Further tests

Standard FEC technique

Realistically you would not perform an egg count from one individual animal and make critical farm management decisions based on the results from that animal. A producer would typically collect ten individual samples from each mob of sheep and submit them for analysis. A faecal egg count would be performed using the technique described above though each sample analysed would be a composite of ten 1g sub-samples.

However, individual counts are important for stud properties and when specific animals are sick. For samples from individuals, 3g of faeces is mixed with 42ml of water (dilution factor = 15).

Larval cultures

Cultures can be helpful in determining the proportion of worm species present in a faeces sample. Faeces are mixed with vermiculite (to aerate the sample) then placed in a plastic container with a small amount of water to maintain appropriate moisture and humidity. Samples are then incubated at ~20oC for 7-14 days in darkness. Containers are then filled with warm water and inverted onto large petri dishes to allow all larvae to wriggle out of the faeces. The water, which contains the larvae, is then collected and larvae concentrated by sedimentation. Using a microscope, it is then possible to identify larvae by species. At least 100 larvae would need to be identified to provide an accurate measure of species proportion. These proportions are not entirely accurate but are useful as a guide.

Different species hatch at different rates and have different preferred conditions. At 20oC, a 7-10 day incubation period is ideal for Ostertagia and Trichostrongylus whereas a 14-day period is required for Haemonchus. Low temperatures affect species differently also. Ostertagia and Trichostrongylus can tolerate short periods at 4oC though Haemonchus cannot. Thus, if interested in locating Haemonchus it is critical that faecal samples are not refrigerated at any time before or during culturing.

Larvae are identified by killing them with a drop of formalin followed by heat treatment. This ensures they remain straight and their internal structures are not degraded. Identification is based on larval length, internal organ structure, head shape, and tail length and shape (Van Wyk, Cabaret & Michael 2004).

Total worm counts

Total worm counts are the most accurate method to identify the number and proportion of each worm species present. Details of the techniques of performing washouts and worm identification can be found in the literature (Anderson 1972). Worms are preserved with a few drops of formalin and identified using a microscope. Worms may be broadly identified by their size and then a more precise identification is performed based on the size and shape of the vulval flap of females and the bursa and spicules of males.

Further information

Niven, Anderson & Vizard (2002) describes the techniques of ‘smart grazing’ where intensive grazing is used along with strategic summer drenching to create parasitologically ‘safe’ paddocks for weaners to graze in autumn and winter.

References

Anderson, N 1972, 'Trichostrongylid infections of sheep in a winter rainfall region 1. Epizootiological studies in the Western District of Victoria 1966-67.' Australian Journal of Agricultural Research, vol. 24, pp. 599-611.

Niven, P, Anderson, N & Vizard, A 2002, 'The integration of grazing management and summer treatments for the control of trichostrongylid infections in merino weaners.' Australian Veterinary Journal, vol. 80, pp. 559-66.

Van Wyk, JA, Cabaret, J & Michael, LM 2004, 'Morphological identification of nematode larvae of small ruminants and cattle simplified.' Veterinary Parasitology, vol. 119, pp. 277-306.