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Two morphologically distinct types of Giardia sp. occur in cotton rats, Sigmodon hispidus.

ABSTRACT. -- A survey of hispid cotton rats, Sigmodon hispidus, for enteric protozoa revealed 16 of 19 (84 percent) rodents in Kansas to be infected with Giardia sp. Six of 19 (32 percent) also harbored Entamoeba muris. Two morphologically distinct types of Giardia were noted, designated as Types I and II. This is the first report of Giardia or E. muris from S. hispidus. Key words: Giardia sp.; Sarcomastigophora; Diplomonadida; Entamoeba muris; Sphaerita sp.; Sigmodon hispidus; cotton rat.


In December 1987, a single fecal sample collected from a hispid cotton rat, Sigmodon hispidus Say and Ord, 1825, from Dallas County, Texas, was submitted by CTM to the parasitology laboratory at Kansas State University (KSU) for routine diagnosis. Giardia sp. cysts were noted and the sample was discarded. It was only later that we discovered Giardia sp. apparently never had been reported from cotton rats (Cameron and Spencer, 1981; Pfaffenberger and de Bruin, 1988). This prompted a survey of S. hispidus so that a proper description of the flagellate from this host could be reported.

In January 1990, 19 cotton rats were captured from various localities in Kansas using Sherman live traps (H. B. Sherman Inc., Tallahassee, Florida). The animals were transported to KSU, killed by an overdose of ether, and duodenal scrappings and caecal contents from them were placed in Merthiolate-Iodine-Formalin (MIF) fixative (Pritchard and Kruse, 1982). Preserved material was smeared onto glass microscope slides, stained in a 1:1 mixture of Polychrome IV (Devetec Inc., Bradenton, Florida) and Mayer's hematoxylin according to the manufacturer's recommendations for Polychrome IV staining, and mounted permanently in Canada balsam. Slides were examined using an Olympus BH-S brightfield photomicroscope equipped with a 100X oil immersion lens. Ten trophozoites were measured from each of four rats using a calibrated ocular micrometer; 20 were of Type I and 20 of Type II. All trophozoites measured possessed median bodies. Measurements of the two types of Giardia were compared statistically using the Wilcoxin Mann-Whitney U-test with the hypothesis that they should not be significantly different. Significance was expressed using a confidence limit of P > 0.05.

Of the 19 rodents examined, 16 (84 percent) were found to be infected with Giardia sp. Two morphologically distinct forms were observed (Table 1, Figs. 1 and 2). Type I trophozoites were broad and contained ellipsoid or teardrop-shaped median bodies. In only five of 11 (45 percent) rats collected in Butler County were these forms observed. Type II trophozoites were more elongate and contained an elongate median body. The prevalence of infection was 1/1 from Pottawatomie County, 1/3 from Osborne County, and 4/11 (36 percent) from Butler County. A single rat from Butler County harbored both forms of the parasite. In addition to Giardia sp., trophozoites of Entamoeba muris were found in the caecae of six animals. The prevalence of infection was 1/4 from Riley County, 1/1 from Pottawatomie County, 3/3 from Osborne County, and 1/11 (nine percent) from Butler County. Some of the trophozoites also were seen to contain the parasitic fungus, Sphaerita sp.


The Giardia sp. observed were designated as Types I or II using the criteria of Grant and Woo (1977). These authors found G. muris in laboratory mice (Mus musculus) and a feral Norway rat (Rattus norvegicus) and G. mesocricetus from golden hamsters (Mesocricetus auratus) to be broadly ovoid (Type I), whereas G. microti from meadow voles (Microtus pennsylvanicus), G. peromysci from deer mice (Peromyscus maniculatus), and G. simoni from R. norvegicus are much more elongate (Type II). Later studies revealed some isolates were capable of infecting different genera of rodents (Grant and Woo, 1978). Whereas G. simoni and G. peromysci failed to establish infections in rodents other than the species from which they were collected, G. microti infected hamsters, G. mesocricetus, laboratory rats (R. norvegicus), and G. muris from the feral wild rat infected laboratory mice and hamsters. A similar isolate of G. muris from laboratory mice was not capable of establishing infections in rats or hamsters, however. These studies suggested the possibility that representatives of both types of Giardia sp. could, in theory, infect the same species of rodent under natural conditions. This was the case in our study.

