Last Updated on July 29, 2021 by Julia Wilson
Feline ischemic encephalopathy (FIE) is a neurological disorder caused by parasitic infection of the Cuterebra larvae common to North America, South America, and Canada.
Cats are accidental hosts; the parasite usually infects rodents and rabbits. Endemic areas include southeastern parts of Canada and northeastern parts of the United States although some species of Cuterebra occur further south, west, and into Southern America.
Infection rates are highest in the summer and autumn months of July, August, and September. The disease only occurs in outdoor cats with young to middle-aged cats most commonly affected. FIE is only found in the United States, Canada, and South America where the botfly is located. Infection with Cuterebra larvae is known as Cuterebriasis.
Three forms of Cuterebriasis may occur in cats:
- Myiasis – This is the most common form of Cuterebriasis where the larvae migrate to the subcutaneous tissue forming localised swelling known as a warble. Common locations include the head, neck, and thorax.
- Respiratory – The larvae enter the respiratory tract including the trachea, pharynx, and lungs resulting in sneezing, nasal discharge, nasal/facial swelling, and difficulty breathing.
- Cerebrospinal – The larvae migrate through the nasal passages into the brain. This type of Cuterebriasis is the cause of feline ischemic encephalopathy in cats.
How do cats become infected with cuterebra larvae?
The natural hosts of cuterebra larvae are rodents and rabbits. The adult botfly lays her eggs around the entrance of burrows and nests. Larvae hatch in response to an increase in temperature or motion of a nearby host. Newly hatched larvae are moist which makes it easier for them to stick to the fur of a passing animal. As a host (your cat or a passing rodent/rabbit) passes, larvae attach to the coat before they enter the body via the mouth or nostrils. Larvae remain localised for 6-8 days before migrating through the trachea, thoracic and abdominal cavities to a subcutaneous location (commonly the head, neck, and trunk) where it sets up home, feeding on surrounding tissue. A warble forms with a fistula (breathing hole) in the skin. While uncomfortable, this form of Cuterebra infection is typically not life-threatening.
In some cases, the larvae follow an abnormal migration pathway and enter the brain with resulting neurological disorders. As the larvae migrate through blood vessels in the brain, the blood vessel constricts resulting in ischemia (disrupted blood flow) and/or infarction (death of tissues due to lack of oxygen).
As the parasite migrates through the nasal cavity, upper respiratory tract infection symptoms may develop such as sneezing and nasal discharge.
7-21 days later, sudden onset of neurological signs develop. The neurological disease occurs as a result of restricted blood flow to the brain. Symptoms can vary depending on the location and extent of the lesions.
Common symptoms may include:
- Anorexia (loss of appetite)
- Increased or abnormal vocalisation
- Head tilt
- Unilateral or bilateral central blindness
- Abnormal gait
- Circling (towards the side the lesion is located)
- Behavioural changes, particularly aggression
- Head pressing
Your veterinarian will perform a complete and thorough physical examination of your cat and obtain a medical history from you including recent trauma, underlying systemic diseases such as kidney or liver disease, any other symptoms you may have noticed (including recent respiratory signs). The examination usually reveals no signs of disease, only neurological disorders which should raise your veterinarian’s index of suspicion in affected areas.
As other disorders can produce similar symptoms diagnostics will be necessary.
- Biochemical profile, complete blood count, and urinalysis typically come back normal but may reveal increased white blood cells and/or eosinophils. These can help to rule out other causes.
- Serology for FIV, FeLV, and FIP.
- Antigen titres for toxoplasmosis or cryptococcosis.
- Fecal examination for the presence of toxoplasmosis oocysts.
- Spinal fluid tap – Removal of a sample of spinal fluid for examination which usually comes back normal but may elevate protein and/or inflammatory cells.
- A definitive diagnosis is made by MRI scan to reveal larval track lesions from the cribriform plate to the brain as well as possible loss of brain tissue.
There are no reports of surgical removal of the Cuterebra larvae from cats and therapy is aimed at managing symptoms and supportive care.
- Fluid therapy to treat dehydration.
- Antiepileptic drugs (phenobarbital) to control seizures.
- Administration of diphenhydramine, an antihistamine to prevent an allergic reaction followed by ivermectin, (an anti-parasitic medication) to kill the larvae. Repeat at 24-48 hour intervals.
- Prednisolone to control inflammation.
- Enrofloxacin (an antibiotic) can be prescribed to treat possible bacterial infection due to the migration of larvae.
The only way to prevent cats from becoming infected with Cuterebra larvae is to keep them indoors, particularly between July – September.
Seek veterinary care if a cat who lives in affected areas develops respiratory symptoms.
Administer monthly worming medication in high-risk areas.