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Larval cultures and "Strongyle" type eggs

Sara Bowers, GYST, preparing a larval culture.
Sara Bowers, GYST, preparing a larval culture.

by Maxine Murphy, ParaBoss News Editor

January 2018

WormTest reports on sheep, goats and cattle usually state the number of ‘strongyle’ type eggs per gram of faeces (epg) present in the submission to the laboratory, but why use such an unfamiliar term as ‘strongyle’ for roundworm eggs?

Roundworms of the gastrointestinal tract, including barber’s poleblack scourbrown stomachsmall intestinal wormhookwormlarge bowel worm and thin necked intestinal worm (Nematodirus), all belong to a group of nematodes (roundworms) called the Strongylida. Their eggs—except for those of Nematodirus—are similar in size and structure and cannot easily be identified to ‘type’ in a WormTest, only counted.

A typical strongyle (or more correctly, strongylid) type egg has a smooth surface, an ellipsoidal shaped shell and contains an embryo in the morula (cluster of cells) stage of development when passed out in the faeces. Fortunately, larvae released from these eggs can be identified to type in a process called the larval culture (or larval differentiation or worm typing). This technique provides conditions suitable for eggs to hatch and mimics what occurs on pasture.

Stongyle type egg
Stongyle type egg
Nematodirus egg. Source: Janssen Animal Health
Nematodirus egg. Source: Janssen Animal Health

Laboratory procedure

The WormTest is really a two-step procedure: firstly, worm eggs in a known amount of faeces are counted, and secondly, if the count 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 bulking (mixing together) of any faeces left over after the worm egg count has been processed. This is placed into a wide-mouth jar with, if necessary, a small amount of water to maintain a relative humidity of about 60–80%, and incubated in darkness at temperatures that range from 22°C to 27°C for 7–10 days. During this incubation period, the eggs hatch, releasing first stage larvae that are then grown through the second stage to the third infective stage (the stage that is also present on pastures).

When the culture jar is again exposed to daylight after the incubation period, larvae can just be seen with the naked eye squirming vigorously in condensation droplets on the walls of the jar, or, if the worm egg count was very high (e.g. from a heavy barber’s pole infection), cloudy tracts of millions of larvae can be seen moving in procession towards the top of the container. Once larvae are washed from the jar, collected and concentrated, a very small drop of Lugol’s iodine is added to straighten, kill and stain the larvae to highlight diagnostic features such as length of the tail sheath, number and shape of the gut cells, and shape of the head region. One hundred larvae are counted and typed under a microscope.

The laboratory report

The larval culture report will show the result as a percentage. For example, 90% of the larvae present were barber’s pole worm and the remaining 10% were black scour worm. If the average worm egg count for the mob was 500 epg, 450 epg of those eggs would have been barber’s pole worm and 50 epg black scour worm.

Larval cultures are not definitive, but rather a very good indication of the proportion of each species or group present because the various nematodes have different optimum conditions for hatching, development and survival—some, such as barber’s pole, produce large numbers of eggs and tend to dominate the culture, if present. This also explains why treatment is indicated at different egg count levels for different types of worms e.g. treatment is recommended when the worm egg count reaches 800 epg for barber’s pole worm, which is a prolific egg-layer versus 300 epg for brown stomach worm.

Your testing laboratory will provide interpretation of your WormTest results, but also check the Drench Decision Guide for your WormBoss region.

By knowing the types of worms present in stock, producers are able to make more informed decisions about when to drench and what type of drench to use, especially if they have DrenchTest results that indicate the efficacy of different drench groups against different worms. It is also necessary to know the type of worms present in stock for drench resistance calculations.