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: All animals were similarly symptomatic at the start of levodopa treatment and had similar therapeutic responses to the drug .
### Output Text:
All animals were similarly symptomatic at the start of <span class="chemical">levodopa</span> treatment and had similar therapeutic responses to the drug . 