You dog can suffer from a condition called caval syndrome, in which large numbers of heartworms invade the right side of the heart and block blood flow. (barbaraspetcareservices.com)
Dogs harboring large numbers of heartworms can have sudden blockages of blood flow to the heart, causing cardiovascular failure. (leesburgvetblog.com)
If large numbers of heartworms are present there can be a massive amount of thrombosis. (lbah.com)