My name is***** and I have over 20 years of experience as a veterinarian.
I understand that you are concerned about your fellow's tense looking face, sensitivity to light and sound and difficulty eating and drinking.
With his symptoms it definitely sounds like a neurotoxin may be involved.
I would be suspicious of tetanus because of the tense muscles
, sensitivity to light and touch and difficulty eating and drinking but no fever.
Tetanus is quite rare in dogs, they are relatively resistant, but it can occur. It is caused by a wound that is contaminated by the bacteria, often a small puncture wound or a migrating foxtail awn is the source of the infection. Unfortunately there is no definitive test for the disease. We treat based upon symptoms and finding a puncture wound, which can be very difficult.
Treatment is simple antibiotic therapy, plain penicillin, that will kill the bacterium and supportive care such as fluid therapy, keeping stimulation to a minimum by keeping them in quiet, dark areas, sedatives and anti-seizure medications and sometimes mechanical ventilators to support the patient until they can breathe on their own again.
Patients do best when treatment is started early, before we see difficulty breathing or seizure activity. In cases where the disease has progressed to seizure activity we may not be successful even with the best therapy.
Other possible causes would be botulism, although that normally leads to flaccid (weak muscles) versus spastic muscles, or chemical toxins like high levels of pyrethrins or organophosphates. If he was running a fever infectious causes like Distemper
would be possible too. If he is not current on Rabies then you need to be very cautious around him as Rabies can have lots of different presentations and may not always present with a fever.
If he was unable to open his mouth and had swollen muscles or loss of muscles on his head and face then an autoimmune disease process, masticatory myositis, where the body attacks
its own muscles would be possible.
Whatever is going on with your fellow this is not something treatable at home. He needs a hands on veterinary exam and directed treatment as soon as possible. If he has not been vaccinated for Rabies virus appropriately I would have him examined immediately.
Please let me know if you have any further questions.