If you think you may be dealing with camallanus, then Praziquantel won't be effective
Agreed, Bt I would also say it doesn't look like Camallanus (I think tht's mispelled, but then we all know what is meant)
I would go with Metrodanizole first, My reccomendation is buy Met powder (Seachem makes and sells it in small vials). dissolve a little in water and soak the food in it. If the met doesn't help, then I'd go with prazi in the tank water or in the food or both.
The primary reason I reccomend met first is it is cheaper and easier to find. Met kills most internals, and unless you can get a positive ID on the worm you have, you'll have to just use the trial and error.
Prazi and met are both really harmless to your fish and other tank life, so Using them won't give you a lot of side effects. Do not follow the instructions to dose metradanizole into your tank water. mix it into the food in heavy doses. It is next to impossible to overdose it, and it works infinately better if it is ingested.
Edit:: IF you decide it's simple constipation, Epsom Salt (Magnesium sulphate) will help. Be careful with it, it does increase hardness, but that can be remedied with water changes after the fish is better, and Platies don't mind hard water at all.
Dave