Rodent Bot Fly - Cuterebra sp.



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This is Skooter - he presented last Saturday with the chief complaint of two wounds that were oozing a pus-like substance.  He was otherwise acting very normal.  A closer inspection of the spots revealed two swollen areas with small holes in the center.  One was on the bottom of his abdomen and the other on the side of his chest.  The lesions looked like this (after clipping the hair):

And a closer look:

Based on the appearance of the lesions and the fact that it is August, we were immediately suspicious of a rodent bot fly larvae under the skin.  After clipping and cleaning the area, we could see the larvae actually move in and out of the hole.  To the owners disgust, we removed two large (1.5 cm) bot fly larvae from Skooter without incident (see one below):

Plenty of questions followed:  What are bot fly larvae?  Where did Skooter pick up them up?  How do you treat this?  How can we prevent it? 

Here is a quick run-down of the rodent bot fly or Cuterebra sp. life cycle.  Rodent bot flies lay their eggs around animal burrows (usually rabbit).  The warmth from a nearby animal stimulates these eggs to hatch.  On the new host, the larvae enter the body in a natural opening (mouth or nose) or through an opening in the skin.  They travel in the host's body to a location under the skin.  The develop a hole through the skin through which they are able to breathe and survive.  After about a month or so, the larvae leave the skin, drop to the soil and pupate - and the life cycle continues.

Dogs, cats, and ferrets pick up these larvae when they live in the same environment of bot flies and natural hosts.  When running around outdoors, they pick up the larvae just as a rodent would.  In Billings, MT, we typically see the lesions in late summer and early fall.  We see swellings around 0.5-1.5 cm in diameter with a well-circumscribed breathing hole.  There is usually a history of fluid or pus drainage from the hole and we often have to remove the matted hair before the hole in the skin is evident.

In the veterinary setting, after clipping the lesion and identifying the larvae, the next step is removal.  Removal must be done very carefully and sometimes the breathing hole must be enlarged to facilitate removal.  If the larvae is ruptured, it can lead to a chronic foreign body reaction and secondary infection.  After removal, the area is cleaned and allowed to heal by granulation.  We typically keep the animal on antibiotics as healing occurs.   

Skooter was a great candidate for picking up Cuterebra, because his owners live on a large piece of property with significant population of rabbits.  We're happy to report that he is doing well and we expect a full recovery.  Unfortunately, he will be at risk for picking up infections next spring, as long as the normal host (rabbit) and bot fly are present.  Yesterday, we also pulled a smaller bot fly from a kitten.  The kitten had been a stray at Lake Elmo and adopted a week ago.  She, too, is doing well after the offending parasite was removed.

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