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This article was published in 1944
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
TREATMENT OF HAEMONCHOSIS UNDER OUTBREAK CONDITIONS
HUGH McL. GORDON. B.V.Sc., McMaster Animal Health Laboratory, Sydney.
Haemonchus contortus usually is regarded as easy to kill, and most anthelmintics are regarded as very effective against it. However, the following story is commonly heard from graziers who have had to deal with a severe outbreak. First drench A was used, then B, then C, then D, with poor results, and finally drench E was used, with success. Had the drenches been used in the reverse order drench A would have been effective simply because the outbreak was over. It is the drench used just before the end of the outbreak that receives the credit for cessation of mortalities and recovery of the sheep.
Consider what is happening to the worm population during an outbreak. The female worms are laying eggs at the rate of 5,000 each per day — and this goes on day in day out for months, irrespective of whether conditions on the pastures are favourable for the further development of the eggs. Under "average" weather conditions there is a tremendous mortality among eggs and larvae on the ground, but when "outbreak" weather occurs — that is, repeated rains with warm, dull, humid conditions — most of the eggs and larvae develop and survive and soon sheep are ingesting hundreds of larvae with their food. Even a lightly infested sheep can contribute considerable numbers of eggs.e.g., a sheep harbouring 200 H. contortus females will not show any clinical symptoms, but will pass 1,000,000 eggs every 24 hours —7,000,000 in a week. A heavily infested sheep with 2,000 females will pass 10,000,000 per day.
There is, therefore, a continuous output of eggs and a continuous intake of larvae during outbreak conditions. In "ordinary" times the continuous output goes on but the intake ceases or is intermittent. Under outbreak conditions sheep harbour worms of all ages from larvae one day old to adult worms perhaps several months old.
When the outbreak is well established there will be large numbers of worms in the sheep and soon the egg output will increase enormously.
What must an anthelmintic do inTREATMENT OF HAEMONCHOSIS UNDER OUTBREAK CONDITIONS
HUGH McL. GORDON. B.V.Sc., McMaster Animal Health Laboratory, Sydney.
Haemonchus contortus usually is regarded as easy to kill, and most anthelmintics are regarded as very effective against it. However, the following story is commonly heard from graziers who have had to deal with a severe outbreak. First drench A was used, then B, then C, then D, with poor results, and finally drench E was used, with success. Had the drenches been used in the reverse order drench A would have been effective simply because the outbreak was over. It is the drench used just before the end of the outbreak that receives the credit for cessation of mortalities and recovery of the sheep.
Consider what is happening to the worm population during an outbreak. The female worms are laying eggs at the rate of 5,000 each per day — and this goes on day in day out for months, irrespective of whether conditions on the pastures are favourable for the further development of the eggs. Under "average" weather conditions there is a tremendous mortality among eggs and larvae on the ground, but when "outbreak" weather occurs — that is, repeated rains with warm, dull, humid conditions — most of the eggs and larvae develop and survive and soon sheep are ingesting hundreds of larvae with their food. Even a lightly infested sheep can contribute considerable numbers of eggs.e.g., a sheep harbouring 200 H. contortus females will not show any clinical symptoms, but will pass 1,000,000 eggs every 24 hours —7,000,000 in a week. A heavily infested sheep with 2,000 females will pass 10,000,000 per day.
There is, therefore, a continuous output of eggs and a continuous intake of larvae during outbreak conditions. In "ordinary" times the continuous output goes on but the intake ceases or is intermittent. Under outbreak conditions sheep harbour worms of all ages from larvae one day old to adult worms perhaps several months old.
When the outbreak is well established there will be large numbers of worms in the sheep and soon the egg output will increase enormously.
What must an anthelmintic do in order to be successful under outbreak conditions? Firstly, it must be effective against immature as well as adult worms. Phenothiazine is very effective against im-mature worms, other drugs are only moderately effective, some are very ineffective. Copper sulphate and carbon tetrachloride are least effective. Mixtures of copper sulphate with sodium arsenite or nicotine are better, but do not approach phenothiazine. Secondly, an anthelmintic must kill a very high proportion of adult worms in a high proportion of sheep. Suppose that a sheep harbours 2,000 H. contortus females and a drench is 90 per cent. efficient—it will remove 1,800. leaving 200. "Wormathematics" show that this sheep still passes 1,000,000 eggs per day — quite sufficient to maintain the outbreak.
