The options for a vaccination programme of Seals against Phocine Distemper Virus (PDV) in response to predicted PDV outbreak in UK in 2002
This paper is a discussion note which explores the options to vaccinate seals against Phocine Distemper Virus (PDV) in the United Kingdom. It is a collaborative document and includes expert contributions from Animal Health, Defra, International Zoo Veterinary Group and RSPCA however it is not intended to be a scientific paper. For further technical information please contact Ian Robinson, RSPCA and Andrew Greenwood, International Zoo Veterinary Group.
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Summary
- A vaccination programme is considered to be a risk to both humans and seals.
- Effective immunisation of the seal population in the wild is unlikely to be feasible.
- An experimental vaccine exists in Holland but will not be available in the UK because it has not been tested to EU standards and there are insufficient supplies.
- Vaccination of seals in sanctuaries is a reasonable possibility but seals may have to remain isolated for a lengthy period of time if the vaccine used has not been tested
- If a tested vaccine was available and safe there is a possibility of vaccinating seals in sanctuaries.
Background
The last outbreak of PDV in 1988 killed over 18,000 seals in Europe. In the UK which has both common and grey seals around its coast, the populations in the Wash and Moray Firth were most effected. Common seal populations are more susceptible to the disease but have largely recovered since 1988. Those surviving exposure in 1988 will have acquired resistance to the disease. However, tests suggest that less than 10% of the population in the UK is likely to be resistant and there is potential for high mortality if an outbreak reaches the UK, particularly in the Wash. Vaccination is a possible option to reduce the number of seals affected by the virus and perhaps contain the outbreak. This paper discusses the possible options for vaccination of seals.
Knowledge of the disease
PDV is a virus similar to Canine Distemper in dogs. The virus is passed between individuals by close contact and the symptoms include respiratory problems. Death is often caused by secondary infections such as pneumonia as PDV affects the immune system. In dogs, where the dam has been exposed to natural infection or has been vaccinated, a temporary natural immunity (maternal antibody) is passed from dams to pups which may last for many weeks. Pups are either vaccinated at 8-10 weeks of age and again at 10 to 12 weeks or given a single dose of vaccine at 10 to 12 weeks of age. The booster interval for UK authorised canine distemper can be up to 2 years. Provided the immune mechanisms are the same in seals, there is transfer of maternal antibody and this passive immunity lasts for a similar period as in dogs, Veterinary Exotic Disease Division suggests that seal pups could be vaccinated to produce some protection to the individual. Whether booster vaccination would be needed would depend on the prevalence of natural infection in the population. Vaccination strategies are usually aimed at protecting populations rather than individuals. If it were required to protect an entire population then, as a rule of thumb a minimum of 60% and ideally 90% of the population would need to be vaccinated. Administering the vaccine would be possible in pups as they are easier to handle on shore than adults. If only 10% of the population of seals were exposed to PDV in the last outbreak the vast majority of pups born since then will not have acquired immunity and there would not be any maternal antibody to effect the vaccination. The pups are likely to respond to a vaccine at an early age but it is not clear what age the pup would need to be vaccinated. The Veterinary Exotic Disease Division is cautious of the unknown impact of administering a man-made vaccine into a wild animal population. It would need careful consideration on ethical grounds.
Practicalities of administering a vaccine
Seals are large, wild animals that spend a substantial proportion of their time at sea and haul out in remote areas to moult and pup. To attempt to catch these animals at sea would be highly dangerous and involves risking physical injury and stress to the seal. The stress of being caught may depress the seal's immune system and make them more susceptible to infection generally. In theory seals could be caught when they are on land. It would be particularly expensive to reach these remote areas and ensure Health and Safety guidelines are followed. The 'hauled out' females will be particularly vulnerable to physical injury whilst giving birth and feeding young pups. The young pups would be easier to catch and handle but it is not known how the stress would affect them and how their immune systems would react to a vaccine. It is also worth noting that the environment in the Wash is not suited to catching seal pups as the sand banks only appear at low tide and common seals pups swim from birth. The RSPCA would only support attempts to pursue/catch seals on shore, if it can be guaranteed that the seals are not injured. In addition adults may flee from the breeding grounds and cause injuries to seal pups and possibly lead to their abandonment. The International Zoo Vet Group believes that vaccination of seals would cause minimal stress, especially if carried out by remote injection and the benefits of vaccination would outweigh the possible impacts of developing PDV. However, the International Zoo Vet Group regards remote injection as impractical with current technology. The RSPCA would support investigation into administering a vaccine orally (as used in wild foxes) by feeding fish or using misting spraying. However, the success of these methods is unreliable as seals do not tend to eat dead fish and the accuracy of the spraying will be variable according to weather conditions. Veterinary Exotic Disease Division have also made the point that it is not known whether existing vaccines would be effective if administered by these route.
