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Metazoan parasites of Peromyscus pectoralis (Rodentia: Muridae) in central Texas.

Abstract.--New records for Peromyscus pectoralis as a host species were confirmed for nine arthropods: three fleas (Jellisonia bullisi, J. ironsi, and Atyphloceras echis echis), one tick (Ornithodoros sp.), two mites (Androlaelaps fahrenholzi (=Haemolaelaps glasgowi) and Omilhonyssus sp.) two chiggers (Euschoengastia fronteriza and Euschoengastia criceticola) and one sucking louse (Hoplopleura hesperomydis) and six helminths: one nematode (Carolinensts tuffi), two trematodes (Scaphiostomum Pancreaticum and Zonorchis Komareki), two cestodes (Catenotaenia dendritica and a larval cewtode, Strobilocercus sp.), and one acanthocephalan (Moniliformis clarki). Carolinensis tujji and Zonorchis Komareki had the highest prevalence and total abundance.

Metazoan parasites have been reported for 19 of the approximately 60 species of Peromyscus (Hall 1981); however, except for P. boylei, P. floridanus, P. leucopus, P. maniculatus, and P. truei, few comprehensive parasite surveys on individual species of white-footed mice (Peromyscus) have been completed (Layne 1963; Whitaker 1968). Few records exist for metazoan parasites of the white-ankled mouse, Peromyscus pectoralis (cf. Schmidly 1974). The first parasite reported for this species was a flea, Pleochaetoides (= Jellisonia, Traub 1950) bullisi Auguston, from Camp Bullis, Bexar County, Texas (Auguston 1944). Barnes et al. (1977) reported another flea, Anomiopsyllus nudatus hiemalis Eades & Menzies (- A. hiemalis), parasitizing P. pectoralis in Big Bend National Park, Brewster County, Texas. The first chigger, Microtrombicula welbourni Loomis & Webb, was described from Val Verde County, Texas (Loomis & Webb 1972). Another chigger, Euschoengastia chisosensis Wrenn, Baccus & Loomis was recorded from Boulder Meadow, Big Bend National Park, Brewster County, Texas (Wrenn et al. 1976). Gingrich & Barrett (1976) conducted laboratory infestations of P, pectoralis with the dipteran, Cuterebra fontinella Clark. Sabrosky (1986) identified a new species, Cuterebra clarki Sabrosky, from P. pectoralis collected at Bustamente, Nuevo Leon, Mexico. Durette-Desset & Santos (2000) identified a new species of trichostrongylid nematode, Carolinensis tuffi Durette-Desset & Santos from the small intestine of Peromyscus pectoralis at Colorado Bend State Park in Lampasas County, Texas. No tick, louse, or mite species have been previously reported for the white-ankled mouse (Schmidly 1974). The objectives of this study were to: 1) survey the metazoan parasites of P. pectoralis in its natural environment, 2) conduct an ecological survey of the prevalence and intensity of metazoan parasites, and 3) examine variation in prevalence and intensity of metazoan parasites with respect to host age, sex, or month of collection.

MATERIALS AND METHODS

Study area.-Colorado Bend State Park (CBSP), a 2,160-ha facility (31[degrees]02'35.9"N, 98[degrees]28'12.3"W) owned and operated by the Texas Parks and Wildlife Department, is located 25.6 km south and 24.8 km east of San Saba, Texas in San Saba and Lampasas counties, Texas. The park is replete with rolling hills, caves, and steep bluffs bordering the Colorado River which bisects the park. The elevation ranges from 305 m to 440 m (Schwausch 1997). Average rainfall is 64.75 cm (Data for 1897 to 1997, NOAA 1998). A drought occurred during most of the study, and rainfall for some months in 1996 was 3.6 cm below the annual mean.

