Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am very sorry to hear that DJ has symptoms consistent with Canine Distemper Virus, including involuntary muscle twitches in his leg and red eyes.
Has he had any vaccines? If not then a blood test called a distemper titer is the best way to diagnose the disease.
Puppies with distemper generally start with runny eyes and nose, sneezing and fever and progress to vomiting, diarrhea and then 2 to 3 weeks later neurologic signs (tremors, muscle fasiculations, seizures). It is possible that the fever and respiratory or gastrointestinal stages occurred before he came to you.
Honestly distemper in young puppies is really tough to recover from. Their immune systems are immature and this is a very hardy, aggressive virus.
It won't be days, it will be weeks to maybe months from the time the virus attacked before we see whether he can recover, this virus waxes and wanes, meaning as it moves and affects the different parts of the body it shows symptoms based upon the part of the body being attacked. He may seem to improve and then worsen again. Dogs that have neurologic symptoms, especially grand mal seizures have a much poorer prognosis then dogs that only have focal tremors or do not show any neurologic symptoms.
This is not a virus that they ever truly recover from. It stays hidden in the body and can become active again and show symptoms later in life in times that the immune system is weakened from other diseases or in old age. Even dogs that seem to recover can continue to shed the virus for 2 to 3 months even if they appear better. So know that he is a danger to any other dog, especially those not properly vaccinated.
In many cases dogs with Distemper can have seizure episodes and muscle tics for life. He will likely need anticonvulsant therapy (phenobarbital) for life.
Your puppy should see your family veterinarian as soon as possible, but do let them know what you are suspicious of before you take him in. They will likely want to have him come in through the back door and thoroughly disinfect any areas he is in. Dogs with Distemper often need antibiotics for secondary bacterial infections, steroids (prednisone) help reduce brain and central nervous system inflammation and phenobarbital may be needed to control the seizures. If his seizures aren't well controlled they can add Gabapentin. Another option is to add methocarbamol (Robaxin) to decrease muscle tremor symptoms. It does have sedative effects so his doses of Gabapentin and Phenobarbital may need to be adjusted.
I am very sorry to hear that your new little one may have a serious disease. It may be best to return him to the breeder now if he does have Distemper as it can be costly to try to treat and you may not be successful.
There are other reasons for a leg "tic" such as low blood sugar, exposure to a pesticide or a congenital liver vessel malformation called a portosystemic shunt, exposure to a virus in the womb that affected the development of his cerebellum or primary brain disease like hydrocephalus.
Whatever is going on with your fellow he needs to see a veterinarian and until you have some sort of diagnosis I would treat him as highly contagious to protect other dogs.
Please let me know if you have any further questions.