Pneumonia - the killer disease
Pneumonia – known as viral pneumonia by farmers – is present in all parts of New Zealand. However, it is the moist, humid areas, particularly in the north where this disease is most devastating. Often losses of 5% will occur over the February/April period. Losses of 10% are not uncommon and it is reported in the veterinary publication “Sheep – Health, Disease and Production” that in one case, 600 lambs were lost out of 2,850 or 21%. In these worst affected areas, up to 95% of lambs will show some lung damage when these lambs are killed in autumn. In most cases, lambs, in time, will make a full recovery, although I am doubtful whether ewes retained in the flock ever reach their true genetic potential. In areas least affected by the disease, there may be a pause in growth rates and only the odd case of lung damage.
The reason for bringing this disease to the attention of farmers is that there will be new research initiatives over the next several years. Dr Kathryn McRae of Invermay (near Dunedin) has already investigated the incidence of this disease at meat processing plants in the Hawkes Bay. The exact focus and direction of this research has yet to be determined. However, the NZ Farmer will endeavor to keep farmers informed.
This disease, known as Enzootic Pneumonia or Acute Fibrinous Pneumonia in scientific and veterinary establishments, has been the subject of considerable research, particularly over the 1970 to 1990 period. Dr Hugh Davies spent many years, both at Wallaceville and in Northland attempting to produce an effective vaccine, but did not succeed due to the complexity of the disease. From memory of discussions I had with Dr Davies about 30 years ago, he informed me that pneumonia was not the real problem. It was the following secondary infections that were the major factors causing illness and deaths. Dr Davies added he had found nine different organisms that could be responsible for these secondary infections, the most common being pasturella.
Some of the most meaningful research into the causes of pneumonia was carried out by Mr Reg Keogh of Grasslands Research Centre (Palmerston North) in the early 2000’s. This research in lower Northland involved a trial of lambs on ten farms. On each property, 50 lambs were grazed on typical rye dominant pastures and 50 on different plant species such as chicory, plantain, clover and brassica crops. Reg recorded, on a regular basis, the lambs’ live weight and temperature. He also recorded both day and night temperatures and humidity. Within a short time, the difference between the two mobs became very noticeable. Those on the traditional rye pasture were lethargic, difficult to move and most showed typical signs of having pneumonia. In contrast, those grazing the alternate pasture were lively and rapidly gaining weight with no outward signs of pneumonia. However, when the lungs of all the lambs were examined at slaughter in April, the healthy lambs on the alternate feed species had only about 2% less lung damage compared to those grazed on rye dominant pasture (the average lung damage of all lambs was about 95%).
My take on this surprising result was that the lambs on the rye dominant pasture were ingesting a “cocktail of toxins” produced by many varieties of fungi that thrive in these pastures. It is my belief that these toxins have an adverse effect on the immune system to a point where the secondary infections following pneumonia are not contained. In contrast, with none of these toxins present in alternate plant species, plus the fact that these plants provided a higher plane of nutrition, an enhanced immune system was able to contain the diseases more successfully.
At the end of the trial, Mr Keogh concluded that night time humidity was the biggest contributing factor in the causation of the severity of pneumonia. This environmental factor provided ideal conditions for the development of toxin producing fungi which, in turn, impact on animal health.
However, there is some very good news. I have found that over the last ten years, the incidence and severity of pneumonia has deceased to a point where it has only been a minor problem over the last couple of seasons.
About 25 years ago, I recorded the sires of all the lambs that died of pneumonia and also those that were treated with antibiotics. I then put these recorded lambs into their sire groups. The sires showed a substantial difference in the numbers of lambs that died or were treated. This would indicate genetic factors were involved. More recently in a meat trial at Lincoln some sires had more lambs with pleurisy than others. In most cases pleurisy is the end result of pneumonia.
So why has the severity of this disease decreased over recent years? I suspect that the main reason would be that we have been unconsciously selecting for resistance to pneumonia. After all, the most susceptible die, the next most susceptible group are “walking skeletons” and will not be selected for the breeding flock. The most likely ewes to be retained are those that were least affected by the pneumonia challenge in the previous autumn.
There may however be another contributing factor in the decreased incidence and severity of pneumonia that I have experienced over recent years. Could strengthening the immune system by breeding for disease and parasite resistance be a factor? Several years ago, Dr Hickford at Lincoln established that sheep which were most susceptible to food diseases (those with DNA foot scores in the 4’s and 5’s) were also more susceptible to internal parasites. This does not surprise me, as the immune system is the determining factor in the health of all animals – including humans.
There may however be another contributing factor in the decreased incidence and severity of pneumonia that I have experienced over recent years. Could strengthening the immune system by breeding for disease and parasite resistance as I have over the years, be a factor too? Several years ago, Dr Hickford at Lincoln University suggested that sheep that were more susceptible to foot diseases (those with DNA foot scores in the 4’s and 5’s) may also be more susceptible to internal parasites and vice-versa. Whether this is a genetic effect or simply that those sheep that get footrot are more run-down and thus more susceptible to parasites needs to be ascertained. Accordingly sheep may also be less susceptible to pneumonia if less susceptible to other diseases, but this too needs to be established. If there was a link, it wouldn’t surprise me, as the immune system is a determining factor in the health of all animals – including humans.
In my own stud flock over recent years only ram lambs that show no signs on pneumonia will be used as stud sires. Like all other disease and parasite problems, I believe that there are genetic factors involved, all linked to the immune system.
So what needs to happen? Those conducting new research on pneumonia need to be aware of all earlier research, and then perhaps tackle the pneumonia problem from a different angle.