Parasites were collected from P. pectoralis inhabiting four distinct habitat types (riparian, grassland, rocky ridge, and Ashe juniper (Juniperus ashei) woodland) (Anonymous 1989; Schwausch 1997). Riparian habitat occurs on steep limestone bluffs along the Colorado River and is dominated by tall trees of Pecan (Carya illinoensis), American elm (Ulmus americana), sugarberry (Celtis laevigata), and sycamore (Platanus occidentalis) (Schwausch 1997). The grassland habitat has Texas wintergrass, little bluestem (Schizachyrium scoparium), King Ranch bluestem (Bothriochloa ischaemum), buffalograss (Buchloe dactyloides), and sideoats grama (Bouteloua curtipendula) as dominants with patches of prickly pear (Opuntia lindheimeri) and shrubs and trees of Ashe juniper and Texas persimmon (Diospyros texana) (Schwausch 1997). The rocky ridge habitat is predominantly a shrubland of prickly pear, Texas persimmon, evergreen sumac (Rhus virens), Ashe juniper and plateau live oak (Quercus fusiformis) (Schwausch 1997). The composition of the Ashe juniper woodland is primarily Ashe juniper shrubs and trees (Schwausch 1997).

Collecting methods.-Mice were live-trapped using Sherman live traps (H. B. Sherman Traps, Tallahassee, FL) baited with oats or birdseed. All mice were euthanized according to Guidelines of the American Society of Mammalogists for the Use of Wild Mammals in Research (Ad Hoc Committee on Acceptable Field Methods in Mammalogy 1987), Texas State University Animal Care and Use Committee Permit Number 1015, and Texas Parks and Wildlife Scientific Collecting Permit Number SPR-0890-234. Total length, tail length, hind foot, ear length, weight, age, and sex were recorded; if a female, the number of fetuses was noted. Age was assigned based on distinct pelage colors for juvenile (gray), subadult (mixture of gray and dull brown), and adult (rich brown) mice (Brown 1963; Schmidly 1974). In addition to P. pectoralis, the rodent community was composed of Reithrodontomys fulvescens, P. maniculatus, Baiomys taylori, and Sigmodon hispidus. Peromyscus pectoralis accounted for 91.7% of rodents collected.

Ectoparasites were preserved in 70% glycerated ethanol. The subcutaneous region was examined for the presence of cuterebrids and filarial nematodes. Commonly parasitized organs (lungs, liver, gall bladder, stomach, small intestine, caecum, and large intestine) were removed and examined for presence of parasites (see Schmidt 1992). All adult and larval stages of cestodes, trematodes, nematodes, and acanthocephalans were removed and washed in deionized water. After this wash, all parasites (with a few exceptions noted later) were stored in 70% ethanol.

Microscopic examination.--'Preparatioa of trcmatode, cestode, and nematode specimens for microscopic examination followed Schmidt (1992). In addition, nematodes were cleared in lacto-phenol and stored in 70% ethanol. Specimens were stained in aceto-carmine, destained in 70% acidulated alcohol until light pink, and then dehydrated in a stepwise manner with 1 h each in the following concentrations of ethanol: 85%, 95%, 100%, and 100% again. After these ethanol dehydrations, worms were cleared in xylene by placing them first in a 50% xylene/absolute ethanol solution for 1 h and then 100% xylene twice for 1 h each. After clearing in xylene, worms were permanently mounted in Canada balsam and dried.

After washing in deionized water, cestodes and trematodes were immediately placed in 10% formalin for overnight fixation. Prior to staining, worms were removed from formalin and placed in distilled water to remove all fixative. Specimens were then stained in a 1:15 stock dilution of Wright's hematoxylin for 24 h, rinsed in distilled water to remove excess stain, and dehydrated in 35%, then 50% ethanol for 1 h each. Worms were then destained in 70% acidulated ethanol until a light pink color. Destained worms were then dehydrated, cleared, mounted, and dried. The single acanthocephalan was stained in aceto-carmine, destained in 70% acidulated alcohol, and stored in 70% ethanol.

