Mycoplasma in pheasants

Key findings

  • In gamebirds, the sensitive PCR test for Mycoplasma gallisepticum (Mg) is much better than the RSA test.
  • It is possible for a pheasant to be carryingMg without giving a positive RSA test.
  • Mg is a definite pathogen causing respiratory disease in gamebirds.
  • Corvids are not common hosts for Mg.
  • A commercial chicken vaccine does not protect day-old pheasant chicks from Mg infection.

This work is part-funded by the National Gamekeepers Organisation and carried out at Liverpool University Veterinary School by Mrs Anne Forester and Dr Janet M Bradbury.

Mycoplasma gallisepticum (Mg), a pathogen of chickens and turkeys, has been a suspected cause of upper respiratory disease in gamebirds. Its role as the primary pathogen was confirmed experimentally in chukar partridges in the USA and in red-legged partridges in the UK, but it has never been proved that Mg is a primary pathogen in pheasants. We therefore conducted a series of trials designed to understand Mg in pheasants.

An accurate test for Mg

We compared different tests for diagnosing Mg in pheasants using paired blood samples and respiratory tract swabs. We found that the most commonly used Rapid Serum Agglutimation (RSA) test was not as accurate as the Polymerase Chain Reaction (PCR) test. Our results show that it is possible for a pheasant to be carrying Mg without a positive RSA test. The PCR test is therefore the method of choice for rapid and accurate diagnosis (see Table 1). There was little evidence of Mycoplasma synoviae, another pathogen associated with current Mycoplasma outbreaks in chickens, throughout this study.

Table 1: Number of pheasants positive for Mg diagnosed by three methods

  Number (and %) positive
Number of pheasants Culture RSA PCR
633 25 (4%) 100 (16%) 139 (22%)

Corvids as a possible source of Mg

Some previous work had suggested that corvids might carry Mg. Therefore, using the PCR test, we tested samples from 232 crows, 128 rooks, 41 jackdaws, 47 magpies and four jays. We found Mg infection in only six (four crows, a rook and a magpie), indicating a very low prevalence. It appears that corvids are not common hosts for Mg. This suggests that at present corvids are not the main source of Mycoplasma infection in pheasants.

Mg as the cause of respiratory disease in pheasants

We infected groups of one-day-old pheasant chicks with five different Mg isolates from pheasants, infected a further group with a turkey Mg strain (Mg/S6), and sham-vaccinated a control group with sterile Mycoplasma broth.

Birds in all the infected groups developed typical signs of respiratory disease (sinusitis, conjunctivitis and nasal exudation). This caused clinical signs of infection in 54-100% of cases. Mg was re-isolated from almost all the infected birds, but not from any of the control group. Birds from the infected groups tested positive for Mg at the end of the trial (see Table 2).

Table 2: Pathogenicity trial: % showing clinical signs of infection, RSA test and number of birds with re-isolation of Mg

Group % showing
clinical signs
No. of birds
Mg-positive by RSA
No. of birds
Mg-positive
by isolation
Controls 0 0/6 0/12
Turkey Mg (Mg/S6) 66 6/6 11/12
Pheasant Mg (G9/01) 54 6/6 11/13
Pheasant Mg (G9/01) 100 5/5 13/13
Pheasant Mg (G87/02) 77 7/7 13/13
Pheasant Mg (G102/02) 85 7/7 13/13
Pheasant Mg (G118/02) 85 7/7 13/13

 

This experiment strongly suggests that Mg is a primary pathogen in pheasants and that these recent UK isolates are pathogenic.

Mg vaccine trial

We tested a vaccine for Mg, recently licensed in the UK for use in chickens, for its efficacy and safety in pheasants for which there were no data available.

We vaccinated six groups of 12 one-day-old pheasant chicks by aerosol and the same number with sterile distilled water (sham-vaccinated). We then challenged all the birds at 10 days old with a field strain of Mg (isolate G87/02). As a control we left two further groups (each of 12 birds) unchallenged - one vaccinated and the other sham-vaccinated.

We observed clinical signs of Mg infection (conjunctivitis, sinusitis and nasal exudation) in all groups that were challenged with Mg including those that were vaccinated (see Table 3), but there were no clinical signs of infection in the controls.

Table 3: Vaccine trial: % showing clinical signs of infection, RSA test and number of birds with re-isolation of Mg

Group % showing
clinical signs
No. of birds
Mg-positive by RSA
No. of birds
Mg-positive
by isolation
Sham-vaccinated/unchallenged 0 0/10 0/10
Vaccinated/unchallenged 0 0/10 0/10
Sham-vaccinated/challenged 93 23/24 52/59
Vaccinated/challenged 95 23/24 49/59

 

The results of this trial indicate that, although the vaccine appeared to be safe for young pheasants, it failed to protect them. This may be because the birds were immunologically immature. Clearly one day old is not a suitable time for using this vaccine. More work is being planned.

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