Early bird catches the worms

Worm management strategies help reduce the cost to lambs of parasites, Tom Ward writes.

Intestinal worms are a significant cost to New Zealand sheep farming. They reduce productivity, are a cost in terms of control, and there is increasing drench resistance.

If the grazing animal ingests sufficient larvae, production declines, caused by suppressed appetite and changes in grazing behaviour. In addition, the animal’s immune response requires energy and protein.

Major clinical signs are weight loss, scouring and dehydration. Inflammation from physical damage can cause secondary bacterial infection and ulcers. In the case of “Barbers Pole” (Haemonchus) blood loss can result.

Age affects immunity

Young animals are very susceptible because they have no immunity and are actively growing. Most sheep have full immunity by 18-20 months of age and continual exposure to worms is thought to be important for developing this immunity. Minimising young animals’ exposure to worms, and minimising stress is important. Adult animals generally cope with worms, however periods of stress like lambing could be exceptions.

A variety of worm types affect sheep in NZ with most being roundworms (nematodes) living in the gut (stomach and/or intestine). Lungworms are also roundworms, but live in the lung and are of lesser importance in sheep. Flukes and tapeworms are also common.

The most common roundworms (worms) affecting sheep and living in the gut are Haemonchus Contortus (Barbers Pole), Ostertagia Circumcinta/Teladorsagia, and Trichostrongylus Axei/Colubriformis, all of which live in the stomach; and Trichostrongylus Vitrinus, T Colubriformis, Nematodirus Filicollis and N Spathiger, which live in the small intestine.

The life cycle is basically eggs passing out of the gut into pasture, developing in dung, maturing to infective larvae stage and then being ingested by livestock. Their numbers are affected by weather with warm, moist conditions speeding development (generally spring and autumn) which takes 21-28 days from ingestion to eggs appearing in dung.

At the peak there can be 85-95% of total worms in the pasture, the rest in the dung and gut. Most will be in the first two centimetres of grass and 1cm of soil. So intensive (close) grazing promotes greater infection.

Different species generally, but not always, infect either sheep or cattle.

For larvae, 20-25C is optimum in the paddock and below 10C few survive. Some do overwinter, however, creating the start of next season’s pasture contamination.

Infective larvae on pasture eventually die; in cooler climes they can survive for eight-12 months, when warmer only two or three months. The longer the grazing spell, the fewer larvae there will be to reinfect animals. More open pasture is less conducive to larvae and dung distribution affects reinfection.

At some points a worm’s development can be stopped (inhibited). Larvae development will not restart until a stimulus is received – for example L3 won’t go to L4 until larvae are eaten by a host.

Another example of inhibited development is a larva in a host sheep may become temporarily ”stalled”, perhaps caused by lowering autumn temps. They typically resume development after a few months, closer to spring, when the eggs would find conditions on pasture more amenable.

Animals with immunity can also arrest the larvae development, or stress on the animal start up development. Generally, this inhibiting has no effect on the animal as the “waking up” happens sequentially not en-masse, however the farmer may be misled as to what is really happening with worms on his farm.

As stated earlier, it is commonly 21 days from ingestion to eggs appearing in dung. This is known as the Prepatent period and is important for two reasons:

1. A 28-day drench period is commonly recommended.
2. Worm egg counts are a snapshot of the level of larvae challenge 21 days prior, which may suggest you are free of worms, when the animals could have picked up a considerable burden.

Numbers of eggs in sheep are generally highest in the autumn, reflecting high pasture levels, and most pass through young sheep which don’t have immunity.

The exception is during periods of stress in adult sheep, i.e. pre lambing/early lactation (peripartum) when their resistance is down and high levels of eggs are passed out in mid lactation. Generally this is sorted by weaning, however by then the larvae are on pastures for lambs to ingest.

Ostertagia is the most common peripartum worm, although it declines in autumn. Peripartum is greater for two-tooth ewes and multiple bearing ewes.

Principles of worm management

The purpose of a worm management strategy is to: minimise contamination of pasture, minimise uptake of larvae, and to monitor success of programme. The main focus is on young animals and keeping drench resistance in mind.

There are many options so planning is very important. You must know what is happening with worms onfarm, the key tools being faecal egg counts (FECs) and faecal larval cultures (to identify the species of worm). Continual monitoring is required of production such as liveweight gain (LWG) to gauge the effect of worms. The situation will be dynamic and should be reviewed regularly.

Worm management strategies could include: well-thought drenching strategies, minimising stress on animals, ensuring animals are well fed, manipulation of pasture and stock management to reduce exposure to worms, and breeding resistant/resilient animals.

A group of farmers, including Robert Peacock, at Orari Gorge Station, South Canterbury, have done some excellent work breeding sheep which require low animal health remedies.

A group of lambs, drenched once at weaning, required no more drenching before being processed in May. The experiment involved grazing to different pasture residuals, and using red clover, which is hard for worms to climb, as lamb feed.

It is important to consider where the worms are, how many there are, what species the worms are, whether they can be removed or reduced (spelling, use of resistant animals), what level of immunity respective animal mobs have, levels of nutrition, and stress.

If possible, utilise longer pastures, use tannin rich species, utilise hay/silage paddocks, fodder crops, and spell pasture for as long as possible (ideally more than three months which is difficult to achieve).

Graze young animals first, alternating grazing between species/class/age, and utilise immune stock. Avoid putting lambs on lambing or weaning blocks. Aim for high growth rates, i.e: minimise the amount of time lambs are onfarm.

Acknowledgement to Wormwise for help preparing this article.

• Tom Ward is an Ashburton-based Farm Management Consultant (027 855 7799)