BY: TREVOR COOK
January is my catch-up month and includes attacking accumulated documents. It’s a slow process because some are forgotten gems, like a page of notes from a workshop I attended in 2008 about enzootic (viral) pneumonia. This is a lamb disease I have commented on for years because it’s a major cost to the industry yet flies under the radar. Back in 2008 it was reported to cost the industry $53 million, or $1.26 to $3.07 per lamb. How that figure was arrived at I cannot understand, but I don’t dispute it. It’s a difficult disease to monitor and our standard indicator is the incidence of pleurisy in killed lambs. Pleurisy reported on killing sheets is the remnants of an active pneumonia, in effect the scar tissue left after pneumonia heals. Unless we know how many repaired pneumonias end up with pleurisy, the incidence of pleurisy on kill sheets is only an indicator.
Often deaths are the expression of an outbreak, but lowered liveweight gain is by far the biggest cost of this disease. The workshop said that lamb growth rates could be reduced by 50%. This reduced weight gain often occurs when other factors that lower weight gain are present. Low pasture quality and worm challenges are two frequent ones. Tail-end lambs are typically an outcome. Enzootic pneumonia is a major factor limiting lamb growth rates throughout New Zealand, especially in the North Island.
The workshop presented the usual factors such as yarding, large mobs, heat, dust, and mouth-breathing. Managing these factors will lower the incidence but hardly reduce it enough in my experience. But what does? It is reported that shearing lambs at weaning increases the incidence sevenfold. Farms that rear their own replacements have higher levels. But in reality it requires something more.
Even growth rates
A feature of the Massey University work looking at lamb growth rates on herbs was how even the growth rates were. The absence of a tail end. Was this because that feed had a protective effect, or feed factors that act as triggers were absent? Other work done in Northland showed that feed factors can be a major trigger. The endophyte toxins in old rye grasses in particular. The improved weight gains we see in lambs on a variety of summer crops will not just be because of higher feed quality, in fact some of those summer brassicas will be only marginally better than pasture. But on those crops a combination of low worm challenge and low pneumonia triggers will be contributing.
There has not been enough focus on spring and the contribution of the ewe to the pneumonia problem. The ewe is the reservoir of the bugs, and by weaning the bugs have been transferred to the lambs. Not necessarily from their mothers but from the mob. We see more pneumonia in lambs following a tough spring. Is that because the ewe transmits more bugs, or provides less protection in some way? Does yarding ewes and lambs a month before weaning increase that transfer? There is mounting evidence, albeit onfarm, of weaning age being a factor. It seems early weaning results in less pneumonia. Is the problem confined to lambs from young mums? In the porcine industry the pneumonia is mostly confined to piglets from gilts. Is that a reason why the lambs from ewe hoggets so often struggle? I have no knowledge of any survey looking at dam age and pneumonia. Is there more pneumonia in twin reared lambs compared with single reared? My hunch is that early flocks have overall healthier lambs than late flocks. Knowing some of these possible relationships could give an insight to how this disease works. Managing the bug once it is in the lamb has proven to be unrewarding.
Study needed
An interesting observation I picked up from sheep breeders who have been selecting for worm resistance for a long time is they get fewer wasting ewes and less pleurisy showing up in killed lambs. A common cause of wasting ewes, especially in the North Island, is chronic pneumonia stemming from when they were a lamb. As a lamb they did not totally get on top of the pneumonia and an active bit was walled off and remained. At some stage, usually mid-life, that walled off infection breaks out and invades the lung and beyond. In selecting for worm resistance, is the immune system boost that occurs also giving some protection in the lung?
There is so much we don’t know. The last substantive study into pneumonia in sheep was in 2005 and included a survey to try to identify risk factors and patterns. It seems some basic study could reveal a lot, as well as the high level DNA work which is being done.
