Matthew Balfour BVM&S MRCVS
With Covid-19 restrictions causing a premature end to the 2020-21 shooting season, preparations for the upcoming rearing and shooting season were brought forwards on many sites. One unfortunate but unavoidable trend on many shoots this year has been the large number of game birds still present. The obvious temptation for many was to catch up surplus pheasants and use them as laying flocks. Although on the face of it this is a cheaper option than overwintering birds in a closed flock, it comes with various risks and extra precautions are needed to limit the spread of disease amongst the birds.
Firstly, there is a substantial risk of introducing Mycoplasma Gallisepticum (MG/Bulgy eye) into a laying flock if birds go untested once they are caught up, particularly those from heavily stocked shoots. This risk is increased further when birds are sourced from several different shoots.
- Once a laying flock is infected with MG it cannot be eliminated without complete depopulation and re-stocking and so it is vital that protocols are followed to limit the risk of disease exposure.
- Vaccination and/or use of antibiotic treatment may reduce the clinical signs in infected birds, as well as transmission from infected to healthy birds and limit vertical egg to chick transmission.
- Veterinary intervention will never 100% eliminate the risk of an infected bird spreading disease. It is therefore important to ensure that any laying flock is MG-free (as much as can be proven) from the beginning.
How do we best achieve MG negative status?
- Keep a permanent closed laying flock of known health status. Although this is seen as the ‘gold standard’ approach testing the disease status of the birds is still strongly advised.
- It is advisable to immediately cull and PCR test any bird, at any point in the year who presents clinical signs for mycoplasma (conjunctivitis, swelling of the head/sinuses, respiratory distress). The PCR test searches for genetic material belonging to MG and is the most sensitive diagnostic method for birds with an active MG infection (Bradbury et al, 2001).
- As an additional precaution, a sample of the flock should be tested annually for the presence of MG antibodies in the blood. The presence of MG antibodies in any bird suggests it has been infected with, and is potentially shedding, the bacteria although it may not be showing clinical signs for mycoplasma.
- Currently the Sci-Tech laboratory offers a modified commercial MG ELISA test which, although not 100% sensitive on an individual bird basis, is able to determine with a high degree of confidence whether a flock contains MG-infected individuals. As a rule of thumb, an appropriate sample size will give a 95% chance of finding a positive if 5% or more of the total population is infected. For a population of 500 or above, this works out as a sample size of 60.
At St David’s we worked with clients to collect blood samples from each shoot for MG antibodies prior to moving the caught-up birds, to provide a good picture of MG prevalence. This gave those clients who were catching-up the confidence that MG was absent, or at least at a low prevalence, within the population of birds that they sourced.
Vaccines…will these help to protect my flock?
- Although different game bird vets will favour different approaches to mycoplasma control, the St David’s Game Bird Services approach has been derived from our experience in dealing with mycoplasma on some of the biggest game farms in the UK over the last decade.
- Our approach is to offer vaccination with a killed injectable MG vaccine in flocks that test negative for MG blood antibodies using the modified ELISA test.
- Killed MG vaccination has been shown repeatedly in chickens to produce a blood antibody response, substantially reduce MG-associated clinical signs (Glisson & Kleven, 1985; Glisson & Kleven, 1984; Bekele & Assefa 2018), substantially reduce MG shedding from infected to healthy birds in the flock (Hildebrand et al, 1983) and substantially reduce MG transmission through the egg to the chick (Sasipreeyajan et al, 1985; Glisson & Kleven, 1985). In pheasants, killed MG vaccines also produce a blood antibody response which, in our combined clinical experience at St David’s, produce similar benefits to those seen in chickens.
- A combination of antibody testing (to ensure nil or very low MG prevalence), combined with a killed vaccine programme, is therefore able to reduce the risk of vertical MG transmission to minimal levels. However, it is important to note that unfortunately no veterinary intervention, including pulse-dosed antibiotic treatment, can completely eliminate the chance of MG transmission from any flock.
The strategy discussed above employs killed MG vaccines (either MG-Bac or autogenous) and it must be stressed that the use of live chicken MG vaccines (eg MG 6/85) in pheasants is not recommended. There is no evidence that live vaccines have affinity for the receptor sites in the upper respiratory tract of the pheasant or produce any immune response. In addition, the importance of good management practices and a low stress laying environment cannot be emphasised highly enough.
For more information and advise on how to prevent, reduce and manage disease risks on your site, please speak to your local St David’s vet by calling 01392 872932 or emailing firstname.lastname@example.org.
References: Bradbury, Janet & Yavari, Christine & Dare, Cynthia. (2001). Mycoplasmas and respiratory disease in pheasants and partridges. Avian pathology : journal of the W.V.P.A. 30. 391-6. Paper Title: Inactivated Vaccine Trial of Mycoplasma gallisepticum in Ethiopia Authors: Legesse Bekele1, Temesgen Assefa2 Date: 2018 Paper Title: Mycoplasma gallisepticum Vaccination: Effects on Egg Transmission and Egg production Authors: John R. Glisson and Stanley H. Kleven Date: 1984 Paper Title: Mycoplasma g. Vaccination: Further Studies on Egg Transmission and Egg Production Authors: John R. Glisson and Stanley H. Kleven Date: 1985 Paper Title: Mycoplasma gallisepticum (MG): Laboratory and Field Studies Evaluating the Safety and Efficacy of an Inactivated MG Bacterin Authors: D. G. Hildebrand, D. E. Page and J. R. Berg Date: 1983 Paper Title: Bacterin to Control the Vertical Transmission of Mycoplasma g. in Chickens Authors: Jiroj Sasipreeyajan, David A. Halvorson and John A. Newman Date: 1985