Design of a vaccination programme - Epizootiological considerations
A vaccination programme should consider the effect of immunization on the population as a whole. The main aim of a vaccination programme should be to establish a level of immunity where the proportion of immune seals is sufficiently high to reduce the transmission of the infection to susceptible seals. The level of immunity required in seals is unknown. To ensure a suitable level is attained following basic principles the Veterinary Directorate of Defra recommends that a high proportion are immunised (60-90%). In Britain the number of common seals has been estimated at a minimum of 36,345 (NERC Special Committee on Seals) from surveys carried out between 1996 and 2000. Approximately 32,000 are found in Scotland and 4,270 in England, of which 2,785 are found in the Wash. To immunise a high proportion, 90% (32,000) of the seal population would be expensive, impractical and may in any case cause undue stress to seals during their breeding season. In theory a vaccine could be used to immunise the population in the Wash which was reduced by 50% in the last outbreak. Approximately 2500 (90%) seals would require vaccination. Seals are capable of moving large distances along the UK coastline and mix with populations from Scandinavia. Thus a European wide vaccination programme would be necessary to ensure the UK population are adequately protected.
Availability and trial status of a possible vaccine
There is no known commercially available vaccine for PDV in seals. Any trial vaccine developed to immunise seals against PDV would need to be authorised for use in the UK via the Animal Test Certificate (ATC) regulations through the Veterinary Medicines Directorate of Defra. Before being used, VMD would need to be satisfied that any vaccine was of sufficient quality and safe. There are a number of other possible authorisation routes depending on the nature and source of the vaccine, but all would require evidence of adequate quality and safety.
Domestic dogs are successfully immunised against the Canine Distemper Virus (CDV). There are a number of commercially available live CDV vaccines but there are no inactivated distemper vaccines licensed in the UK, or possibly even in Europe. The use of live vaccines carries some risk of inducing the disease in the animal post vaccination. Inactivated vaccines often require adjuvants to produce an adequate immune response and often have a shorter duration of immunity compared to live vaccines i.e. there is no guarantee they will work for life (seals may live for 20-30 years). A genetically modified CDV vaccine has been used successfully in America to immunise susceptible non-canine species in zoos and sanctuaries. However, this group does not include phocids. A GMO could be authorised for use via an Animal Test Certificate providing the requirements of Directive 90/220 had been demonstrated. There would also be a requirement to consult with the Chemicals and GM Policy Division.
There is provision in the UK for a veterinary surgeon to administer a licensed product (under the cascade) without any specific safety studies i.e. this would permit the use of a licensed live canine distemper vaccine in seals if there was no authorised product for use in seals (VMD). However, the Institute of Animal Health, Pirbright would not recommend this course of action. A live CDV vaccine was used on the rare black-footed ferret in America with disastrous consequences (all the females of the breeding programme died following vaccination). It would be highly risky to attempt to use live CDV vaccine on seals outside controlled conditions, as it is not known how they would react. It is possible the vaccine strain itself may cause death or even cause a new epidemic amongst seals.
