The best bet is to have the worst dog's stool checked for parasites by the vet. Its inexpensive to do and will tell you if the problem is intestinal parasites or not.
If this has been going on for a week or so only this might actually be a viral disease such as kennel cough
www.marvistavet.com/html/kennel_cough.html
As this seems to be being passed around parasites are a good guess as is a viral disease. If it were only one of your senior dogs I'd worry about heart disease or a trachea problem. But with all of them doing this I'd expect you are right to think of it as something contagious like worms.
Another possibility is the new canine flu virus
www.marvistavet.com/html/body_canine_influenza.html
If the dogs seem really sick you should see the vet with them.
I'd not do the worm med until I was 100% sure that was the problem as putting a poison in their system might have a negative impact if the problem is something else.
If it is hook or roundworms then any of the over the counter meds with pyrantel pamoate should work fine and be fairly safe to use.
Hope this helps you!
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