The 2009 Chihuahua Club of America health survey revealed an alarming number of "seizures of unknown origin", technically referred to as Idiopathic Epilepsy. Idiopathic is defined as "Arising from an unknown cause". 45% of survey participants had bred or owned a Chihuahua with seizures of unknown origin with an average of 2 dogs per breeder/owner affected by the condition. One breeder had 20 Chihuahuas affected out of 30 litters. Another two affected Chihuahuas were noted as being sired by the same dog.
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:
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
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!