Surgical Removal of a Cuterebra Larva From the Cervical Trachea of a Cat

Introduction

Cuterebra spp. are large flies resembling bees with vestigial mouth parts. The adult female often lays eggs near the dwellings of natural hosts, such as rabbits and rodents. Other mammals, such as cats, dogs, and humans, have also been reported to be parasitized by Cuterebra spp. larvae.^1–5 Since these mammals are unnatural hosts, the larval migration patterns are aberrant and may cause damage to a variety of tissues.⁶ The eyes, pharynx, trachea, thorax, and central nervous system have all been previously reported as locations where Cuterebra spp. larvae have been found.^1–4,6,7 Treatment of cuterebriasis includes removal of the intact larva and treatment of the concurrent wound.

Three cases of intrathoracic intratracheal cuterebriasis have been reported.^2,3 Of these, only two were reported to be successfully treated.² This report describes a case of cervical intratracheal myiasis of a cat that was successfully treated through surgical exploration. In this case, a second instar larva of a Cuterebra spp. was identified.

Case Report

An 18-month-old, 4.6-kg, castrated male, domestic shorthair cat was referred for a 3-day history of dyspnea induced by minimal activity. The referring veterinarian observed intermittent inspiratory dyspnea with increased upper respiratory sounds during examination. Initial treatment included 2 mg of dexamethasone sodium phosphate^a and 20 mg of enrofloxacin.^b The cat was stabilized and subsequently discharged, but it was returned the following day with a worsening of clinical signs. The referring veterinarian sedated the cat and performed an oral examination followed by tracheoscopy. A small mass was observed on the tracheal mucosa approximately 4 cm caudal to the larynx.

Upon referral presentation, physical examination revealed inspiratory dyspnea with stridor. The remainder of the physical examination was normal. The cat was placed in an oxygen cage^c to provide a calm, oxygen-rich environment and alleviate the cat’s respiratory distress resulting from the physical examination. A complete minimum database was not completed at that time because of the cat’s fragile condition. The cat remained stable during the following 24-hour period with the aid of oxygen supplementation.

An oral examination, rhinoscopy, and tracheoscopy were scheduled following this 24-hour period. Anesthetic induction and maintenance were achieved using propofol^d intravenously [IV] via a cephalic catheter. The cat was anesthetized uneventfully, and blood was collected to complete a minimum database. Oral examination and rhinoscopy were normal. Tracheoscopy of the cat using a flexible endoscope^e revealed an extremely small, raised mucosal defect approximately 2 mm in size located at the seven o’clock position approximately 4 to 6 cm caudal to the larynx. During visualization of the mass, a small, parasitic- like organism was observed to intermittently protrude from the mass. Several unsuccessful attempts were made to retrieve the organism. The cat was recovered from anesthesia in an oxygen cage. Results of the complete blood cell count (CBC) and serum biochemical analysis were normal. Feline leukemia virus and feline immunodeficiency virus test results were negative.

The cat was reanesthetized, using the same protocol mentioned previously, for exploration of the cervical region. Preoperatively the cat received cefazolin^f (22 mg/kg IV once), diphenhydramine^g (4 mg/kg IV once), and dexamethasone sodium phosphate (0.25 mg/kg IV once). In addition, hydromorphone^h (0.03 mg/kg IV once) was administered for analgesia.

A ventral midline incision was made over the median raphe of the cervical region. The sternohyoid and sternocephalicus muscles were divided along their respective midlines and retracted laterally. The outer tracheal surface was thoroughly and carefully inspected, but no grossly observable lesion was identified. The cat was extubated, and a flexible endoscope was used to perform the tracheoscopy. As visualization of the tracheal lumen was observed on the monitor, transillumination of the trachea was observed at the surgical site. The mucosal lesion was observed at the caudal extent of the surgical site. The incision was extended caudally approximately 2 cm, and a small, worm-like organism approximately 2 to 3 mm in length was observed on the extraluminal tracheal surface. The organism was collected, placed in formalin, and submitted for identification. The cat was reintubated.

The surgical site was carefully inspected for additional organisms as well as for additional lesions. Only a small, 1.5-mm, raised defect was noted on the outer tracheal surface corresponding to the mucosal lesion previously observed. The lesion did not appear to compromise tracheal patency. No other abnormalities were identified. The surgical site was lavaged and routinely closed in layers. The cat recovered uneventfully in an oxygen cage. Normosol-Mⁱ (60 mL/kg per 24 hours IV) was administered to maintain hydration. Buprenorphine^j (0.01 mg/kg q 6 hours sublingually) was provided for postoperative analgesia.

During the next 12 hours, the cat responded well to room air challenges and was subsequently removed from the oxygen cage. The cat was eating and drinking well 24 hours after surgery; therefore, fluid therapy was discontinued. No further evidence of dyspnea was observed, and the cat was discharged the following day. The recovered parasitic organism was identified as a Cuterebra sp. second instar larva by a parasitologist.

