Myiasis (fly-strike) in Rabbits
Esther van Praag Ph.D.
Warning: this file contains pictures and videos that may be distressing
for people.
Myiasis, also called fly-strike, is more frequently
observed during the hot humid summer months. It is caused by several kinds of
insects that lay their eggs in the wounded skin of mammals. Rabbits suffer in
particular from the blowflies Lucilia sericata, Calliphora
sp., the grey flesh fly Wohlfahrtia sp., the
common screwworm fly Callitroga sp., and
from the botfly Cuterebra sp, which is seen in the USA only. A maggot
attack is often linked to poor hygiene, with rabbits kept on litter soiled
with urine and excrements, or poor-cleaned litter pans, but can also relate
to health problems. A particular attention must also be given to rabbits
suffering from dental (malocclusion, removal of incisors) or digestive
diseases, from obesity, untreated infected wounds, or that are disabled
(fracture of the spine, limb, arthritis, spondylosis).
Indeed, the inability to groom the perianal and tail regions, or eat their
cecotropes feces can lead to the appearance of a smell that will inevitably
attract flies.
Myiasis flies lay eggs in the skin soiled with feces or diarrhea,
on skin irritated by urine or in untreated infected wounds. The larvae that
emerge from the hatched eggs will immediately start burrowing themselves
through the skin, into the flesh of the host animal. A consequence is
septicemia and shock, which lead to the rapid death of the rabbit.
The use of prophylactic solutions is not recommended as
adverse fatal effects have been observed in rabbits (Frontline). Some
veterinary professionals use the prophylactic product Dicyclanil
(Novartis), which protects sheep against the blowfly Lucilia
sp. The product is not registered for use in rabbits, and a safe use in
rabbits can thus not be guaranteed. Clinical signsThe early stages of myiasis are often subclinical. With
time, a rabbit becomes depressed, weak, loses weight and shows paresis. At
this stage, the infection becomes visible; the larvae are about 1 cm long and
their hind part protruding from the respiratory hole
(spiracle) in the skin.
In a severe case, alopecia is observed. The skin is
inflamed, injured with signs of necrosis, and is often accompanied by the
smell of ammonia. The later is excreted by the larvae, in order to cause cell death and decomposition, will cause an
intoxication of the rabbit.
Aberrant migration of the larvae is possible. Migration
into the trachea has been observed. This leads to the formation of a
laryngeal edema, blocking the air supply to the lungs. It may be accompanied
by concurrent accumulation of mucus and swelling of the esophagus.
DiagnosisThe history of the rabbit and the clinical signs are
generally sufficient for a proper diagnosis.
TreatmentThe hair is delicately clipped away around the infected area and each
larva is removed individually and entirely with the aid of forceps, without
crushing it, to prevent skin irritation or the development of an allergic
reaction. The wounds are cleaned with a sterile saline solution, an
antiseptic solution (e.g. povidone-iodine or chlorhexiderme). There is no
need to use an insecticidal solution, if all the maggots have been removed. Video: Fly Strike In Rabbits (1):
Symptoms and Treatment Aberrant migration brings the larvae deep under the skin or in vital
organs. Three options are available here: • Injection of ivermectin (400 mg/kg, SC). The rabbit must be closely monitored as the dying larvae
excrete a toxin that can be fatal to animals, including rabbits. Although
controversial, corticosteroids are sometimes given to the affected animal, in
order to reduce the swelling. • Injection of doramectin (0.5 mg/kg, SC). • Surgical removal, under anesthesia, in case
of aberrant migration or infection by Cuterebra sp.. Use of antibiotics is indicated, if the myiasis infection is severe.
They help fight a secondary bacterial infection of the wounds and prevent
sepsis, which can be fatal in rabbits. The administration of non-steroidal pain medication is necessary (e.g.
meloxicam, carprofen). When the affected rabbit has stopped to eat, it must be hand-fed and
given SC fluid therapy, in order to avoid the onset of fatal hepatic
lipidosis and dehydratation. Depending on the situation, the affected rabbit
can furthermore be administered appetite stimulants, or gut motility
medication (e.g. cisapride, metoclopramide). Bathing the rabbit with antiseptic or insecticide solution is not
indicated. This procedure is stressful for the rabbit, and often ends in a
panic reaction as soon as the fur is wetted or death by heart-arrest. A jump
out of the bathtub has led to broken limbs or fracture of the spinal cord. If
this method is nevertheless chosen, the rabbit should be dried with a towel
and a hair-dryer or placed under a heat lamp. The heat will indeed bring the
remaining worms to the surface of the skin, from where they can be easily
discarded. If a rabbit is heavily affected by myiasis,
euthanasia should be considered. Prevention of myiasis can be done by addressing the
causes of fecal or urine contamination of the skin, and by keeping the rabbit
in a clean hygienic environment. Daily inspection of the perianal region is
necessary in rabbits prone to suffer from digestive disorders, that are obese
or that are disabled. The fur should be combed with a flea-comb, in order to
detect the eventual presence of eggs and/or maggots. The windows of the apartment
or the cage of the rabbit can furthermore be covered with a mosquito net, in
order to avoid the insects to have contact with the rabbit. Video: Fly Strike In Rabbits (2):
Prevention For detailed
information on fly strike infestation in rabbits, see:
“Skin
Diseases of Rabbits” by
E. van Praag, A. Maurer and T. Saarony, 408
pages, 2010.
AcknowledgementMany
thanks to Kerry Su-Lin Leow (Singapore) for sharing
her pictures. Further ReadingsBaird CR. Biology of Cuterebra lepusculi Townsend (Diptera: Cuterebridae) in cottontail rabbits in Idaho. J Wildl Dis. 1983;19(3):214-218. Harcourt-Brown F.: Textbook of Rabbit
Medicine. 1st ed., Butterworth-Heinemann, Oxford, England, 2002. Hess L. Dermatologic diseases. In:
Ferrets Rabbits and Rodents. Clinical Medicine and Surgery, 2nd ed., (Quesenberry K.F., Carpenter J.W.) Saunders, St-Louis, USA., 2004. Jacobson HA, McGinnes
BS, Catts EP. Bot fly
myiasis of the cottontail rabbit, Sylvilagus floridanus
mallurus in Virginia with some biology of the
parasite, Cuterebra buccata. J Wildl
Dis. 1978;14(1):56-66. Newell GB. Dermal myiasis caused by
the rabbit botfly (Cuterebra sp). Arch Dermatol.
1979;115(1):101.
Schumann H, Schuster R, Lange J. The
warble fly Oestromyia leporina
(Diptera, Hypodermatidae)
as a parasite of the wild rabbit (Oryctolagus cuniculus).
Angew Parasitol. 1985;26(1):51-52. Weisbroth SH, Wang R, Sacher
S. Cuterebra buccata: immune response in myiasis of
domestic rabbits. Exp Parasitol. 1973; 34(1):22-31.
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