In dogs, we do indeed want to make sure they are heartworm negative before starting preventative - specifically that they do not have circulating microfilaria (baby heartworms, from here on out referred to as MF). Why? Because the preventative meds are very good at killing MF, and doing it quickly. For a dog with lots of MF in the bloodstream, this is a Bad Thing (essentially the dog gets a massive allergic reaction to all the dying MF). (testing for dogs is a two-step process - one test for antigen, or proteins from the adult worms, and if that is positive, then the blood is checked for MF)
Cats, OTOH, pretty much don't get MF in the bloodstream. The diagnostic testing in cats is for antibodies. And a cat that has antibodies against HW? Well, they've had exposure, and presumably will keep being exposed, so it's considered a pretty good thing to have 'em on preventative, even if testing positive. Especially as there are no treatments to kill the adult heartworms that are safe for cats.
This leaves out the whole mess of how long to have the kitties (or dogs) on HWP; I prefer to have them on preventative 12 months a year - you don't have to worry about when they've been dosed relative to when the mosquitoes are about and capable of transmitting the parasite, and you are controlling other parasites as well. If an owner chooses to skip preventative in the winter months, I will remind them that heartworm preventative is a bit of a misnomer for the oral drugs - it actually kills off any of the heartworm larvae picked up in a window of 30 to 40 days BEFORE you give the meds. It DOES NOT stick around in the system for the next 30 days :-) IOW, if you are going to skip the "winter months" please dose thru mid-December or so.
(yeah, day job and all that. Hey, Sharon, if you want and specific comments offline, just drop me a note via my LJ address)
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mmmag
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Date: 2008-10-04 03:51 am (UTC)In dogs, we do indeed want to make sure they are heartworm negative before starting preventative - specifically that they do not have circulating microfilaria (baby heartworms, from here on out referred to as MF). Why? Because the preventative meds are very good at killing MF, and doing it quickly. For a dog with lots of MF in the bloodstream, this is a Bad Thing (essentially the dog gets a massive allergic reaction to all the dying MF). (testing for dogs is a two-step process - one test for antigen, or proteins from the adult worms, and if that is positive, then the blood is checked for MF)
Cats, OTOH, pretty much don't get MF in the bloodstream. The diagnostic testing in cats is for antibodies. And a cat that has antibodies against HW? Well, they've had exposure, and presumably will keep being exposed, so it's considered a pretty good thing to have 'em on preventative, even if testing positive. Especially as there are no treatments to kill the adult heartworms that are safe for cats.
This leaves out the whole mess of how long to have the kitties (or dogs) on HWP; I prefer to have them on preventative 12 months a year - you don't have to worry about when they've been dosed relative to when the mosquitoes are about and capable of transmitting the parasite, and you are controlling other parasites as well. If an owner chooses to skip preventative in the winter months, I will remind them that heartworm preventative is a bit of a misnomer for the oral drugs - it actually kills off any of the heartworm larvae picked up in a window of 30 to 40 days BEFORE you give the meds. It DOES NOT stick around in the system for the next 30 days :-) IOW, if you are going to skip the "winter months" please dose thru mid-December or so.
(yeah, day job and all that. Hey, Sharon, if you want and specific comments offline, just drop me a note via my LJ address)