VET CHECK: SEIZURES IN SLED DOGS
Also described as convulsions, epilepsy, or “fits,” the term seizure is used to describe any onset of involuntary, and usually recurring, episodes of loss of consciousness, deranged consciousness, loss of balance, increased voluntary muscle contractions or weakness. The first order of business in diagnosis of the cause of a seizure is confirmation that the abnormal activity you observed was, in fact, a seizure. This can often be done when described in detail to your veterinarian, but occasionally partial seizures can be hard to differentiate from other disorders. Grand mal seizures usually include collapse and loss of consciousness, dramatic muscle activity with paddling and snapping, loss of bladder and bowel control, and a slow return to normalcy. Partial seizures can involve any part of the body, abnormal behavior, and can be easily confused with other disorders. In these cases having your veterinarian actually witness the event, or making a video-recording of the event may be necessary. All seizure episodes have three periods: the first is the aura phase that is a prelude to the actual seizure. This period is often undetectable, but some dogs will display behaviors that can indicate a seizure is imminent: they may appear nervous, distressed, or dazed. The actual seizure, called the ictus, follows. When the seizure ceases there is a postictal period in which the dog is not actually seizuring, but has not returned to normal yet. Many dogs appear dazed and disoriented, but a few will be agitated, excited, or even aggressive. The length of the postictal period is highly variable, from less than a minute to hours long.If you suspect your dog may have had a seizure, a thorough physical exam, in particular looking for any persistent neurological abnormalities, is the first order of business. Many, many dogs that suffer from seizures are completely normal in between episodes. The next step is to perform a Complete Blood Count (CBC), chemistry panel, electrolyte test and thyroid test (these can all be taken in a single blood sample). Quite often, these are normal as well. If this is the case, further diagnostics to ascertain the cause of the seizures become quite expensive, invasive, or both. An MRI will help rule out space occupying lesions (such as tumors or hematomas), degenerative diseases (for example, subacute necrotizing encephalopathy) and vascular disorders. A spinal tap for cerebrospinal fluid can identify some of the infectious or inflammatory diseases that can trigger seizures. Treatment of seizures depends upon the origin. Not every dog that seizures requires anticonvulsant therapy; remember that your veterinarian will be weighing the risks associated with having a seizures versus the risks caused by anticonvulsant medication. All anticonvulsants have side effects and if given on a routine basis, will require periodic monitoring of blood levels and serum chemistries to minimize chances of toxicity and permanent problems. Infrequent, brief, and mild seizures are usually treated with benign neglect; the risks associated with drugs will affect the dog more than the seizures. Prolonged, violent, frequent seizures, or “cluster seizures” (the seizures are brief but there will be more than one in the span of a day) warrant medication if underlying causes have been ruled out or brought under control. You may need to make some changes in your basis husbandry for a dog that suffers from seizures. A seizure can evoke excited, aggressive behavior in other dogs, so it is not recommended that a dog who is at risk of seizuring be kenneled with another dog that could potentially harm him if he has a convulsion. Keep the dog in his own run or tethered out of reach of neighbors. Unless all genetic causes of seizures have been ruled out, a seizuring dog should not be used for breeding and should be spayed or neutered.Below is a description of some potential causes of seizures in sled dogs. It is by no means complete. A seizuring dog will absolutely need to be examined by your veterinarian; an emergency visit is warranted if a seizure persists for ten minutes or longer, or if cluster seizures occur. Idiopathic Epilepsy. By definition, idiopathic epilepsy is a seizure of unknown origin, and is only definitively diagnosed when all other causes of seizures have been ruled out. Epileptic dogs are completely normal on physical exam as well as normal on all routine screening tests. It is an inherited disorder in dogs; breeding a dog with this problem is likely to produce offspring with the same problem. It is frequently seen in various breeds of sled dogs, including hybrids. “Hybrid” sled dogs are often as tightly inbred as many registered purebreds, so it really isn’t surprising that problems like this should arise. Age of onset is usually in dogs less than five years old, although it occasionally will begin at a little older than that.Hypothyroidism. This disorder, again frequently diagnosed (and misdiagnosed) in racing sled dogs, is associated with seizures. This includes the “classic” intracranial grand mal seizure, but there is another seizure-like syndrome observed in dogs with severe hypothyroidism. These dogs have very low, even undetectable, levels of thyroid hormone (T4) when tested, and will show loss of balance, weakness, and poor coordination immediately after exercise. The situation resolves quickly with rest. Any dog with hypothyroidism severe enough to trigger seizures should be ruled out as a breeding candidate.Alaskan Husky Encephalopathy (Subacute Necrotizing Encephalomyelopathy). This rare problem has been described in sled dogs, it is an inherited disorder that results in degeneration of neurons in the brain. The first symptoms are usually observed between seven and twelve months of age, although it has started as late as two and a half years. There is a sudden onset of various neurological symptoms (depending on which part of the brain is affected), including seizures, blindness, ataxia (incoordination), difficulty eating food or drinking water, and bizarre behaviors. There is no treatment and the disease is usually fatal. Hypoglycemia. “Hunting Dog hypoglycemia” is described as an exercise associated onset of low blood sugar. After running hard, affected dogs will become weak, and if the hypoglycemia is not corrected, may begin to seizure. It has been associated with overly thin body weight, poor diet, and poor athletic conditioning in hunting dogs; their body stores of glycogen are depleted and their blood sugar falls dramatically. Often these dogs have other health problems that make them susceptible. Occurrence in sled dogs is extremely rare and I have only seen a few instances of this problem; an underlying problem should always be ruled out in these cases.Portal Systemic Shunts. This is another inherited disease; it is an anatomic defect that directs nutrient-rich blood flowing from the intestine around the liver and into general blood circulation via a venous “shunt.” In normal dogs the liver receives the blood flow directly from the intestine and begins the process of metabolizing various nutrients before they reach other tissues. Since the liver does not get a chance to metabolize the nutrients before they reach the brain, dogs with a PSS will often exhibit bizarre behaviors or seizures. Age of onset is usually very young, puppies under six months old that seizure are commonly screened for this problem. However, it has been diagnosed in dogs up to six years old. A blood test called a serum bile acids test is used as an initial test; special imaging of the blood flow from the intestine is used to definitely diagnose the problem. Surgery is required to permanently correct the problem; it is a risky procedure that can lead to life threatening complications and is typically performed by board certified surgical specialists. In the sled dog breeds, Siberian Huskies seem to be overrepresented with this problem.Infectious diseases. Certainly Rabies is the most well known disease that can cause neurological symptoms (including seizures); but there are many others. Canine Distemper outbreaks occur in situations where vaccination is inadequate. Occasionally, parvovirus, canine infectious hepatitis, and canine parainfluenza have caused encephalitis resulting in seizures. Tick borne diseases such as Erlichiosis and Rocky Mountain Spotted Fever have been known to cause seizures. Fungal diseases, such as blastomycosis, cryptococcosis, histoplasmosis, and coccidiomycosis can affect central nervous system as well.Dawn Brown D.M.V. graduated from the New York State College of Veterinary Medicine at Cornell University and practices small animal medicine and surgery at Carthage, New York. She has been racing since 1982, currently in 6 and 8-dog limited class speed racing in the lower 48, Canada, and Alaska.