The weather wouldn't worry me tho time of year but I would check your vets policies to make sure they won't require a retest due to a time lapse Of the medication. If it has been too long in their view (which I think one month is fine but who knows) it may be their policy to run a heart worm test before they will giv you your pups prescription- which is a good idea for a lengthy lapse but one month in February?! Bah, I would say you should be fine...course I'm not a vet
