Learning more about a devastating disease - Veterinary Practice
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Learning more about a devastating disease

Veterinary Practice reports on a recent international seminar on bluetongue.

Bluetongue (BTV) serotype 8 is different.

That was the main message from a seminar hosted by Intervet/ScheringPlough Animal Health in Amsterdam last month, which attracted world experts from the Netherlands, France, the USA, Spain, Portugal, Belgium and the UK. Acknowledging that everyone – from the scientific community to farmers – is constantly learning about the bluetongue virus, delegates were told about some of the latest R&D underway and also given first-hand experiences of trying to minimise its impact on livestock farming.

Professor James Maclachlan from the School of Veterinary Medicine at the University of California stressed how different BTV8 is in terms of both prevalence and transplacental infection.

He stated that no one could afford to observe countries with endemic BTV infection (serotypes 1, 2, 4, 9, 11 or 16) and assume that the variable levels of virulence will be replicated in countries affected with BTV8, which is markedly different.

Lack of understanding

The lack of understanding about how it over-winters and the role of the Culicoides midge leaves the industry vulnerable to attack and the devastating consequences that follow.

Speakers from the Netherlands and France reported studies into transplacental transmission and their own countries’ experiences of vaccination and vector control.

Utilising the Netherlands’ existing surveillance system, information was gathered from January to July 2007 and again from April 2008 with the aim of better understanding vertical transmission. While the first study reached the conclusion that vertical transmission was not a serious threat, the subsequent study (conducted after the Northern Ireland case where a newborn calf from an uninfected dam, but from a group with BTV positive animals, became infected) differed.

Of the sample group of 385 calves from 43 dairy farms in which cows and calves were tested by ELISA and PCR, results showed >50% seroprevalence in both cows and calves. They also showed that calves from seronegative cows tested seropositive and that PCR-positive calves were born to PCR-negative dams.

Newborns susceptible

In addition, it demonstrated that by PCR testing, BTV prevalence was higher in newborns than older calves. It also revealed that PCR-positive calves became PCR-negative before four to five months of age and also that, alarmingly, one calf that was euthanased at eight months of age as a “dummy” calf had survived to that age with hydranencephaly (a common factor associated with transplacental transmission).

The serious nature of BTV8 was further expressed by Dr H. Petit from France who reported a study into the economic impact of the virus. The average number of veterinary visits to BTV-infected dairy units was nine, with treatment per animal ranging from €26- 207, based on a study of 45 dairy herds.

Milk yield was reduced by 8% in the more seriously affected herds; calf mortality was 9.5%, cow mortality 3.3%, but mortality in bulls was 33% and between 25-100% of breeding males showed clinical signs.

A similar study on beef units showed a reduction in performance of 17.7% on the most seriously affected units, with one of the main areas affected being bull fertility. Significantly, all infected animals also had reduced feed intakes, which resulted in an extended fattening period.

The speaker also outlined the results of BTV infection on 58 sheep flocks. Mortality ranged from 7.9% in ewes to 10% in rams, and morbidity was 18% in ewes, 30% in rams. As with the beef study, the impact on reduced male fertility is yet to be quantified but is expected to be significant. Also speaking about the French experience, Dr S. Zientara explained that of the 32,702 confirmed cases, 27,766 were BTV8 while 4,837 were BTV1 and 99 tested positive for BTV8 and 1, indicating concurrent infection with both serotypes.

Taking into account the economic impact expressed by a previous speaker, Dr Zientara outlined the 2009 strategy for France. Based on the fact that 26 million doses of Bovilis BTV8 were used in 2008 with no significant negative effects, guidelines to farmers are to vaccinate all susceptible stock and combine this with vector control.

Best practice on farm

Examining vector control specifically, Professor J. Lucientes from Spain discussed best practice at farm level. Experts still do not fully understand many aspects of the Cullicoides midge’s lifecycle – from breeding habits to distribution patterns and from their survival over winter to varying habitats during a 24-hour period.

Concluding that, through various studies, control of the midge is impossible and that killing larvae or adults chemically may have wider negative repercussions, he advised that good management in terms of reducing breeding habitats, draining wet areas on the farm combined with vector control in the form of the licensed insecticides plus vaccination is the only option.

He also stated that, as insecticides are not serotype-specific, there can be benefits in areas where BTV8 vaccination is taking place, but producers worry about BTV1.

Policy developer Dr C. Posthumus Meijes from the Netherlands shared the Dutch experience with the group, and outlined how the different methods of control have been considered. These include culling, hygiene, movement control and vaccination.

At the end of 2006, the Netherlands had 457 cases confirmed, this rose to 6,472 in 2007 but fell to 66 in 2008. Mass vaccination paid off and all of the infected premises in 2008 had failed to vaccinate for BTV.

Voluntary vaccination to stay

Looking forward to 2009, and assuming a vaccination uptake of above 60% but below the target 80%, the Dutch policy will involve voluntary vaccination with no financial compensation to the producer.

An education programme will stress the importance of vaccination to all producers but the Netherlands, as with many other countries, believes that there will be some resistance to vaccination so it remains in a “control” and not “eradication” phase.

Concluding the meeting, Dr Birgit Makoschey of Intervet/Schering-Plough Animal Health gave an update on recent laboratory studies to determine the safety and efficacy of Bovilis BTV8. Tests in pregnant heifers and ewes demonstrated that the product did not have a negative effect on the outcome of the pregnancy.

The difficulty to reproduce clinical signs of BTV8 under laboratory conditions had already been acknowledged by previous speakers. In this context, the development of a challenge model in sheep in which the animals succumb to bluetongue disease can be regarded as a major achievement of the scientists at the Intervet/Schering-Plough research department.

Vaccination studies in lambs with maternal antibodies revealed that vaccination in the face of MDA strongly reduced viraemia after challenge. The summary was that vaccination in the face of maternal antibodies protected lambs against challenge with live BTV8 three months after vaccination.

An additional study also looked into the duration of immunity in sheep (using the BTV8 PCR test) and clearly showed that none of the vaccinated sheep was viraemic six months postvaccine, concluding that vaccination with Bovilis BTV8 prevents BTV8 viraemia for at least this period of time.

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