Diseases / List of Parasitic Diseases / Disease description:

Cuterebra (Warble) Infection in Lagomorphs 

INFORMATION AVAILABLE

GENERAL INFORMATION

CLINICAL CHARACTERISTICS & PATHOLOGY

INVESTIGATION & DIAGNOSIS

TREATMENT & CONTROL

SUSCEPTIBILITY & TRANSMISSION

ENVIRONMENT & GEOGRAPHY

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General and References

Disease Summary
Subcutaneous fly larvae infection usually seen in rodents and lagomorphs in the Western Hemisphere. (B272.11.w11)
Lagomorphs Subcutaneous fly larvae infection of lagomorphs in the Western Hemisphere. (B272.11.w11)

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Alternative Names (Synonyms)
  • Warble infection
  • Bot fly myiasis

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Disease Type
Parasitic Infection

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Infectious/Non-Infectious Agent associated with the Disease
Botflies
  • Cuterebra spp. (J213.4.w4)
    • The genus of Cuterebra is only known to occur in North and South America. (J1.15.w13)
  • These are obligate myiasis-producing parasites that commonly infect wild rodents and wild rabbits. (B602.19.w19, J1.14.w12, J15.28.w1).
  • "Cuterebra larvae occasionally are found in a variety of animals including dogs, cats, cattle, deer, and man. These incidental infections are most often caused by rabbit bot fly larvae, rarely by rodent-infecting species. Larval development in unusual hosts is usually unsuccessful". (J1.15.w13)
  • Adult botflies do not have mouthparts and so cannot feed. They only live for a short time, to mate and deposit their eggs on or close to a suitable host for larval development. Cuterebra spp. that affect rabbits lay their eggs in or around burrows. (J213.4.w4)
  • In other forms of myiasis, flies lay eggs on decaying flesh in exposed wounds. The eggs then hatch and the larvae are found locally and superficially. However in bot-flies such as Cuterebra, the larvae are buried subcutaneously each with their own breathing hole, after migrating randomly from their entry point. (B604.5.w5)
  • Bot flies; their larvae may be known as warbles. (B602.19.w19)
  • Warble fly (B600.9.w9)
  • Eggs hatch in response to a sudden increase in environmental temperature and possibly moisture. They enter the host via any opening, usually the mouth ot nares, sometimes a minute skin abrasion. They may remain at the site of entrance for several days, then move to their preferred subcutaneous site, where they form a cyst, containing the warble, with a pore to the outside. Development takes up to 55-60 days. After emergence, the larvae pupate in e.g. loose soil or debris, mate soon after emergence as adults, and live up to about 14 days. (B272.11.w11)
In North America
In pikas (Ochotona - (Genus))
  • Cuterebra spp. has been found in Ochotona princeps - American pika. (J1.15.w13)
    • Species unknown but the Cuterebra larvae that were found were thought to be rodent bot fly larvae because they had multiple-pointed spines rather than the single pointed spines of rabbit bot fly larvae. The posterior spiracular arrangement and cuticular spine structure of the larvae were similar to larvae of Chipmunks that were in the same area. The American pika was not thought to be the normal host of this species of Cuterebra. (J1.15.w13)
  • Oestromyia spp.
  • Oestroderma spp. 
  • Portschinskia spp. 
  • Hypoderma bovis in Ochotona dauurica - Daurian pika in Japan. (J511.47.w1)

(B208.3.w3, B336.42.w42)

Infective "Taxa"
  • Cuterebra spp. 
  • Oestromyia spp.
  • Oestroderma spp. 
  • Portschinskia spp. 
  • Hypoderma bovis 

Non-infective agents

 --

Physical agents

 -- Indirect / Secondary

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References

Disease Author

 Nikki Fox BVSc MRCVS (V.w103)
Click image for main Reference Section

Referees

 Anna Meredith MA VetMB CertLAS DZooMed (Mammalian) MRCVS (V.w128); Richard Saunders BVSc BSc CertZooMed MRCVS (V.w121); Lord Soulsby of Swaffham Prior MA,PhD,DSc (H c: Mult) , AM(H c),DVM&S (H.c),DVMS(H.c) ,DVM(H,c) F.Med Sci.,Hon FRCVS F.Inst Biol. F.R.C.Path (H.c), DVSM, MRCVS (V.w135)

