Defence forces: the horse’s immune system

In part one of a series, James Hart takes a look at the horse's immune system, how it works, and what we can do to support it

(Image: Dusty Perin)
(Image: Dusty Perin)

The immune system is one of the most important of the body’s many and complex systems. It works unseen but is vital to the continued wellbeing of any animal. As with all things though, occasionally it doesn’t work quite as it should and may need some help. If the system is ultimately overwhelmed, the outcome is likely to be fatal.

Before we look at how herbal medicine can assist in immune disorders, it is important to have a broad understanding of what is involved.

We live in a world in which we are outnumbered many times over by much smaller organisms, some of which are helpful to us, and others that will do us harm – these are known as pathogens.

Small, smaller, smallest

It is convenient to look at pathogens by size; the smallest are the viruses. These are basically packets of genetic material that need a host in order to replicate. They can gain access to a new host in a number of ways, but probably the most common is via the respiratory or digestive system.

Familiar viruses are those that cause many respiratory problems, scours in young animals and, more seriously, foot and mouth disease.

Bacteria are quite a lot bigger than viruses – in some cases they can be a viral host themselves. It is important to remember that only a handful of bacteria are bad, and that we could not function at all without help from quite a number of different beneficial bacteria, particularly in the gut, as they are basically responsible for the digestion process.

The most common pathogenic bacteria include Staphylococcus, Streptococcus and E. coli. These bacteria are all around us all the time, and cause illnesses such as strangles in horses (Streptococcus equi), mastitis in cows and sore throats in humans.

In many cases, the problem caused by a bacterial infection is not so much the presence of the bacteria themselves, but the build-up of toxins produced by the bacteria.

Larger than bacteria but still microscopic are fungi, amoeba and protozoa. These are not generally as significant a problem as bacteria and viruses, but are still pathogens that the immune system has to deal with.

Finally, there is a special group of pathogens that we term parasites. These include not only the gut parasites which cause modern farming so many problems, but also the lice and ticks that attack horses from outside.

And although they are not considered parasites, we can justify the inclusion of sand flies and mosquitoes here, because they elicit an immune response by their biting and blood-sucking actions.

Hidden defences

It can be quite helpful to think of the body as a nation state with the immune system as its defence force. At the border are Customs and Immigration: the hair, skin and mucus membranes. They allow in desirables, turn away others and look for smugglers. In conjunction with the beneficial bacteria on the skin, they are very effective and able to keep out most of the nasties.

Occasionally, an undesirable will get past the border patrol and enter the body. This now becomes the responsibility of local security. In the body, we call these the white blood cells. There are billions in circulation with another million being made each second in the bone marrow.

What can affect the immune system?

The immune system is quite sensitive; it can be depressed by a range of factors, many of which the horse’s owner can influence. An easy thing you can do is be aware of your horse’s stress level. Keep this in mind and if necessary, change your routine or activities as appropriate.

Chronic mental or physical stress tends to elevate the cortisol levels – this has a detrimental effect on the immune system (incidentally, this is why Cushings sufferers with their elevated cortisol levels tend to have reduced immune function). When cortisol is elevated, pathogens that would normally be readily controlled by the immune system can become a problem.

The herpes viruses that are problems for both horses and humans are particularly adept at remaining dormant until the host is under stress and then erupting when the opportunity presents itself.

Mud-fever is another good example of pathogens that are dormant until the host is under stress, and we have found great success in treating the immune system as well as the external symptoms.

Having said that we should try to manage stress, it is important not to forget that it is a natural and very necessary reaction. From a broad view, stress is the body’s reaction to any stimulus that disturbs its balance or equilibrium (vets refer to this as ‘homeostasis’). When the equilibrium of various hormones is altered, the effect of these changes can be detrimental to the immune system. This is the price we pay for the familiar ‘fight or flight’ mechanism that is the most obvious manifestation of acute stress.

Under those circumstances, a short-term lessening of immune system response is of little consequence – far more important is the preservation of life.  

Chronic stress, however, is a far more insidious and serious issue. The way we house and look after our horses can predispose them to chronic stress.

By weaning early, keeping them alone, or in small groups, shutting them in stables and restricting their access to food can all be stressful and made worse because, for the horse, the situation cannot be resolved.

It is not uncommon to see horses who cannot cope with being permanently boxed with no companions around. They might lose weight, develop coping mechanisms like weaving or box-walking and will probably develop ulcers.

Quite a range of other things, including some that as humans we may find trivial, can be stressful for horses. There will be other triggers for individual horses but the common range includes:

  • bad weather; very cold, wet, windy or hot conditions
  • excessive, prolonged exercise
  • long-distance travel; this may be the effort required by the horse to stand up during the journey as well as the mental stress that may be associated with going away from home
  • disrupted meal and sleep patterns away from home, as well as separation from the peer group and exposure to strange horses and people
  • picking up on nervousness and tension from handlers and /or riders 
  • certain types of medication 
  • poor feeding and loss of body condition.

Other influences

Although most can be linked to stress, there are a few other influences that are independent. Unfortunately, most of the research is carried out on humans, but it is probably relevant to horses given that the immune system works in the same way. Bacterial balance on the skin and in the gut can have a remarkable effect.

In both cases, the balance of beneficial and pathogenic bacteria can tilt the wrong way – in favour of the pathogens – and if not rectified can become a challenge to the immune system. Of course it is never that simple, and recent work published in August last year has found that the immune system itself can influence the skin flora, as well as vice versa.

The horse’s age and nutritional status also have a large effect on the healthy functioning of the immune system.

– This article was first published in the April 2014 issue of NZ Horse & Pony