Comparing the forms we observed with measurements provided by Grant and Woo (1977), Type I trophozoites from S. hispidus most closely resembled G. mesocricetus in size. Type II trophozoites from S. hispidus are similar to G. simoni. In light of experimental transmission studies by Grant and Woo (1978), and considering that natural hosts for both species discussed above are prevalent at our study sites, it seems plausible that we are dealing with previously described species of Giardia and thus we refrain from naming new taxa at this time.

This research was supported by Kansas Agricultural Experiment Station grant no. KAN081865. This is Kansas Agricultural Experiment Station Contribution no. 91-85-J.
TABLE 1. Measurements (in micrometers) comparing two types of Giardia
from Sigmodon hispidus.

Measurement Type I (1)

Body length 12.9 (12.0-13.6) [+ or -] 0.5
Body width 9.5 (9.0-10.4) [+ or -] 0.4
Disk length 7.2 (6.4-7.6) [+ or -] 0.3
Disk width 7.4 (7.0-8.0) [+ or -] 0.3
Median body length 2.4 (1.6-3.2) [+ or -] 0.5
Median body width 1.6 (1.0-2.6) [+ or -] 0.4
Nucleus length 3.8 (3.0-4.8) [+ or -] 0.5
Nucleus width 2.4 (2.0-3.0) [+ or -] 0.2
Distance between nuclei 3.7 (2.6-4.0) [+ or -] 0.5
Distance from posterior 1.1 (0.0-1.8) [+ or -] 0.7 (2)
 end of disk to middle
 of median body

Measurement Type II (1)

Body length 15.3 (14.4-17.6) [+ or -] 0.8
Body width 8.2 (8.0-9.0) [+ or -] 0.3
Disk length 6.6 (5.6-8.0) [+ or -] 0.6
Disk width 7.1 (6.4-7.6) [+ or -] 0.3
Median body length 3.4 (2.8-4.2) [+ or -] 0.4
Median body width 1.3 (0.8-2.2) [+ or -] 0.5
Nucleus length 3.5 (3.0-4.8) [+ or -] 0.6
Nucleus width 2.2 (1.6-2.6) [+ or -] 0.3
Distance between nuclei 3.1 (2.6-3.6) [+ or -] 0.3
Distance from posterior 0.4 (0.0-1.2) [+ or -] 0.4 (2)
 end of disk to middle
 of median body

(1) Mean (range) [+ or -] standard deviation. All corresponding
measurements between the two types of trophozoites are significant at P
< 0.05, except for nucleus lengths and widths where no significant
differences were found to exist. N=20 for each measurement.
(2) Type I trophozoites have a median body anterior to the posterior end
of the adhesive disk whereas Type II trophozoites have a median body
posterior to the disk.


Cameron, G. N., and S. R. Spencer. 1981. Sigmodon hispidus. Mammalian Species, 158:1-9.

Grant, D. R., and P. T. K. Woo. 1977. Comparative studies of Giardia spp. in small mammals in southern Ontario. I. Prevalence and identity of the parasites with a taxonomic discussion of the genus. Canadian J. Zool., 56:1348-1359.

_____. 1978. Comparative studies of Giardia spp. in small mammals in southern Ontario. II. Host specificity and infectivity of stored cysts. Canadian J. Zool., 56:1360-1366.

Pfaffenberger, G. S., and D. de Bruin. 1988. Parasites of the hispid cotton rat, Sigmodon hispidus (Cricetidae), and population biology of the cotton rat louse, Hoplopleura hirsuta (Hoplopleuridae: Anoplura), in eastern New Mexico, including an annotated host-parasite bibliography. Texas J. Sci., 40:369-399.

Pritchard, M. H., and G. O. Kruse. 1982. The collection and preservation of animal parasites. Tech. Bull. 1, Harold W. Manter Lab., Univ. Nebraska Press, Lincoln, 141 pp.


Division of Biology, Ackert Hall, Kansas State University, Manhattan, Kansas 66506, and Renal-Metabollic Laboratory (151-G), Department of Veterans Affairs Medical Center, 4500 South Lancaster Road, Dallas, Texas 75216
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Author:Upton, Steve J.; Brillhart, Dianne B.; McAllister, Chris T.
Publication:The Texas Journal of Science
Geographic Code:1U4KS
Date:Nov 1, 1991
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