Major References / Reviews

Code and Title List

 B10.45.w47, B64.22.w8, B272.11.w11, B600.9.w9, B602.19.w19, B604.5.w5, B608.21.w21, B610.23.w23, B615.6.w6
J1.14.w12, J40.19.w1, J213.7.w2

Other References

Code and Title List

 J1.15.w13, J1.19.w13, J40.7.w1, J11.26.w1, J524.60.w1, J469.55.w1, J469.125.w1 J469.360.w1, J511.47.w1

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Clinical Characteristics and Pathology

Detailed Clinical and Pathological Characteristics
General  

Clinical Characteristics
  • Development of palpable/visible subcutaneous nodules. (B272.11.w11)
  • Obvious problems with locomotion in some smaller hosts. (B272.11.w11)
  • Painful on palpation. (B272.11.w11)
  • The host may lose weight. (B272.11.w11)
Lagomorphs Pathogenesis
  • Larvae pupate in the subdermis which causes multiple swellings particularly over the dorsum. (B602.19.w19)
  • In other forms of myiasis, flies lay eggs on decaying flesh in exposed wounds. The eggs then hatch and the larvae are found locally and superficially. However in bot-flies such as Cuterebra, the larvae are buried subcutaneously each with their own breathing hole, after migrating randomly from their entry point. (B604.5.w5)
  • "Larvae of some species can penetrate the skin. Others enter via natural openings and penetrate the mucosa or enter through breaks in the skin...After a period in quiescence, the larvae migrate to the trachea and from there penetrate through the pleural and abdominal cavitiesto preferred subcutaneous locations, where a furunculoid cyst forms". (B614.11.w11)
  • There are three larval stages and they are all pathogenic. (B604.5.w5)
  • Some larvae may migrate aberrantly from the nasal passages, sinuses, eyes and ear canals through to the central nervous system. (B602.19.w19)
Clinical findings
  • Initially, subcutaneous cyst-like lesions. (B600.9.w9, B608.21.w21, J213.7.w2)
  • Usually on the head or neck, a localised swelling with a visible breathing hole. (J29.5.w1)
  • There may be swelling and ulceration. (B10.45.w47)
  • Fistulous nodules develop, 
    • each nodule is one to three centimetres in size, encapsulating a single larva that has produced a fistula in the skin surface ("breathing hole"). (B602.19.w19, B608.21.w21, B610.23.w23, J213.4.w4)
    • The grub can be up to two to three centimetres in length with large black spines. (J213.4.w4)
  • These lesions are often painful. (B600.9.w9, B608.21.w21)
  • Matted and moist fur surrounding the lesions. (B600.9.w9, B608.21.w21)
  • Secondary bacterial infection is common. (B600.9.w9, B608.21.w21)
  • General signs:
    • Some rabbits may become weak, dehydrated, anorexic, lame and develop severe toxic shock. (B602.19.w19)
    • Juvenile rabbits with more than nine to ten bot-flies may be overwhelmed and develop toxic signs. 
    • Reduced fertility (B604.5.w5, J1.14.w12)
    • "anaemia, changes in leukocyte count, plasma protein alteration, splenomegaly and lymph node enlargement, lowered body weight". (J1.14.w12)

    • In a study of Cuterebra lepusculi in Sylvilagus nuttallii - Mountain Cottontail,hosts often supported seven to nine larvae without obvious systemic effects, although repeated infestation with this number of larvae resulted in emaciation and death of some rabbits. (J1.19.w13)

