Which form of arsenic is used in heartworm therapy?

• A. Inorganic pentavalent arsenic.
• B. Trivalent organic arsenic.
• C. Elemental arsenic.
• D. Pentavalent arsenic in feed additives.

Answer: (B)

The treatment relies on a trivalent organic arsenic compound—the organoarsenical form. This specific form is effective against adult heartworms because it disrupts the parasite’s energy metabolism by targeting enzymes that rely on thiol groups, such as those in the lipoic acid–dependent pyruvate dehydrogenase pathway, leading to worm death. The organic moiety helps the drug reach the worms and improves pharmacokinetics, while the trivalent state provides the necessary potency against the parasite with a relative safety profile for the host when used as directed. Inorganic forms, elemental arsenic, or inorganic pentavalent arsenic (including forms in feed additives) are not used for heartworm therapy due to lower efficacy and greater host toxicity.