Overview of 'Lumpy' Squirrels


      'Lumpy' tree squirrels are a common sight during mid- and late-summer throughout much of eastern and midwestern North America, from southern Canada to Florida. People who observe these squirrels in their backyards, nature parks and elsewhere but are not familiar with the cause of this situation often wonder if pets or people can catch whatever it is that the infested squirrels have. Other common concerns include whether there is anything one can do to help cure the 'lumpy' squirrels and to prevent uninfested squirrels from getting infested. Here we address these questions, including:






What causes 'lumpy' squirrels ?


      The annually occurring 'lumps' on tree squirrels (see pictures above) are most likely the result of infestation by the tree squirrel bot fly, Cuterebra emasculator, a natural parasite of eastern gray and fox squirrels , as well as American red squirrels and eastern chipmunks (flying squirrels appear to be only rarely parasitized)(Fig. 1). However, there is a another 'lump-causing' affliction, a viral disease called squirrel fibromatosis that can occur in the same geographic areas as bot flies. In some cases, the swellings produced by fibromatosis can resemble those caused by bot flies.

Fig. 1: Gray (left) & fox (center) squirrels are two common natural hosts of both the tree squirrel bot fly and squirrel fibroma virus. However, southern flying squirrels (right) appear to be rarely parasitized by bot flies, and we are aware of no reports of them being infected with fibroma virus.


      The 'grub-like' larvae, which are also called 'bots' (Fig. 2), live under the hide of these animals and produce the noteworthy swellings or 'warbles' (Figs. 3 & 4). Each warble typically contains only one larva and at its tip is an opening, the 'warble pore', which is cut through a squirrel's hide by the larva so that it can breathe and excrete liquid (see Figs. 3 & 4). The terms 'bots' and 'warbles' give rise to the names 'bot flies' and 'warble flies' for these and closely related species of flies.

Fig. 2: Nearly mature
larva or bot removed
from its warble in an
eastern gray squirrel
('head' end at upper left) 		Fig. 3: One-week-old
warble with fluid draining
& posterior tip of pale,
immature larva showing 		Fig. 4: Posterior end
of a dark-colored, nearly
mature larva fills the pore
of its warble


      Adult tree squirrel bot flies are black and pale yellow with broad bodies, somewhat resembling bumble bees (Fig. 5; note that about one third of the length of this fly's dark wings have been broken off, due to its flying around in a jar prior to the photo). They are very secretive, and their egg-laying behavior has not been observed in nature, but after mating, a female presumably lays her eggs
Fig. 5: Adult tree
squirrel bot fly
(head to left) 		Fig. 6: Bot fly eggs
laid on a twig
on twigs, branches, etc. (Fig. 6; note that although these look similar to the eggs of the tree squirrel bot fly, they are from a different species, C. americana, which parasitizes eastern wood rats). This 'indirect' laying of eggs on habitat substrates rather than directly on the host animal has been documented for other Cuterebra species that also parasitize rodents (such as pocket gophers) or rabbits, and it is presumed to be characteristic of this group of bot flies (Catts,1982). In contrast, females of other bot fly species, such as the horse stomach bot fly (Gasterophilus intestinalis) and the cattle grub (Hypoderma bovis), and of other parasitic flies, such as blow flies, attach their eggs (or in some cases, the female fly deposits maggots) directly to the host animal (Wall & Shearer, 1997).

      Squirrels thus apparently get infested when they move about in their habitat and contact some of the bot fly eggs (Fig. 7). The warmth from a squirrel's breath or body causes the eggs to hatch
Fig. 7: Squirrel moving
along a branch may
become infested with
tiny bot fly larvae
rapidly, and the tiny, infective-stage larvae (about 1/16" [1.5 mm] long) that emerge attempt to crawl onto the animal. If a larva gets on a squirrel, it may enter one of the animal's body openings, burrow through the membranes of the eye or be inadvertently licked into the mouth by the squirrel as it grooms itself. The larva then moves through the squirrel's body for about a week before settling under the host's skin and using its two, pointed mouth hooks (Fig. 8) to cut the warble pore (Catts, 1982).


