5 months old is when I started mine at a 1/4 of a cc. They are 6 months now and I will start doing 1/2 cc at 7 months. I normally jump up to 1cc at 9 to 10 months. Not sure if it's right or wrong, but it's been working.
8 weeks seems way to early. how much are you giving them.
Ivomec 50 ml bottle - 1% injectable for cattle
one-tenth of a cc per 10 pounds. of dog
example; a 100 pound dog would get 1cc
a 40 pound dog would get four-tenths of a cc
I give pups one-tenth of a cc
Mosquitoes can infect a pup as easy as it can infect a grown dog.
Check this link to a chart which shows the life cycle of heartworms.
http://www.google.com/imgres?imgurl=http://heartworm.com/wp-content/uploads/2011/06/life_cycle-Image.jpg&imgrefurl=http://www.heartworm.com/research/life-cycle&h=2481&w=3231&sz=1075&tbnid=L5JveOOInPc-lM:&tbnh=90&tbnw=117&zoom=1&usg=__pO1WgEfR7uqlNEzFAu2KbkY6gMk=&docid=cF1pniV1xD35iM&sa=X&ei=gFIAUqSEJMWyygGJxIHQCw&ved=0CDwQ9QEwAg&dur=3985#imgdii=L5JveOOInPc-lM%3A%3B5PiDveu4gwMO_M%3BL5JveOOInPc-lM%3A
