SALMONELLA IN NZ DAIRY COWS
Over the past few weeks, a number of vets have been reporting Salmonella outbreaks on dairy farms. Our investigations on some of these farms indicate that these outbreaks seem to correlate to use of pelleted Magnesium Oxide being fed in palm kernel based feeds. Although one of the farmers we interviewed, reported some salmonella before feeding the PKE containing the coarsely ground magnesium oxide, he reported that after introducing the feed the disease took off. On discussions with some in the feed industry it would seem that coarsely ground magnesium supplements may have been widely used throughout the feed mill industry over the past 2 seasons. This magnesium is a little different to that used in the past and the pelleted magnesium associated with the outbreaks of salmonella reported in Taranaki. Prior to the last two seasons this coarsely ground form of magnesium oxide was formerly only sold into the fertiliser market and has large particle sizing, ranging from 2mm – 5mm. We believe the same risk factors are involved as those reported in the Taranaki outbreak of 2011 at that time associated to a prilled magnesium oxide product used in those feeds.
A case controlled study was performed in 2011 – 2012 to identify herd-level risk factors for acute Salmonellosis. It is our belief that the most recent outbreak has many of the same issues in common with the earlier outbreak. This summary, identified a number of risk factors that contributed to this sudden outbreak of the infection across a large number of herds, most particularly highlighted was the strong connection between feeding granulated magnesium supplements and prevalence of the disease.
CONTRIBUTING FACTORS IDENTIFIED IN THIS REPORT WERE AS FOLLOWS:
1) Continuous feed troughs – consumption of supplementary feed from shared troughs varies amongst individual cows where dominant cows will consume more than their allotted daily feed allowance, with submissive cows consuming less. Fluctuations in feed intake could affect the rumen microflora balance, allowing salmonella to multiply and cause infection. Reduce risk by using individual troughs.
2) Use of Palm Kernel – a possible vehicle by which salmonella organisms are introduced onto farms. Appropriate storage and handling of supplementary feeds is recommended to reduce contamination from pests.
3) By far the single strongest factor reported for the spread of the disease into these herds was the use of the pelletised magnesium supplement. One of the reasons theorised being that pelletised magnesium was known to be poorly soluble in the digestive tract thus influencing the growth of salmonella in the rumen as it had an alkalinising effect on it. Should the pH in the rumen increase, salmonella growth increases more vigorously. In the study there was no significant association between risk of salmonella and the use of magnesium in the form of powder, or magnesium dosed into drinking water.
COARSE GRADE MAGNESIUM OXIDE
Magnesium has been clearly identified as a mineral ion that acts to potentiate the spread and development of salmonella in all mammals. In a ruminant the bacteria could be expected to multiply in the favourable pH of the rumen environment, however, once it reached the acid environment within the abomasum the bacteria would normally be destroyed by the acidity. We speculate that the use of large magnesium granules allows the bacteria a suitably hospitable alkaline environment, allowing the spread of the disease into the intestine.
We would highly recommend that should farmers be supplementing magnesium into feeds that they use only magnesium oxide with a fine particle sizing of at least 200 mesh. This mesh size has been used for many years in the supplementation for magnesium in ruminants with no complicating issues. Using a micronized form will ensure that the magnesium is fully absorbed within the rumen, as well as delivering magnesium more efficiently. The fine particle sizing will ensure that bacteria should be unable to attach to the particle and should not therefore survive the acidic environment of the abomasum. Other acidic forms of magnesium such as magnesium sulphate and chloride, being acidic should offer little risk.