Bluetongue has been found at a dairy farm near Canterbury, according to a government warning. This is a serious notifiable disease of ruminants (cattle, sheep, goats, deer, llama, alpaca), although, fortunately, it does not affect humans. In dairy farms, it causes the death of some animals and the loss of milk yields.
What is bluetongue?
The last outbreak in the UK was in 2007, but it has been resurging recently on the near continent, in the Netherlands, for example, which in September reported outbreaks among sheep at four farms near Utrecht. Vets there are reporting sheep with high fever and lesions around the face, mouth and udder.
The bluetongue virus lives in infected biting midges (Culicoides imicola), which can be carried across the sea lurking in cargo or other transport, or possibly even by the wind. The alert will now affect border sanitary controls for those trading in livestock, sperm or eggs. It can have a major effect on livestock trade.
Sheep are most likely to show the signs of the disease. But other ruminants may show no early signs, and these animals are the most likely spreaders of the disease. The symptoms in cattle are:
- crusty erosions around the nostrils and muzzle
- redness of the mouth, eyes and nose
- reddening of the skin above the hoof
- nasal discharge
- reddening and erosions on the teats
- milk drop
- torsion of the neck
The virus produces lameness sores on the feet, which result in knee-walking sheep. Infected cattle shift their feet constantly, which gives the disease its nickname ‘the dancing disease’. The incubation period is 5–20 days, and all signs usually develop within a month. The mortality rate is normally low in most animals, but recovery of sick animals may take months. Those that die of it mostly do so within a week of symptoms appearing.
The disease was identified in a single animal on a farm near Canterbury during an annual surveillance programme for Bluetongue (BTV-3). The animal was culled to prevent further spread. There is now a 10 km control area around the farm, which restricts movement of susceptible farm animals. Trade in these animals between the UK and Northern Ireland has also been cut.
Changing patterns of disease
Bluetongue used to be thought of as an animal disease of the tropics or subtropics. It was identified in South Africa in the early 19th century, and a South African, Arnold Theiler, created the first vaccine for it in 1906. In Africa, local breeds of sheep may have developed resistance, but in imported breeds, up to 90% of a flock may die. In Africa, it is endemic in wild ruminants such as elk, but mostly not fatal. It is fatal in red deer.
Sheep-farmers in endemic regions can now protect against it by using vaccines. But as we learned with Covid, the disease evolves with different strains, and so the vaccines have to change in response. Indian scientists, for instance, in 2015 developed a vaccine that is potent against the 15 strains of the virus prevalent in that country. Microbiologists identify at least 27 different strains of the virus worldwide.
Evolving for warmer climates
It used to be thought that the virus could not over-winter in colder regions. The midge Culicoides imicola is killed off by hard frost, so the disease is endemic, but seasonal, in some Mediterranean countries. Now, new vectors for the virus have evolved, C.obsoletus and C.pulicaris, which can withstand higher latitudes. From 2007, these midges began to infect animals in northern Europe. Climate change also means that these regions are getting warmer. Maybe the midges can also survive the winter well in the warm premises of the barns of intensive farms.
All this is another instance of the serious new challenges humanity faces because of climate change. So, please bear that in mind if you see any dancing cows near Canterbury.