GENETIC DISEASES

 

The study of genetics is fascinating. It is quite predictable, and a field I enjoyed studying at university and teaching to students.

 

Genetic Diseases are those that can be passed on from parents to offspring. They are inherited from either one or both parents. Highly heritable diseases are present if the genes are present in both parents. Lowly heritable diseases may only be present if both the gene and environmental factors are involved.

 

While Border Collies are a robust breed, there are some genetic diseases that can be present. All of the possible Border Collie genetic diseases are recessive or complex. Being recessive means that both parents must possess the gene for the disease to present. Being complex means factors other than genes (eg. environment) influence the expression of the disease. A dog that possesses the recessive gene for a disease is called a carrier. This means they don't show signs of the disease themselves, but can pass it on to their offspring.

 

While having a dog that is a carrier of a genetic disease is not a problem, it is important for breeders to know what their dogs may be carriers of. The procedure for definitely knowing whether a dog is a carrier of a disease is simple. It involves rubbing a small brush inside the cheek of the dog and sending the sample off to a laboratory for testing. The sample can be tested for one or more diseases or even for other genetic factors such as colouring and breed specific markers. If a breeder does not know the genetics of their dogs, it is possible to breed two carriers of a disease and produce puppies that suffer from the disease. When breeders refer to puppies being clear of genetic diseases due to parentage it usually means the puppy's parents or grandparents have been genetically tested and found to be clear, meaning the puppy's parents cannot have the diseases. Hence, the puppies have to be clear of the diseases too.

 

So, if you choose a responsible breeder and ask the right questions you should have no concerns about the recessive diseases of Border Collies.

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There are six main categories genetic disorders can be divided into:

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• Neurologic (Associated with the Brain, Spinal and Nerves) - Degenerative Myelopathy - Ivermectin Sensitivity MDR1 (Multi Drug Resistance) - Neuronal Ceroid Lipofuscinosis 5 (Border Collie Type).

• Ophthalmologic (Associated with the Eyes) - Primary Lens Luxation - Collie Eye Anomaly/Choroidal Hypoplasia

• Metabolic (Associated with the Body's Enzymes and Cell Metabolism) - Cobalamin Malabsorption: Cubilin Deficiency (Border Collie Type)

• Immunological (Associated with the Immune System)  - Trapped Neutrophil Syndrome (Border Collie Type)

• Haemolymphatic (Associated with the Circulatory System) - von Willebrand's Disease Type II

• Musculoskeletal (Associated with Bones and Muscles) - Myotonia Hereditaria (Cattle Dog Type)

 

There three well known genetic diseases that can be tested for are: Collie Eye Anomaly (CEA), Neuronal Ceroid Lipofuscinosis (CL) and Trapped Neutrophil Syndrome (TNS).

 

Collie Eye Anomaly (CEA):

 

CEA refers to an inherited abnormality in the development of the retina, optic nerve and choroid of the eye. The severity of the disease ranges from no visual impairment to blindness. It is not a progressive disease and affected dogs usually have only mildly impaired vision.

 

It is known to affect Collies, Border Collies, Australian Shepherds and Shetland Sheepdogs.

 

Neuronal Ceroid Lipofuscinosis (CL):

 

Affected dogs lack a certain enzyme important for cellular metabolism. This leads to an abnormal accumulation of cellular waste product in cells (particularly those of the nervous system) causing disruption of normal cellular function. Affected dogs appear normal until around fifteen to twenty months of age. The following neurological symptoms may then appear:

 

* behavioural changes,

* disorientation,

* aggression and irrational fears,

* blindness,

* abnormal gait,

* seizures, and

* difficulty with balance.

 

Affected dogs progressively deteriorate and most die or are euthanised by three years of age.

 

Trapped Neutrophil Syndrome (TNS):

 

TNS is a disease in which white blood cells (neutrophils) are produced but are not able to move from the bone marrow into the blood stream. Affected dogs have a compromised immune system and are not able to fight off infection. Pups are very prone to  infections and often die or are euthanized within their first few months.

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The Less Common Genetic Diseases:

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Degenerative Myelopathy:

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The disease affects the White Matter tissue of the spinal cord. Affected dogs usually present in adulthood with gradual muscle atrophy and loss of coordination typically beginning in the hind limbs due to degeneration of the nerves. The condition is not typically painful for the dog, but will progress until the dog is no longer able to walk. The gait of dogs affected with degenerative myelopathy can be difficult to distinguish from the gait of dogs with hip dysplasia, arthritis of other joints of the hind limbs, or intervertebral disc disease. Late in the progression of disease, dogs may lose faecal and urinary continence and the forelimbs may be affected. Affected dogs may fully lose the ability to walk six months to two years after the onset of symptoms.