Identification.--All parasites were identified to species when possible. If parasites could not be matched with species currently reported in the literature, an attempt was made to determine whether the organism was new to science, or its apparent uniqueness could be attributed to geographical or host-induced differences. Specimens were sent to recognized authorities for conformation of identifications and in some cases, identification. Fleas were sent to R. E. Lewis (retired), Iowa State University. The single specimen of a tick and all adult mites were sent to F. J. Radovsky, Oregon State University, for identification after mounting in CMC-S 10. Chiggers were also mounted in CMC-S 10. Chiggers were sent to William J. Wrenn (retired), University of North Dakota for identification. Lice were sent to L. A. Durden, Institute of Arthropodology and Parasitology, Georgia Southern University. Helminths, excluding the acanthocephalan, were sent to J. M. Kinsella (retired), University of Montana, for confirmation. B. B. Nickol, University of Nebraska at Lincoln, identified the single specimen of acanthocephalan.

Voucher specimens.--Voucher specimens are deposited in various collections. R. E. Lewis assigned voucher numbers 61978 through 62006 to flcas identified and retained in his personal collection. Lice are in the ectoparasitic collection of the Institute of Arthropodology & Parasitology at Georgia Southern University and have the following accession number: RML 122,473. The tick, all adult mites, and all helminths, including the acanthocephalan are in the invertebrate collection at Texas State University.

Analysis of data.--The following ecological parameters were examined: 1) variations in endoparasitcs prevalence and intensity with respect to season of collection, 2) variations in endoparasites abundance due to the host sex, 3) variations in endoparasites ecology due to host age, 4) variations in endoparasites ecology due to host locality, and 5) variations between endoparasites prevalence and intensity as they relate to host body length. Ecological terminology was based on Bush et al. (1997). Prevalence, expressed as a percent, was defined as number of hosts parasitized by 1 or more individuals of a particular parasite species divided by number of hosts examined multiplied by 100. Intensity of infection was the number of individuals of a particular parasite species on/in a single infected host. Locality referred to the geographical locale of the external environment where the parasite occurred and a host captured; whereas, location of a parasite was the topological or spatial location in/on the host where a particular sample of parasites was collected. Mean abundance referred to the total number of individuals of a particular parasite species in a sample of a particular host divided by the total number of hosts of that species examined, both infected and uninfected.

The Significance Test for Comparing Two Proportions (Moore & McCabe 1993) was used to test prevalence of parasites between populations of mice. There was no difference in helminthes infecting males (n = 98) and females (n = 91) in prevalence, so males and females were pooled in further analyses. Intensity between populations was compared by the Mann-Whitney U Test (Zar 1984). The notations for one- or two-tail tests and degrees of freedom in reporting the results of statistical tests followed the subscript format of Zar (1984).

RESULTS

Trematoda.-Twenty-three specimens of the pancreatic fluke, Scaphiostomum pancrealicum McIntosh, occurred exclusively in the pancreatic duct of 15 mice. Flukes were assigned to the family Brachylamidae based on long and filiform bodies with the genital pore and associated reproductive structures in the posterior-most region. Peromyscus pectoralis is a new host record.

A total of 589 specimens of the liver fluke, Zonorchis komareki Mcintosh were collected from bile ducts or gall bladders (McIntosh 1939) and occasionally in the small intestine proximate to the bile duct opening. The largest of these flukes was approximately 3 mm in length and about 0.75 mm in width at its widest point. Peromyscus pectoralis is a new host record.

Cestoda.--Thirty-six mice contained the adult cestode, Catenotaenia dendritica (Goeze). This craspedate cestode has four suckers on an unarmed scolex. Mature proglottids contained between 100 and 120 testes. The cestodes recovered had fewer testes than reported for C. dendritica, but too many to meet the description of the alternative species, C. peromysci Smith (Yamaguti 1971). In addition to this tapeworm, a single mouse had one larval cestode on the surface of the liver. It was a strobilocercus type and incomplete organogenesis of the specimen prohibited any further identification. Peromyscus pectoralis is a new host record.

