Can't Seem to Find the Solution

[dnnisburt]

I have a small chihuahua, short haired and by all appearances is a very healthy little guy! He is very active all the time and nothing appears wrong with him except he has a streak of his hair running from the back of his neck to his tailbone gone. He doesn't itch or appear to be tender in any way shap or form. It's like someone painted a streak right down the middle of his back! When we found him he was somewhat emaciated and with this streak. He has gained weight but still has this loss of hair. Can anybody give me any ideas what might be the cause of this little guy? If you respond I can send you a picture of his back. One would think if it's dry skin it would itch....Idon't know. Would love to hear from you guys on this matter