In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: INTRODUCTION : Ethambutol is used in the treatment of tuberculosis , which is still prevalent in Southeast Asia , and can be associated with permanent visual loss .
### Output Text:
INTRODUCTION : <span class="chemical">Ethambutol</span> is used in the treatment of tuberculosis , which is still prevalent in Southeast Asia , and can be associated with permanent visual loss . 