Acanthocephala.--A single male specimen of the acanthocephalan Moniliformis clarki (Ward) was recovered from the small intestine of one mouse. Peromyscus pectoralis is a new host record.

Nematoda.--One trichostrongylid nematode species, Carolinensis tuffi Durctte-Desset & Santos, was recovered from the small intestine (Durette-Desset & Santos 2000). Peromyscus pectoralis is currently the only known host for this nematode.

Another nematode was found in one mouse but did not have enough visible internal morphology for identification. This nematode, also found in the small intestine, may be a juvenile form of C. tuffi, or some other species.

Arthropoda.--Three species of fleas were recovered from P. pectoralis: Jellsonia bullisi (Augustson), J. ironsi (Eads), and Atyphloceras echis echis (Jordan & Rothschild). Most host individuals had no fleas. Peromyscus pectoralis is a new host for J. ironsi and A. echis echis.

One damaged larval tick, Ornilhodoros sp. was recovered from one host. Peromyscus pectoralis is a new host record, since no tick has been previously reported for P. pectoralis.

Two mites, Androlaelaps fahrenholzi (Berlese) (= Haemolaelaps glasgowi) and Ornithonyssus sp., were recovered from two host individuals. Peromyscus pectoralis is a new host record for both mites.

The chiggers, Euschoengastia fronteriza Wrenn, Baccus & Loomis and E. criceticola Brennan were found on 10 hosts. Peromyscus pectoralis is a new host record for both chigger species.

Ecology.--Peromyscus pectoralis infected with parasites inhabited the riparian, grassland, rocky ridge, and Ashe juniper woodland habitats. The prevalence of parasites was not skewed toward a specific habitat type. Prevalence, intensity, mean abundance, and total number of individual parasites associated with P. pectoralis were skewed by the abundance of two species, C. tuffi and Z. komareki (Table 1). There was no difference in prevalence [P (Z[greater than or equal to] 0.43) > 0.05] or intensity [P([U.sub.a(2) 53,54] > 1511.5) > 0.05] for helminths infecting males (n = 98) and females (n = 91). Adult mice were more likely to be infected than subadults [p (Z[greater than or equal to] -1.55) < 0.10]. Also, adult mice were more likely to be infected than juvenile mice [P (Z [greater than or equal to] -2.19) < 0.05]. No difference [P (Z [greater than or equal to]-l. 18) > 0.10] was found in prevalence between juveniles and subadults, but the sample size for juveniles was small (n = 15). Adult mice were infected at a higher rate (prevalence) and with an increased intensity compared to juvenile mice. Although prevalence varied somewhat by age group, there was no difference in intensity between juveniles and subadults [P (U.sub.a(1) 7.38] [greater than or equal to] 144) > 0.05] and between juveniles and adults [P (U.sub.a(1) 762] [greater than or equal to] 297) > 0.05]. Prevalence varied by month with most helminths less prevalent in late summer and fall. Overall, there was a sharp decrease in the prevalence of helminthes in fall.
Table 1. Prevalence, intensity, mean abundance and total number of
helminths collected from 189 Peromyscus pectoralis at Colorado Bend
State Park, San Saba County, Texas from February 1996 through
March 1997.

                Prevalence      Intensity
                (a)             (b)

Helminth taxon  n           %   [bar.X][+   Range  Mean       Total
                                or -]SE            abundance
                                                   (c)

Trematoda

Scaphiostomum   15          8   1.5 [+ or   1-4    0.12       23
pancreaticum                    -] 0.3

Zonorchis       53          28  11.1 [+ or  1-61   3.12       589
komareki                        -] 1.5

Cestoda

Catenotaenia    36          19  1.8[+ or    1-5    0.35
dendritica                      -]0.2

Acanthocephala

Moniliformis    1           1   1.0 [+ or          0.01       1
clarki                          -] 0.0

