I'd say you're right on both accounts. Dogs aren't treated the way they are in more developed countries, and it's a cultural difference overall. But Nicaragua's poverty also plays a role - there's just less food around for everyone. The Nicaraguans living in Miami I know have little fat dogs though, so I guess as they get better off they treat their animals better as well.
The best place in the country for helping out is Donna Tabor's
Building New Hope (106 Overton Lane, Pittsburgh, PA 15217, 412-421-1625,
www.buildingnewhope.com). Their Casa Lupita project is spaying/neutering stray animals and providing free veterinary service to the rest. They have some heartwarming stories about animals they help, but function on donated time and resources, so I'm sure they would be glad to help you find a way to make a difference, particularly if you are a vet with some free time.