This workshop provided a brief overview of bovine viral diarrhoea (BVD) and sheep scab, with a focus on how the Gwaredu BVD and Gwaredu Scab programmes have helped on the path to disease eradication and how veterinarians can use the ideas developed in these schemes in their own practice.
Bovine viral diarrhoea
BVD virus is in the Flaviviridae family. There are two genotypes, BVD 1 and 2. BVD 1 occurs in the UK, and there are two biotypes: cytopathic and non-cytopathic. Most persistently infected calves have non-cytopathic BVD at the point of diagnosis. Cytopathic virus causes mucosal disease. BVD is an immunosuppressive disease of cattle that causes increased pneumonia, diarrhoea and abortions, and increased disease overall. There is also an increase in greenhouse gas emissions in comparison to uninfected populations. BVD causes a persistent and long-term drag on productivity.
The most important agent of spread is the persistently infected calf. Persistently infected calves are born infected – BVD virus infects the calf in utero (at 40–120 days gestation) and is viewed by the immune system as part of the calf and therefore these calves never have an immune response and they spread virus in large amounts for the rest of their lives.
Screening for BVD
If we want to eradicate the disease, we need to know where we are to start with – do we have it on the farm? There are several strategies for screening:
- Management group antibody screening is a useful tool to allow us to separate herds into those that need attention for BVD eradication and those that do not. It involves a blood sample from 5–10 animals per management group
- Bulk milk sampling for antibodies. This is useful for negative dairy herds. Antibodies in bulk milk will persist for a number of years
- Whole-herd antigen screening. This involves testing every animal. Non-viable calves must be included in the screening programme, in case it was a persistently infected calf.
Once it has been determined that there is BVD on the farm, we have to think about how to get rid of it. The aim is to find and remove the persistently infected animals from the herd as quickly as possible. This can involve tagging every animal, blood sampling them all on the same day, bulk milk PCR to determine if there are any virus-positive animals. It is also important to determine whether there are any animals that didn't contribute that day: those with mastitis; high somatic cell counts; milk withdrawal animals. The reality is that a combination of testing may occur on each farm.
Biosecurity
Once the infected animals have been removed, it is important to think about risk factors for bringing the disease back on farm.
Buying in
Where are farmers getting their animals from and what do they know about them? Do they quarantine and test as they come in? For BVD specifically, farmers can buy from accredited BVD-free farms. Quarantining animals once they arrive on farm can be useful.
Over the fence contact
Over the fence contact is another biosecurity risk for BVD. Generally, the recommendation is a 3-metre separation between a herd and animals of an unknown status. This can be difficult, but electric fencing, hedges and double fencing can all help. Timing of grazing can also help, so coordinating by moving animals away from neighbouring pastures when your neighbour brings a herd close to your farm can reduce the risk.
Vaccination
Vaccination is important. The aim is not to get rid of the disease but to stop it getting back on the farm, buying more time, and stopping over the fence contact from ruining the hard work of disease eradication. Vaccination may not help if a persistently infected animal is bought in, but it may slow the spread from a transiently infected animal and give more time.
Farm visits
Veterinary visits and visits from other professionals are another risk factor. Good biosecurity protocols are needed from everyone.
Gwaredu BVD
Gwaredu BVD was the eradication scheme that was run in Wales. This gave farmers in Wales a chance to screen their animals so they could be assigned as BVD free or BVD present farms. For those determined to be BVD free, veterinarians would give advice on biosecurity to minimise the chances of disease impacting the farm.
For those farms with BVD present, they were offered financial support to hunt for the persistently infected animals, and they would also get the herd health advice once the infected animals were eliminated.
In total, 85% of farms in Wales were screened and 1296 PI hunts were carried out. The percentage of positive farms dropped from 27% to 23%. The Welsh government then carried out a consultation on mandatory screening, and there are hopes that this may happen in the near future.
Sheep scab
Sheep scab is caused by the parasite Psoroptes ovis. Anecdotally it is an increasing problem in the sheep industry. The life cycle of the parasite occurs purely on the sheep, although it can live off the sheep for 17 days. It can take a few weeks to months of infestation before clinical signs can be seen.
Clinical signs include scratching and rubbing, nibbling and biting. Tags of fleece may be found all over the farm due to animals rubbing against fences.
The wool needs to be cut away to get to the affected areas, so a scrap can be taken. Proper diagnosis is important, as a lot of problems are caused by assuming or guessing diagnosis. Scab can be mistaken for lice and it should be remembered that mixed infections do happen.
Skin scraping is the mainstay of investigation. Other tests include antibody tests. These can be used as a surveillance tool and we only need to take samples from 12 animals per management group. Antibody tests can be a useful quarantine tool.
Treatment of sheep scab includes macrocyclic lactones. These will interfere with worming programmes. The other treatment option is organophosphate dip. This is toxic to invertebrates, so it is important that this does not go into the water course. Disposal should ideally be by licensed contractor.
Gwaredu scab
Sheep scab was identified as a priority disease by Welsh government and the Gwaredu Scab scheme was set up. Index farms self identify that they have itchy sheep, and the local veterinarian then makes the diagnosis as discussed previously. In-contact farms are identified by the scheme and these farms are tested through antibody testing. Farms with positive results are treated. This can allow farms to catch the scab early, before clinical signs start, reducing further spread to neighbouring farms.
Biosecurity is again key, with quarantine and testing for bought-in sheep. Avoiding over the fence contact and having good biosecurity for veterinary visits and other professional visits, such as those from shearers and scanners, are essential.
There is current legislation across the UK that means farmers are legally obliged to treat sheep scab. The Gwaredu Scab programme helps farmers to treat effectively by working in clusters.
Common themes from both diseases
Both BVD and sheep scab have been well studied and there are good diagnostics and treatments where appropriate. The risk factors for reinfection are broadly similar and the same sort of advice is applicable. Control of infected animal movement is possible in theory.
Human factors
A key factor to consider with an eradication programme is the people involved and getting them on board.
For BVD there were two professional industries involved: farmers and veterinarians. This made it easier to pass on the correct messages and get them trained and up to speed. Veterinary delivery partnerships in Wales also already existed that helped to assist co-ordination and disseminate messages, as well as allowing veterinarians to pass information back up the chain. BVD testing was bundled up with TB visits and this meant there was little extra work for farmers and veterinarians.
The BVD eradication plan took a long time to set up. A massive amount of time and mileage went into visiting farmers and vets and educating them on the topic.
In comparison, for sheep scab eradication, four industries were involved in the field delivery: veterinarians, farmers, contract dippers and tanker companies to dispose used dip. This doubled the complexity. With sheep, there is no regular opportunity to collect surveillance samples, which makes it more expensive. Traditionally, there is also some stigma associated with the disease and because of the nature of the disease, with over the fence contact, we need lots of neighbours to work cooperatively. Also, with scab, we are reacting to active disease. The industry was less well-prepared with information before hand, which may have impacted effectiveness of the scheme.
Suggestions to take to farms for other diseases
It is important to be realistic about what diseases can be eradicated. Some diseases are not as feasible. There needs to be a reasonable level of understanding about the disease in question, so that a sensible plan can be made. Then trust needs to be built with the farmer(s), depending on how it is being tackled – on individual farms or in clusters. Finally, for the BVD eradication programme, the multiple meetings with farmers and vets were key to the success of the programme and this should be considered going forwards.
Conclusions
The Gwaredu BVD and Gwaredu Scab schemes can be used as examples of disease eradication schemes that use a variety of measures including vaccination, screening and biosecurity measures. These measures could be used for the eradication of other suitable diseases on farm.