Squirrel fibromatosis vs. bot fly infestation:
External appearance
[Note: clicking on some of the figures below will provide larger images
for viewing. Credits for the photos of eastern gray and fox squirrels with fibromatosis are indicated with each image; all photos of bot fly-infested squirrels (eastern grays) were taken by F. Slansky & L. R. Kenyon.]
NUMBER, LOCATION & SIZE OF 'LUMPS'
Squirrels with fibromatosis can have a few to more commonly many (upwards of 100) lesions and nodules over much of their body, including on the head (especially around the
eyes and on the eyelids), torso, legs, toes and tail (Fig. 1), as well as internally (Kilham, 1955; O'Connor et al., 1980). In contrast, bot fly-infested squirrels typically have only a few warbles (in our experience, seldom more than 5-6), usually on their shoulders, back and sides (Fig. 2). Occasionally, warbles are located on the squirrel's head or limbs, but we have never seen them on the toes or tail (Slansky & Kenyon, 2000).
|
Fig. 1: Eastern gray squirrel whose death was associated with fibromatosis, with
fibromas on its head, torso, legs and paws (Photo: Dr. S.P. Terrell)
|
Fig. 2: Eastern gray squirrel with differently sized warbles around its shoulder and on its back
|
|---|
|
|
|
The size of the 'lumps' caused by each of these afflictions ranges
from a few millimeters to over 2.5 cm (= an inch or more; Kilham et al., 1953; Kenyon & Slansky, 2000) and thus
is of limited diagnostic value. However, during much of the
approximately three week developmental period of bot fly larvae
under the hide of a squirrel, the warbles are relatively
large (approximately 25 - 38 mm [1 - 1 1/2"] in length, and 13 - 25 mm [1/2 - 1"] in width and height) compared with many of
the fibromas seen on a squirrel with fibromatosis (up to around 13 mm [1/2"] in diameter and 2-3 mm [1/16 - 1/8"] high). More distinguishing than size are the shape and other aspects of the appearance of the lumps caused by these two afflictions (see below).
FIBROMATOSIS
The external manifestation of fibromatosis can be quite variable; the following is a synthesis of the descriptions provided by Hirth et al. (1969), King et al. (1972), O'Connor et al. (1980), Wolf (1991), Miller (1992, 2000), Surratt (2000), and Terrell (2000). A more technical description of lesion development in the various poxvirus diseases is given in Jubb et al. (1993).
The hide of squirrels with fibromatosis may be thickened ("corrugated") and reddened, especially on the face and around
|
Fig. 3: Lesions of the face and ear of a fox squirrel with
fibromatosis (Photo: Dr. E.A. Miller)
|
Fig. 4: Fibroma under the fur of a gray squirrel (Photo: Dr. E.A. Miller)
|
|
|
|
the genitals; distinct, slightly raised, often circular, flattened, wart-like lesions may also be present (Figs. 1 & 3). In addition to these skin surface abnormalities, outgrowths of the tissue underlying the outer surface of the hide occur; these fibroma nodules are solid masses that initially occur under the fur (Fig. 4) but will frequently protrude through it as they increase in size. Their shape varies from round to irregular, and they can be gray, whitish gray, brown or black (Figs. 1, 4, 5 & 6). These nodules grade into round, oblong or irregular-shaped tumor-like growths (Figs. 1 & 7).
|
Fig. 5: Fibromas protruding through the fur
near the base of the tail of a fox squirrel (Photo: Dr. E.A. Miller)
|
Fig. 6: Fox squirrel with several fibromas around its shoulders and on its head (Photo: Dr. E.A. Miller)
|
|
|
|
The affected animal's fur "is usually only missing at the site of the nodules, unless mange is also involved; the nodules may occur singly or in clusters and as the disease progresses, they can pile up on top of each other" (Surratt, 2000; see especially Fig. 7, and Fig. 1 in Exposure &
transmission). Miller (2000) describes the progression of fibroma growth and healing: "Nodules grow and become dry/scaley and are white-grey (see Figs. 5 & 6), but then the 'crusty' surface falls off (usually from the animal scratching) and the tissue exposed is white and/or red and raw. This then scabs over and the lesion eventually heals."
