Flock and Herd logo


This article was published in 1998
See the original document

An Observational Study On Sheep Internal Parasites In The Narrabri District

SM Slattery, Narrabri Rural Lands Protection Board


This project was developed and undertaken in direct response to inquiries from local sheep graziers which could not be answered competently with the then knowledge of sheep internal parasites in the Narrabri District. Further, there was no regional strategic plan, that is, easy to follow drench program, which could be extended to producers.

The ultimate aims of this project were to gain knowledge that would enable local producers to more effectively and efficiently control sheep internal parasites and to develop a strategic drench program for the Narrabri district.

Local Sheep Industry

The local sheep industry is very mixed as well as being interspersed with cropping and cattle producers who make up the majority of ratepayers in the Narrabri Board district. There are four broad groups of sheep producers;

1. Merino wether weaner producers. These are mostly in the western Board area with a spring lambing, selling wether weaners as stores around 10/11 months.
2. First cross ewe producers. Purchased merino ewes joined to Border Leicester rams. The male lambs are sold as fat or store lambs with the female portion kept to near 12 months for sale as maiden first cross ewes. Autumn lambing.
3. Self replacing merino wool producers. These are mostly found on the lighter Pilliga soils and are winter/spring lambers.
4. Lamb producers. These are mostly found in the eastern districts of the Board in conjunction with cropping. The majority run large framed merino ewes crossed with Dorset rams. Lambing occurs at a variety of times but spring is favoured.

Traditional control of internal parasites has been based around drenching with Levamisole or BZ drench three to four times a year. That is, drenching when sheep are yarded for other management procedures or when clinical signs of haemonchosis are noted.

Haemonchus is traditionally the major worm of concern to graziers with clinical signs of sub-mandibular oedema and pale membranes seen two years in three in traditional management systems. Most sheep graziers drench to prevent Haemonchus "crashes" where substantial numbers of sheep die quickly. Most graziers have had first hand experience with this syndrome, and in the flat western areas major sheep losses during floods have been recorded.

Clinical haemonchosis usually starts to appear in autumn but can be seen as early as late December. It is not uncommon for flocks, especially those on lighter soils, to experience sporadic deaths and illthrift through to June, no doubt due to heavy autumn pasture contamination and decreasing nutrition.

Local graziers have very limited knowledge of the epidemiology of sheep parasites and the advantages and disadvantages of various drench groups. Not only was traditional drench usage often found to be ineffective in preventing deaths and losses, it was also encouraging the development of anthelmintic resistance. Drench tests have indicated that most properties have resistance to their "favourite" broad spectrum drench.

Pasture management is minimal in the western pastoral area being set stocked at 2.5 DSE per hectare.

In the eastern areas lambs are a secondary operation to cropping, and are grazed on stubble or lucerne cropping rotations. The sheep are shifted from paddock to paddock, with the purpose being maximising cropping returns and lamb growth rather than internal parasite control.

Other Sections of the Project

This observational study was undertaken as part of a Masters of Rural Science through New England University. The other important section of the project (which is not covered in this paper) is to review and examine field experience and monitoring data in the Narrabri district.

Study Design

One hundred merino ewe hoggets were grazed on forty acres of Travelling Stock Route (this corresponds with the traditional stocking rate of one hogget per acre).

Previous to this, the paddock had been only grazed by cattle for a number of years. The project site consists of heavy black self mulching soils, growing a variety of forbs in summer and medics in winter between clumps of perennial grasses. Grass predominance varies with runs of wet/dry seasons from almost nil to 90% cover.

Of this group of a hundred hoggets, twenty were tagged and monitored by faecal egg counts and culture every two weeks.

Tracer sheep consisting of lambs or hoggets donated by a variety of local graziers were drenched with a Levamisole/BZ combination (Salvo® Hoechst) and grazed for two weeks in the paddock. They were then placed in off the ground pens for a further two weeks after which they were slaughtered for total worm counts. A faecal egg count at entry to pens confirmed effectiveness of drench. It is recognised that the variations in levels of immunity of the tracer sheep is a major complicating factor. However, this is part of the realities of running a research trial on a limited budget.

