Options for vaccination of seals in seal sanctuaries for protection against PDV
This paper is a discussion note which explores the options of vaccinating seals in sanctuaries against Phocine Distemper Virus (PDV) in the United Kingdom. It is a collaborative document and includes expert contributions from Animal Health, DEFRA, Sea Mammal Research Unit, Institute of Zoology, International Zoo Veterinary Group and RSPCA however it is not intended to be a scientific paper.
Vaccination of seals against PDV in seal sanctuaries
Background
1. A guidance note has recently been produced in collaboration with Animal Health, Defra, International Zoo Veterinary Group and RSPCA on options to vaccinate seals in their natural environment and the risks involved in doing so. This note (which can be found at www.defra.gov.uk/wildlife-countryside/ewd/seals/vaccination.htm ) concluded:
a) A vaccination programme is considered to be a risk to both humans
and seals.
b) Effective immunisation of the seal population in the wild is unlikely
to be feasible.
c) 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.
d) 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
e) If a tested vaccine was available and safe there is a possibility of
vaccinating seals in sanctuaries.
2. However, while it is impractical to vaccinate seals in the wild, it may be possible to vaccinate healthy or recovering seals in sanctuaries or treatment centres so they might be released back into the wild without fear of infection from PDV. The rescue organisations, such as the RSPCA and the International Zoo Vet Group are also proposing to vaccinate seals residing in treatment centres as a means to promote recovery. This paper discusses the possibilities of a limited vaccination programme taking into account the testing of a vaccine, procedures required, the risks involved and evaluation of the overall benefits to the seal population.
Objectives of a PDV vaccination programme of seals in sanctuaries
3. The objectives and scale of a PDV vaccination programme in sanctuaries should be clearly defined. Seals might be admitted into sanctuaries or rescue centres suffering from a range of conditions other than PDV - for example they may have been injured during storms. Once recovered it is important that seals are released back into the wild as soon as possible to minimise human contact and ensure that they are able to survive on their own. Vaccination in a treatment centre might offer protection in the wild after release or help protect those seals that reside permanently in treatment centres from becoming infected with PDV brought in by rescued pups. Without this protection sanctuaries with residents may find it difficult to function.
4. The size of the programme will depend on the number of seals held in sanctuaries in UK. In the UK (in a normal year) there are around 300/350 seals taken in by a number of sanctuaries and hospitals around the country, about 150 of these are likely to be common seals. There will also be an average level of mortality depending upon the circumstances of individual seals.
5. It is uncertain how the vaccination of seals in sanctuaries will aid the conservation of the wild seal population. The current UK population is at least 34,000. Even if this population suffers 50-60% mortality from PDV, the release of, say, 100 vaccinated seals from rescue centres would only increase the surviving population by less that 1%. Thus the objectives of the vaccination programme are unlikely to be for the long-term conservation of the species.
6. Vaccination might also help protect seals entering sanctuaries with PDV. Injection of a vaccine could boost their immunity and ability to fight the disease. A trial would need to be carried out and most likely require Home Office approval. However, vaccination is unlikely to provide necessary protection or boost immunity in seals already suffering from PDV as seals often die of the side effects of the disease and not the disease itself.
7. Thus the benefits of vaccinating a small number of seals in sanctuaries for the conservation of the wild population are limited. Vaccination of seals in sanctuaries will largely be carried out to benefit the welfare of healthy seals or those suffering non-PDV related symptoms.
The virus and the type of vaccine required
8. The Phocine Distemper Virus is a Morbillivirus genus within
the family Paramyxoviridae. Morbilliviruses infections have had
a huge impact on human beings and animals such as measles in humans, rinderpest
in domestic stock and wild species in Africa and canine distemper in carnivores.
New morbilliviruses have been discovered in the last decade including
Cetacean morrilliviruses (CMV) in dolphins, whales and porpoises and PDV.
These viruses have had significant ecological consequences for marine
mammals. Further Information on morbilliviruses may be found on the Animal
Health website;
www.iah.bbsrc.ac.uk/virus/Paramyxoviridae/index.html
9. The types and availability of vaccines appropriate for protection against
PDV have been discussed previously in a note at www.defra.gov.uk/wildlife-countryside/ewd/seals/vaccination.htm.
