11 January 2013 | EN
Carp are at greater risk of infection in ponds with lots of snails
[HANOI] Controlling populations of unwanted snails on fish farms plays a "crucial role" in reducing parasite prevalence and making raw fish safer to eat, a study has found.
Certain species of snails are key hosts of fish parasites called trematodes, which are classed as an emerging infectious disease that infects millions of people around the world though consumption of undercooked fish.
Researchers from Denmark and Vietnam wanted to quantify the relationship between snail density and the infection levels in farmed fish to help assess whether intervention against snails in ponds would be sufficient to control infections in fish.
In a survey of 48 carp nurseries in north Vietnam, the study published last month (20 December) in PLoS Neglected Tropical Diseases, found that the prevalence of infected fish was around 11 times higher in ponds with a high snail density compared with those without any snails.
The intensity of fish infections increased with the density of infected snails, but fish were also infected even when they were no snails in the ponds. This implies that parasites may be entering the nurseries from external water sources.
As a result, the study suggests not only controlling snail numbers in the ponds, but also in the water sources that supply water to them and snail habitats outside the nurseries, such as rice fields and ponds around the fish farms.
One option for large commercial nurseries is to treat water supplied to ponds, for example using sand filters, it says. However, this may be too costly for smaller nurseries.
About 18 million people worldwide are infected with the fish-borne trematodes that can cause stomach irritation in the short term or liver cancer after prolonged exposure. Residents of South-East Asia are especially susceptible to the parasites because raw or undercooked fish is a staple of many cuisines across the region.
Jesper Hedegaard Clausen, researcher from the University of Copenhagen in Denmark, the study's lead researcher, says it can require up to 15 days of work a year to remove snails from fish ponds in Vietnam.
"It's quite a big job to remove the snails," he tells SciDev.Net. "We wanted to make sure there actually was a correlation [between the snails and fish infection] so we weren't asking farmers to do something that's not relevant."
Because no country in South-East Asia has a programme to address parasites of farmed fish, the study offers promising lessons for researchers and governments across the region, says Paiboon Sithithaworn, head of the parasitology department at Khon Kaen University in northeast Thailand.
Although it is extremely difficult to persuade people in South-East Asia to stop eating raw fish, it is possible to control parasites on fish farms using appropriate technologies, he says.
But Sithithaworn says that the cost of mitigation measures could be a barrier. For example, lining a fish pond with concrete that deters the snails costs about US$500 to US$1,000 in Thailand, which is too expensive for many farmers.
He adds that chemical spraying only has a short-term impact on the number of snails. "They are quite resistant and they can come back very quickly," he says. As a result, more research is needed on how to use natural predators to eradicate snails, he adds.
Vietnam is a major exporter of frozen fish to Europe and the United States, according to the UN's Food and Agriculture Organization. Clausen says that any parasites are killed when the fish are frozen.
This article has been produced by SciDev.Net's South-East Asia & Pacific desk.
PLoS Neglected Tropical Diseases doi:10.1371/journal.pntd.0001945 (2012)
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