Definition

• Seasonal neurologic disease that occurs in outdoor cats or those with access to the outdoors in North America during late spring, summer, and early fall; usually results in sudden onset of seizures, circling, altered mentation, and/or blindness.
• Aberrant migration of a Cuterebra larva in the brain of a cat that often causes thrombosis or vasospasm of the middle cerebral artery with ensuing ischemic necrosis; degeneration of the superficial layers of the cerebral cortex and parenchymal destruction associated with the physical migration of the larva in the brain parenchyma.
• Must be differentiated from other causes of vascular diseases affecting the brains of cats as well as other neurologic diseases of cats.

Pathophysiology

• In FIE, the Cuterebra larva enters the nasal passage of the cat, migrates through the cribriform plate into the olfactory bulb of the brain, then along the olfactory peduncle and sometimes continues in the parenchyma of the brain or alternatively in the subarachnoid space; the parasite then, if still alive, migrates caudally, where it may compromise the MCA physically through the spines on the larvae's body, or possibly via a chemical agent secreted by the parasite causing vasospasm to the vessel; or the vasospasm may be secondary to the hemorrhage caused by the parasite; the parasite may then die in the SAS or within the parenchyma.
• The adult botfly lays eggs by the entrance of a rodent's den. The eggs hatch into the L1 stage of the larva, which attaches to the hair of the mouse or rabbit and enters the body through a normal orifice (mouth, nose, eye, or anus) and migrates into the associated tissues (nasopharynx, trachea, thoracic cavity, diaphragm, abdominal cavity). Then the larva continues its migration to reach a subcutaneous site in the inguinal or thoracic region where it matures first to the L2 stage and then within the warble to the L3 larval stage, emerges through the skin, drops off the host, pupates in the soil over the winter, and then emerges in the spring as the adult botfly; when the cat hunts near the rodent's den, the L1 larva attaches to the cat's hair and gains access to the nasal passage of the cat and begins the catastrophic pathway of feline ischemic encephalopathy.
• Occasionally the larva does not migrate through the cribriform plate but embeds in the nasal passage or respiratory tract of the cat, causing focal respiratory signs. The larva has also been found in the eye and oropharyngeal region.

Systems Affected

• Brain and less commonly spinal cord.
• Larvae can also be found in the skin, nasal passage, pharynx, larynx, eye, trachea, and thorax of cats.

Genetics

None

Incidence/Prevalence

Not known

Geographic Distribution

• North America only.
• Same distribution as the Cuterebra botfly.
• Disease is not recognized in locations that do not have the Cuterebra botfly, such as Australia and Japan.

Signalment

Signs

• Sudden onset of neurologic signs.
• Often preceded by upper respiratory signs 1–3 weeks prior to the neurologic signs (due to the migration of the parasite in the nasal passage).
• Often prosencephalic signs.
• Most commonly-seizures, circling, altered mentation, blindness.
• Sometimes multifocal neurologic signs.
• Rarely spinal cord signs.

Causes

Cuterebra larvae

Risk Factors

• Outdoor cats; access to outdoors
• July, August, and September in the northeast United States and southeast Canada. Can be found in May and June, in the southeast United States
• Hunting cats

Differential Diagnosis

• Other causes of vascular accidents in the brain such as renal disease with hypertension.
• External trauma.
• Tumors-progressive rather than sudden in onset.
• Infectious/inflammatory diseases-such as those caused by Cryptococcus sp., Toxoplasma gondii, feline infectious peritonitis virus, and feline immunodeficiency virus.

CBC/Biochemistry/Urinalysis

• Usually normal
• CBC-occasionally neutrophilia, leukocytosis, or eosinophilia
• Chemistry-occasionally elevated globulins or hyperglycemia

Other Laboratory Tests

• Serology for FeLV, FIV, FIP-negative
• Cryptococcal antigen titers-negative
• Toxoplasma IgG and IgM-negative

Imaging

• MRI-diagnostic modality of choice; detect track lesion extending from the cribriform plate into the olfactory bulb and frontal, parietal, and temporal lobes best recognized on dorsal sequences; may also see area of ischemic infarction; if performed soon after onset of signs, increased signal intensity on T2-WI, PD-WI, and FLAIR images associated with ischemia of the superficial layers of the cerebral cortex, or the area supplied by the MCA; scant parenchymal enhancement in the area of infarction after administration of contrast. If MRI is done more then 2–3 weeks after onset of signs, may find loss of overlying gray matter in the region supplied by the MCA and associated hydrocephalus ex vacuo. MRA (time of flight or post-contrast) may be of some utility in some cases.
• CT-limited value.

Diagnostic Procedures

Cerebrospinal fluid-normal or non-suppurative inflammation with macrophages, lymphocytes, or eosinophils.

