Ophiocordyceps sinensis (also known as caterpillar fungus) is a parasitic fungus that has been used in traditional medicine for centuries. The fungus is typically found in high-altitude regions of the Himalayas and Tibetan Plateau. It is extremely rare, expensive, and nearly impossible to cultivate (although there have been recent advancements).

For this reason, researchers have developed alternatives by cultivating mycelium (the fungus roots) isolated from this fungus. One of these isolates is known as CS-4, which is typically cultivated in liquid mediums for the production of bioactive compounds. It has been heavily studied and shown to be an effective alternative to the traditional wild-harvested fungus.

While it was historically considered a strain of Ophiocordyceps sinensis, research has shown that strain CS-4 is not actually this same species but a different fungus, Paecilomyces hepiali (Chioza, 2014). Paecilomyces hepiali is another parasitic fungus that is believed to coinhabit the wild caterpillar fungus. It is suggested that the effects of the wild caterpillar fungus may be attributed to various distinct species that occur together.

Despite being a different fungi, CS-4 is well known to be medicinal and has similar effects to Ophiocordyceps sinensis and it’s cousin Cordyceps militaris. A portion of the research that supports the use of these other related fungi is based on research conducted on the CS-4 strain of Paecilomyces hepiali.