Every two weeks, five pasture plots at the site were examined for pasture composition, height, percentage dry and coverage. Records were kept of rainfall and number of rain days. After grazing for one year the hoggets returned to their home property and were replaced by the corresponding year younger group of merino hogget ewes. In November 1996, whilst the author was on leave, faecal egg counts in the hoggets rose quite dramatically. In response to this the staff responsible took the prudent action of drenching the entire mob with a BZ/Levamisole combination (Salvo® Hoechst).

The faecal egg counts, cultures and total egg counts were performed at NSW Agriculture's Armidale Regional Veterinary Laboratory until its closure after which the work was performed at the Menangle Regional Veterinary Laboratory.

Conditions over the 24 months of the trial were conducive to sheep internal parasites with cases of haemonchosis occurring throughout the Narrabri RLPB in both autumn/summer periods and production limiting Trichostrongylosis also being diagnosed.

The average annual rainfall at the project site is 500 mm. Rainfall has a substantial peak in January and a smaller one in June. Further, rainfall in any month is extremely variable with no real season.

In qualitative terms, spring 1996 started very dry and was followed by a very wet summer, drying out through March-April. This was followed by a wet winter. Spring 1997 was again dryish and that summer very wet. Autumn and winter 1997 was dry.


See graphs.

The only internal parasites that appeared significantly in the faecal egg counts were Haemonchus and Trichostrongylus. Although a number of other species commonly appeared in significant numbers in tracer sheep, notably Ostertagia and Nematodirus, this larval pick-up failed to translate into sustained infections.

Total worm counts confirmed that the Trichostrongylus eggs involved were small intestinal species.

Graphs were qualitatively examined for relationships between FEC and larval pick-up and the following factors; pasture composition, height, percentage dry, coverage, rainfall in a two week period and rain days in a two week period.

The only apparent relationships were between FEC/larval pick-up and time of year and rainfall.



  1. There was little Haemonchus larval pick-up over December and January of both summers despite good rainfall over this period.
  2. Spring and autumn were periods of rise in faecal egg counts for Haemonchus.
  3. Haemonchus faecal egg counts peaked in autumn.
  4. From the above though there is little larval pick-up over December and January, Haemonchus eggs can hatch and larvae develop in this period and/or larvae present prior to this period can easily survive across it.
  5. The sudden autumn rise in faecal egg counts and larval pick-up in both years, indicates that if only one of these factors is operating it is very likely to be the former.
  6. Haemonchus faecal egg counts decrease from autumn on into winter. In moderate worm years FECs decrease to nothing. However in high worm years substantial burdens are carried through winter.
  7. There is a suggestion that the size of the spring rise is strongly related to the degree of previous autumn/winter contamination.
  8. The size of autumn rise is strongly related to size spring rise.
  9. Little rain is required in autumn to have a rise in faecal egg counts.
  10. A single short acting broad spectrum in spring has little effect on decreasing pasture contamination and level of autumn rise.
  11. Hugh McL.Gordon's classic "self cure" phenomenon with dramatic reduction of faecal egg counts following rainfall episode to be followed by a dramatic rise was seen.

While rainfall plays a role, the crucial factor in the size of a spring or autumn rise is the amount of contamination due to the size of the relevant previous autumn or spring rise.


  1. Possibly production affecting Trichostrongylus faecal egg counts can be present in weaners and hoggets.
  2. Trichostrongylus can be present all through the year.
  3. Trichostrongylus faecal egg counts decrease as young sheep gain immunity with age, to the extent there are negligible levels by the time of their second autumn (19 months).
  4. Trichostrongylus faecal egg counts at any time of year are a result of the level of immunity, immediately recent mob Trichostrongylus faecal egg counts and recent rain.
  5. A single effective broad spectrum drench of hoggets in November resulted in a dramatic, near absolute suppression of Trichostrongylus over the following summer and autumn.
  6. From above there is a suggestion that pasture survival of Trichostrongylus larvae is short term.

