The effect of climate change on some parasitic diseases of livestock is far more complex than previously thought, and farmers may have to look to historic farming practices to control future diseases, according to new research from scientists at the University of Bristol.
Parasitic gastroenteritis, a disease caused by worms infecting the digestive tract of livestock, costs the UK sheep industry more than £80 million per year. The worms inhabit the guts of livestock, shedding eggs in their faeces. The eggs hatch on the pasture and the larvae re-infect more animals when they are eaten.
The resistance of some worms to the available drugs has sparked fears that soon there will be no treatments available. In addition, the warmer temperatures caused by climate change could dramatically change the geographic ranges and abundance of the worms, with devastating impacts.
Research has shown that the eggs hatch and larvae develop when the temperature rises above a lower threshold in the spring. Larvae then die off later in the summer, which was previously attributed to increased mortality at higher temperatures. But fluctuations in the number of larvae on the pasture throughout the year cannot be fully explained by temperature.
Experiments led by Dr Eric Morgan, of the Veterinary Parasitology Group in the School of Biological Sciences at the University of Bristol, and conducted by Dr Jan van Dijk and colleagues, have shown that UV (ultra violet) light – as produced by sunlight – also kills worm larvae and may explain these fluctuations. The research is published today in the International Journal for Parasitology.
Dr Morgan said: “Our findings mean that climate change models based on temperature alone may not be sufficient to predict future infestations. However, there is a silver lining. Understanding the effect that future climate change will have on the survival of worm larvae on pastures means that farmers can change their husbandry procedures. By avoiding grazing their animals on contaminated pasture at times of high risk, farmers can reduce the risk of disease in their animals without resorting to drugs.
“Of course this is no new trick. Farmers have historically relied on their knowledge of the weather to decide when and where to graze their animals, and when to shear, breed, feed and house them. It seems the key to the future sustainability of farming may lie in the practices of our forefathers.”
Gastrointestinal nematode parasites (‘gutworms’) are common and the major constraint on livestock production worldwide. At high levels of infection, they cause diarrhoea, weight loss and death, but even at low levels of infection they reduce growth rates, even in animals that appear healthy. Because the impact on the individual animals and on herd productivity is related to the number of worms present, and the development of worms on the pasture accelerates at higher temperatures, there is concern that global warming will lead to increased disease and production loss.
Although drugs are available for treating these worms, resistance to them is increasing at an alarming rate, such that worms on some sheep farms are resistant to all commercially available drug classes. There is therefore great renewed interest in ‘old’ methods of control, such as rotational grazing and targeted treatment according to the weather. The hitch is that research into this area has been very slow in the past decades (drugs were available so there was no perceived need), and the many complex factors affecting infection rates at pasture are not well understood. UV light, although an obvious candidate for affecting larval survival on pasture, has never previously been studied at natural levels, or used to explain seasonal patterns of infection at pasture.
The experiments reported in the paper took infective larvae of three major gutworms of sheep, and exposed them to continuous artificial UV light over a 6 day period. Control larvae were protected by UV-impermeable Perspex. Mortality in the exposed larvae increased 100-fold. In a second experiment, larvae were placed on the roof of the Biology building in Bristol, on grass turf, for a 24 hour period on several days in spring and summer. On cloudy days, exposed larvae died at the same rate as protected larvae. But on sunny days, exposed larvae died around 3 times quicker. This would be further exacerbated over a long period of exposure. The larvae of Haemonchus contortus, a tropical-adapted species that is just getting established in the UK, was less affected than the other species.
The results help to explain patterns of larval availability at pasture, which are known to decline rapidly in spring, at a rate faster than can be explained by increased temperature alone. It also explains why larvae do not always climb to the top of grass stems, which would give them the best chance of being eaten by the sheep and cattle hosts, since in this position they would suffer from greater risk of death by UV irradiation.
In terms of parasite control, farmers can expect larvae to die quicker in sunny weather, especially if grass length is short, providing little shade to larvae. Pastures that have been previously grazed and contaminated with worms will then be safe to graze sooner, helping farmers to make the most of natural weather to disinfect their pastures. This fits in with traditional farming practices, but with new support from science that will enable refined methods and greater success in facing the challenges of climate change to farm animal productivity and welfare.