Bluetongue (BT) is a non-contagious viral disease transmitted by biting midges, which affects both domestic and wild ruminants. BT is caused by infection with bluetongue virus (BTV). BTV does not pose a threat to human health.
The disease can cause significant outbreaks and is a World Organisation of Animal Health (WOAH) listed disease (WOAH, 2023a). In Great Britain (GB), BT is a notifiable disease, so anyone suspecting the disease must report it to the Animal and Plant Health Agency (APHA).
The bluetongue virus can affect cattle, sheep and other ruminants, with varying clinical signs depending upon the strain and species involved. The disease is generally characterised by inflammation of the mucous membranes, congestion, swelling and haemorrhages. It can also result in congenital malformations and abortions.
The current BTV-8 strain circulating in northern Europe causes milder clinical signs in sheep and cattle compared to the 2007 BTV-8 strain that was present in the UK. Research has, however, shown that the current strain can still compromise animal health and welfare in susceptible species that have not been vaccinated or previously exposed to the virus. Additionally, BTV-4 is circulating in northern Europe – this strain has resulted in up to 30 percent mortality in some meat-producing sheep breeds.
Transmission is primarily via biological vectors, for example the bite of certain species of Culicoides biting midges. It is not transmitted by direct or indirect contact between animals in the absence of the insects. As a result, virus transmission to livestock will predominantly take place during the spring, summer and autumn months when midge vectors are most active.
Certain strains, including the BTV-8 strain circulating in Europe, can also be transmitted from infected dam to foetus via the placenta.
Current management strategies for BTV
In the European Union, the management of BTV infection is regulated under Regulation (EU) 2016/429. This regulation has changed the approach from a disease of immediate eradication (category A) to one that may be subject to optional eradication programmes (category C).
Although Belgium and parts of Germany were under restrictions until early 2023, they are now BTV-free. Luxembourg, France and The Netherlands remain the closest countries to the UK with ongoing active BTV infection events, according to WOAH. (Restricted zones for BTV in Europe are shown in Figure 1.)
In early September 2023, the Netherlands reported the presence of BTV (for the first time since 2009) in four sheep farms in the municipality of Wijdemeren (province of North Holland) and the municipality of Stichtse Vecht (province of Utrecht) (Rijksoverheid, 2023). The virus has been serotyped as BTV-3, a strain last reported in Europe in southern Italy (Mons, 2023). The source of BTV-3 in The Netherlands is currently unknown.
During the vector-active season, APHA conducts fortnightly assessments into the risk of virus-infected vectors being transported by wind from adjacent countries under restrictions (currently France and the Netherlands). These assessments involve experts from the APHA, the Pirbright institute, the Met Office and the UK Office for Sanitary and Phytosanitary Trade Assurance. They use information on temperatures on the continent and in GB, simulation modelling (the Met Office’s “NAME” atmospheric dispersion model; Figure 2) and vector activity levels.
The UK has been officially BTV-free since 2011, but passive and active surveillance is still in place to detect possible incursions of BT. Surveillance includes the sampling of live animals imported to the UK from BT-affected and neighbouring regions.
Additionally, GB carries out active laboratory risk-based surveillance for BTV on an annual basis, targeting cattle that may have been infected during the previous vector season as potential reservoirs of the disease (Figure 3). Atmospheric dispersion modelling is used to identify “target” counties, ie GB counties at a higher risk of windborne introduction via BTV-infected midges during the BTV vector season from the nearby European countries under restrictions. Farmer participation is voluntary, and the samples (taken by Veterinary Delivery Partners) are tested at the Pirbright Institute by real-time RT-PCR.
All samples collected under this annual risk-based active surveillance programme have so far tested negative for the presence of BTV RNA.
This level of surveillance is sufficient to be 95 percent confident that at least one positive animal would have been identified if the prevalence in cattle had been over 5 percent in each of the geographical units investigated during the part of the year when an incursion was most likely to have occurred. The surveillance aims to provide confidence that there has been no undetected incursion of BTV via vectors into GB and build upon other evidence streams demonstrating the UK’s BTV disease-free status, thus facilitating trade.
