In Mesa Verde the greatest incidence of infestations is in late September and early October. This agrees with the finding of other investigators (Sealander, 1961:58).
Sealander (1961) investigated hematological values in deer mice infected with botflies, and found that infected mice had significantly lower concentrations of hemoglobin than non-infected mice. Myiasis, associated with infection by Cuterebra, is likely to lead to a lowering of the physiological resistance of a segment of the population, and perhaps to a subsequent decline in the population (Sealander, 1961:60).
Mice infected by warbles were less agile than non-infected mice. Other investigators also have reported awkwardness in locomotion in infected mice (Scott and Snead, 1942:95; Sealander, 1961:58). Test and Test (1943:507) noted that parasitized mice did not appear to be emaciated, and this was also true of parasitized mice at Mesa Verde. Healed wounds, where warbles had emerged, were apparent on a number of mice. The warbles, and wounds, usually were found on the flanks and backs of the mice. The large, third instar larvae weighed about one gram apiece; there is little doubt that such large larvae induce trauma in their hosts.
The highest rate of infestation by botflies occurred in 1961, the year in which the population density of P. maniculatus was near its peak. The population of this species was reduced considerably in 1962, and remained low through 1964. In 1965, the density of P. maniculatus appeared to be increasing. Other investigators have reported that increased incidence of Cuterebra infestation in deer mice coincides with lower population densities and with a downward trend in the population (Scott and Snead, 1942:95; Wilson, 1945). My data indicate that this may not be the situation in Mesa Verde.
The intestines or stomachs of almost all individuals of P. maniculatus contained parasites. Endoparasites were less abundant in individuals of P. truei. This heavier infestation of P. maniculatus by tapeworms, roundworms, and spiny-headed worms probably reflects the larger proportion of insects eaten by P. maniculatus than by P. truei.
The most common endoparasite encountered was the nematode, Mastophorus numidica Seurat, 1914; it was found in the stomachs of many individuals of both species of Peromyscus. This nematode has been reported from Felis ocreata in Algeria, Bitis arietans in the Congo, and from the following mammals in the United States: Canis latrans, Peromyscus crinitus, P. gossypinus, P. maniculatus, P. truei, Onychomys leucogaster, Dipodomys ordii, Reithrodontomys megalotis, and Eutamias minimus.
Individuals of P. maniculatus obtained on the northern end of Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At this time, the population of P. maniculatus was high, but on a downward trend.
My data and observations lead me to conclude that individuals of P. maniculatus are more heavily parasitized by both botflies and endoparasites than are individuals of P. truei. The reasons for this unequal amount of parasitism in two species of mice occurring in the same general area remain obscure.
The kinds of endoparasites and ectoparasites collected from P. maniculatus and from P. truei are listed below (m = present in P. maniculatus, t = present in P. truei).
Acarina: Ixodidae: Dermacentor andersoni mt, Ixodes angustus mt, Ixodes spinipalpis m. Laelaptidae: Androlaelaps glasgowi m. Myobiidae: Blarinobia sp. m. Trombiculidae: Euschoengastia lanei mt, Euschoengastia criceticola m, Euschoengastia dicipiens t, Euschoengastia peromysci m, Leewenhoekia americana m, Trombicula loomisi m.