    • In Lepus californicus - Black-tailed Jackrabbit from Nevada, animals parasitised by several bot larvae were weakened and blinded (eyes closed due to adjacent larvae); they were considered to be "in obvious distress", behaved abnormally and were seen wandering around at midday. (J40.19.w1)
  • With aberrant migration:
    • Neurological signs may be seen if there has been aberrant migration of the larvae. (B602.19.w19, B604.5.w5)
    • Ophthalmomyiasis may occur if there has been aberrant migration of the larvae. (B614.11.w11)
  • In a female Ochotona dauurica - Daurian pika in captivity in Japan, anorexia and severe weight loss. (J511.47.w1)
Distribution of lesions
  • The commonest area affected is the area around the base of the spine that is between the tail and the dorsum. (J213.7.w2)
  • Scrotum, nose, and other dermal areas. (B64.22.w8)
  • Shoulder, throat or neck area and ventral abdomen. (B604.5.w5)
  • Neck or throat area. (B615.6.w6, J213.4.w4)
  • Cuterebra horripilum: ventral cervical region (B602.19.w19, B608.21.w21, J1.14.w12)
  • Cuterebra buccata:
    • axillary 
    • interscapular 
    • rump 
    • inguinal area  
      (B602.19.w19, B608.21.w21)
    • This species is found in the following areas in decreasing order of prevalence: genital, belly and rib-cage. (J1.14.w12)
  • In Ochotona princeps - American pikas, larvae have been found dorsally over the scapulae and also ventrally near the sternum. (J1.15.w13)
  • In a study of cottontails [probably Sylvilagus floridanus - Eastern Cottontail] in the eastern USA, Cuterebra larvae on four rabbits were mainly near the anal region, but one was above the right shoulder. (J40.7.w1)
  • In Sylvilagus floridanus - Eastern Cottontail in Georgia, USA, the most common site for Cuterebra buccata  was the genital area, particularly the scrotum, but larvae were also found on other parts of the body, but not the head. Cuterebra cuniculi on Sylvilagus palustris - Marsh rabbit were found on the lateral or ventral neck. (J11.26.w1)
  • In wild Romerolagus diazi - Volcano rabbit, one larva per rabbit, up to 2.7 cm long and 1.8 cm wide, between the skin and muscle of the neck. (J469.360.w1)
Clinical pathology:
  • Anaemia, alteration of complete blood count, white blood cell counts, and plasma proteins. (B604.5.w5)

Incubation

 Development of the larvae in the host takes 19 - 60 days. (B272.11.w11)
Lagomorphs

Mortality / Morbidity
  •  
Lagomorphs

Pathology

 --
Lagomorphs
Gross lesions due to Cuterebra
  • The gross lesions are mainly found in the subcutis and are associated with the dead or live larva and the breathing pores. 
  • In aberrant migration the larvae may be found almost anywhere, particularly in the middle ear, brain or nasal passages. 
  • Death is often caused by the toxins produced by the larvae so there may be no other gross lesions in other areas of the body. 
  • Rabbits that die from chronic infection may be emaciated and have splenomegaly, cahcexia and lymph node enlargement.

(B604.5.w5)  

  • In Lepus californicus - Black-tailed Jackrabbit from Nevada, bot larvae of all stages were found under the skin. In cystic bot cavities, pus and sero-sanguinous fluid was found. Where young bot larvae were migrating under the skin, inflammation and oedema were noted. Sometimes disintegrating larvae were found under the skin. Where bots had matured and departed, healing lesions were found. (J40.19.w1)
Microscopic lesions in Cuterebra infection
  • Marked inflammation and fibroblast proliferation are associated with the aberrant migration of the larvae in some organs. 

(B604.5.w5)

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Human Health Considerations
  • Cuterebra larvae have occasionally been reported from humans. (B272.11.w11)
  • Rabbit bot flies have been known to occasionally infect humans, particularly the elderly and children. There have been cases of humans with intracranial, intranasal and intraocular larval migration. (B604.5.w5)

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Susceptibility / Transmission

General information on Susceptibility / Transmission
--
Lagomorphs
  • Younger animals
    • "The incidence of infestation decreases with age, which correlates with the development of immediate and delayed hypersensitivity reactions to larval antigens". (B608.21.w21)
  • Cuterebra spp. is reported to occasionally cause myiasis in the domestic rabbit reared in non-screened enclosures or outdoors. (B608.21.w21)
Transmission
  • Cuterebra are obligate myiasis flies that lay their eggs in logs, nests and burrows that are frequented by wild lagomorphs. The eggs hatch out into first stage larvae which then cling to a passing hosts or are mechanically transferred to other contact hosts. 

