Which hepatitis virus can only be transmitted if the patient is already infected with hepatitis B?

Hepatitis D virus (HDV) is unique among the hepatitis viruses in that it requires the presence of hepatitis B virus (HBV) for its replication and transmission. This is because HDV is a satellite virus that cannot exist or replicate independently without the helper function provided by HBV. Therefore, a person must already be infected with HBV in order for them to subsequently contract an HDV infection.

In clinical practice, co-infection with HBV and HDV can complicate the disease course and management of hepatitis, as HDV can exacerbate liver damage caused by HBV. This interdependence is what makes hepatitis D distinct from the other hepatitis viruses listed, which do not have such a transmission requirement related to hepatitis B.

Hepatitis C, hepatitis E, and hepatitis A can be transmitted independently of hepatitis B. Specifically, hepatitis C is spread primarily through blood-to-blood contact, hepatitis E typically through fecal-oral transmission often linked to contaminated water, and hepatitis A is also transmitted via the fecal-oral route, predominantly through food or water.