Following the last outbreak in 1988 a batch of dead CDV vaccine was used to successfully immunise seals at the RSPCA sanctuary in Norfolk. The batch was a one-off and there are no further supplies available. Researchers at the Seal Rehabilitation and Research Centre (SRRC), Pieterburen in Holland have been experimenting with different PDV vaccines to immunise seals in sanctuaries in Holland (Visser et.al.,1992). The vaccine is currently unavailable outside Holland possibly as a result of supplies being used to deal with the current PDV outbreak in Scandinavia. The PDV vaccines have been tested under experimental conditions and are not commercially available under licence. In normal circumstances the VMD would often require the safety of an overdose to be demonstrated before an ATC could be authorised permitting the vaccine to be used in a trial under field conditions. If it did become available there may not be sufficient quantities and it would be very expensive to vaccinate sufficient numbers of seals.
Limited vaccination within seal sanctuaries
Vaccination of a limited number of seals held in sanctuaries in the UK is an option supported by the RSPCA and International Zoo Veterinary Group. The used of dead vaccines would allow permanent seal residents to be protected and also seal pups undergoing rehabilitation to be released into the wild without risk to PDV infection and free up valuable recovery units. Experiments in Holland found that the dead vaccine affords protection from PDV and leads to long term immunity but may need to be repeated in the future. However, the seals would need to be kept in quarantine, vaccinated three times and then finally blood tested to check for antibodies. This vaccination programme would take a minimum of 2 months to turn around and it would rely on sufficient space to isolate the seals (they would need to be isolated anyway if diseased seals were brought into the sanctuary). It is not clear how this option would buy time and space in sanctuaries needed for diseased seals. The use of a live vaccine would require the seals to be kept in isolation for much longer until it is clear they do not excrete the virus. . However, while there is some support for this approach the availability of an approved vaccine remains an issue.
Further Research
Further research into vaccination of seals and the effect of vaccines on seal populations would be desirable. However, research will depend on future priorities and availability of funding. This is likely to involve trials of vaccines in isolation. The International Zoo Veterinary Group would support trials of a less virulent live CDV vaccine which contains only a single strain of the virus (currently marketed vaccines only have one distemper strain but are often multi-component vaccines with other antigens such as parvovirus and adenovirus). However, the RSPCA would need to carefully consider testing different vaccine strains in seals.
The International Zoo vet Group is exploring the possibilities of using vaccination for biosecurity measures and will keep interested parties informed of their progress. This would involve vaccination of all resident seals before moving them to disease free sanctuaries and then setting up disease only sanctuaries. However they recognise there are no guarantees that the vaccine will be effective and there are risks involved. DEFRA will keep abreast of developments in this area.
Conclusions
This paper has set out the current options for vaccination of seals to protect them from Phocine Distemper Virus. Administering a vaccine in the wild is largely ruled out by the practicalities and ethical considerations. Resident seals in sanctuaries could be vaccinated but a PDV vaccine is not commercially available. Canine versions of the vaccines are commercially available and in theory may be administered in isolation by a veterinary surgeon. However, there are no guarantees that the vaccine will be effective and there is a risk (in the case of live attenuated vaccines) of killing the seal.
References
Hall, A. and Harwood, J. (1995). The Intervet Guidelines to vaccinating Wildlife. NERC, Sea Mammal Research Unit, High Cross, Cambridge.
NERC Special Committee on Seals (2001). Scientific Advice on Matters Related to the Management of Seal Populations:2001
Osterhaus, A.D.M.E.; Uytdehaage, F.G.C.M.; Visser, I.K.G.; Vedder, E.J.; Reijnders, P.J.H.; Kuiper, J.; Brugge, H.N. (1988). Seal vaccination success. Nature (Lond) 337, 21.
Visser, I.K.G.; Vedder, E.J.; van de Bildt, M.W.G.; Orvell, C.; Barrett, T.; Osterhaus, A.D.M.E. (1992). Canine distemper virus ISCOMs induce protection in harbour seals (Phoca vitulina) against phocid distemper but still allow subsequent infection with phocid distemper virus-1. Vaccine; Vol.10, Issue 7.
Jensen, T.; van de Bildt, M.W.G.; Dietz, H.H.; Anderson, T. H. ; Hammer, A.S.; Kuiken, T.; Osterhaus, A.D.M.E. (2002). Another Phocine distemper Outbreak in Europe. Science. Vol 297.
Page last modified: 27 November 2002