(B604.5.w5)

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Disease has been reported in either the wild or in captivity in:

Further information on Host species has only been incorporated for species groups for which a full Wildpro "Health and Management" module has been completed (i.e. for which a comprehensive literature review has been undertaken). Host species with further information available are listed below:

Host Species List

(List does not contain all other species groups affected by this disease)

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Disease has been specifically reported in Free-ranging populations of:

Further information on Host species has only been incorporated for species groups for which a full Wildpro "Health and Management" module has been completed (i.e. for which a comprehensive literature review has been undertaken). Host species with further information available are listed below:

Host Species List

(List does not contain all other species groups affected by this disease)

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Environment/Geography

General Information on Environmental Factors/Events and Seasonality

In lagomorphs:

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Regions / Countries where the Infectious Agent or Disease has been recorded
  • Cuterebra spp. are found only in the Western hemisphere (North, Central and South America). (J1.15.w13, J213.4.w4)

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Regions / Countries where the Infectious Agent or Disease has been recorded in Free-ranging populations

  • --

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General Investigation / Diagnosis

General Information on Investigation / Diagnosis
Usually detected when the growing warble becomes palpable. In small hosts, they may cause a notable locomotor problem. (B272.11.w11)
Lagomorphs
  • If the swelling is squeezed, a proboscis can usually be seen protruding from the area. (B64.22.w8)
    • Furuncular lesions that are associated with a fly larva. (B604.5.w5)
    • Cone shaped swelling that is one to three cm in diameter with an apical pore. Dissection of the pore may reveal a first, second or third stage larva which is club-shaped with tapered ends and a "segmented body with circumferential rows of caudally projecting, spinelike denticles used for orientation". There are a pair of hooks in the mouth and the tail end has the stigmatic plates which are used for breathing through the pores in the subcutaneous layer. (B604.5.w5)
Related Techniques
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Similar Diseases (Differential Diagnosis)
--
Lagomorphs
  • Aberrant Cuterebra infection may cause severe CNS signs that should be distinguished from that of rabies. (B604.5.w5)

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Treatment and Control

Specific Medical Treatment
--
Lagomorphs --
Related Techniques

 
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General Nursing and Surgical Techniques
--
Lagomorphs
  • Surgically remove the larvae - it is important to remove the whole larvae intact, being careful not to crush or cause any damage to them as this may result in anaphylaxis. (B272.11.w11, B600.9.w9, B601.13.w13, B602.19.w19, B604.5.w5, B608.21.w21, J29.5.w1, J213.7.w2)
    • The rabbit will need to be sedated or anaesthetised for this procedure and the skin area that is affected must be clipped and disinfected. (B601.13.w13)
    • The larva can be killed prior to removal by chloroform or other anaesthetic (B604.5.w5) or ether. (B615.6.w6). However, most vets discourage this practice now. (P62.7.w1)
    • The breathing hole needs to be enlarged by using haemostats and then the larva gently removed. (B601.13.w13, B602.19.w19)
    • It is necessary to debride the necrotic fistulous tract. (B601.13.w13)
    • After debridement, the swelling usually resolves. If the swelling does not resolve or if the skin has become abscessed, surgical excision of the affected tissue is necessary. (B602.19.w19)
    • Swabbing out the hole with 2% povidone iodine is suggested. (B10.45.w47)
    • Give systemic antibiotics to cover against secondary bacterial infection. (B601.13.w13, B602.19.w19, J29.5.w1)
    • In uncomplicated cases, the cavity left by the larvae can rapidly close over and heal by secondary intention. (B614.11.w11, J29.5.w1) A scar or hairless areas of skin remains. (B614.11.w11)
Related Techniques
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Preventative Measures
Vaccination --
Lagomorphs --
Prophylactic Treatment

--
Lagomorphs --
Related Techniques
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Environmental and Population Control Measures
General Environment Changes, Cleaning and Disinfection --

Lagomorphs
  • Eliminating contact with Cuterebra spp.. (B600.9.w9)
  • Infected dead rabbits should be disposed of immediately to prevent Cuterebra larvae from migrating to other rabbits. (B604.5.w5)
Population Control Measures --
Lagomorphs --
Isolation, Quarantine and Screening --
Lagomorphs --
Related Techniques
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