|
Fig. 7: Abdomen of a fox squirrel
with numerous fibromas, including
larger, tumor-like types (Photo: Dr. E.A. Miller)
|
|---|
|
|
In extreme cases, the squirrel's hide may occur in 'folds', on the order of a Shar-Pei dog, making the animal appear old and wrinkled (Kuruziak, 2000; Figs. 8 & 9). Note that fibroma nodules, as virus-induced tissue masses, generally do not contain pus. However, pus may be present if there is secondary infection with bacteria, such as may often occur
when a squirrel's eyes become overgrown by the surrounding fibromas and those on the eyelids (Surratt, 2000; see Fig. 9); in severe cases, the eye may ulcerate and be destroyed (Terrell, 2000). When fibroma tumors occur on a squirrel's legs, there may be "sloughing and death of the limb distal to the tumor" (Terrell, 2000). When the paws are affected, the toes can swell to extreme proportions (Surratt, 2000; see Figs. 1 & 8).
|
Fig. 8: Immature eastern gray squirrel with a severe case of fibromatosis; note the grossly swollen digits (Photo: M.C. Kuruziak)
|
Fig. 9: Same squirrel as in Fig. 8; note that its eye is swollen shut from fibromas and that the hide occurs in folds, esp. at the base of the legs and tail (Photo: M.C. Kuruziak)
|
|
|
|
Fibromatosis may co-occur with mange (caused by mites) and with bacterial and fungal infections (Hirth et al., 1969; Novilla et al., 1981; Miller, 1992; Johnson, 2000; Surratt, 2000), and it may be misdiagnosed as these or other skin problems, especially in its early stages (Wolf, 1991). Whether individual squirrels have been simultaneously subjected to fibromatosis and bot fly infestation has apparently not been reported.
BOT FLY INFESTATION
In contrast to fibromas, warbles start out as small, bloody lesions
caused by the bot fly larvae cutting through a squirrel's hide to
create the warble pore (Fig. 10). Within a few days the pinkish to
reddened warble begins to swell (Fig. 11), such that by the end of
the first week a moderate-sized lump is present (Fig. 12). If the
squirrel can reach the warble site with its hind paws, it typically
will repeatedly scratch the area, causing localized fur loss.
During the first week, fluid begins to drain from the warble pore,
and the posterior end of the light-colored larva may be seen
protruding slightly from it (see Figs. 11 & 12).
Fig. 10: Initial
wound made by a
bot fly larva in a
squirrel's hide
|
Fig. 11: Early-forming
warble with fluid draining
& the posterior end of the
larva protruding from the
warble pore
|
Fig. 12: 1-week-old warble
with fluid draining & the
posterior end of the light-
colored larva protruding
from the warble pore
|
|---|
|
|
|
|
As larval development continues during weeks 2 and 3, the warble becomes a very prominent lump in the squirrel's hide (Fig. 13) and the tip of the now darkened
larva fills the warble pore (Fig. 14). Warbles generally are oblong, with their long axis more or less parallel to, or angled slightly out from, the outer surface of the squirrel (note that some of the 'tumor-type' fibromas of fibromatosis may also be oblong; see Fig. 7). A broad range of warble sizes is often seen in heavily infested animals (see Fig. 2). This suggests that an individual squirrel may become infested on more than one occasion (perhaps over a period of a week or more), such that it contains various-sized larvae at different stages of development. After the larvae complete their development, they leave the
warble, and the squirrel is left with large holes in its hide (Fig.
15). The empty warbles close up within a week or so, although healing may be prolonged if they become infected. The hide at the site tightens up
and the fur grows back, such that within a few weeks after the
larvae have exited, there is little or no sign of the previous
infestation (Kenyon & Slansky, 2000).
Fig. 13: 2-week-old
warble on the side of
an eastern gray squirrel
|
Fig. 14: Posterior
end of a dark larva
fills the pore of a
3-week-old warble
|
Fig. 15: Eastern gray
squirrel with empty
warbles soon after
the larvae exited
|
|---|
|
|
|
|
From the descriptions above, it is evident
that a key difference between warbles and fibroma nodules is that the latter lack a 'warble pore'-- the opening in a squirrel's hide created by a bot fly larva for breathing and excretion. However, some fibromas "may ulcerate, become encrusted with cellular debris, and become infected with secondary bacterial or fungal agents" (Miller, 1992). Such
nodules (Figs. 16 & 17) may thus superficially resemble a warble on which fluid discharge has become encrusted (Fig. 18).
|
Fig. 16: Ulcerated fibroma on the front
leg of a fox squirrel (Photo: Dr. E.A. Miller)
|
Fig. 17: Ulcerated, scabbed-over fibromas on
the side of a fox squirrel (Photo: Dr. E.A. Miller)
|
Fig. 18: Encrusted bot fly warble
on an eastern gray squirrel
|
|---|
|
|
|
|
Frank Slansky & Lou Rea Kenyon ||
fslansky@ufl.edu
Version 1.1 (updated July 26, 2001)