Larval cultures

It is important to know which types of worms are present in worm egg count reduction tests, as different worms typically show different resistance levels to drenches and this will guide your decision on which drenches to use.

The eggs of many roundworm species look alike (Figure 1). For this reason WEC tests usually report on the number of ‘strongyle’ type eggs per gram of faeces (epg) present in the sample. Strongyle type eggs include small brown stomach worm (Ostertagia ostertagi), barber’s pole worm (Haemonchus placei), stomach hair worm (Trichostrongylus axei), small intestinal worm (Cooperia species), hookworm (Bunostomum phlebotomum) and large-mouthed bowel worm (Chabertia ovina). The type of worm cannot be determined from the egg stage, but can be identified when they hatch into larvae (click here to see a roundworm life cycle), through a process called ‘larval culture and differentiation’. This laboratory technique mimics conditions suitable for eggs to hatch as they would on pasture. If the WEC is at or above a target number, faeces are cultured to hatch the eggs and the resultant larvae are identified and counted.

The culturing process involves mixing faeces after the faecal worm egg count and incubating them in jars held at around 25°C for a week. During incubation, the eggs hatch and release first stage larvae, which develop to the third infective stage in the jar (in a paddock the third stage would migrate out of the faeces and onto vegetation for ingestion by grazing cattle, see the stomach worm life cycle). As larvae develop, they can just be seen with the naked eye on the walls of the jar, squirming vigorously in condensation droplets.

The larvae are washed from the jar, collected and concentrated, and a very small drop of iodine is added to straighten, kill and stain the larvae to highlight diagnostic features. Identifying the larvae of different worm types is based on their dimensions and morphological features and is a highly skilled job.

Figure 1. Example of a strongyle-type worm egg.

The laboratory report

The larval culture report displays the result as a percentage. For example, 90% of the larvae present were small intestinal worm (Cooperia species) and the remaining 10% were small brown stomach worm (Ostertagia ostertagi). If the average worm egg count for the mob was 500 epg, the result would be given as 450 epg of small intestinal worm and 50 epg of small brown stomach worm.

Larval cultures provide a good indication of the proportion of each species or group present but are not completely accurate as different worms have different culture requirements.

The testing laboratory will provide an interpretation of the WEC and larval culture results.

By knowing the types of worms present in stock, producers are able to make more informed decisions about the need for a drench and which chemical actives to use. Different worm types vary in their impact on animal welfare, and their economic effect on cattle production.

An alternative to a larval culture to identify the worm species present in faecal samples is to use a DNA test.