@Plow, I just read your story, my thoughts/reactions as a reader:
Very vivid description - by reading I "saw" what you descibed.
language in direct speech - hey, I really liked that, very natural. The way you let Adam speak in his persistant way as rather small kids do if they turn stubborn. Same for the way you described Harold and let him speak - both came alive in my mind: a frightened, bewildered little boy trying to cope with the unknown in a sort of fatalism and stupor - the gruff, somewhat distanced father, who still cares for his son. It was sketched (purpose, hm? As focus goes to the threat) - but enough for me to build up
building up tension and reader curiosity - starting with a strange happening and then extending the weirdness step by step to show the threat, first short culmination in the ambulance scene => <img src="/ubbthreads/images/graemlins/up.gif" alt="" />
exact description of the environment, so I was not desoriented.
atmosphere - weird, therefore a good ground is prepared to build upon. I'd not put in a lot of horror effects if you continue - like beads on a thread following quickly. Tension can build up better if a silent threat is there and if Adam's full metamorphosis will not occur in your next part IMO. Or do you wish to keep a fast pace in development? I'm just asking, because Hohlbein e.g. (German fantasy/horror author) has a tremendous pace in action, leaving me (the reader) breathless. Too many chases/dangers, following too quickly, so I can't build up pics and delve into a story.
Suggestions to help me (the reader with a vivid imagination):
a description of Adam (if you mean to keep him as the main protagonist) - I saw his hand, arm and what happened to it - but my inner pic wavered a bit: hair/eye colour? Age? Figure? Clothes? Or did you skip this on purpose, as he will complete his metamorphosis?
More exact description of the people you already mentioned, so the persons form more in my mind. Or is this your style, as you want the reader to focus on the black bumps/blisters and the strange creature forming? And a description of these people might distract then? Though, I still would like to see the dog-woman and hope, she will pop up later, need to read more for this. You described Sparky very well and distinctive - so, why not his mistress?
The episode, about how Adam's Mom ran wild and became violent, needs a separation/distinction/inclusion IMO. Either a phrase such as: "Adam' eyes fell upon a dirty wool cap in the corner and he suddenly remembered...." Or "by passing the corridor, Adam threw a glance at the tiny photograph of a woman on the wall and... <knitting attack memory comes>" Or "Adam was scared and instinctively touched his tummy for comfort... <2 scars and knitting attack>" Or "A sharp twang, his scars hurt again...<memory>"
Don't know how to phrase this clearer => it was like a rupture from the current action surrounding it - and my concentration sort of broke. As this memory is a flashback (correct?) - and probably a pretty traumatic one for Adam - it might need a bit of flesh around it, to put it into place. A sort of going to and coming back, you know what I mean? Or maybe an empty line at the beginning and the end - so, the reader knows: aha, I'm in the house again and will follow Adam.
Please, keep on writing and inform, so I can read more <img src="/ubbthreads/images/graemlins/wave.gif" alt="" />