On a farm in Coleambally in South Western NSW there have been ongoing issues with Mycoplasma ovis after the 2012 flood.
M. ovis is a bacterial infection which infects the erythrocytes of sheep and goats. Infection may not cause any clinical signs but when numbers are large enough it leads to massive destruction of erythrocytes and subsequently anaemia and jaundice. Deaths often occur in young stock, especially if stressed for example when yarding them.
The bacteria are normally spread at marking/mulesing time but shearing and blood feeding insects may also be a source of infection. In this case the assumption was that after the flood there was a large increase in the amount of mosquitoes to become vectors to spread the disease.
It has always been known that the organism stays in blood and that a stressful event which lowers the immune system can cause clinical M. ovis recrudescence. However it has also been noted that older sheep tend to be more resistant as they develop immunity after exposure.
On the 3/4/12 the mob of 6,000 weaners which had been shorn in early March were moved by the farmer a distance of 3-4km. During this movement a decided tail to the mob was noticeable and about 15 animals dropped down and died. The private veterinarian in Jerilderie, Des McRae was called by the owner to post-mortem the dead animals. He found signs of anaemia but no signs of Haemonchus, coccidiosis, chronic infection or large worm burden. He did note that at this time there was an abnormally large amount of mosquitoes present due to the flood waters. He sent samples to the lab being suspicious of a M. ovis infection. Lab results confirmed a diagnosis of M. ovis and showed a small amount of Haemonchus in the abomasum and a non significant amount of strongyle worms.
Des consulted Dan Salmon about the case and was advised that oxytetracycline treatment may be of benefit as long as handling stress was minimal. Des and the farmer discussed the many factors which can influence the severity of M ovis: stress, shearing/mosquito vectors, and oxytetracycline treatment. The farmer stated at the time that the affected animals had lost a lot of condition despite their still being an adequate feed present.
The ewe lambs were kept and lambed for the first time in 2013. A small mortality rate was noticed in their offspring but nothing great enough to make the farmer call for veterinary assistance.
In May/June of 2012 another generation of lambs was born. These lambs were separated, the ewe lambs were kept on the property to become the next generation of the flock and the male lambs were sold at the Hay saleyards in September 2012.
Oral history of the brothers in the Hay saleyard is that a high proportion either died or became symptomatic at the saleyards. There was no history of disease in their sisters who were not subjected to the same amount of stress. These ewes were classed in December 2013 and joined to lamb in May/June of 2014 which they did without incident.
On the 25/8/14 Des was called back to the property due to a 1.5-2% death rate in the lambs of a group of 800 of these maiden ewes. The lambs had been mulesed about 1 month earlier. The deaths did not seem to be occurring in lambs born to mixed aged ewes, only the maiden ewes. The mob had been moved several days earlier into a new paddock which contained better feed. It was noticed that the lambs which were affected more severely tended to be those which were born as part of a set of twins rather than as a single. The affected mob had been scanned as twin-bearing. There was not the same incidence of the problem in mobs of single-bearing ewes of the same age.
At the time Des speculated that twins may be more likely to suffer due to mismothering, decreased amounts of colostrum or nutrition. Transmission of M. ovis into the lambs was postulated to be due to either in utero infection from carrier mothers or mosquito vectors (there had been a large number of mosquitoes present again that winter before they had been killed off by frosts).
Farmer spoke to Elizabeth Braddon the next day to gain more information on M. ovis. She established that the biggest losses appeared to occur when there was a few very cold nights in a row (-2 and -3 degrees C) which probably added the extra stress.
Approximately a week later the farmer spoke to Dan Salmon about the ongoing issues with M. ovis. It was decided to find out if M. ovis carriers were detectable in the maiden ewes. PCR test was performed on 20 of the ewes which had been found to be carrying twins by ultrasound scanning. The results came back that 9/20 showed signs of M. ovis infection.
Based on these results it was decided to do a clinical trial of the effectiveness of the antibiotic oxytetracycline given as a one off long acting injection in causing clearance of the organism from these carrier animals. Effectiveness of oxytetracycline in treating M ovis has long been postulated (especially since the reclassification of this bacterium from an Eperythorozoon to a mycoplasma). The biggest issue in treating severely infected animals is that the stress caused by mustering the animals to administer any kind of treatment is usually enough to cause death in severely anaemic animals. Another speculated way of using the drug is as a preventative measure used at the time of procedures such as mulesing which could lead to transmission of the bacterium.
It was decided to get a new sample of sheep from the same group and individually identify them with numbered ear tags. Nineteen sheep were presented the second time and given an injection of long acting oxytetracyline. Blood was then collected from 16 animals (due to not being able to collect blood from 3 individuals) and sent off for testing.
Of the 16 samples taken for PCR testing 8 of them were found to be positive for M. ovis.
Seven of the sheep which had been positive on the initial testing where then retested 48 days later to see if the M ovis had been cleared from their system. The eight individual was not tested because a few had lost their eartags and it was impossible to identify correctly the eigth individual.
Only 2 of the 7 known to be positive were found to be negative at this subsequent testing. Although this was not a large sample size these results were not as promising as originally hoped. This corresponds with assertions made that tetracycline does not eliminate this agent from chronically infected animals (Neimark, Hoff & Ganter, 2004).
It has now been proposed to trial the difference in the growth rate and wool clip of affected individuals versus unaffected flock mates to see if this carrier state is causing a production loss as well as potential lamb deaths.
Most of the data has been extrapolated from case notes and lab reports generously supplied by Des McRae - Jerilderie Vet Clinic, Dan Salmon - Murray LLS and Elizabeth Braddon - Riverina LLS.