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Ivermectin Sensitivity MDR1 (Multi Drug Resistance):

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This gene mutation causes dysfunction of P-glycoprotein, which is responsible for removing certain drugs and toxins from the body. Clinical signs are most commonly associated with distribution of the drug in the central nervous system. MDR1 is inherited in an autosomal incomplete dominant manner in dogs meaning that dogs only need to inherit one copy of the mutated gene to be at an increased risk of developing adverse reactions to certain medications. Though adverse reactions to certain drugs are most commonly seen in dogs having two copies of the mutated gene, carrier dogs can also experience drug sensitivities and dosages need to be adjusted accordingly. Thus, dogs that have one or two copies of the mutation are considered at-risk for adverse drug reactions. If an at-risk dog is treated with one of several common drugs, they are at risk of developing neurologic symptoms that could range from tremors, excess salivation, anorexia, and blindness to coma and even death. Because of the defective ability to metabolize specific drugs, these drugs can be lethal even at low doses. The MDR1 mutation does not cause adverse effects in dogs unless the dog is exposed to these drugs. Therefore, veterinarians should be notified when a dog is at risk for multidrug resistance 1 prior to administration of any medications.

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Drugs known to cause neurological signs related to the MDR1 mutation include:
Acepromazine, butorphanol, doxorubicin, emodepside, erythromycin, ivermectin, loperamide, milbemycin, moxidectin, rifampin, selamectin, vinblastine and vincristine.

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Ivermectin is commonly used against parasites.

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Primary Lens Luxation:

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Primary Lens Luxation (PLL) is an inherited abnormality of the eye affecting dogs. It is characterized by dislocation of the lens in the eye due to the breakage of the ligaments that hold the lens in place. The age of onset is variable depending on whether a dog has one or two copies of the mutation, but affected dogs typically present between two to eight years of age with sudden signs of eye irritation. Symptoms of lens luxation include excessive blinking, squinting and tearing of the eye. Dislocation of the lens can occur in both the forward and backward position within the eye, but dislocation in the forward position is more common and serious. If not treated immediately, lens dislocation can lead to glaucoma and vision loss.

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Cobalamin Malabsorption: Cubilin Deficiency (Border Collie Type):

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Intestinal cobalamin malabsorption (border collie type) is an inherited disease affecting border collies. Affected dogs are unable to make adequate amounts of a protein that plays a role in absorption of certain nutrients from the intestinal tract and kidneys, including the B vitamin, cobalamin. Affected dogs have increased levels of methylmalonic acid in their urine from as early as fourteen weeks of age, but symptoms of disease may not be recognized by owners for months or years. Symptoms of disease include anorexia, lethargy, poor weight gain, poor muscle mass, and in rare circumstances, a severe neurological dysfunction called hepatic encephalopathy that can lead to altered mental state, seizures, coma and death. Affected dogs have an increase in certain proteins in their urine, and have decreased synthesis of blood cells resulting in anaemia and decreased numbers of neutrophils. Affected dogs require cobalamin supplementation for life that results in disease remission for most animals within a few weeks. Though not associated with clinical disease, affected dogs will continue to pass increased amounts of certain proteins in the urine even with cobalamin supplementation.

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Von Willebrand's Disease Type II:

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Von Willebrand disease type II is an inherited bleeding disorder affecting dogs. Dogs affected with VWDII have decreased levels and abnormal function of von Willebrand coagulation factor (vWf), which is an essential protein needed for normal blood clotting. Affected dogs generally have moderate to severe signs of a bleeding disorder. Affected dogs may bruise easily, have frequent nosebleeds, bleed from the mouth when juvenile teeth are lost and experience prolonged bleeding after surgery or trauma. The bleeding may be severe enough to cause death. Due to variable severity of the disorder, affected dogs may not be identified until a surgery is performed or trauma occurs at which time excessive bleeding is noted. Veterinarians performing surgery on known affected dogs should have ready access to blood banked for transfusions. Dogs can have a normal lifespan with this condition although they are susceptible to life-threatening bleeding with an accidental injury or any surgical procedure.

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Myotonia congenita (Australian cattle dog type):

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Myotonia congenita (Australian cattle dog type) is an inherited muscle disorder affecting dogs. The muscle cells of an affected dog are over-excitable, which causes muscles to remain contracted rather than relaxing after voluntary activity. Signs of the disorder usually present when puppies begin to walk. Their gait may appear stiff and uncoordinated and they may fall frequently. Affected dogs frequently have a “bunny hop” type gait. Episodes do not appear to be painful and the muscle stiffness may improve with increased exercise. Episodes can worsen with hot weather and excitement. Other features include enlargement of the muscles, especially of the neck and limbs, abnormal posture, and an upper jaw that is much longer than the lower jaw. Dogs with this disorder typically have a normal lifespan.

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