Nematoda

Carolinensis    57          30  11.2 [+ or  1-175  3.38       640
tuffi                           -] 3.3

(a) 1Prevalence = [(# Infected Hosts)/(# Hosts Examined)] * 100
(b) Intensity = (Total # Parasites)/(# Hosts Infected) [+ or -]
Standard Error
(c) Mean abundance = (Total # Parasites)/(# Hosts Examined)


DISCUSSION

Whitaker (1968) suggested the predominantly vegetarian diet of rodents in the genus Peromyscus explains the relatively small number of parasitic helminths. Kennedy (1968) proposed differences in infection patterns of helminths in fish could be related to feeding behavior. Watve & Sukumar (1995) identified host diet as an ecological factor influencing the parasite community by carnivores having higher parasite loads and species richness compared to herbivores, as the diet of the former contained intermediate hosts of a variety of parasites. Diet may in part explain the low prevalence and diversity of trematodes and other endoparasitic species parasitizing the P. pectoralis population sampled at CBSP. Baccus et al. (2009) found substantial amounts of plant matter (range = 62.5%-73.3%) in the seasonal diet of P. pectoralis in central Texas. Pfaffenberger et al. (1985) and Grundmann (1957) discovered low prevalence and intensity of infections by helminthes in primarily herbivorous rodents similar to findings for this study on P. pectoralis. The results of this study suggest the vegetarian diet of P. pectoralis is a prime ecological factor influencing a lower parasitic species richness and prevalence.

Zonorchis komareki had low prevalence (28%) across all age groups, sex, and season variables. Although it had the second highest prevalence and abundance of all helminth species (Table 1), it still rarely or infrequently occurred in the host sample. The intermediate host for Z. komareki is the terrestrial snail, Polygyra texasiana (Moricand). However, gastropod matter formed a miniscule portion of the diet of P. pectoralis in central Texas (Baccus et al. 2009). The presence of this snail is the limiting factor for the incidence or newly acquired infections of Z komareki by P. pectoralis. A study of snails in the park could help in understanding the function of snails in the parasitism of P. pectoralis. Zonorchis komareki also parasitizes Peromyscus gossypinus (Mcintosh 1939, Kinsella 1991) and Peromyscus polionotus (Kinsella 1991).

Scaphiostomum pancreaticum was not as successful a parasite of P. pectoralis as Z. komareki. The terrestrial snail, Anguispora alternate! (Say) is an intermediate host for this trematode, although two other snails, Triodopsis albolabris (Say) and Haplotrema concavum (Say), may serve as secondary intermediate hosts to the microcercous cercaria as well as A. alternata; but only A. alternata can function as the primary intermediate host for the miracidia (Jenson 1972). Thus, the population of A. alternata may be the limiting factor in S. pancreaticum prevalence and intensity. Scaphiostomum pancreaticum spends about nine months of its life cycle in a snail host, and Jenson (1972) postulated this phase may be an overwintering adaptation. Although this assumption may have application in northern North America where Jenson's work was done, P. pectoralis at CBSP were active every month and parasitized by 5. pancreaticum in winter. Snails in the park should be examined to expand on Jenson's work and to determine their function in the pancreatic fluke's life cycle. Scaphiostomum pancreaticum also parasitizes P. gossypinus (Mcintosh 1935).

The low prevalence of the tapeworm, C. dendritica, in summer and fall may be attributed to fluctuations in the population of the tyroglyphid mite, which serves as the intermediate host for the merocercoid larvae. The best method for confirming the cause of the low prevalence would be surveys of mites for the presence of the larvae. Catenotaenia dendritica also parasitizes P. maniculatus (Leiby 1962).

