Squirrel fibromatosis vs. bot fly infestation:

Hosts


FIBROMATOSIS       Squirrel fibromatosis frequently has been reported in eastern gray squirrels (Sciurus carolinensis) and also in fox squirrels (S. niger; Fig. 1), with juveniles appearing to be more susceptible than adults (Kilham, 1955; Wolf, 1991; Miller, 1992;
Fig. 1: Gray (left) & fox squirrels are natural hosts of both squirrel fibroma virus and the tree squirrel bot fly
see Geographic distribution & seasonality for additional references [Link not yet available]). One published report describes two western gray squirrels (S. griseus) in California with skin lesions containing "pox-like particles" but whether this was the same virus that causes squirrel fibromatosis in the eastern and midwestern states was not addressed (Regnert, 1975). Wildlife rehabilitators contacted by us who work with squirrels west of the Great Plains have not seen fibromas in these animals (see Geographic distribution & seasonality for details [Link not yet available]). Further, we are not aware of any confirmed reports of fibromatosis in the other eastern tree-living squirrels (i.e., the American red or pine squirrel [Tamiasciurus hudsonicus] and the flying squirrels [the northern flying squirrel, Glaucomys sabrinus, and the southern, G. volans]; Fig. 2) or in any
Fig. 2: We are aware of no reports of southern flying squirrels infected with fibroma virus, & they appear to be rarely parasitized by bot fly larvae
other tree squirrel species in North America. Whether this apparent variation in susceptibility of different squirrel species to fibroma virus is caused by differences in their innate resistance to this virus and/or in their propensity to be fed upon by the presumed mosquito vectors, or by other factors, is unknown.

      In laboratory studies, other animals to which squirrel fibroma virus has been successfully transmitted via experimental inoculations include woodchucks (Marmota monax) and domestic rabbits (Oryctolagus cuniculus), although in the latter, progression of the disease was limited and the fibromas regressed a few days after they appeared (Kilham, 1955; Kirschstein et al., 1958; Hirth et al. 1969). Animals in which no lesions were produced after inoculation include laboratory mice, guinea pigs, chinchillas and golden hamsters (Hirth et al. 1969; Kirschstein et al., 1958). There is one report (from 1938) of a porcupine (Erethizon dorsatum) that may have had some form of fibromatosis (Herman & Reilly, 1955). Although some poxviruses, especially certain members of the Orthopoxvirus and Parapoxvirus groups, can infect a relatively broad range of animals (e.g., hosts of cowpox virus include rodents, felines, bovines and humans), most, including squirrel fibroma virus, are more host-specific and have not been reported from pets (cats and dogs) or people (Fraser et al., 1991; Jubb et al., 1993).

BOT FLY INFESTATION       The tree squirrel bot fly belongs to a group of over 30 species of bot flies that infest either rodents (mice, rats, voles, tree squirrels, etc.) or lagomorphs (rabbits and hares), with each species of bot fly infesting a restricted group of host species (Sabrosky, 1986). As with squirrel fibromatosis, the natural hosts of the tree squirrel bot fly are eastern gray and fox squirrels (see Fig. 1), but unlike fibromatosis, this bot fly also infests American red (pine) squirrels and eastern chipmunks (Tamias striatus; Sabrosky, 1986). Flying squirrels (see Fig. 2) seem to be only rarely infested (Kenyon & Slansky, 2000), for unknown reasons. Apparently, no other species of North American tree-dwelling squirrels serve as natural hosts, most likely because they are located outside of the range of this species of bot fly, which occurs from southern Canada to Florida east of the Great Plains (Sabrosky, 1986).

Fig. 3: Four- to five-week-old suckling eastern gray squirrel with a bot fly warble on its side near the base of its right front leg, infested prior to leaving its nest
      Our observations of eastern gray squirrels in northcentral Florida indicate that it is primarily subadults and adults which are infested by bot fly larvae. Juvenile squirrels are less likely to become parasitized because the summer bot fly 'season' (i.e., the period when eggs and infective-stage larvae are present) ends before most of these begin to venture forth from their nests (Kenyon & Slansky, 2000). However, suckling squirrels that have yet to leave their nest sometimes become infested with bot flies (Fig. 3; see Exposure & transmission for discussion of this phenomenon). Occasionally, animals other than rodents and lagomorphs become infested by representatives of this group of parasitic flies; these have included raccoons, cats, dogs, humans and other atypical hosts (for further discussion, see Exposure & transmission).

     Frank Slansky & Lou Rea Kenyon || fslansky@ufl.edu
     Version 1.1 (July 26, 2001)