One reason might be because it makes people weak, but doesn't kill a lot of people quickly in a dramatic fasion.
After they built the three Gorges dam in China, the number of snails decreased, as did the rate of snail related schistosomiasis.
Rice growing in paddies requires a lot of contact with muddy water, so did people catch the disease from planting rice in the mud, or because they washed in the river water? One is curious.
Following the opening of the dam, they found, the volume of annual runoff into Dongting Lake declined by 20.85% and the sediment volume discharged into the lake declined by 73.9%. In turn, the mean density of living snails decreased by 94.35% and human rates of schistosomiasis decreased from 3.38% in 2003 to 0.44% in 2015, a reduction of 86.98%. The researchers hypothesize that low water levels in the summer and high levels in the winter led the lake to become an unsuitable environment for snails.
this article suggests that growing rice in paddies in Uganda (and fishing with baskets) has increased the intestinal type of parasite. article is from 1994
In Eastern Uganda, paddy-rice growing, which has only become popular in recent years, seems to be associated with the emergence of schistosomiasis mansoni as a new problem in public health.
the article suggests the farmers should wear boots, which gave me an LOL moment: the boots would sink into the thick mud. That is why we use handtractors or waterbuffalo instead of regular tractors in our rice fields.
this is a slightly different parasite than what we saw in Afro, but it makes me wonder if a similar drop was seen after other dams were built..
1975 article on the Aswan dam (Egypt).
An increase in the prevalence of schistomiasis in Egypt, as a result of the Aswan High Dam, has for long time been predicted because of reclamation of land and conversion of annual flooding (basin irrigation) into perennial irrigation in Upper Egypt.
Two aspects, neglected in the predictions, have been investigated in the present study, and these are: the ecological changes, relevant to the transmission of the disease, in the Nile itself, and those in irrigation canals and drains in areas which had been under perennial irrigation before the constriction of the dam.
The findings indicate that the Nile, at least in the Delta, has become a favorite habitat for breeding of the snail hosts of both urinary and intestinal schistosomiasis, a center for human activities and evidently for transmission, and that irrigation canals and drains now harbor throughout the year undisturbed and stable populations of these snail hosts.
This has resulted from the elimination in these canals, of the so-called 'winter closure', because it is no more needed. Before the dam the closure was enforced for about 40 days, during which the canals were closed and dried up, and the silt deposited on their beds during the Nile flood dredged out together with the snails and aquatic weeds.
translation: The irrigation canals no longer dry up so there are more snails.
here is a 1979 Lancet reference that compared the population data with data from 1935 and found the urinary parasite decreased but the intestinal form (that can affect the liver) increased quite a bit.
The prevalence of Schistosoma mansoni infection had increased from 3.2% to 73%, whereas S. haematobium infection, which had been very common in 1935 (74%), had almost disappeared (2.2%).
So what about other areas of Africa? we had quite a few small irrigation dams in Zimbabwe (the veldt is dry and with irrigation you can grow two crops a year).
But ppeople tended to get the urinary type infection from swimming or bathing or washing things in the rivers so changes could also be from digging wells to supply water..
Here is a study in Senagal, Zambia and Mali (sub saharan Africa).
Recent ecological changes caused by the construction of dams at Diama and Mananatali on the Senegal River, such as reduction in salinity, more stable water flow, creation of irrigation canals and development and extension of rice culture, have contributed towards the occurrence of new outbreaks of both intestinal and urinary schistosomiasis in the Senegal River Basin
In Mali, the four main areas of high prevalence of S. haematobium are Office du Niger (irrigation areas), Bandiagara (small dams), Selingué (dam areas) and Baguineda (irrigation areas). Apart from the Office du Niger, S. mansoni infections are rare.again, one wonders if the infection is caught during planting/weeding in rice paddies (or in mud) or just from bathing or contact with the water from the canals.
The entire article is not available.
finally, the Uganda article notes that the rice growing was a new project.
In more recent years, China (and Saudi) are buying up land in Africa for agriculture. Will this include rice growing and increase intestinal schisosomiasis?
related article: Chinese returnees are bringing back the intestinal and urinary forms to their native country.
heh. the article blames climate change.
but it does show that globalization can spread diseases that are "below the fold" and pretty well ignored by the world.
nor is this a new problem:
here is an article about the British soldiers getting infected during the Boer war and the Yanks getting infeccted in the Philippines during World War II.
prevention was the key:
a report on the problem in the Philippines: and it is worse in the southern regions.
3. * * *. The following precautions will be observed: (a) All water used by troops will be obtained from approved water points * * *. This is meant to include water used for drinking, bathing, laundry, and washing of vehicles or floors. (b) Wading, bathing, and washing of clothing by troops in any fresh water river, swamp, pond, or rice field is prohibited * * *.
Schistosomiasis japonica has a long history in the Philippines. In 1975, 24 endemic provinces were identified in the northern, central, and southern islands of the Philippines. More than five million people were at risk, with approximately one million infected. In 2003, new foci of infection were found in two provinces in the north and central areas. For the past 30 years, human mass drug administration (MDA), utilizing the drug praziquantel, has been the mainstay of control in the country. Recent studies have shown that the schistosomiasis prevalence ranges from 1% to 50% within different endemic zones. Severe end-organ morbidity is still present in many endemic areas, particularly in remote villages with poor treatment coverage. Moreover, subtle morbidities such as growth retardation, malnutrition, anemia, and poor cognitive function in infected children persist.
There is now strong evidence that large mammals (e.g. water buffaloes, cattle) contribute significantly to disease transmission, complicating control efforts. Given the zoonotic nature of schistosomiasis in the Philippines, it is evident that the incidence, prevalence, and morbidity of the disease will not be controlled by MDA alone. There is a need for innovative cost-effective strategies to control schistosomiasis in the long term.
so in the Asian parasite, water buffalo are part of the problem...discussion here.
hmm... wonder if anyone has done a study comparing areas tat use waterbuffalo vs handplows.