Understanding the Immune System
The immune system is vital in ensuring the health of all animals – including humans – and has a direct influence on all productive traits. It is therefore important that all farmers, and particularly stud breeders, have a good understanding of how it works and the factors that strengthen – or weaken – its effectiveness.
In the medical world “Immunetherapy” is the byword heralding new research on enabling the body’s own immune system to combat diseases such as cancer. Cancer cells have a protective mechanism which prevents the T-cells of the immune system recognizing them as a threat to the body. Scientists have now discovered drugs which strip these cells of their defenses, thus allowing the T-cells to destroy them completely. Although still being trialed, this new approach, immunetherapy, is being acknowledged as a major new treatment for cancer and other diseases.
In domestic and production animals, we do not need to take this track, as there is another option not available in the human domain. We can breed animals with immune systems that can control disease and parasite challenges. Before I cover this genetic option, allow me to canvass general aspects of the immune system.
Before I ventured down the course of endeavouring to breed sheep that have an inherent resistance to worms 29 years ago, my understanding of the workings of the immune system was very basic. This understanding was that it developed in young animals and reached a peak at maturity. I understood that vaccines triggered the immune system to target and control some infectious diseases that caused deaths.
What I have learned over the past 3 decades, from my own practical experience and talking with scientists, is that the immune system can be strengthened – or weakened – by natural causes like climate, environment and management. For instance, in the big North Island drought of several years ago, veterinarian Trevor Cook, noted that many adult ewe flocks suffered greatly from challenges from the Barbers’ Pole worm (Haemonchus contortus). Why? Because their immune systems were not firing on all cylinders due to lack of fuel (nutrition or grass). The strength and effectiveness of the immune system is not constant, but fluctuates as a result of external circumstances. A major factor in its effectiveness is nutrition. Various diseases will weaken immunity. For example, lambs suffering from pneumonia will be less able to cope with worm challenges. Likewise, blood sucking parasites like the cattle tick and the Barbers’ Pole worm will weaken immune responses to other diseases. Skin irritation caused by lice will also have a negative impact. (My father used to say “skinny sheep breed lice” or in other words, the impact of lice will weaken an animal’s immunity). Mineral deficiencies will also have the same effect.
Management plays a part in ensuring good immune responses. Regular shifting of stock, even under adverse conditions, is far better than set stocking for long periods. Even when grass is abundant, animals will immediately graze when shifted. Leaving rams in the “ram paddock” for long periods between mating is a sure way to have high worm burdens. When nutrition levels fall, particularly as a result of drought, farmers should pre-empt loss of body weight and deaths caused by higher levels of worms by drenching sooner rather than later.
Our current farming practices are contributing to our national flock having weaker immune systems. Our effective drenches ensure the survival of the lambs that are most susceptible to worms and therefore have weak immune responses. Such animals – be they rams or ewes – retained in the national flock will contribute to a weakening trend.
Furthermore, we have the best programme ever devised by man to breed super bugs and resistant parasites. We are constantly told that household cleaners and drenches, kill 99.9% of germs and worms. This leaves the top 0.1% - or 1 in 1000 – of the ultra-resistant to survive, mate and produce the next generation of resistant organisms.
It also needs to be realised that over drenching of young animals, and virtually keeping them free of worms, will delay the development of the immune system. Immune systems only develop when exposed to disease and parasites. The greater the challenges, the strong the immune system becomes.
Now for the genetic option.
Without a doubt, the biggest lesson I have learned since breeding for worm resistance is that the immune system can be greatly enhanced by this genetic approach. Originally, I did not realise that breeding for worm resistance was in fact breeding for a stronger immune system, which in time, would contain worm challenges. Now we all understand that we can improve production and physical traits (including FE tolerance) because of genetic variation. However, it may not be understood that the same genetic variation also exists in the immune framework. Some immune systems will react early and aggressively, while in others the development will be slower and more passive. This genetic variation enables progress by the simple but traditional method of “selecting the best and culling the worst”.
Worldwide, all our endeavours have been focused on breeding to improve productive and physical traits, and in some cases breed characteristics. The possibility of breeding for an enhanced and proactive immune system has never been on the radar. It has always been my belief, and experience, that if a trait is not being positively selected for, it will slowly but surely be lost. For the immune system it is even worse. The advice from some advisors is that to select for stronger immune systems by breeding for worm resistance would result in “diversion of energy away from productive traits”, and therefore is not a good option. If this is so, then selection for productive traits will divert energy away from an animal’s immune responses. Is this desirable? Are we prepared to compromise an animal’s health to achieve production targets. Those promoting this thinking obviously do not realise that scientists promoting the Worm FEC programme (breeding for worm resistance) have always insisted that production traits had also to be selected for. In practice a breeder in the Worm FEC programme would aim to select sires in the top 5% for all traits.
So what are our options for the future?
Currently the only way to breed for a more active immune system is to breed for resistance to parasites and disease, namely worms, foot diseases – using DNA – and possibly pneumonia.
Commercial farms have few practical options to achieve the second option, so the ball is in the stud breeders’ court. For stud breeders to make the necessary changes in thinking and then in practice will be very difficult. We humans prefer to stick with the status quo, and do as we have always done.