Fig. 8: Microscopic view of shiny black, pointed mouth hooks of a nearly mature bot fly larva
      Over about a three week period, the warble becomes larger as the host animal's skin is stretched by the larva growing under it (Fig. 9), reaching a maximum size of about 1 1/2" [38 mm] long and 1" [25 mm] wide. The warble pore also gets wider and the posterior end of the dark brown larva may be seen protruding slightly from it (see Fig. 4). When fully grown, the larva backs out through the warble pore, falls to the ground and burrows a few inches into the soil. It then turns into a 'pupa' (Fig. 10), which hibernates until next summer when it transforms into an adult fly that emerges to once again begin the life cycle. After the larvae have exited the squirrel, it is left with large holes in its hide (Fig. 11). If the empty warbles do not become secondarily infected with bacteria, they begin to close up and heal shut within a week or so. The hide at the site tighens up and the fur grows back such that within a few weeks after the larvae have left, there is little or no sign of the previous infestation

Fig. 9: 2-week-old
warble on the side of
an eastern gray squirrel 		Fig. 10: A bot fly pupa ('head'
end to right) is formed in the
soil after a larva becomes fully
grown & exits from its warble 		Fig. 11: Eastern gray
squirrel with empty
warbles soon after
the larvae exited



Can this affliction spread from infested squirrels to uninfested squirrels, pets or people ?


      From the previous discussion (What causes 'lumpy' squirrels?), it is evident that these often gruesome-appearing, fluid-draining lesions most likely are not the symptoms of an infectious disease that can be 'caught' by other squirrels, pets or people from an infested squirrel. Furthermore, once the bot fly larvae settle under an animal's hide, they do not leave to transfer to another host. Therefore, uninfested squirrels coming into close proximity with these 'lumpy' squirrels will not become infested with the larvae already under the skin of the parasitized animals [but note that there is a different 'lump-causing' affliction, squirrel fibromatosis, a viral disease that can be spread from infected to uninfected squirrels by mosquitoes, and perhaps by ectoparasites (e.g., fleas) or direct squirrel-to-squirrel passage].

      There are, however, a few situations in which bot fly larvae could possibly transfer from a squirrel to another animal. When invading larvae are crawling on a squirrel, another animal contacting this squirrel could get some of these on its body. However, such transfers between adult squirrels are unlikely because these territorial animals typically maintain their distance, the main exception being during the breeding season. Although this has not been established, predators (i.e., raccoons, cats, etc.) attacking an animal with infective-stage larvae might become parasitized, such as can occur when cats and dogs get infected with roundworms (Toxocara species) after eating mice containing these ascarid nematodes (click here for an offsite link to more information about the life cycles of parasites of cats and dogs).

      A mother squirrel that gets infective-stage larvae on her while out foraging might transfer some of these to her nursing offspring when she returns to the nest. Although unproven, this is a likely explanation for how infant and juvenile squirrels that have never left their nest occasionally get infested with bots (Slansky & Kenyon, unpublished manuscript). Also, infective-stage larvae, perhaps from eggs laid on twigs or leaves comprising the squirrel's nest, might crawl into the nest and parasitize the squirrels within. Other undocumented scenarios to explain bot fly parasitization of nest-bound squirrels involve the fetuses becoming infested by the tiny larvae moving within the mother's body or entering through her vagina, and the nursing babies ingesting infective-stage larvae that passed through their mother's nipples; such modes of infestation can occur with roundworm parasites (Toxocara species) of cats and dogs (click here for an offsite link to more information about the life cycles of parasites of cats and dogs).

      Occasionally, cats, dogs and people become parasitized by Cuterebra bot flies. This is an atypical situation because these insects usually attack a narrow range of animals that, depending on the species of bot fly, includes only certain species of rodents (squirrels, mice, rats, voles, pocket gophers, etc.) or lagomorphs (rabbits and hares). For example, the tree squirrel bot fly normally parasitizes only tree squirrels and chipmunks, other Cuterebra bot fly species attack only particular species of mice or rats, others only infest rabbits, etc. (Sabrosky, 1986). Exposure of a person or pet to a rodent- or rabbit-attacking bot fly presumably occurs when they walk through vegetation, touch tree branches or otherwise contact bot fly eggs or infective-stage larvae. Which bot fly species have been responsible for human infestations has seldom been determined because the young larvae removed from patients are difficult to identify as to species, unlike the adult flies (for more information, see 'Human infestation by bot flies' [Link not yet available]).




