Hope your vet gets there as quickly as possible. It sure sounds serious. WNV came to mind when I read about your boy. You may find this helpful, and you can mention the other disorders as well to your vet to help him/her possibly isolate the exact cause of your boy's symptoms. Good Luck!
What to Look for in Your Horse:
Horses and other members of the equine species become infected following being bitten by a mosquito carrying WNV. The symptoms relate to the central nervous system, because the virus causes an encephalomyelitis or brain and spinal cord infection. Symptoms include listlessness, depression, loss of appetite, stumbling and incoordination, weakness of limbs, muscle twitching, partial paralysis, inability to swallow, head pressing, recumbency, convulsions and death. Fever may or may not be present. The symptoms will progress from the very early, mild symptoms through to recumbency and death in a period of five to seven days.
These symptoms are easily confused with other nervous system disorders in the horse. Diseases to be considered when making a diagnosis include rabies, sleeping sickness, equine viral rhinopneumonitis, equine protozoal myeloencephalitis, and tetanus.
