Veterinarians will often ask pet owners whether they have done, or will do, any travelling with their pet, and if so, where to, & what times of the year. Part of the reasoning behind asking these questions is to assess the pet's risk for contracting heartworm disease. This parasite infestation is easy to prevent, but very difficult (expensive and risky) to treat. As always, we'd much rather prevent this.
Heartworm disease is the infestation by a worm that develops in the bloodstream of the dog and eventually matures to the adult form that lives in the vessels of the lungs and in the heart. For obvious reasons, this infestation can be a serious health threat to the patient.