The single male acanthocephalan recovered, Moniliformis clarki, may occur in the park in low numbers due to low populations of necessary intermediate hosts or the mice are not eating substantial quantities of insects. Baccus ct al. (2009) determined insects composed as much as 36.2% of the diet of P. pectoralis in Hays County, Texas depending on season. If this also applies to the diet of mice at CBSP, more acanthocephalans should have been recovered. Moniliformis clarki also parasitizes P. manicnlatus (Grundmann & Frandsen 1960).

Why the nematode C. tuffl has not been previously reported is puzzling. Perhaps, it affects only P. pectoralis and no helminth data existed for this species prior to this study, or similarity of C. tuffi with the closely related Longistriata carolinensis Dikmans (Dikmans 1935) or Nippostrongylus brasiliensis (Travassos 1914) caused misidentification. It is suggested specimens previously reported as L. carolinensis or N. brasiliensis from Peromyscus be reexamined to verify the authenticity of their identification.

The absence of C. tuffi from mice collected in October through December was also puzzling. Since this nematode, as others, is probably spread through direct contact with ova, it should be found in the population year round. Possibly the trichostrongylid exists in another vertebrate host also found in the park, although this conjecture has not been examined. Two mice had an over-dispersion of the parasite load (90 and 175) of this nematode, but they did not appear malnourished.

The overall pattern of endoparasitism for P. pectoralis was a low diversity of species with a limited number of other species of Peromyscus being infected by the endoparasites infecting P. pectoralis.

The distribution of ectoparasites was characterized by their aggregation or overdispersion (Krasnov et al. 2002) on a few host individuals; whereas, most micc had few ectoparasites or none at all (Anderson & May 1978; Poulin 1993).

The genus Jellisonia is a group of ceratophyllid fleas essentially restricted to peromyscine rodents (Peromyscus, Baiomys, Reithrodontomys, Scotinomys) from the foothills of southern Texas to montane Mexico and Central America (Traub et al. 1983). In addition to an association with P. pectoralis, J. bullisi only parasitizes P. eremicus (Traub 1950). Jellisonia ironsi is closely associated with species of the genus Baiomys (Traub et al. 1983, Morrone & Gutierrez 2005). This record from P. pectoralis at CBSP is the first and only host record of this flea from a species of Peromyscus (Whitaker 1968, Morales 1990). Atyphloceras echis is also an ectoparasite of P. boylii (cf. Morlan 1955), P. maniculatus (cf Stark 1958). P. nasutus (cf. Morlan 1955),and P. truei (cf. Morlan 1955) in addition to P. pectoralis.

Ticks of the genus Ornithodoros also occur on P. maniculatus (cf. Cooley & Kohls 1941; Kohls & Clifford 1963), and P. crinitus (cf. Egoscue 1964) in addition to P. pectoralis.

