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This article was published in 1980
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Control of Resistant Nematodes
C.A. Hall, M.R.C.V.S. Glenfield Research Station
Several systems for the control of gastro-intestinal nematodes in grazing livestock have been recommended and there is a wide variation in the practices adopted by graziers, particularly in the frequency of drenching. They can be divided into those depending on anthelmintics alone and those integrating anthelmintics with grazing management.
Those using anthelmintics adopt either the:-
Curative approach - treating symptoms as they appear.
Strategic or Preventative approach - treating particular classes of stock at specific times based on epidemiological knowledge.
Suppressive approach - treating young stock at monthly intervals irrespective of the parasite burden.
In the C.A. the number of worms in the host and larvae on pasture are usually high at the time of treatment. The change made in the population of larvae from the eggs of the survivors after treatment is therefore small. Reinfection is unlikely to be affected whilst the population of susceptible larvae on pasture remains high. It is less likely to select quickly for resistance.
In the P.A., with even fewer treatments than in the C.A. there can be a strong selection towards resistance. Anderson, recommended in winter rainfall areas to give 2 summer drenches when conditions for helminth transmission are least favourable, aiming to reduce pasture contamination and worm burdens in autumn and winter. However, a large proportion of the larvae picked up at this time will be the progeny from eggs of the adults surviving treatment, than if the same number of treatments had been given later during times of high transmission.
Le Jambre has pointed out that if sheep were drenched and moved to a pasture containing few larvae, a high proportion of worms in the next generation are likely to be the progeny of those surviving treatment. Using 3 or 4 treatments/year the selection could be as rapid as with the S.A.
In general, the more effective the control of parasites with dependence on anthelmintics the greater the potential for developing resistance. Use of pasture management and less dependence on treatment is one of the most desirable methods for reducing the chance for resistance to develop.
Delaying Resistance
Use anthelmintics sparingly and augment with other methods of control.
If a narrow spectrum drench will suffice then its use will reduce the selection pressure to one of the broad spectrum anthelmintics.
If broad spectrum drenches are required then both the BZ and levamisole/morantel type drugs should be used. However, Le Jambre suggested that alternation within the life span of the adult within the host may hasten the development of resistance to both compounds. The ad hoc committee convened by the N.S.W. Stock Medicines Board suggest rotation as a desirable policy e.g. 6 monthly for H. contortus and 12-15 months for Trichostrongylus and Ostertagia spp.
Control in the Presence of Resistance
After resistance is confirmed use of that compound and related drugs should not be used in regular drenching programmes. Another anthelmintic belonging to the same group, may at first, give reasonable results, but evidence has been produced indicating a level of side resistance after few generations of selection.
When resistance precludes the use of say the BZ's then the levamisole/morantel group will need to be used for broad spectrum activity. During this period some recession will occur towards the BZ's enabling them to be re-introduced for a limited period i.e. in a slow rotation. However, the effectiveness of the re-introduced compound should be monitored.
Stock owners should show some responsibility and limit the spread of parasites showing multiple resistance, should it occur.
If the time comes when none of the available anthelmintics are of any use, it may be necessary to consider using PTZ carbon tetrachloride, copper/arsenic etc.
There is no hope that resistance will disappear and like insecticidal resistance the problem will be present forevermore and will need considering even if new anthelmintics are produced which initially affect the result obtained in the field.