In the medical world “Immunetherapy” is the byword heralding new research on enabling the body’s own immune system to combat diseases such as cancer. Cancer cells have a protective mechanism which prevents the T-cells of the immune system recognizing them as a threat to the body. Scientists have now discovered drugs which strip these cells of their defenses, thus allowing the T-cells to destroy them completely. Although still being trialed, this new approach, immunetherapy, is being acknowledged as a major new treatment for cancer and other diseases.
In domestic and production animals, we do not need to take this track, as there is another option not available in the human domain. We can breed animals with immune systems that can control disease and parasite challenges. Before I cover this genetic option, allow me to canvass general aspects of the immune system.
Before I ventured down the course of endeavouring to breed sheep that have an inherent resistance to worms 29 years ago, my understanding of the workings of the immune system was very basic. This understanding was that it developed in young animals and reached a peak at maturity. I understood that vaccines triggered the immune system to target and control some infectious diseases that caused deaths.
What I have learned over the past 3 decades, from my own practical experience and talking with scientists, is that the immune system can be strengthened – or weakened – by natural causes like climate, environment and management. For instance, in the big North Island drought of several years ago, veterinarian Trevor Cook, noted that many adult ewe flocks suffered greatly from challenges from the Barbers’ Pole worm (Haemonchus contortus). Why? Because their immune systems were not firing on all cylinders due to lack of fuel (nutrition or grass). The strength and effectiveness of the immune system is not constant, but fluctuates as a result of external circumstances. A major factor in its effectiveness is nutrition. Various diseases will weaken immunity. For example, lambs suffering from pneumonia will be less able to cope with worm challenges. Likewise, blood sucking parasites like the cattle tick and the Barbers’ Pole worm will weaken immune responses to other diseases. Skin irritation caused by lice will also have a negative impact. (My father used to say “skinny sheep breed lice” or in other words, the impact of lice will weaken an animal’s immunity). Mineral deficiencies will also have the same effect.
Management plays a part in ensuring good immune responses. Regular shifting of stock, even under adverse conditions, is far better than set stocking for long periods. Even when grass is abundant, animals will immediately graze when shifted. Leaving rams in the “ram paddock” for long periods between mating is a sure way to have high worm burdens. When nutrition levels fall, particularly as a result of drought, farmers should pre-empt loss of body weight and deaths caused by higher levels of worms by drenching sooner rather than later.
Our current farming practices are contributing to our national flock having weaker immune systems. Our effective drenches ensure the survival of the lambs that are most susceptible to worms and therefore have weak immune responses. Such animals – be they rams or ewes – retained in the national flock will contribute to a weakening trend.
Furthermore, we have the best programme ever devised by man to breed super bugs and resistant parasites. We are constantly told that household cleaners and drenches, kill 99.9% of germs and worms. This leaves the top 0.1% - or 1 in 1000 – of the ultra-resistant to survive, mate and produce the next generation of resistant organisms.
It also needs to be realised that over drenching of young animals, and virtually keeping them free of worms, will delay the development of the immune system. Immune systems only develop when exposed to disease and parasites. The greater the challenges, the strong the immune system becomes.
Now for the genetic option.
Without a doubt, the biggest lesson I have learned since breeding for worm resistance is that the immune system can be greatly enhanced by this genetic approach. Originally, I did not realise that breeding for worm resistance was in fact breeding for a stronger immune system, which in time, would contain worm challenges. Now we all understand that we can improve production and physical traits (including FE tolerance) because of genetic variation. However, it may not be understood that the same genetic variation also exists in the immune framework. Some immune systems will react early and aggressively, while in others the development will be slower and more passive. This genetic variation enables progress by the simple but traditional method of “selecting the best and culling the worst”.
Worldwide, all our endeavours have been focused on breeding to improve productive and physical traits, and in some cases breed characteristics. The possibility of breeding for an enhanced and proactive immune system has never been on the radar. It has always been my belief, and experience, that if a trait is not being positively selected for, it will slowly but surely be lost. For the immune system it is even worse. The advice from some advisors is that to select for stronger immune systems by breeding for worm resistance would result in “diversion of energy away from productive traits”, and therefore is not a good option. If this is so, then selection for productive traits will divert energy away from an animal’s immune responses. Is this desirable? Are we prepared to compromise an animal’s health to achieve production targets. Those promoting this thinking obviously do not realise that scientists promoting the Worm FEC programme (breeding for worm resistance) have always insisted that production traits had also to be selected for. In practice a breeder in the Worm FEC programme would aim to select sires in the top 5% for all traits.
So what are our options for the future?
- Continue to breed solely for production traits while completely ignoring the potentially powerful and game changing immune system, which will require more chemical intervention to ensure stock health and production.
- Breed animals that have more aggressive immune responses to control diseases and parasites which will reduce our dependence on new and more costly chemicals.
Currently the only way to breed for a more active immune system is to breed for resistance to parasites and disease, namely worms, foot diseases – using DNA – and possibly pneumonia.
Commercial farms have few practical options to achieve the second option, so the ball is in the stud breeders’ court. For stud breeders to make the necessary changes in thinking and then in practice will be very difficult. We humans prefer to stick with the status quo, and do as we have always done.