As a vaccine specific for seals is unlikely to be available in the UK
due to lack of investment and commercial demand a substitute is required.
At present the only available live vaccines are attenuated Canine Distemper
Virus (CDV) strains (to protect dogs from distemper) which are likely
to provide effective cross-protection to seals.
The risks associated with a vaccine
10. All vaccination involves some risk which should be minimised as far as possible. The risks are not only to the vaccinated seal itself but also to other seals it comes into contact with in the sanctuaries and when it is released into the wild. These risks include the side effects of the vaccine harming the seal and also excretion of the virus post vaccination and possible transfer of infection to other seals. To ensure the risks are assessed a vaccine should undergo a trial. The trial may also investigate the appropriate level of protection required.
Assessing and dealing with the risks
11. A vaccine trial needs to be conducted to assess the risks but also ensure that procedures are standardised and repeatable and thus allow comparisons of results from other trials. An animal test certificate (ATC) is required from Defra in order to carry out a trial. An ATC authorises the field trial and the procurement and supply of the veterinary medicine used in the trial. An application is required to describe the trial and justify the use of live animals. The applicant will be required to demonstrate that the veterinary medicine is safe in the target species and of sufficient quality as is appropriate in the circumstances.
12. This field safety or efficacy trial authorised under an ATC would permit the testing of a vaccine under field conditions but would only permit investigations which would be expected within normal veterinary practice. A trial may also involve challenge testing by exposing the seals to the disease. A challenge experiment would need to be authorised by the Home Office. However, this type of experiment will require the use of control seals as a sacrifice which has practical and ethical implications.
13. A trial requires isolation of a number of seals. For example a seal is isolated and blood samples taken to verify the seal does not already carry antibodies to the virus (seronegative) and is healthy. The seal is then injected with the vaccine and then external signs of the disease are monitored. Regular blood tests are taken to measure antibody production, the titre level. The titre level required to ensure adequate protection may be assessed against known titre responses in dogs. If the appropriate titre level is attained and there are no signs of the vaccine inducing PDV seals may be added to the trial sequentially. The likelihood of vaccine excretion will vary with type of vaccine, host response, infectivity of the passaged virus and the length of time, if any during which the virus is excreted. After a period of time sufficient to ensure effective protection and absence of virus excretion the seals may be released from isolation.
14. Research into vaccination of seals in sanctuaries has been carried out by the Seal Rehabilitation and Research Centre (SRRC), Pieterburen in Holland and noted in the previous paper www.defra.gov.uk/wildlife-countryside/ewd/seals/vaccination.htm. Researchers have conducted trials of different PDV subunit vaccines to immunise seals in sanctuaries in Holland. However, as previously noted these vaccines are not available in the UK and most likely in short supply as they are often produced in a single batch. Further information may be found on the SRRC website; http://www.zeehondencreche.nl/english/
Vaccination of seals in sanctuaries following a trial
15. If a vaccine trial is not successful (i.e. the seal(s) become ill during the trial) the trial must be halted and the seal treated. If after vaccination the seal(s) continue to excrete the virus the animals will be isolated and kept in permanent captivity. If a vaccine trial were successful (i.e. the vaccine provides effective protection and the virus is not excreted from the seal) it would be possible to vaccinate other seals in sanctuaries. The decision to carry this out would need to be considered following advice from veterinary experts. The number of seals vaccinated will depend on the availability and cost of further supplies of the vaccine as a trial vaccine is often produced in a one-off batch.
Implications of vaccinating seals in sanctuaries and the PDV outbreak in the UK
16. Vaccination in sanctuaries will largely benefit the welfare of seals and not the conservation of the species in the long term. A vaccination trial may, if properly monitored, provide valuable information about the mobillivirus infection in seals and possibly aid understanding of PDV outbreaks in the future. However, vaccination of seals in sanctuaries will not help prevent a future outbreak of PDV.
Page last modified:
22 December 2003
Page published: 16 October 2002