This issue rears it´s rather ugly head at around this time every year.   Here, in southern Spain, it is particularly contentious given the climate we live in.  Day´s can start -5ºC, zoom up to 23 or more, down to 8, up again to 22, and all before lunchtime!  So rugging your horse is a full time job - unless you want to risk your horse suffering heatstroke - as you put on and take off their rug all day long. 

Well of course that isn´t practical.  Do you, therefore, enjoy your chimney or central heating whilst riddled with guilt about your naked  equine out in the "cold"?  Of course not, and here is an excellent article explaining why.

http://mhwf.websitetoolbox.com/post/equine-thermoregulation-in-cold-weather-6697515

You must consider the article focus´s on horses being correctly maintained, naturally.  Horses that have forage available 24/7 (just as it should be) and are not filled on 2 or 3 meals of highly processed feeds that consist of short fiber´s, meaning they are processed very quickly through the gastrointestinal tract, unable to provide any natural internal warmth as they pass on their speedy way.  This also does not take into consideration horses with PPID o

Our protocol has, for many years now, been to do counts before worming,, to not worm just for the sake of it, to respect seasonal changes before worming, to poop-pick and rotate paddocks, to rotate worm products in the past, but for some time now to use natural plant-based wormers - when necessary!
This is very interesting reading.  Over 150 different species of internal parasites! - <<Gulp>>
http://horsesidevetguide.com/?p=4714