Treatment of HaemonehosIs Linder Outbreak Conditions-4 continued from previous pager. Drenches which depend upon copper sulphate to carry them into the abomasum fail on the average in 10 per cent. of sheep, so that even were such drenches 100 per cent. efficient in the !peep in which they were swallowed into the abomasum there still would be serious contamination of pastures with eggs from the sheep in which the oesophageal reflex did not respond. The greater the number of worms harboured by a sheep the greater Should be the efficiency of an anthelmintic if it is to bring about resolution of an outbreak. If a sheep harbours 200 H. conlortus females and a drug is 80 per cent, efficient. 40 worms will remain. If a sheep harbours 2.000 females and a drug is 80 per cent. efficient 400 will remain. The respective egg outputs are 200.000 and 2.000,000 per day. If the worm burden of the second sheep is to be reduced so that the egg output will be as low as that of the first sheep the drug must remove 98 per cent. of the worms — this is asking a great deal of any anthelmintic other than phenothiazine. The following table, which records the average egg output per sheep per day, will serve to give the general picture of what occurs.
TREATMENT OF HAEMONCHOSIS UNDER OUTBREAK CONDITIONS.
MEAN DAILY OUTPUT H. CONTORTUS EGGS PER SHEEP.
(100,000's)
| Faecal Examinations | 1. Phenothiazine A | 2. Phenothiazine B | 3. Copper Sulphate Sodium Arsenite | 4. Carbon Tetrachloride | 5. Copper Sulphate | 6. Controls |
|---|---|---|---|---|---|---|
| 6.7 | 8.0 | 10.4 | 7.2 | 10.1 | 9.1 | |
| 6.7 | 7.1 | 7.4 | 9.0 | 7.4 | 7.6 | |
| 17.2 | 16.8 | 12.9 | 15.3 | 10.8 | 10.0 | |
| 7th Day after Treatment | 0.003 | 0.007 | 25.5 | 15.7 | 18.1 | 35.6 |
| 13th Day after Treatment | 0.0003 | 0.009 | 16.5 | 10.6 | 19.8 | 26.5 |
| 16th Day after Treatment | 0.003 | 0.33 | 18.7 | 24.4 | 26.0 | 28.1 |
| 22nd Day after Treatment | 3.3 | 3.5 | 18.9 | 48.1 | 70.3 | 52.6 |
| 28th Day after Treatment | 12.6 | 20.9 | 29.1 | 69.7 | 78.5 | 69.3 |
GROUPS
1. Phenothiazine A. Treated, then moved to a paddock spelled for three weeks
2. Phenothiazine B. Treated, then run with groups 4, 5 and 6.
3. Copper Sulphate-Sodium Arsenite. Treated, then moved to a paddock spelled for three weeks. (Paddock separate from group 1).
4. Carbon Tetrachloride. Treated, then run with groups 2 and 6.
5. Copper Sulphate Treated, then run with groups 2 and 6.
6. Controls. Not treated, run with groups 2, 4 and 5.
These figures show, in no uncertain manner, that anthelmintics fail under outbreak conditions. The reasons for failures are now fairly clear:—
1. Insufficient adult worms are killed to reduce the egg output effectively.
2. Many immature worms escape.
3. With drugs depending on copper sulphate, treatment is quite ineffective in 10 per cent. of sheep.
The remedy may be discussed in terms of tactical measures, in which case the more effective steps, in order of decreasing value are:— in
1. Phenothiazine and move sheep to a spelled paddock.
2. Phenothiazine.
3. Other drugs and move sheep to a spelled paddock.
4. When copper tulphate either alone or mixed with arsenical compounds or nicotine is used, alternate with carbon tetrachloride; the action of which is independent of the oesophageal groove reflex.
In terms of a strategic remedy a drenching plus management plan must be developed with these points as a basis:—
1. Not to rely on drenching alone.
2. Use epidemiological findings to draw up seasonal plans.
3. Strategic drenching based on epidemiology.
4. Rotational grazing.
Elaboration of these points must await the future.