Domestic pets and other animals can get distemper if they come in contact with other animals that have it. (petpremium.com)
Since your pets tend to be outdoors longer in the summer months, they may come into contact with other animals more frequently while at dog parks, dog beaches, etc.. (morenapethospital.com)
They get them through contact with other animals also contact with fleas in the environment (trails, dog parks, tall grass). (roverstayover.net)