Reading the veterinarian articles on these sites I found that they don't put anything toxic into a mare once bred. No deworming or vaccinating of any kind.
Until day 50 of pregnancy... then we revaccinate everything and return to our normal worming schedule. This is the treatment directed by some of the top reproduction vets in the US.
Like someone brought up on the parallel rhino vaccine thread, just because you haven't ever had problems doesn't mean you won't. If one horse goes to a show and gets infected with rhinopneumonitis 1 and goes back to your herd of broodmares, the ENTIRE herd can abort. 70% isn't uncommon. Just because its always served you well not to vaccinate doesn't mean you are immune, just lucky. Now if you have a 100% closed herd (NOBODY coming or going) you could probably get away without vaccinating, but that's exactly what it is, getting away with it. If you understand that risk and still choose not to vaccinate, that's absolutely fine, but the key is understanding the risk and how the disease works and is transmitted.
Also remember that rhino and rhino aren't the same thing
Pneumabort-K (and there is another brandname too, a live-virus version that isn't quite as good) is rhinopneumonitis 1a and 1b. The "normal" rhino you give traveling horses every three months (you do do every three months, right? Once a year isn't covering your horse unless you only travel during the few months after the shot
) is Rhino 1 and 4, different strains of the same disease. One does not replace the other. Pregnant mares need Pneumabort-K, NOT the normal flu/rhino or plain rhino 1/4 that you give to the other horses.
I generally give my mares their annual shot about 30 days before foaling, in order to give the foal some immunity as well, and then give the mares their rhino and West Nile shots before being bred back, if they are going to be.
Remember that rhino is only a 2-3 month vaccine, and so are most West Nile vaccines. If you choose to do this, realize that a) that rhino won't protect pregnant mares from late term abortion, b) its only good for 3 months, and c) I'd suggest you use Recombitek as your west nile vaccine, its an annual vaccine as opposed to Fort Dodge's, which is three months.