Trichostrongylus are a problem in young sheep but strong immunity exists by 14 months. The crucial factors in Trichostrongylus levels are recent faecal egg counts, recent rainfall and age.

An ancillary finding of this study was the faecal egg count reduction testing of a number of donated tracer sheep from a wide variety of properties. In all cases there was no detection of resistance to Levamisole/BZ combination drenches. This suggests such resistance is not prevalent in the Narrabri district.

Extension of Information Gained

During the observational study two field days were held where the interim findings and suggested parasite control programs were extended to local graziers. At the second of these field days, a suggested strategic drenching program for the Narrabri district 'Narrabri Worm' was presented.

This program revolves around the use of closantel in all sheep in spring to suppress Haemonchus spring rise, relying on the harsh environmental conditions of December and January to further ensure that the autumn rise is negligible. In addition to this, at the spring drench, weaner or hogget sheep are given an effective broad spectrum to control Trichostrongylus. To give the program flexibility, monitoring is undertaken in late February to assess the need for additional closantel and/or broad spectrum drenches.

This program was designed with the following factors in mind;

a) It must work.

b) It should be hardy enough to allow drench and monitor times to vary by up to a month either side the ideal October closantel and late February monitor. This allows internal parasite control to slot in with other management procedures.

c) The program must he simple and easy to follow.

d) It must be adaptable to wet and dry seasons.

e) It should minimise the use of drench, thus reducing the development of resistance and costs. The use of broad spectrum in young sheep further reduces the likelihood of resistance.


This project was run on a shoe string budget, with many of the solutions to logistic problems demonstrating the adaptability and practicability of Board staff.

This project would not have been possible without the support of James Hunt, "Elmore" Wee Waa, various local graziers, the work of laboratory staff at Armidale RVL and Menangle RVL and the support, encouragement and work of the staff and Directors of the Narrabri RLPB.



Drench all sheep with closantel (Seponvere/Razar/0 Closicare®)

Drench lambs and hoggets with broad spectrum as well (Levamisole / White / Ivomec® type / Levamisole-White combination/Rametin®). (Closal® is a combination of closantel and white)


If above drench not at weaning, then at weaning closantel and broad spectrum to weaners.


Wormtest lambs and hoggets and decide on results

Either: 1. Nothing 2. Broad spectrum 3. Closantel 4. Broad spectrum and closantel


Wormtest late July


If resistance status of broadspectrum drench not known, Wormtest 10-15 days after drench to check all worms killed.

Always drench all introduced sheep with Cydectin® to prevent introducing resistant worms.

The Narrabri Worm Program is based on five years of experience and monitoring on properties in all districts of the Board.

This experience is backed up by a two year research project.



Barbers Pole is a summer worm starting from low numbers in October. As temperatures rise, these small numbers multiply and more importantly contaminate pastures for the whole summer. Problems in late summer and autumn are due to failure to stop this rise and contamination. Closantel is a Barbers Pole, bot and fluke only drench that works for four weeks. This long term protection blots out much of the summer rise and greatly prevents contamination. (MUST BE DONE EVERY YEAR)

SPRING BROAD SPECTRUM DRENCH vBlack Scour Worms cause unseen production losses in lambs and hoggets Spring born lambs and hoggets show rising burdens over spring causing production losses through to autumn. A broad spectrum drench in spring (at the same time as the closantel) cleans these worms out. Experience has shown that this works. Older sheep develop immunity and usually don't need drenching.


This gives the program it's flexibility to work in dry and wet years, in different areas and operations. In many years no further drench is needed. THIS WORMTEST IS ESSENTIAL TO THE PROGRAM.


In some seasons autumn lambs carry costly unseen Black Scour burdens over winter. A late July Wormtest solves this problem.


Drench tests carried out over the last few years show White and Clear drenches are just not working on most properties. To check a drench is working, Wormtest 10-15 days after using it. Resistance is due to over drenching and introducing resistant worms.

Don't Drench More Than The Suggested Program Unless Wormtests Indicate. Use A Cydectin® Quarantine Drench To Clean Out Resistant Worms From All Introduced Sheep.


Site contents and design Copyright 2006-19©