed to Mexico by Guzmán-Torres and Cano-Santana (2021) (e.g., Enderleinellus suturalis from Chihuahua). However, this record was originally cited by Ritzi (2014) in a study on ectoparasites of mammals from the northern extent of the Chihuahuan Desert in the United States. Therefore, such records were excluded from this manuscript.

On the other hand, in the initial stage of knowledge about Mexican arthropods, the contribution of foreign researchers was notable (Grant 1947; Furman 1955; Fain 1973; Loomis 1969; Genoways 1973; Werneck 1948, among others), while for helminths this has been developed mainly by national researchers in a punctual manner, and recently in a more systematic aproach: Pulido-Flores et al. (2005); Preisser and Falcón-Ordaz (2019); Panti-May et al. (2023); Falcón-Ordaz et al. (2024).

The nomenclatural revision of both parasites and hosts aimed to achieve the greatest possible accuracy in the records we present. However, some were problematic, such as Peromyscus maniculatus, since according to the database we followed (The Mammal Diversity Database 2025) this species is not distributed in Mexico. Notwithstanding, a recent study (Boria and Blois 2023) based on populations of this species in the USA found that P. maniculatus split into two regionally distinct subspecies: P. m. gambelii and P. m. sonorensis, both distributed in Northwestern Mexico. The inclusion of records of “P. maniculatus” in our study needs to be confirmed with a similar analysis to that referred to previously. Likewise, in accordance with Rose (2025), S. hispidus is distributed exclusively in northern Mexico; however, we found several records of parasites in the central and southern part of Mexico, which do not correspond to the current reported distribution of this rodent species. Unfortunately, the preservation of symbiotypes (sensu Frey et al. 1992) was not a common practice among parasitologists, so the identity of the specimens cannot be clarified. Some studies refer to this same problem (Light et al. 2020). To address this limitation, some efforts have included the collection of host tissue for subsequent DNA-based identification or the deposition of specimens in mammal collections.

Regarding the distribution of these groups of parasites, we can point out that they are distributed throughout all 32 states of Mexico; however, we also observed a significant asymmetry in sampling effort, with more than 220 records in states such as Estado de México, Querétaro, and Oaxaca, while only four have been recorded in other states such as Aguascalientes and Tabasco. Furthermore, it highlights that although sampling effort has been similar in Querétaro (where 30 rodent species have been analyzed), in Estado de México and Oaxaca, the richness of metazoans is practically half of what is reported in these last two states (80 species each). This can be explained by a probable lower regional richness of parasites in Querétaro than in the two states mentioned above, particularly in Oaxaca, which is in the Neotropical portion of the country, and occupies the first place in number of mammal species in Mexico (Briones-Salas et al. 2015). To establish the true current distribution of metazoans associated with wild Mexican rodents, a detailed study mapping the collection sites would be necessary; however, many of the older records are difficult to locate on maps due to their imprecise descriptions. Fortunately, many recent works include the precise geographic coordinates [e.g., Herrera-Mares et al. (2022); Panti-May et al. 2023)] of the sites where the material was obtained among their collection data.

On the other hand, as Herrera-Mares et al. (2022) highlighted, some regions still remain unexplored or poorly investigated because they face problems related to violence associated with drug cartels which can limit sampling in these particular areas.

According to the results obtained from the list presented, we found that the eight families of wild rodents have records of parasitic metazoans. However, the two families with the greatest parasite richness are Cricetidae (110 species) and Heteromyidae (40), which is likely related to the high diversity of both groups: 150 species of Cricetidae and 40 of Heteromyidae (Light et al. 2020), and to the extensive sampling of species belonging to both families.

In accordance with The Mammal Diversity Database (2025) V.2.0 (MDD2) the number of wild rodent species distributed in Mexico is 270; as result of our study, 204 have been reported as hosts of metazoans, representing 75.5% of the total. This could indicate that the knowledge of parasitic metazoans in this group of hosts is quite advanced in Mexico; notwithstan-ding, analyzing the individual host records, we can see that the number of metazoan species associated with these mammals varies widely, ranging from one species in Dipodomys ornatus (Falcón-Ordaz et al. 2024) to 73 in P. difficilis.

Parasites and rodents are crucial components of biodiversity and the ecosystems they inhabit. Furthermore, several of these parasitic species are of medical and veterinary importance, either as disease-causing agents by feeding on hosts infected with worms (e.g., Rodentolepis nana) or as potential vectors of disease-causing pathogens, since some rodent species are reservoirs of microorganisms (Keesing and Ostfeld 2024). Therefore, we consider it essential to continue monitoring the population dynamics of these parasites in their hosts, not only in relatively well-studied areas, but especially in less explored ones. This task is urgent considering the accelerating impact of human activities and global climate change, which in certain circumstances favor the survival of some species of public health importance.

