Objective: To determine frequency of initial clinical signs and risk factors for acquired myasthenia gravis (MG) in dogs.
Design: Retrospective study.
Sample population: 1,154 dogs residing within the United States from 1991 to 1995 with a confirmed diagnosis of acquired MG and 7,176 dogs with other neuromuscular disorders, including generalized weakness, megaesophagus, and dysphagia (control group).
Procedure: Records were retrieved from a database containing results of serum samples tested for acetylcholine receptor antibodies. Signalment, breed, age, state of origin, and month of onset of clinical signs were obtained. An antibody titer > 0.6 nmol/L was diagnostic for acquired MG. Unconditional logistic regression was used for statistical analysis.
Results: In comparison with mixed-breed dogs, dogs with the highest risk of acquired MG were Akitas, terrier group, Scottish Terriers, German Shorthaired Pointers, and Chihuahuas. Rottweilers, Doberman Pinschers, Dalmatians, and Jack Russell Terriers had low relative risks. Sexually intact males and dogs less than 1 year old had some protection from risk. Generalized weakness with megaesophagus and megaesophagus alone were the most common initial clinical signs.
Clinical implications: Breed predispositions for acquired MG were demonstrated. Age and sex were contributing factors. Although most dogs had generalized clinical signs, a substantial proportion of dogs had focal signs.