Animal and Plant Health Agency (APHA) veterinary investigation officer Harriet McFadzean presented findings from her 2021 MSc project from the University of Edinburgh, on surveillance and risk analysis for bovine babesiosis at a recent COWS Steering Group meeting.
Babesiosis, colloquially known as red water disease because of the colour of affected animals’ urine, is caused by infection with the protozoal parasite Babesia divergens. This is spread to cattle through tick bites, primarily in the UK from the species Ixodes ricinus (Figure 1).
Infected animals can also present with fever, abortion, reduced milk production and death if left untreated. The disease usually occurs where ticks are present, potentially causing significant economic and welfare issues on affected farms.
It has been known for humans to become infected and to fall very ill, but at the moment this is rare.
Factors for change
The lifecycle of ticks is inherently linked to their environment. Factors such as climate change and differing land use options, such as re-wilding or burgeoning biodiversity in pastures, may have un-intended consequences in tick number dynamics in future.
“Ticks are reliant on a number of climatic factors including humidity, temperature, light, the amount of vegetation and the availability of a host so they can have a blood meal and reproduce,” says Ms McFadzean. “All are intertwined and dictate when and where ticks are likely to be active.”
Ms McFadzean set out to investigate transmission and to ascertain hotspots for animal and potentially human disease. This is important so that awareness campaigns can be put in place in high-risk areas to alert farmers and vets and those working in Public Health. The work was funded by Defra and the Welsh Government.
“At the moment there are gaps in tick-borne disease knowledge. My project set out to gather baseline surveillance data for babesiosis and assess two new diagnostic methods.”
The surveillance project offered farmers free blood testing for babesiosis for up to three animals showing clinical signs. The farmers had to fill out a form detailing animal and grazing history to accompany the samples.
The diagnostic tests used a blood smear, which is the traditional diagnosis method which requires a lab technician to examine it under a microscope, and two new polymerase chain reaction (PCR) tests developed by APHA, which can detect multiple Babesia species and tick-borne fever infection.
Animals that were showing suspected signs from 95 holdings took part and over half were confirmed to be infected with a tick-borne disease.
In previous years, spring and autumn disease peaks were typical, with hot sunny weather causing ticks to shrivel and retreat down to the moist ground, lessening summer risk. But in this project, Ms McFadzean saw consistently high levels from May through to December with no summer dip.
The highest levels of babesiosis were seen in the South West, Wales and the North West which have always been problem areas. But cases were also being seen in the North East of England, which could be an emerging threat.
The PCR diagnostic methods had advantages over the traditional microscope tests, detecting different species and at lower parasite levels. The results, being computer-derived rather than being dependent on a technician, reduced human error.
Lack of licensed products
“While I feel there are useful conclusions to draw from my work, we must not forget that there are no products licenced for the control of ticks on cattle,” advised Ms McFadzean.
“Farmers in high-risk areas need to accept that they are there, build up their cattle’s immunity to infected bites and aim for endemic stability. This means that cattle can withstand low levels of infection with no clinical symptoms.
“Fortunately, cattle have innate immunity to infected tick bites up to six to nine months of age, so it is important they are challenged by ticks during this time. In addition, farmers need to be aware that when animals that have grazed clean pasture, where immunity can wane, are re-introduced into infested pasture, can develop disease problems.
“In high-risk areas I would advise farmers to build and retain immunity in their herds. Where animals are bought in with no knowledge of where they have been grazing, putting them onto cleaner lowland pasture and avoiding upland, ‘ticky’ pasture would be sensible.”