Hi, I'm sorry your little one is poorly.
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It sounds like your vet has taken the right approach this time- a diuretic and two pronged antibiotics.
My only worry would be that the one dose of diuretic wouldnt be enough to keep the fluid off his lungs. It may be worth contacting them asking for some diuretic to give orally at home over the next few days if his breathing is very laboured like you describe. If they want to see him again before they give you diuretics to take home, i would comply and take him in- although extra stress isnt ideal, he really sounds like he needs diuretics regularly at the moment.
The other possibility would be a heart problem, as i would expect him to be extremely unwell and not eating at all if this was a URI/pneumonia that had been going on this long. A chest xray to check for fluid and any enlargement of the heart can be enough to diagnose a piggy with a heart issue, as well as an ongoing history of related issues. Heart issues are ultimately fatal sadly, especially once it is at the stage of fluid in the chest.. But it can be managed with heart meds such as fortekor, as well as regular diuretics for the rest of the life granted fluid is the issue. I have a 7.5 year old piggy who has made her 1 year anniversary after being diagnosed with heart failure. Other symptoms of a heart issue include (try and think about how he was before this bout of ill health began):
Lethargy (more inactive than usual)
Cold extremeties
Pale/blue/purple tinge to tongue/lips/feet
Difficult to rouse from sleep
Hooting when breathing
Coughing
How is his weight doing? If he has lost weight or continually losing, you need to step in with syringe feeding to give him a fighting chance. Please only syringe a very small amount into the mouth at a time as he is at risk of aspirating with having such labored breathing.
Complete Syringe Feeding Guide
Is he on probiotics? Antibiotics are harsh on the guts and probiotics help balance this. Examples include fibreplex, pro-c or avipro plus. They should ge administered 1-2 hours after each antibiotic dose.
If you arent happy with your vet, there is a recommended uk vet locator on the forum (if thats where you are). It may be an option for a second opinion.
For now i would step in with syringe feeding if needed, get him some probiotics and most importantly, speak to the vet about giving him diuretics regularly at home to try and ease his breathing.