From what I've been told, tetanus doesn't usually cause fever until the very last stages, when it may go very high, near 110. Your filly does sound to have very typical WNV symptoms--I had 2 cases here in 2002--but of course there is a whole list of things with similar symptoms. It's so difficult when you pretty much have to treat what you see & hope it's the right choice--test results just take too long to get back in many cases, because like my vet told me when my big mare was so sick, we'd know within 48 hours which way she was going to go. I think it was a good 2 weeks that time before I got the test results back for this mare.
Good luck, & prayers for your pretty baby!