The annual risk-based active surveillance programme is due to take place again this year in November and December, and farmer recruitment has commenced. The authors would like to acknowledge and thank the farmers who volunteered their time and cattle for inclusion in this surveillance programme in previous years, as this work has allowed them to collect invaluable information in support of our continued BTV-free status. The authors would be very grateful if cattle veterinary practitioners could encourage their clients to volunteer to take part in this surveillance again if approached.
Epizootic haemorrhagic disease
An exotic notifiable disease very similar to BTV in terms of vectors and routes of transmission that has recently spread to Europe is epizootic haemorrhagic disease (EHD).
Recognition and transmission
EHD is caused by the epizootic haemorrhagic disease virus (EHDV), a virus closely related to BTV that also affects wild and domestic ruminants.
Acute outbreaks of EHD in cattle are similar to BT and are characterised by fever, anorexia, reduced milk production, swollen conjunctivae, redness and scaling of the nose and lips, nasal and ocular discharge, stomatitis, salivation, lameness, swelling of the tongue, oral/nasal erosions and dyspnoea. Abortions and stillbirths have also been reported in some epidemics, and some affected cattle die.
Differential diagnoses for EHD in cattle include BTV, FMD, bovine viral diarrhoea, infectious bovine rhinotracheitis, vesicular stomatitis and malignant catarrhal fever.
EHD is considered an emerging disease in cattle and was added to the WOAH list of notifiable diseases in May 2008. EHD is classified as a notifiable disease, and suspicion of it in GB must be reported to APHA immediately. EHD is not a zoonotic disease and does not pose any risk to human health.
EHDV is transmitted by vectors, predominantly Culicoides biting midge species. As for bluetongue, the distribution of EHD is therefore largely limited to the distribution of competent Culicoides vectors. In temperate regions, outbreaks typically occur in late summer to early autumn during peak vector population.
Current outbreaks and surveillance
Three outbreaks of EHD in cattle (with five cases and one death) were reported for the first time in Sardinia, Italy, in November 2022 (DEFRA, 2023a). The outbreaks were initially suspected to be caused by BTV but subsequently tested positive for EHDV. Following the EHDV outbreaks in Sardinia, three cattle in Sicily, Italy, tested EHDV-positive a week later. The positive cattle in Sicily were identified through increased surveillance along the west coast of the island following the confirmations of EHDV in Sardinia. EHDV is also present in countries neighbouring Europe, across the Middle East and North Africa.
Genome sequencing on the strain detected in Italy identified it as originating from Tunisia, suggesting vectors carrying EHDV may have been blown into Sardinia via desert winds, which could be plausible as the north coast of Tunisia is less than 200km from the south coast of Sardinia. The introduction of BTV strains, such as BTV-3, in the last few years has followed a similar pathway of introduction into southern Italy from North Africa.
Since the initial reports of EHDV from Sardinia and Sicily, two EHDV outbreaks were confirmed in southern Spain, initially in November 2022. Following further outbreaks in cattle and wild red deer in Sardinia, EHDV spread in southern and central Spain and Portugal (Figure 4). On 18 September 2023, EHD was confirmed by ANSES, the reference laboratory for this disease in France and WOAH, in three cattle in France (in the Pyrénées-Atlantiques and Hautes-Pyrénées départéments). These are the very first cases to have occurred in this country.
EHDV is not currently present in the UK. The main route of entry would be via the movement of live viraemic animals and/or infected vectors into the UK from an affected area. But at present, APHA considers the risk of the introduction of EHDV into the UK via the movement of live animals and/or vectors to be negligible.
EHD is diagnosed by real-time RT-PCR on blood and tissues. There is no licensed vaccine available.
Considering the similarities in the clinical picture between EHD and BT, the emergence of BTV-3 in the Netherlands and the emerging situation with the spread of EHDV-8 in Europe, care is needed when trading with affected regions, together with vigilance on-farm for signs of BT and EHD and prompt reporting of any suspicion of the disease to APHA.
|This article was written by Christina Papadopoulou, with contributions from Martin Ashby, Alex Mastin, Claire Wade, Simon Gubbins and Carrie Batten.|