Feline ischemic encephalopathy (FIE) is a neurological disorder caused by parasitic infection of the Cuterebra larvae which are located in North America, South America, and Canada. There are 12 species of Cuterebra Cuterebra and 22 subspecies of Cuterebra Trypoderma. Cuterebra Cuterebra are parasites of rabbits (lagomorphs) and Cuterebra Trypoderma are parasites of rodents.
Feline - Cat, Ischemic - Restriction of blood flow in a vessel supplying blood to the brain, Encephalopathy - Disease of the brain
Cats are accidental hosts, the parasite normally infects rodents and rabbits. It is most commonly found in southeastern parts of Canada, northeastern parts of the United states although some species of Cuterebra are located 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.
There are three forms of Cuterebriasis which 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 fur 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. The larvae should, however, be removed by a veterinarian.
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). It has been suggested that the vascular constriction is a result of toxins released by the Cuterebra larvae.
What are the symptoms of feline ischemic encephalopathy?
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)
- Behavioral changes, particularly aggression
- Head pressing
How is it diagnosed?
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). Examination usually reveals no signs of disease, only neurological disorders which should raise your veterinarian's index of suspicion in affected areas.
As there are other disorders which can produce similar symptoms he will need to perform some diagnostic tests do determine the underlying cause, these may include:
- Complete blood count, biochemical profile, and urinalysis which 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 normally 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 in more advanced cases.
How is it treated?
There are no reports of surgical removal of the Cuterebra larvae from cats and treatment is aimed at managing symptoms and supportive care. This may include:
- Fluid therapy to treat dehydration.
- Antiepileptic drugs (phenobarbital) to control seizures.
- Administration of diphenhydramine which is an antihistamine to prevent an allergic reaction followed by Ivermectin an anti-parasitic medication which may be administered to kill the larvae. This treatment (along with prednisolone) is to be repeated at 24 and 48 hours. It should be noted that Ivermectin is not approved for treatment of Cuterebra in cats.
- Prednisolone will also be prescribed to control inflammation.
- Enrofloxacin (an antibiotic) may also be administered to treat possible bacterial infection caused by larval migration.
How is feline ischemic encephalopathy prevented?
The only way to prevent cats becoming infected with Cuterebra larvae is to keep them indoors, particularly between July - September.
Cats who develop respiratory disease in affected areas should be seen by a veterinarian and carefully monitored.
Monthly administration of worming medication should also be carried out in affected areas.