The two mites associated with P. pectoralis inhabit several other species of Peromyscus. Androlaelaps fahrenholzi parasitizes P. boylii (cf. Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1974), Keegan 1953; Witaker & Wilson 1974 ), P. eremicus (cf. Allred 1957 Allred & Beck 1966; Whitaker & Wilson 1974), P.(= podomys)floridamus(cf. Laync 1950a; Morlan 1952;Hays & Guyton 1958 Whitaker & Wilson 1974; Clark & Durden 2002)P leucopus (cf Drummond 1957; Hays & Guytoon 1958 Florsuhutz & Datrsie 1960 Thidale & 1974 Clark & Durden 2002 Ritizi & Whitaker 2003), P maniculatus (cf Keegn 1953;p Allreed 1957; Rapp 1962 Scholten et al 1962 Allred & Goates 1964 Elzinga & Rees 1964;r & Wilson 1968; Whitaker & Wilson 1974; Clark & Durden 2002; Ritzi & Whitaker 2003). P. maniculatus (cf. Keegan 1953; Allred 1957; Rapp 1962; Scholten et al. 1962;1962; Allred & Goates 1964; Elzinga & Rees 1964; Hansen 1964; Lawrence et al. 1965; Allred & Bcck 1966; Whitaker & Wilson 1968; Whitaker & Wilson 1974; Durden & Wilson 1991; Ritzi & Whitaker 2003), P. melanotis (cf. Alvarez-Castaneda 2005), P. polionotus (cf. Morlan 1952; Hays & Guyton 1958; Whitaker & Wilson 1974), P. truei (cf. Keegan 1953; Holdenricd & Morlan 1955; Allred 1957; Allred & Goates 1964; Allred & Beck 1966; Whitaker & Wilson 1974), P. zarhynchus (cf. McClellan & Rogers 1997), and P. spp. (cf. Judd 1950). The genus Ornithonyssus has been associated with P. boy/ii (cf. Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1974), P. crinitus (cf. Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1974), P. eremicus (cf. Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1974), P. floridanus (cf. Layne 1963; Whitaker & Wilson 1974), P. gossypinus (cf. Worth 1950a, 1950b; Morlan 1952; Whitaker & Wilson 1974; Clark & Durden 2002), P. leucopus (cf. Drummond 1957; Whitaker & Wilson 1968; Whitaker & Wilson 1974; Clark & Durden 2002; Durden & Wilson 1991), P. maniculatus (cf. Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1968; Whitaker & Wilson 1974), P. polionotus (cf. Morlan 1952; Whitaker & Wilson 1974 ), P. truei (cf. Holdenried & Morlan 1955; Allred 1957; Allred & Beck 1966; Whitaker & Wilson 1974), and P. spp. (cf. Ellis 1955).

The sucking louse Hoplopleura hesperomydis infects P. californicus (cf. Ferris, 1951; Kim 1965), P. gossypinus (cf. Morlan 1952; Kim 1965), P. leucopus (cf. Osbom 1891; Kellogg & Ferris 1915; Bell & Chalgren 1943; Morlan & Hoff 1957; Wilson 1957; Cook & Beer 1959; Scanlon 1960; Tindale & Darsie 1961; Wilson 1961; Mathcwson & Hyland 1962; Parsons 1962; Kim 1965; Kim et al. 1966), P. maniculatus (cf. Kellogg & Ferris 1915; Ferris 1916; Augustson 1941; Morlan & Hoff 1957; Cook & Beer 1959; Scanlon 1960; Wilson 1961; Scholten et al. 1962; Elzinga & Rees 1964; Hansen 1964; Kim 1965; Lawrence et al. 1965; Kim et al. 1966), P. nasutus (cf. Kim 1965), P. polionotus (cf. Morlan 1952), and P. truei (cf. Morlan & Hoff 1957; Kim 1965) in addition to P. pectoralis.

Other than P. pectoralis, the chigger Euschoengastia fronteriza parasitizes P. difficilis (= P. nasutus) (cf. Hoffmann 1990). Euschoengastia criceticola infests P. boylii (cf. Allred 1952; Brennan & Jones 1954; Brennan & Beck 1955; Gould 1956; Jameson & Brennan 1957; Allred & Beck 1966; Hoffmann 1990), P. leucopus (cf. Brennan & Jones 1954; Gould 1956; Loomis & Bunnell 1962), P. eremicus (cf. Brennan & Beck 1955; Gould 1956; Allred & Beck 1966; Hoffmann 1990), P. leucopus (cf. Loomis 1956), P. maniculatus (cf. Brennan 1948; Brown & Brennan 1952; Brennan & Jones 1954; Brennan & Beck 1955; Gould 1956; Allred 1957; Jameson & Brennan 1957; Allred & Goates 1964; Elzinga & Rees 1964; Allred & Beck 1966; Hoffmann 1990), P. sitkensis (= P. keeni) (cf. Hogan et al. 1993; Kim ct al. 1986), P. melanotis (cf. Loomis & Somerby 1966), P. truei (cf. Brennan & Jones 1954; Gould 1956; Allred & Beck 1966; Hoffmann 1990), and P. spp. (cf. Farrell 1956) in addition to P. pectoralis. Although E. chisosensis was not collected from P. pectoralis in this study, this chigger was collected from P. pectoralis, P. boylei, and P. difficilis (= P. nasutus) (cf. Wrenn et al. 1976).

