Humane societies usually don't have the "power" to investigate and seize animals anyway.
When you are dealing with neglect/abuse cases, some spca's in some larger cities have animal control officers on staff that will investigate the situation. In rural areas, it's almost always handled by animal control. If your animal control officers have failed to check in to the situation, you can report them to their governing body. In rural counties, it's often the health department (because one of animal control's main objectives is rabies control) or the sherriff's department (because of public safety).
Neglect is difficult to prove and, unfortunately, unless the horses are near starvation, animal control can only issue an order to comply. This means that they can order the owners to have a vet look over the horses and they can give a deadline by which it has to be done. This means that the owners might be given a second chance to get the horses to a healthy state which would not solve your problem.
I suggest that you call animal control in your county and report the case. Ask them when it will be followed up. Ask them if they will call you to tell you when the case has been satisfied. They cannot report to you any of the findings, only that they have investigated and whether the case is still open or closed.
If animal control fails to act, find out who the governing body is and call them. Ask to speak with the sherriff himself if the sherriff's department or the health director if it's the health department.
If you have any questions, feel free to ask. I have been rescuing animals for years and help to investigate cruelty cases in my county. I might have the information that you need.