National Farmers

Apart from improving nutrition and therefore resistance to disease, I can only think of reasons why a switch from failed dryland cultivation of say, corn, to wet rice would increase disease. For example. wet places breed mosquitoes which spread malaria, and also breed a lot of snail vectors for diseases such as schistosoma (I think that is the right one…things with worms).

That is an interesting question. You are right, based on all the studies I have read & done, rice cultivation doesn’t really help prevent tropica diseases.
It is much perferable to do a dry land cultivation if the environment allows it. Unfortunately, this is not the case in much of southeast China/Asia where rice are primarily grown.

The only reason for rice cultvation I could think of is crop diversity (which reduce pests, weeds etc.) but I am not sure that would do so well against tropical diseases.

However, correct me if I am wrong, I believe the keyword here is in wet rice cultivation. That’s an interesting area of research. As you probably know, wet rice cultvation requires irrigation. This is typically done using domestic animals like ox in much of the third world country.(In North America we use mechanized machines) In addition to ox, many farmers also grow chickens, ducks, pigs and other animals to take advantage of the water nearby.

Having domestic animals around is good because it widens the the preys for vectors like mosquitos that typically transmit these diseases. This is a tactic those in the field usually call zooprophylaxis (which means the use of animals to protect human health- sorry we are very selfish creatures). This means that mosquitos and others are less likely bite human beings and this decrease the likelyhood of catching those tropical diseases.

Here is a study done, it is very long and filled with technical jargons. I will spare you the pain of reading though it and copy & paste the most important part of this study..

"It was apparent that in rice irrigated villages, only a small proportion of mosquitoes that entered the house actually fed on humans. Anthropophily for both species was lowest in the planned rice village, moderate in the unplanned rice village, and highest in the non-irrigated village. These findings suggest that the extent of rice cultivation has a significant impact on the blood-feeding behavior of malaria vectors. These findings are consistent with those of previous reports that rice cultivation decreases the degree of human-vector contact by diverting mosquitoes to other hosts.11,12,14 It has been suggested that rice cultivation empowers the surrounding communities economically, enabling them to protect themselves against mosquito bites"

This was the source of the article.
http://www.ajtmh.org/cgi/content/full/78/2/270#R12
(You probably need to be associated with a research institute or university to access the full article)

Now, another possibility is the loss of habitats for many vectors after the field became a wet rice field..

Again, it came from this article

"The reason for this is still unknown but could be the result of loss of larval habitats after transformation of natural habitats into rice fields."

I hope this answer your question, which is very interesting & challenging , I must admit.

- Isotope