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.

www.city.nagoya.jp/10eisei/ngyeiken/insect/d_callip/ls.htm

Left to Right: Lucilia sericata fly, larvae and pupa.

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.

 

Kerry Su-Lin Leow

 

Rescued rabbit with bite wound heavily infected by fly maggots. In spite good medical care and signs of recovery, the rabbit died.

 

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 signs

The 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.

 

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Paralyzed rabbit and contamination of the skin by feces (arrows).

Diagnosis

The history of the rabbit and the clinical signs are generally sufficient for a proper diagnosis.

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Paralysis or severe arthritis can lead to incontinence and skin soiled with urine and cecotropes.

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The smell of urine soiled skin and soft cecotropes will attract Lucilia sp. female flies. These will lay eggs on the damaged skin.

Treatment

The 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 (0.4 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.

Acknowledgement

Many thanks to Kerry Su-Lin Leow (Singapore) for sharing her pictures.

Further Readings

Baird 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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