I had a runt in my last litter of Springers... I contemplated putting him down several times, but just couldn't do it so I nursed him along... Tube fed him, had him to my vet a couple of times, etc. My vet didn't think the pup had much hope. At 4 weeks of age I brought him to my vet and he weighed 15.2 ounces.. Here's a photo of him at 4 weeks of age...
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I continued to supplement his feeding and finally had to remove him from his littermates as they were getting rough with him... A fellow I'd placed a pup with a couple of years ago was adamant that he wanted this pup... I told him I'd give him the pup free of charge if he promised to continue to give him whatever care he needed... He named him "Hank". Here's Hank at about 7 weeks of age with one of his littermates...
I held "Hank" back a couple of extra weeks and before he went to his new home... Hank appears to be doing well. Here's a couple of photos of Hank at about 12 weeks of age... Looks like he's finally growing. I'm waiting for some new photos of him. He'll be 5 months old this week. All reports so far is that he's developing normally.... It will be interesting to see what he's like when mature....