By Dr Clive Dalton
What is immunity?
- The body can chemically recognise things which are either 'self' (which is accepts), or 'non-self' or foreign (which it rejects). It then tries to destroy the invaders with antibodies produced from white blood cells in organs like the spleen and lymph nodes.
- Antibodies produced by a first invader challenge may not be as high as that from a second challenge (a booster), which results in much more antibody being produced.
- This is the principle behind vaccination against disease. One vaccination is followed later by a booster.
- Animals to develop immunity to internal parasites just as they do to bacterial and virus infections, and they use this as a defence mechanism. At one time that’s all we needed to know.
- But now we need to understand more about how immunity works which is not easy, as the ‘immune system’ is not located in a single organ. It’s described as being one of the most ‘proliferative organs of the body', meaning that it operates in many different cells so clearly must be complex.
- There is 'passive' immunity made up of pre-formed antibodies, which an offspring gets from it's dam via colostrum, and then there's 'active' immunity produced from antibodies the animal develops itself.
- Developing active immunity takes about 10 days after a challenge, but can last for the rest of the animal' lifetime.
- Immunity to parasites develops more slowly than to bacteria and there is a nutritional cost involved.
- Part of the immune response to internal parasites is to shut down the female worms’ ability to produce eggs before it gets rid of them from the gut.
- Immunity increases with age, remembering that a worm challenge is good, but a massive worm burden will swamp the animal’s immunity until full immunity is established.
- In sheep, immunity is normally fully developed in their second year of life but an ideal is to have them with at least around 50-60% immunity by 6-8 months of age.
- The ideal situation (best practice) would be to let a sheep’s immune system develop without the need for anthelmintics. Unfortunately in practice, farmers are forced into drenching to control the worms.
- To maintain good immunity, animals need periodic or continual challenge by the parasites – so sheep must have some worms in the system for a continuous (but manageable) challenge.
- But an immune response can get too high which has a cost in nutrients and will most likely reduce the other performance traits of the animal. So SIL breeders are encouraged to use the Dual Purpose Overall (DPO) Index which incorporates disease resistance Breeding Values (e.g. WormFec or Facial Eczema) to balance up these genetic conflicts.
- Protein is very important to immunity. Damage to the gut by worms causes protein leakage, and the immune response which occurs in the gut uses lots of protein. This loss then means there are less nutrients available for the rest of the body – especially the valuable carcass.
- Deprived of protein, the animal cannot then mount an effective immune response to worms so better feeding, especially increased protein at this time is important. Feeding clover-dominant pasture is the best option to do this.
- The late Mrs Gladys Reid, OBE, pointed out research in humans and animals showing that zinc, selenium and vitamin B12 are important to develop and maintain a healthy immune system. She also pointed out that zinc and selenium are lost from the gut when enteric infections and upsets like scouring occur.
What harms immunity
- The young lamb before and immediately after weaning is most vulnerable. It needs to develop immunity as soon as possible at a time when there is a high demand for nutrients for growth. But at the same time it can be full of parasites and pouring out worm eggs. So it’s very easy for these things to get out of phase and internal parasites take a great toll.
- Research has shown that anthelmintic drenching of young lambs pre-weaning can delay the advancement of their own inherent genetic immunity. This is an enormously important finding in the drive to identify worm-resistant sheep – but has not been widely broadcast.
- Bloodsucking lice in young lambs (infected from their mothers) can cause anaemia and this will also reduce the build up of immunity. Check with your vet for an appropriate treatment and timing.
The periparturient rise
- Immunity levels drop in ewes for about 4 weeks around lambing so worm populations can increase rapidly. This is called the ‘periparturient rise’ or PPR.
- Also around lambing and early lactation, ewes like dairy cows are in ‘negative nutritional balance’ when their energy output cannot be met by feed intake – so they are under added stress.
- The periparturient rise is higher in twin-bearing ewes than in those having singles, and it’s higher in two-tooths than older ewes.
- If the animal’s immune system is stressed by other factors such as hunger, poor feeding, bad weather, low body condition, or carrying and feeding multiple lambs, then the worm burden will have a greater impact on the immune system and animal performance, so the periparturient rise will be greater.
- Despite recognising that mature stock have the highest immunity against internal parasites, farmers still claim benefits from drenching ewes.
- The general view now is that this should be avoided unless there is an established parasitism problem, especially in young ewes in poor condition where feed is short and hence animal welfare may be compromised.
Genetic improvement of immunity
- There is great individual variation between animals in their inherent level of immunity, and at the age this occurs. This is especially important when we start looking at selection opportunities to improve a trait like host resistance.
- Research and farmer experience has shown that fortunately immunity can be genetically improved. Two lines of AgResearch sheep were selected for over 20 years for ‘resistance to worms’ and ‘susceptibility to worms’.
- By 8 months of age the resistant line had developed 80% immunity to parasite burdens, while the susceptible line were only 20% immune. A ‘normal’ line used as a control showed 50% immunity by 8 months.
- But the dilemma was that the high immune sheep were less productive. This is a common finding in other farm animals such as beef cattle and pigs and is always disappointing to farmers.
- An important finding from breeding research is that ewes selected for low FEC as lambs retain their status around lambing, and are not hit as hard by the periparturient rise and record much lower FEC than susceptible ewes.
- Key point: Anthelmintics have a major negative effect on the developing immune system in the lamb. So to really help a lamb develop its immunity to worms, the less drench it gets the better.
Do anthelmintics harm the immune system?
This very important question was never asked in the past; we took it for granted that if drenches killed worms, then all the news had to be good. The concern today is that this question is still not being asked enough. Sheep farming was all about 'killing things' and after drenching with the latest product, everyone involved in the job felt better!
Thinking has changed in some quarters, although the research showing the bad effects of anthelmintics is still not widely publicised, especially in veterinary circles where they have to sell drench. Here are some important points to ponder.
- When a lamb's delicate immune system is developing, some very complex things are going on at the cellular level. The authorities on the subject will tell you that little of this is understood at present.
- We know that animals vary greatly in how they handle this process.
- Some handle it with ease in a very short time (e.g. 6 months), while in others it can take twice as long and also inflict a considerable nutritional cost. Such sheep will need extra care and a greater protein intake – all adding to costs.
- So getting a lamb to develop its natural immunity early is a money saver as a lamb, and also in its later life, when it won’t need as much drench (if any). Beyond 9-10 months of age, little if any immunity develops.
- So it makes sense to keep anthelmintics out of lambs. They should be used to help vulnerable animals during the critical 2-3 months after weaning, and at other times when you think a massive worm load may impair production and welfare. Consult your vet about this.
- At all costs, avoid the old schedule of monthly drenching by the calendar, when lambs after weaning and through winter used to get 7 drenches or more. The more drench they got the more their immune system was damaged - and the more money was wasted.
- Some advisers now recommend that products containing albendazole should not be given to ewes in early pregnancy as it may damage the developing embryo. That tells you something about the drugs used in drenches!
The store attendant will assure you that all these products are good, because that's what the company rep told them. And remember 'the special offers' to consider.And why do they put products in bright coloured packs?
