Idiopathic Epilepsy in the Chihuahua
A number of researchers have conducted pedigree analyses on specific breeds and these analyses have found strong evidence that idiopathic epilepsy is in fact an inherited genetic condition2. A genetic factor may be highly suspected when seizures occur in dogs 1 to 3 years of age. In Vizlas it appears to be inherited as a simple autosomal recessive trait although polygenic inheritance has not been ruled out completely.3 Test breedings of epileptic dams and sires done by veterinary researchers have produced incidences of epilepsy in the offspring ranging from between 38 percent (a study of three different epileptic bitches outcrossed to a single epileptic sire) to 100 percent (a mating of two epileptic siblings).5 Epilepsy in the Belgian Tervueren and Belgian Sheepdog has been found to be polygenic with a single locus of large effect. 9
In addition to external metabolic influences, there are internal factors in a neuron that regulate how excitable each cell is in the brain. The makeup of all the internal machinery of the neuron and its interactions with its neighbors is determined by the genetics of the animal. A mutation in certain genes can cause these cells to be more excitable and thus more likely to slip over the threshold into seizures. This is presumed to be the basis of hereditary epilepsy. 10
All dogs have a seizure threshold. A seizure occurs when this threshold is exceeded. Seizures may be caused by a stimulus such as hormones (estrogen can lower the threshold to seizures in parts of the brain5), fatigue, injury, a fight with another dog, or even breeding. Seizures may also occur spontaneously.
Dr. Alexander de Lahunta of Cornell University and others suggest that each animal inherits a “genetically determined predisposition to seizures”, and that seizures occur when this threshold is exceeded. 6 In other words, a physical condition which may cause seizures in a low-threshold animal may not cause seizures in a “normal” animal. The seizure threshold is apparently exceptionally low in animals that suffer from idiopathic epilepsy. 7
In order to be diagnosed with Idiopathic Epilepsy, other conditions must first be ruled out as the cause of seizures. Seizure inducing conditions include:
- Brain tumor- Most common cause of seizures that begin after age 58 MRI can rule out.
- Distemper- Can determine by CSF analysis. Suggested to do while pet is already under anesthesia for MRI.
- Encephalitis- Can be diagnosed by CSF analysis.
- Exposure to toxins- rat poison, some plants, pesticides, herbicides, drugs, etc
- Garbage poisoning- Staphylococcal or botulism toxins can cause central nervous system stimulation and result in seizures.
- Head injury- An injury to the skull, possibly hard enough to crack the skull would require a significant impact to cause an intracranial disturbance. If such an event did occur, then, if seizures were to occur, they will not be observed immediately. The dog would not have seizures until weeks, or even months after the event. An MRI would show an abrasion on the skull. Anticonvulsant drugs would be ineffective. 5
- Hydrocephalus-Ultrasound can be performed by scanning through the fontanel to detect excessive accumulation of fluid within the brain.
- Hyperkalemia- High concentration of potassium in the blood. Bloodwork can rule out.
- Hypocalcemia-Can be detected with a simple blood test.
- Hypoglycemia- responds to glucose, epilepsy does not.
- Hypothyroidism- Jean Dodd recommends a thorough panel: T4, free T4, T3, free T3, T3 and T4 autoantibodies plus thyroglobulin autoantibody and TSH where indicated.
- Hypoxia- Lack of oxygen. Can be result of heart or lung disease which can be ruled out with bloodwork and x-rays.
- Kidney disease- Acidosis can result in buildup of toxins in the blood which may cause seizures. Kidney health can be determined through blood tests.
- Liver disease- Portosystemic Shunt falls into this category. Excess ammonia in the blood can cause seizures. Blood ammonia levels can be measured.
- Lead poisoning- can be from tiny particles of dust from degenerating paint. Blood can be tested for lead levels.
- Lyme disease and Rocky Mountain Spotted Fever- Can be detected with blood tests.
- Toxoplasmosis- Blood test can rule out.
Hopefully Chihuahua breeders will not fall prey to the ostrich syndrome described in C.A.Sharp’s The Road To Hell: Epilepsy and the Australian Shepherd. “It’s something in your water”…”You gave him a vaccination”…”She got stung or bitten by something”…”You’re feeding the wrong food”…”The garden shed door was open, he must have gotten into something”…”She hit her head” …And so on… There are many things that can cause seizures. Thorough veterinary follow-up is required so that the dog can receive proper treatment. While the excuses above do indicate other possible causes, most are unlikely to cause long-term repeated seizure episodes, would have additional symptoms, and/ or could be identified through testing. The “idiopathic” label has played directly into the hands of those suffering from Ostrich syndrome. Since there is no positive test, a dog with recurring seizures surely must have something else. No matter what tests have been run, there will be something Ostriches can point to that wasn’t done or can’t be followed up on allowing them to exonerate their dogs’ genes.9 The list above clearly states how to rule out most of the conditions as the causes of seizure activity in a dog. Epileptic dogs should never be used for breeding. Until we know how epilepsy is inherited, serious thought should also be given as to whether or not to continue breeding parents or siblings of an affected dog.
The Canine Epilepsy Project is a study supported by grants from the AKC Canine Health Foundation, National Institutes of Health, Individual breed clubs and private donations. The project’s goal is to find the genes responsible for epilepsy, the mode of inheritance and hopefully a DNA marker test, so that wise breeding can decrease the incidence of the disease in dogs.
The Canine Epilepsy Project researchers need samples from dogs that have experienced seizures and immediate relatives, both normal and affected. Specifically they need samples from all available siblings, parents and grandparents and also any offspring and mates if bred. Participation in the study is free and confidential- the names of individual owners or dogs will not be revealed.
Unfortunately to date no Chihuahua samples have been received. According to the organizers of the project “The level of participation by any breed should not be interpreted as an indication of the frequency of this problem within the breed, but can serve to demonstrate the commitment by fanciers of that breed to help researchers solve this problem”4
Our health survey has revealed that without a doubt idiopathic epilepsy is an issue that needs to be addressed in our breed. Now it is time for Chihuahua fanciers to demonstrate their commitment to help researchers solve this problem. Please support the Canine Epilepsy project. Send an email to firstname.lastname@example.org to have data and sample collection forms and instructions mailed or faxed to you. Forms may also be downloaded from the website at www.canine-epilepsy.net/forms.html.
Donations may be made to the Canine Health Foundation www.akcchf.org/. If you specify that your donation be used towards epilepsy research they will honor your request. We CAN make a difference!
- Clinical Characteristics and inheritance of idiopathic epilepsy in Vizlas Patterson EE, Mickelson JR, Da Y, Roberts MC, McVey AS, O’Brien DP. Johnson GS, Armstrong PJ Department of Small Animal Clinical Sciences, University of Minnesota, College of Veterinary Medicine, St Paul, MN J Vet Intern Med. 2003 May-Jun;17(3):319-25
- Cunningham, James G. DVM Ph.D. and George C. Farnbach VMD Ph.D.: Inheritance and Idiopathic Canine Epilepsy. Journal of the American Animal Hospital Association, February 1987, pp. 421-424.
- De Lahunta, Alexander DVM Ph.D: Special Report – Seizures: How They Happen and What You Can Do. (Handout).
- The Road to Hell: Epilepsy and the Australian Shepherd by C. A. Sharp. Australian Shepherd Journal, vol 13 issue 4, July/August, 2003