A computer program helps make grazing decisions by predicting the likelihood of conditions conducive to the parasite
Liver fluke, a disease of cattle and sheep, appears to be on the rise in a warmer, wetter climate.
The parasite Fasciola hepatica lays eggs that pass into the pasture from the feces of cattle or sheep. Emerging from the egg, the miracidium seeks a mud snail to continue its development to the next stage. The snail is usually found in poorly drained, slightly acidic soils.
The snail can rapidly multiply, increasing the potential for the parasite to also multiply. Continuing through its growth stages, the parasite becomes a tadpole-like cercaria that emerges from the snail when the temperature is greater than 10 C and moisture is just right. It migrates onto wet grass where it encrusts itself as metacercaria and waits to be consumed by a grazing animal where it makes its way to the liver and starts the cycle as an adult all over again.
Controlling liver fluke, or fasciolosis, is a worldwide problem. The disease contributes to lost productivity at a rate of $3 billion annually. Symptoms can include weight loss, reduced body condition and milk production, and sometimes chronic diarrhea.
The disease appears to be on the increase.
However, researchers with the University of Bristol, United Kingdom, have developed new software that may help farmers mitigate the potential risk to their livestock.
“In the U.K., increases in disease prevalence and shifts in its spatial distribution have been reported in recent decades,” said Ludovica Beltrame with Bristol’s School of Civil, Aerospace and Mechanical Engineering.
“In England, prevalence of infection in dairy herds has increased from 48 percent in 2003 to 72 percent in 2006. Moreover, fasciolosis has been reported in previously unaffected parts of East Anglia and Scotland. The disease has been reported to be emerging or re-emerging in many other regions of the world, including Latin America, Africa and Asia, both at animal and human levels. The importance of human fasciolosis has been recognized only in the last decades, with numbers of reported cases increasing from less than 3,000 to 17 million globally.”
The team developed a computer program called the Hydro-Epidemiological model for Liver Fluke (HELF), which captures the mechanisms underlying the transmission of fasciolosis,
“HELF can be used to help farmers decide where and when to graze their livestock based on predictions of soil moisture,” said Beltrame. “Its main value would be looking ahead in time. This could be a few months ahead or even longer for assessing potential climate change impacts rather than monitoring current conditions. Given topographic information about the area under study, predictions of rainfall and temperature, and a potential parasite control strategy, the computer program simulates risk of infection for grazing livestock in time and space.”
Beltrame said that previous liver fluke risk forecasts and projections were based on relationships originally estimated in the 1950s between rainfall and temperature. The problem with that approach is that the direct and most important driver of the parasite’s life cycle is not just the mud snail and rainfall but soil moisture.
When the software program becomes available, HELF will predict the risk of infection based on actual environmental drivers, such as soil moisture, and assess that impact on the life cycle of the parasite. The program will account for how the environment modifies the impact of temperature and rain on disease transmission so that it can be used to predict disease risk across a region and potentially into future climate changes.
Currently, HELF is still a research tool at two U.K. study sites and the hope is to create a suitable interface for farmers.
In the U.K., warmer weather conditions have led many parts of the country to experience conditions for fasciolosis to extend from being a seasonal risk to a year-round threat. Changes in temperature and rainfall patterns have allowed the parasite to expand to previously unaffected areas.
The warmer climate is also increasing the growing season with stock turned out earlier in spring and brought in later in fall. This leads to intensive grazing, more dependence on worming treatments and the risk of resistance to drugs that undermines treatments for disease management.
Beltrame said changes to herd management practices are also required to reduce the disease risk in the future.