How do humans acquire infections with Diphyllobothrium latum adult worms?

Diphyllobothrium latum, also known as the fish tapeworm, is primarily transmitted to humans through the consumption of raw or undercooked freshwater fish that harbor the larvae. The lifecycle of this parasite begins when eggs are shed in the feces of an infected host, typically a fish-eating mammal, and subsequently enter freshwater environments. After hatching, the larvae become free-swimming and can infect various species of freshwater fish, which then serve as intermediate hosts.

When humans ingest these infected fish without proper cooking, the larvae can develop into adult worms within the human intestine, leading to infection. This method of transmission is well-documented in the epidemiology of Diphyllobothrium latum, making ingestion of raw freshwater fish the correct answer for how humans acquire this particular parasite.

In contrast, the other options do not reflect the actual modes of transmission associated with this parasite. While freshwater crabs and water chestnuts can be involved in other types of parasitic infections, they do not play a role in the lifecycle of Diphyllobothrium latum. Skin penetration by cercariae is associated with different parasites, particularly those belonging to the Trematoda class, and is not relevant to this tapew