The prevalence and intensity of parasites fluctuates seasonally on the basis of such factors as sex, month of collection, parasite specificity to a host, predator pressure, or host age, size, body size, gregariousness, or population density (Stock & Holmes 1988, Waive & Sukumar 1995, Little et al. 2006). This study did not reveal a sex bias in the prevalence of helminth infections in P. pectoralis at CBSP (Snyder & Fitzgerald 1987; Pistole 1988; Perez et al. 1995). Older, adult white-ankled mice were parasitized with higher intensities than subadults or juveniles (Anderson & Gordon 1982, Rousset et al. 1996, Soliman et al. 2001), yet larger adult mice ([less then] 95 mm body length) appeared to be infected at a higher rate and with increased intensity, but most noteworthy was the overdispersion in a few host individuals with extraordinarily large total lengths (< 180 mm), while most host individuals had only a few parasites or none at all (Anderson & May 1978, Poulin 1993, Krasnov et al. 2002). Krasnov et al. (2006) found the relationships between prevalence and host size in flea infestations of larger size, older-age class Apodemus agrarius and Microtis arvalis had either a concave or positive linear relationship. In host-parasite systems, a parasite's selection of habitat, host species or host size, may influence parasite-host dynamics (Mangel & Roithberg 1992), which can produce cascading effects on populations that shape the structure of communities (Petchey et al. 2008). A parasite's host selection can be described using a variation of the optimal diet breadth models (Charnov 1976, Stephens & Krebs 1986) where the diet breadth exhibited by parasites is the number of size classes used and intensity of infection (Henry et al. 2006). Larger, older individuals eat more food and therefore have a higher probability of consuming the infective life stage of a parasite. Larger mice can probably accommodate and tolerate more worms. Peromyscus pectoralis with high intensity infections may simply have been older adults with a broader diet, which were at greater risk or had more exposure for infection (Soliman et al. 2001; Hawlena et al. 2005, Krasnov et al. 2006) or suffered from immunoscenescence (Moller & de Lope 1999; Cattadori et al. 2005). However, the herbivorous diet of P. pectoralis may have protected smaller, younger individuals in that many plant parts, ingested by animals as dietary supplements, act as prophylactics or remedials for parasite infection (Dogiel 1964). Another explanation for the higher intensity of infection of larger, older mice may be host quality because large hosts (older mice) contain more resources; therefore, they are supposed to be higher quality and represent a greater resource than small hosts (Charnov et al. 1981; Charnov 1982; Jones 1982; Liu 1985; Mackauer 1986; Opp & Luck 1986; King 1988).

Most parasites became less prevalent in late summer and fall. Intermediate hosts populations may have influenced the prevalence of trematodes and cestodes but failed to explain fluctuations in nematode populations in P. pectoralis where no intermediate host was involved in transmission of the parasite. Overall, there was a sharp decrease in the prevalence of helminthes in October shortly after the first frost of the season.

ACKNOWLEDGEMENTS

We appreciate the interest and help from D. G. Huffman, R. W. Manning, T. R. Simpson, D. E. Lemke, and two anonymous reviewers. We thank the Department of Biology at Texas State University for financial support. Thanks to Drs. J. M. Kinsella, B. B. Nickol, M. C. Durette-Desset, R. E. Lewis, L. A. Durden, F. J. Radovsky, and W. J. Wrenn for determining the species of parasites. K. Schwausch, T. W. Schwertner, D. O. Zamora, L. Russell, and T. Pilcik assisted with field work. Thanks, also, to the staff at Colorado Bend State Park, especially R. Basse.

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JTB at: jb02@txstate.edu

Alberto Santos, III, Donald W. Tuff, and John T. Baccus

Department of Biology, Texas State University

San Marcos, Texas 78666
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