January 31, 2009

Drench resistant worms in sheep – a worm's life

Sheep, farming, husbandry, worms, internal parasites, health, definitions, genus & species, life cycles, signs of wormy sheep, effects of worms on sheep

By Dr Clive Dalton

What are ‘worms’?

‘Worms’ is a general term used for internal parasites that affect sheep. Plenty of other terms are used such as:
  • Roundworms (the main species that are round in cross section)
  • Nematodes (a general name for roundworms)
  • Cestodes (a general name for tapeworms)
  • Helminths (includes round worms, flukes and tape worms.
  • Internal parasites (live inside the animal which is their host).
  • Gastrointestinal worms (inhabit the sheep’s four stomachs and intestines

Worms in New Zealand

  • Apparently twenty nine species of internal parasites got a free ride to New Zealand with livestock but the most important ones in sheep ended up being the round worms - Haemonchus, Ostertagia, Trichostrongylus and Nematodirus.
  • All the roundworms are found throughout New Zealand but their relative abundance varies between districts, and with the age of the sheep and time of year.
  • Only sheep and goats share the same species of worms. Some sheep and goat worms may survive going through other species, e.g. cattle, but they are not viable after their journey.
  • All worms are classified by the Latin name of their genus and species. A few have common names but some of these are as ugly as the Latin names so they are not in common use.
  • The table below gives some examples. Within each genus there are many species only some of which cause problems. The subject can be complex when discussing any problems they cause. Often the genus is only used to describe the species adding to the confusion.

  • The abomasum is the 4th stomach where final gastric digestion takes place. In the previous three stomachs (rumen, reticulum and omasum) fermentation by bacteria and micro-organisms takes place.
  • When using the species name the first letter of the genus is used to save writing it out in full.
  • Key point: In the past, farmers didn’t need to worry about these names but things have changed, so use this table as a quick reference when talking to your vet or consultant. Hopefully you won’t ever have to write them down!

Can you see them?
Barber's Pole (Haemonchus) can be seen by eye once concentrated in a jar

  • You can’t see round worms with the naked eye when mixed in among stomach and gut contents.
  • Once the gut contents have been washed through a fine sieve and the worms collected and suspended in alcohol, then you can see them en masse. Some are so fine you can only see them when the specimen jar is shaken.
  • Haemonchus are easy to see in a specimen jar.
  • Tape worms sections are easy to see when passed out in the dung. These regularly panic farmers into drenching lambs.
  • Tapeworms are a very minor issue in stock health and plenty of research shows they can be ignored.
  • However, some farmers believe they are associated with other health problems in lambs such as pulpy kidney, flystrike and trace element deficiencies and so drench lambs as a protection.
The sections of tape worms are easy to see in faeces

Worm life cycles

Round worms:

  • Both sexes of the mature worms live inside the sheep (the host) where they mate and produce eggs.
  • These then pass out on to the pasture in the dung which protects them, and where they feed off bacteria.
  • The eggs hatch inside the faeces and start to develop into larvae. They go from the L1 to L2 stages, and then to the L3 stage which is the critical one, as it’s eaten by another sheep to start the cycle all over again.
  • The ecology of these larvae is not fully understood, but it’s known that L3 larvae can spend a long time in the ground. They can go 10-20cm down and move back to the surface when sufficient moisture is present. They are aquatic creatures.
  • Warm moist conditions encourage the L3 larvae to migrate up plant leaves in a film of moisture to be eaten by the new host.
  • Larvae need a minimum of 2-6 weeks of warm moist conditions to develop successfully.
  • It takes about 21 days for ingested larvae to develop into adults capable of laying eggs.
  • When developed, larvae survive longest in moist and cool (even frosty) conditions. Larval peaks occur in autumn (the autumn rise), with a more variable peak in spring (the spring rise).
  • Some larvae can live for 6-8 months in cooler temperatures (sometimes even longer) and in warmer conditions they will die after 2-3 months.
  • These facts have come as a shock to most of us who were taught that a 3-6 weeks grazing rotation would kill most of them. It won’t.
  • Key point: In New Zealand, 85-95% of the worm population is found as larvae on pasture, so only a small proportion is inside the animal at any one time.
  • Lambs are the real culprits by ingesting larvae and putting out large numbers of eggs, and this continues through to the hogget and early two-tooth stage.
  • Worms love lambs and the earlier they can infect them the better.

Liver flukes:
  • After ingestion the fluke larvae bore their way through the liver tissue causing damage which then affects liver function.
  • Mature flukes are hermaphrodites (having both male and female sex organs) and live in the bile ducts from where their eggs pass out.
  • These eggs need to go through a fresh water snail to hatch into larvae which are eaten by the sheep again.
  • They reach maturity in the lungs where they mate and lay eggs.
  • These hatch and L1 larvae move up in the mucus and are swallowed to pass out in faeces.
  • On the pasture they moult until the L3 infective stage and are eaten by the sheep off the pastures to complete the cycle.
  • Their heads attach by hooks to the small intestine wall, and with no mouth they absorb nutrients through their skin. The egg-rich (gravid) rectangular sections pass out in the faeces where the eggs after hatching (either inside or outside the sheep) are eaten by a pasture mite.
  • After 15-30 weeks they develop into larvae and are eaten again by a sheep. Lambs are most prone to tapeworms but seem to self-cure themselves by 8 months of age.
  • A large evacuation of tape worm sections in the faeces of lambs is certainly impressive. It’s easy to panic when seeing them, assuming they block the gut so must have a terrible effect on lamb growth.
  • The experience of some farmers shows that the very distinctive smell of lambs with tapeworms and their scour attracts blowfly and drenching avoids all this extra work.
What do worms feed on?
  • Roundworms live off the proteins in the mucus lining the gut wall.
  • Lungworms feed off similar mucus in the lungs.
  • Blood suckers (Haemonchus only) attach themselves to the gut wall with savage-looking mouth parts and feed on blood.
What do worms do?
They cause a range of problems, either singly or in combinations. Here’s a selection:
  • Key point. Partial or complete loss of appetite.
  • Damage to the gut lining causing inflammation.
  • Interference with the production and absorption of digestive juices.
  • Leakage of fluid from the gut and an increase in mucus production.
  • Dehydration through scouring.
  • Blood loss from the gut wall (Haemonchus only).

Have worms any benefits?
  • All animals have worms so it’s important that their immune systems can respond to deal with them.
  • So they actually need a worm burden to stimulate the very complex immune system to respond to the challenge.
  • This takes time, and when a worm burden swamps the immune system problems will arise.

What are the signs of ‘wormy’ sheep?
There are plenty of signs and you may see some or all of the following:
  • General unthrift and looking tucked up and lethargic.
  • They are easy to catch as they generally fall over!
  • They can show a massive weight loss.
  • Pot belly and low body condition. They are very skinny and their backbones will be visible from a distance despite their wool covering.
  • Diarrhoea (scouring) instead of well-formed faecal pellets or ‘marbles’.
  • Mucus in the faeces. Note this does not always mean worms.
  • Anaemia. The mucous membranes of the eyes and gums are very pale. There is no skin pigment anywhere.
  • White rectangular sections in the faeces (tapeworms).
  • A ‘bottle jaw’ – skin pouch hanging down from lower jaw (liver fluke).
  • Lack of ‘bloom’ in the wool. Wool with good bloom has high grease content.
In the past, when you saw scouring (diarrhoea), you automatically suspected worms and grabbed the drench gun. Not any more! There are many reasons for scouring, so get a veterinary diagnosis before drenching.

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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