Pathologic Findings

• Local areas of malacia and hemorrhage involving the olfactory bulbs and peduncles and brain transverse sections. Thorough examination of the cribriform plate, olfactory bulbs, and peduncles, as well as the remaining parenchyma, meninges, and overlying calvaria, may reveal the larva, which is approximately 5–10 mm in length (stage 2 larva), tan and with concentric rings of spines along the length of the body.
• Histopathologic features may include necrosis and hemorrhage of the parasitic track or less specific findings such as superficial laminar cerebrocortical necrosis, cerebral infarction, subependymal rarefaction, and astrogliosis and subpial astrogliosis.

Appropriate Health Care

N/A

Nursing Care

• Padded cage may be necessary if the cat is having seizures.
• Swivel IV line can be used if the patient exhibits propulsive circling or loss of balance.

Activity

N/A

Diet

N/A

Client Education

• Only occurs in outdoor cats and those with access to the outdoors; strictly indoor cats do not develop FIE.
• Only occurs in summer months with the majority of patients seen during July, August, and September in the northeast United States and southeast Canada.
• May not occur in major metropolitan areas that do not have the normal appropriate hosts such as the cottontail rabbit.

Surgical Considerations

Successful removal of the parasite from the brain/spinal cord has not been reported in cats but may be possible if neuroimaging is available early after onset of clinical signs.

Drug(s) Of Choice

• Supportive care including antiepileptic drugs and appropriate fluid supplementation, which may include thiamine administration and additional potassium intravenously depending on the nutritional status of the patient; typically phenobarbital is used at a maintenance dose of 7.5–15 mg PO, IM, or IV q12h/cat; in addition phenobarbital can be loaded at a total loading dose of 16 mg/kg IV, PO, or IM; this dose is usually divided over 24–48 hours (e.g., 4 mg/kg q12h for 2 days); then maintenance dose is started; diazepam can be used at 2.5–5 mg IV to stop cluster seizures or status epilepticus. Other anticonvulsants, such as levetiracetam, may also be utilized.
• A cocktail treatment has been proposed for recently affected cats-diphenhydramine IM at 4 mg/kg, 1–2 hours before giving ivermectin SC at 200–500 µg/kg and prednisolone sodium succinate at 30 mg/kg IV; treatment is repeated 24 and 48 hours after the first injection of ivermectin; in addition patients receive prednisone at 5 mg/cat q12h PO for 14 days and enrofloxacin at 22.7 mg PO q12h for 14 days; ivermectin is not approved for use against Cuterebra larva so appropriate client permission must be obtained prior to administration; the above cocktail treatment is not for patients with clinical signs >1 week as the parasite is likely already dead.

Contraindications

Do not use ivermectin in cats with known sensitivity.

Precautions

No adverse effects from ivermectin have been noted; however, an anaphylactic or allergic reaction could occur if the Cuterebra larva suddenly dies and releases possible foreign antigens.

Possible Interactions

N/A

Alternative Drug(s)

Can use dexamethasone instead of prednisone.

Patient Monitoring

Sequential neurologic evaluations

Prevention/Avoidance

• Keep cat indoors.
• Use of monthly fipronil, imidacloprid, selamectin or ivermectin has been suggested to prevent infections with the parasite.

Possible Complications

• May continue to have uncontrolled seizures.
• May continue to circle compulsively.
• May have behavioral changes such as aggression.

Expected Course and Prognosis

After initial onset, many patients improve and become acceptable pets; there may be persistent deficits, seizures, circling, and undesirable behavior such as aggression; persistent clinical signs depend on damage caused by the infarction and parasitic migration.

Associated Conditions

None

Age-Related Factors

N/A

Zoonotic Potential

None; however, aberrant Cuterebra larva migration has been reported in humans, most commonly as an ocular form in children.

Pregnancy/Fertility/Breeding

N/A

Synonyms

CNS cuterebriasis

Abbreviations

• CT = computed tomography
• FeLV = feline leukemia virus
• FIE = feline ischemic encephalopathy
• FIP = feline infectious peritonitis
• FIV = feline immunodeficiency virus
• FLAIR = fluid attenuated inversion recovery
• MCA = middle cerebral artery
• MRA = magnetic resonance angiography
• MRI = magnetic resonance imaging
• PD-WI = proton density weighted images
• SAS = subarachnoid space
• T2-WI = T2-weighted images

Internet Resources

• http://Botfly.ifas.ufl.edu/links1.htm.
• http://Cal.vet.upenn.edu/dxendopar/parasitepages/unknown/cutebra.html.
• http://web.vet.cornell.edu/public/oed/neuropathology.
• http://www.neurovideos.vet.cornell.edu/ Videos 14-8, 14-9, 12-20

Client Education Handout Available Online