Discussion

The diagnosis of cuterebriasis should be based on a thorough collection of historical information and physical, radiographic, and endoscopic examinations.^2–4,6 Historical evidence commonly encountered may include indoor/outdoor housing conditions and progressive or nonresponsive inspiratory dyspnea. Clinical signs consist of inspiratory dyspnea, stridor, cyanosis, coughing, subcutaneous emphysema, inappetence, lethargy, and pyrexia.^2–4,6 Cuterebriasis should be considered as a differential diagnosis if the cat is presented with clinical signs consistent with upper respiratory distress in the late summer or fall months.^2–4,6 Previously reported radiographic evidence associated with cuterebriasis consists of suspicion of an intraluminal tracheal mass and/or ill-defined tracheal wall margins.² Endoscopic examination routinely provides the most pertinent information for diagnosis and treatment.² Lesion localization, visualization of the larva, identification of tracheal defects, and potential retrieval of the larva with forceps passed through the operating channel are all possible via tracheoscopy or bronchoscopy. ² Alternatively, surgical exploration of the cervical or thoracic region may be necessary to retrieve the larva.

The larval stages of Cuterebra spp. are the parasitic form.³ The adult female fly lays eggs near the burrows and tunnels of natural hosts. The first instar larva, or infective first-stage larva, emerges from the egg in response to body heat from the host, and it crawls immediately onto the host.¹ Entrance into the host is commonly facilitated by inhalation or ingestion.² Occasionally, the larva will enter the host through a wound.^1,2,6 In rabbits, the larva penetrates the mucosa of the upper respiratory tract—most commonly the nasopharyngeal mucosa or tracheal mucosa. The larva migrates throughout the host’s pleural and peritoneal cavities until reaching the subcutaneous tissues.^2,3,6 The larva penetrates the epidermis and maintains a small “breathing pore.”^2,3,6 The larva maintains this position and undergoes molts, progressing to the second and third instar stages.² Once reaching the third instar stage, the larva emerges from the host’s skin and drops to the ground to later undergo pupation in the soil.

The approximate time required for a first-stage larva to migrate and molt into a third instar is approximately 30 days.² In domestic species, the exact route of migration is unknown. A reasonable assumption is that the route of entry is the same oronasal route seen in natural hosts.^1–3,6,8 The migration and penetration of oronasal or tracheal mucosa are also distinct possibilities, as the case in this report would suggest, because no historical evidence suggested wound migration as a possible route of entry. Interestingly, a second instar larva was identified in this report, indicating that larva migration, penetration, and molting might have previously occurred.

Previous reports of upper respiratory infestation by Cuterebra spp. have included intranasal and pharyngeal myiasis.^7,9 Intrathoracic cuterebriasis in a cat diagnosed upon postmortem examination has been described.³ Two cases of successful management of intrathoracic tracheal cuterebriasis have also been previously reported.² However, to the authors’ knowledge, no report of successfully treated cervical intratracheal cuterebriasis is contained in current literature.

Clinicopathological information has not been shown to provide any specific details leading to confirmation of a diagnosis of tracheal cuterebriasis.^2–4 In the two cats with intratracheal myiasis, only a mild, mature leukocytosis was evident.² In this particular case, no abnormalities were observed in the CBC and serum biochemical panel. No evidence of eosinophilia has been seen in any case of intratracheal cuterebriasis.^2,3 One of two previously reported cases had an increase in globulin concentration on the serum biochemical profile.² This was also noted in three of 11 cats in the study performed by Glass et al.⁴ No biochemical abnormalities were evident in this case. It may be reasonable to expect an increase in globulin concentration as a result of acute-phase protein production in response to larval migration, tissue injury, antigens, and toxins.^2,3,6

Treatment of intratracheal cuterebriasis should include careful removal of the larva with repair of any possible tracheal wall defect.^2,3 In this case, the cat was pretreated using dexamethasone sodium phosphate and diphenhydramine prior to surgical exploration to reduce the risk of anaphylactic reactions associated with larval rupture. Hypersensitivity reactions have been reported to occur in cases of accidental larval rupture during removal.^6–8 The response to the parasitic proteases is thought to enhance both eosinophilic activity and immunoglobulin E activity. In addition, activation of the complement cascade may also occur.^10,11

Conclusion

Cervical intratracheal cuterebriasis was diagnosed in a cat with intermittent dyspnea of 3 days’ duration. The larva was located during tracheoscopy, but it was not retrievable. Surgical exploration of the cervical region was performed, and the larva was removed. All clinical signs resolved upon recovery from surgery. Cuterebriasis should be considered as a differential diagnosis in cats presented with clinical signs consistent with upper respiratory distress in the late summer or fall months.