Is there anything one can do to help cure bot fly-infested squirrels, and to prevent other squirrels from getting infested ?


      A common reaction upon seeing a squirrel infested with bots is to wonder if there is anything one can do to help the afflicted animal. While such a reaction is understandable, it is important to realize that bot flies are a natural part of the ecology of squirrels, along with many other parasites that are not so readily apparent, including fleas, mites and internally living species such as tapeworms and roundworms. A wild animal's parasites are but one of many natural factors with which it must cope; others include unfavorable weather, inadequate food supply and the presence of predators. All of these factors can contribute to the regulation of animal populations in the wild, and each species has a long evolutionary history of adapting to endure these stresses. Thus, people who work with wildlife generally do not attempt to interfere with the parasites or other aspects of the ecology of animals in the wild (exceptions include threatened or endangered species, and injured or orphaned wildlife, where a 'helping hand' may be offered).

      Furthermore, in our opinion the trauma of capturing and treating a wild animal with a generally non-life threatening infestation of natural parasites imposes more stress than does the infestation itself. It is our impression from almost 20 years of observation that unless an adult gray squirrel has more than 5-6 warbles, it will usually survive the ordeal without apparent longterm consequences (for further information, see Effects of bot fly infestation on the host animal [Link not yet available]). Thus, capturing otherwise-healthy wild squirrels to extract their bots is not routinely done.

      If, however, an injured adult squirrel that also has bots is brought to a wildlife rehabilitator or veterinarian for treatment, and if the animal is going to be kept in captivity for at least several days (to allow for a course of antibiotics), we believe the best approach would be to extract the larvae. This would remove one source of stress on the animal such that it could better cope with its other injury. Similarly, bots in orphaned infant and juvenile squirrels in the care of a rehabilitator should be removed because the stress of the larvae could significantly hamper the growth of the baby or prove fatal.

      Bot fly larvae can be relatively easily removed from an infested squirrel by carefully pulling them out through the warble pore using forceps (without the need and risks of anesthesia or a surgical incision), followed by rinsing out the empty warble with an antiseptic and treating with topical and oral antibiotics. This procedure is relatively straight-forward and poses little risk to the animal if done properly by a trained wildlife rehabilitator or veterinarian (for further details, see Non-surgical removal of larvae from squirrels [Link not yet available]).

      Manual removal of the bots is currently the only treatment for bot fly-infested squirrels. Antiparasitic drugs such as ivermectin are used to kill non-insect arthropod (mites) and nematode pests of dogs, cattle, etc., but their use against bot fly larvae in squirrels has not been studied and thus these drugs should not be used. Even if such treatments killed the larvae, if they were not physically removed from the warbles, infection would likely occur, possibly causing the squirrel's tissue to become necrotic, requiring surgery and antibiotic treatment.

      There is currently no way to prevent squirrels in the wild from getting infested. Adult tree squirrel bot flies are seldom seen and they are not known to be attracted to particular odors or colors, so a trap could not be designed to attract and catch them. Spraying an area with a broad spectrum insecticide would not be useful as it is not known exactly where the flies are located in the habitat, and many beneficial insects would likely be killed by the toxic spray. Trying to coat the squirrels with some repellant of the infective-stage larvae probably wouldn't work because squirrels spend a lot of time licking their fur to keep 'well groomed' and they would quickly lick off and ingest any such chemical, reducing its effectiveness and possibly incurring harmful side-effects. Also, to our knowledge no information exists on what compounds are repellant to the larvae. Finally, no vaccine is available to prevent squirrels from becoming infested. Thus, one should generally "let nature take its course" with wild squirrels infested with bots.

      Frank Slansky & Lou Rea Kenyon // fslansky@ufl.edu
Version 1.0 (July 26 2001)