If there are no advices for another origin (like fever, lesions of vesicular stomatitis, chemical irritation, rabies, dysphagia or inability to swallow) and the horse is despite of the mentioned signs normal, you could consider slaframine intoxication and exam the hay, pasture and so on for red clover or other legumes with black patch disease. It is also called "Slobber-Syndrome".

Clinical signs disappear after removal of contaminated forage within 48-72 hours. Some horses may need some electrolyt supplemetation because of extrem losses via saliva.

Maybe the hepatomegaly in your case is due to ingestion of poisonous plants, mould or infection with another virus like hepacivirus or equine parvovirus.

Good day

We suspect it was actually West Nile Fever. The blood did not test positive, but it could be nerve damage as a result of an infection quite a long time before signs of disease. Also, a blocked salivary gland followed paresis with secondary bacteriological infection. Intensive care and treatment with antibiotics, anti-inflammatories and cortisone cured the mare.

Thanks for the info on black clover disease!!!!