Acknowledgments

This work is dedicated to Dr. Livia León-Paniagua for her career over 38 years studying Mexican mammalogy, in which she has made important contributions to its knowledge, and for the great support she has given us in the field collections that we have carried out together. We thank Georgina Ortega-Leite for providing important bibliographic references and to Lázaro Gervara López for their comments abouth the species of Peromyscus. RP-L thanks to program UNAM-PAPIIT IA204525.

Declaration of Artificial Intelligence use

Google translator was used to correct the grammar of the writing.

Author contributions

Carmen Guzmán-Cornejo conceptualization, investigation, data curation, methodology, writing-original draft; Angel Herrera-Mares conceptualization, investigation, data curation, methodology, draft revision; Roxana Acosta-Gutiérrez conceptualization, investigation, data curation, methodology, draft revision; Ricardo Paredes-León conceptualization, investigation, data curation, methodology, draft revision; Rosario Mata-López conceptualization, investigation, data curation, methodology, draft revision; Luis García-Prieto conceptualization, investigation, data curation, methodology, writing-original draft.

Supplementary data

SD1. Records of parasitic metazoans associated with wild rodents in Mexico.

SD2. Literature of parasitic metazoans associated with wild rodents of Mexico.

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Associated editors: Giovani Hernández Canchola and Pablo Colunga Salas

Submitted: October 31, 2025 ; Reviewed: December 12, 2025

Accepted: March 19, 2026; Published online: May 29, 2026

Table 2. Richness of parasitic metazoans in rodent species by Mexican state. Tre = Trematoda; Ces= Cestoda; Acn = Acantocefala; Nem = Nematoda; Aca = Acari; Ano = Anoplura; Isc= Ischnocera; Sip = Siphonaptera.

State	Tre	Ces	Acn	Nem	Aca	Ano	Isc	Sip	Total parasite richness	Richness of hosts studied
Aguascalientes	0	0	0	0	0	0	2	0	2	1
Baja California	0	0	0	0	27	2	5	10	44	27
Baja California Sur	0	0	0	0	15	2	2	0	19	13
Campeche	0	1	0	4	11	0	2	2	20	12
Chiapas	1	0	0	2	26	5	1	18	53	31
Chihuahua	0	0	0	6	5	8	11	31	61	38
Ciudad de México	0	0	0	0	19	4	5	36	64	23
Coahuila	0	0	0	3	8	4	14	4	33	15
Colima	0	1	0	1	14	2	3	0	21	15
Durango	0	0	0	1	27	3	10	15	56	31
Estado de México	0	2	0	5	20	5	11	37	80	31
Guanajuato	0	0	0	8	0	1	4	3	16	10
Guerrero	0	0	0	0	17	5	0	31	53	26
Hidalgo	1	4	0	13	7	2	6	21	54	30
Jalisco	2	0	1	3	19	8	13	12	58	20
Michoacán	0	0	0	3	14	1	6	22	46	25
Morelos	0	0	0	7	10	4	2	23	46	18
Nayarit	0	0	0	1	7	5	4	1	18	9
Nuevo León	1	2	0	2	10	6	5	31	57	30
Oaxaca	0	0	0	1	35	9	3	32	80	36
Puebla	1	0	0	2	18	3	11	23	58	29
Querétaro	0	0	0	0	0	1	2	40	43	30
Quintana Roo	0	0	0	4	11	0	1	1	17	11
San Luis Potosí	0	0	0	7	19	2	7	3	38	24
Sinaloa	0	0	0	0	13	5	5	3	26	20
Sonora	0	0	0	1	32	2	6	9	50	30
Tabasco	0	0	0	0	0	2	1	0	3	2
Tamaulipas	0	0	0	0	2	3	6	0	11	9
Tlaxcala	1	0	0	2	0	0	8	22	33	24
Veracruz	0	1	0	9	20	8	10	15	63	31
Yucatán	0	2	0	6	16	3	2	3	32	13
Zacatecas	0	0	0	6	6	4	9	1	26	16

Parasite taxa	Number of families	Number of genera	Number of species
Trematoda	3	4	5
Cestoda	5	7	11
Acanthocephala	1	1	1
Nematoda	13	29	53
Acari	16	58	204
Anoplura	2	6	40
Ischnocera	1	3	67
Siphonaptera	9	49	136