Larvae of Cuterebra spp. (Diptera: Oestridae) typically infest mice, rats, chipmunks, rabbits, hares, and certain other rodents or lagomorphs native to the Americas (Sabrosky 1986). However, non-native members of these orders, as well as mammals in other orders such as artiodactyls (e.g., deer and pigs), carnivores (raccoons, domestic cats and dogs, etc.), and primates (humans), can be parasitized by these bot flies (Hall 1925; Sabrosky 1986; Baird et al. 1989; Glass et al. 1998; Harris et al. 2000; Suedmeyer et al. 2000; Safdar et al. 2003; Slansky & Huckabee 2006; Slansky 2007a). Infestation of these atypical or accidental hosts, an affliction termed cuterebrosis or cuterebriasis, occurs when they contact first instars of Cuterebra spp. newly hatched from eggs oviposited in the habitats of the typical hosts (Catts 1982; Slansky & Kenyon 2003).
Published cases of cuterebrosis in perissodactyls (donkeys, horses, zebras, etc.) are few and mostly unsubstantiated. Knipling & Bruce (1937) reported that a second instar of Cuterebra buccata (Fabricius), a lagomorph-infesting species, was removed from a mule in South Carolina (16-VII-1935), but they provided no justification for this species assignment. Second instar Cuterebra spp. are very difficult to identify visually to species (even currently) and there were no descriptions of second instars of other Cuterebra spp. for comparison at the time of Knipling & Bruce's (1937) report. Thus, I consider the assignment of C. buccata for their specimen as unsubstantiated. Dalmat (1942) indicated that asses (donkeys) were atypical hosts of Cuterebra spp., but no details were given. Similarly, Sabrosky (1986) mentioned mules as atypical hosts of these bot flies, but supportive information was not provided.
During curation of the immature insect collection in the Department of Entomology & Nematology at the University of Florida, an isopropanol-preserved larva that appeared to belong to a Cuterebra species was encountered. This specimen had been collected from beneath the hide of a horse in Marion Co., Florida (11-VII-1991). This specimen possesses characteristics typical of a third instar Cuterebra spp., including dark brown cuticular platelets covering its body (Fig. 1a), 2 pair of sensory tubercles and 2 sharply pointed mouth hooks on its head (Fig. 1b) and on its anal segment (Fig. 1c), 2 kidney-shaped spiracular plates, each containing 3 wedge-shaped sections with serpentine spiracular slits (Fig. 1c inset). The larva is coated with a substantial amount of white material, which most likely is purulent matter from the host, as is often seen when these larvae infest atypical hosts (Slansky 2007b).
The species of Cuterebra that this specimen belongs to is unclear, as there are no taxonomic keys for species identification of larvae in this genus. However, the platelets covering its exterior either are ovoid, have blunt, single tips, or are multipointed, indicating this is a larva of a rodent-infesting Cuterebra species rather than that of a lagomorph-infesting species, third instars of which lack flattened, multipoint platelets and have many single-pointed, spine-like platelets (Knipling & Brody 1940; Dalmat 1942; Bennett 1955; Haas & Dicke 1958; Baird & Graham 1973; Slansky 2007a). There are three rodent-infesting species of Cuterebra in Florida: C. americana (Fabricius) (its main typical hosts are Neotoma woodrats), C. fontinella (Clark) (Peromyscus mice) and C. emasculator Fitch (Sciurus squirrels) (Slansky 2006). Cuticular platelet patterns are available only for the latter two species (Bennett 1955; Slansky & Huckabee 2006), and of these, this specimen more closely resembles C. fontinella (F.S., unpublished observations). However, determining the species of this specimen must await a morphological description of the third instar of C. americana, knowledge of intraspecific variation in morphological characteristics of these larvae (see Slansky & Huckabee 2006), and/or molecular analyses (Otranto et al. 2003; Noel et al. 2004) of the candidate species.
This appears to be the first record for Florida of cuterebrosis in a horse or other perissodactyl, and the first such record for a horse in North America. Based on the scarcity of published reports, infestation of these animals by larvae of Cuterebra appears to be a very rare phenomenon that should pose little concern to horse owners. This contrasts with the common occurrence of larvae of Gasterophilus spp. bot flies, which use horses and other perissodactyls as typical hosts and can require treatment to avoid pathological effects (Wall & Shearer 1997; Catts & Mullen 2002). One might expect that these animals would be exposed to Cuterebra spp. larvae both while grazing, especially lagomorph-infesting species that likely oviposit in grassy areas (e.g., Suedmeyer et al. 2000), and in their stalls, especially rodent-infesting species that may inhabit barns (F. S., unpublished observations). Thus, the rarity of published reports of cuterebrosis in perissodactyls may indicate that these atypical hosts exhibit resistance to parasitism by Cuterebra spp. larvae. Laboratory studies involving controlled infestations have shown that certain mammals outside the typical host spectrum of the Cuterebra spp. being tested can exhibit partial to complete resistance (reviewed in Slansky 2007b). Of course, failure in publishing cases of cuterebrosis involving these animals could also contribute to the scarcity of reports. A survey of large-animal veterinarians would be required to investigate the prevalence of cuterebrosis in these animals.
I thank Dr. Dale Habeck for bringing this specimen to my attention, Dr. Craig Welch for taking the photomicrographs used in this study, and Douglas Beckner for assistance in preparing the images. This work was supported in part by an instructional improvement grant from the College of Agricultural and Life Sciences at the University of Florida.
A third instar of a rodent-infesting Cuterebra spp. removed from under the hide of a horse in Marion Co., Florida, is described and illustrated. This appears to be the first record for Florida of cuterebrosis in a horse or other perissodactyl, and the first such record for a horse in North America.