Dog Parvo Symptoms

Congenital form

The congenital form of myasthenia gravis has been reported in Jack Russel terriers, smooth haired fox terriers, Springer spaniels, Gammel Dansk honsehunds, and Samoyeds.  Clinical Signs become evident in these dogs around 6-8 weeks of age.  The congenital form is considered a disease of young dogs.  The main sign is general muscle weakness and some show the presence of megaesophagus (smooth haired fox terriers only).

megaesophagus - great enlargement of the lower portion of the esophagus.  The clinical signs include regurgitation, increased drooling or salvation, and repeated attempts at swallowing with extension or twisting of the head or neck.

Acquired form
 
The acquired form of myasthenia gravis occurs in adult dogs.  Mainly larger breed dogs such as German shepherds, Golden retrievers, and Labrador retrievers are affected.  Clinical signs usually become evident in affected dogs that are older than two years of age.  The main sign is progressive muscle weakness with exercise.  Thoracic or pelvic limbs are predominantly affected which progressively shortens the stride of the animal and may result in refusion to move.    Facial muscles, eyelids, and ear flaps may droop, and the animal may have troubles holding its head up after fatigue.  The weakness may be restricted to certain muscle groups such as the esophagus, pharyngeal and laryngeal muscles, and ocular muscles.  Megaesophagus is frequently seen with impaired swallowing, drooling, choking, and retching.  Along with Megaesophagus, apiration pneumonia is commonly seen.  In older dogs, myasthenia gravis is usually connected with thymoma.

apiration pneumonia – bronchopneumonia resulting from the inhalation of foreign material, usually food particles or vomit, into the bronchi.
thymoma - a neoplasm in the anterior mediatinum, originating from thymic tissue, usually benign, and frequently encapsulated; occasionally invasive, but metastases are extremely rare; histologically, consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant.

Diagnosis

Myasthenia gravis is said to be a clinically obvious disease which does not normally require laboratory tests, however, there are many laboratory tests that can be done to confirm myasthenia gravis.
 
Edrophonium chloride (Tensilon) response test – One test that can be done is a endrophonium chloride (Tensilon) response test.  Endrophonium chloride is a short acting anticholinesterase. Animals that are not already collapsed are made to do some type of exercise until weakness develops.  After the exercise, the animal is  given an injection of the anticholinesterase endrophonium chloride.  Muscle strength following the injection of endrophonium chloride is evaluated.  If there is no response after the first injection, a second can be given 1 – 2 minutes later, but the total dose should range from  0.5 – 5.0 mg IV depending on body weight.  With a positive test, muscle strength improves within 10 – 30 sec after injection and decreases within 5 minutes.  If no benefit is seen after 2 minutes, the test is negative.  This test is not specific only for myasthenia gravis and other tests should be performed to confirm the disease.

Neostigmine methylsulfate (Prostigmin) response test – Other tests can be done in the same fashion as the endrophonium chloride response test using longer acting anticholinesterase drugs such as neostigmine methylsulfate.  The dose is 0.05 mg/kg IM.  Clinical improvements occur 15 – 30 minutes after the injection.  A prior administration of atropine is required.

Electrophysiologic testing - A electrophysiologic test is done by repetitive supramaximal electrical stimulation to peripheral nerves at 3 – 5 stimuli per second.  The decreasing response (action potentials) of the animal to the stimuli is observed.  Normal animals will show a decreased response if greater than 20 stimuli per second is present.  If there is a 10% decrease in response, it is suggestive that the animal has myasthenia gravis.  If there is a 20% decrease, it is very reliable that the diagnosis is myasthenia gravis.  If the decrease in response ceases after tensilon is injected, the diagnosis of myasthenia gravis is confirmed even more.

Measurement of antibody titer – There is a significant correlation between the amounts of antibodies to the acetylcholine receptors and acquired myasthenia gravis.  However, there is no correlation between amounts of antibodies to acetylcholine receptors in the congenital form of myasthenia gravis.  Animals with thymoma also tend to have high amounts of antibodies.

Immunocytochemical  testing – Immunocytochemical testing can provide evidence of acquired myasthenia gravis.  All that is needed for this test is a sample of the serum from the animal suspected to have myasthenia gravis.  The serum of the animal is incubated with sections of normal muscle tissue.  Antibodies from the patient’s serum that bind to the end-plates within the muscle are detected by enzyme- or flourescence-linked secondary reagents.  This shows the amount of antibodies that are aimed against the proteins on the end-plate of the muscle.  However, this test is not specific only for the proteins on the acetylcholine receptors.

Radiographs  all cases of myasthenia gravis should have chest radiographs due to the great incident of megaesophagus and resulting apiration pneumonia.  Also, the radiograph should be checked for the presence of cranial mediastinal masses due to the association of thymoma with myasthenia gravis.