新托福听写训练 第15期（在线收听）

Listen to part of the lecture in a health science class.

The professor will be discussing a solution to a health problem caused by a certain parasite sometimes invade humans.

A parasite is an organism that live in or on another organism called the host. During the talk, listen for:

Where the parasite normally lives?

How the parasite affects and harms humans?

How the proposed solution is supposed to work?

Let's look at another way that health agencies fighting parasitic diseases. One alternative to drugs and pesticides is the use of biologic agency. Another word, if the parasite is causing a problem, you find something that eats the parasite itself or something that eats the parasite's normal host. Then you turn that something lose in the parasite's habitat. For example, considering the schistosomisasis. Schistosomisasis is the infection with the parasite worm called schistosome. Normally the host for the schistosome worm is a kind of freshwater snail. But in the African country Kenya the worm frequently infects humans who are exposed to water from ponds where the snail lives. The contact commonly occurs when people get their cattle water or wash their clothes in the ponds. In fact about 2 million Kenyans mostly children are infected. Once inside the human host, the worm lays eggs and these can result internal breeding, fever, fatigue and sometimes death. The snails are still necessary as hosts though because schistosome eggs only can hatch inside the snails. If you are already infected, you can take a drug to kill the worms. But the drug is expensive and it's easy to get re-infected when you go back to the ponds. Instead, researchers are trying to eliminate the worm's normal host the snail by introducing Louisiana crayfish into the ponds where worms and the snails live. Now as you can see, a crayfish is something like miniature lobsters and it loves to eat snails. Snail shells are rich in calcium which crayfishes need for their own shell. And the crayfishes are already abundant to their natural Kenya rivers, they were imported there in early 1930s, and they are easy to find. So crayfish could end up being a cheap and effective way to reduce receive schistosomiasis. If they do though, it would be the first time a biological agent has been successfully used to control a parasite.