14 minute read
THE IMMUNE SYSTEM –a mighty defence force©
Dr Jennifer Stewart BVSc BSc PhD Equine Veterinarian, Equine Clinical Nutrition, Consultant Nutritionist, CEO Jenquine
The immune system - a complex network of organs, cells and proteins – protects the body from damage and defends the body against infection. A deficiency of key vitamins, amino acids and trace minerals can suppress the immune system and increase the risk of disease. A wide variety of veterinary conditions - ranging from mild to debilitating conditions like chronic liver failure and life-threatening diseases such as tetanus and Hendra – are caused by infection with bacteria, fungi, viruses or parasites. Each invader uses a different tactic to infect horses and each is thwarted by different components of the immune system.
The front line
Firstly, the microbe must move past the external armour - the skin, penetrable only through cuts or tiny abrasions; the gut with mucus and acid to entangle and destroy pathogens swallowed with food, and mucus, coughs and sneezes that eject and expel microbes attempting to enter via the respiratory tract. If the would-be invaders survive the body’s frontline defenses, they confront specific weapons of the immune system, tailored just for them - and when challenged by an invader, the immune system has many weapons to choose.
Specialised white blood cells lie in wait to ‘eat’ the invaders (Figure 1.) and natural killer cells - programmed to attack and kill - patrol the body.
The second line
The immune system - an elaborate and dynamic communications network is the second line of defence. In response to an invader, millions of cells, organised into sets and subsets, mass together and pass information back and forth. Once they receive the alarm, the cells become activated and produce powerful chemicals that enable them to regulate their growth and behavior, enlist other immune cells, and deploy recruits to trouble spots. Extraordinarily complex, the immune system can identify and remember millions of different invaders, and produce fluids and cells to match up with and wipe out nearly all.
Positioned throughout the body, the immune system includes lymph nodes, bone marrow, the spleen, thymus gland and clumps of tissue in the linings of the gut, airways, and lungs. These depots stockpile a huge arsenal of cells – some of which take on all intruders, others trained on highly specific targets. Working together, immune cells communicate by direct physical contact and by releasing chemical messengers.
When the immune response is triggered into action it releases chemicals that act as alarm signals, warnings and messages. These chemicals initiate an inflammatory response, which raises body temperature (causing fever); increases blood flow to the affected area (seen as redness, heat, swelling and pain); summons white blood cells, and recruits immune cells to the war zone. The battle that follows initiates a cascade of chemicals that coat the enemy cells and mark them for destruction, disrupts their protective outer cell wall and deploys millions of white cells to kill the invaders.
White blood cells are the individual soldiers or police - always on patrol, identifying, attacking and killing invading microorganisms. They have special chemicals that can kill on contact, or they can swallow, engulf and kill invaders by digestion (Figure 1.). Special ‘killer’ cells release poisons that burn holes in the outer membrane of enemy cells, allowing water to flood in; ‘helper’ cells search for invaders and then alert, direct and deploy killer cells. Other white blood cells display on their surface, copies of the invading organism‘s structure.
Other killer cells, identify and destroy tumour cells and cells infected with viruses (Figure 2).
As well as defending the body against foreign invaders, the immune system cleans up waste products, such as reactive forms of oxygen (known as oxidants or free radicals) and removes abnormal cells (eg cancer cells). Electrically charged free oxygen molecules are a waste product of energy production and they weaken the immune system. Production increases with illness, ageing and exercise. The immune system provides antioxidants to scavenge and mop up oxidants.
The anti-oxidant/free radical defence force
The elaborate antioxidant defence system depends on the daily intake of vitamins and minerals, including vitamin E, vitamin A, copper, manganese, selenium and zinc and vitamin C. Because more oxygen is used, the generation of reactive oxygen waste products increases 10 to 30 fold during strenuous exercise. While rare in humans, vitamin E deficiencies are common in horses just on hay. Vitamin E varies markedly in feedstuffs - highest in fresh grass and declining with processing and storage by around 50% a month. Up to 5000IU vitamin E/kg per day is needed to maintain blood and muscle levels. Again it’s important to check feed labels - if the feed bag analysis lists vitamin E at 1000iu or mg, it would need to be fed at 5kg/day to meet vitamin E requirements. Magnesium also plays a very important role in immune system balance and most equine diets are low in magnesium.
Feeding the immune system -The immune system demands a lot of nutrients and energy. Nearly every nutrient has a role or effect on the immune system. Most diets provide enough vitamin A, a healthy gut generates abundant B vitamins, while the liver generates vitamin C. But common diets may not provide enough copper, magnesium, zinc, selenium, vitamin E, Omega-3 and amino acids.
Horses produce their own vitamin C – and unlike humans – have no need for it in their diet. However, older horses and those with fractures, severe infections, overtraining, or a compromised immune system may have a reduced ability to create vitamin C, and in horses under stress, demand may exceed supply. Vitamin C is not easily absorbed in the equine intestinal tract and they need 10 – 20g (10000 to 20000mg) a day to make a difference in blood levels. This is a good example of why reading labels and knowing how much your horse needs can help you make the best decisions. For example, if a feed lists vitamin C at 4000mg then you need to feed 2.5 - 5kg day to provide recommended levels of vitamin C.
Omega 3 -Omega-6 and omega-3 fatty acids have immune-boosting properties. They can’t be synthesized and must be included in the diet. Flaxseed, linseed, sea algae and cold-water fish oils are good sources of omega-3s. Horses with respiratory disease and osteoarthritis receiving both omega-3s and -6s have reduced inflammatory markers. The American College of Veterinary Internal Medicine recommends adding the omega-3 to the diets of horses with inflammatory airway disease.
The gut biome
Did you know that a large percentage of the equine immune system is located in the gut? The gut biome harvests nutrients, extracts energy from the diet, resists colonization by pathogens, develops the immune system and removes toxic compounds. An imbalance in the biome allows overgrowth of ‘bad’ microbes that damage the intestinal lining and contribute to or cause a ‘leaky gut; so we want to keep the delicate dance between gut microflora and the immune system happy with appropriate nutrition and management.
Functional fibres (prebiotics such as beet pulp, soyhulls and oat hulls) help bacteria in the gut produce energy and reduce pathogenic bacteria. They support immunity by feeding beneficial bacteria, binding to pathogens, supporting the intestinal lining and interacting with immune cells. The opposite occurs with diets that are high in starch/sugar or based on grains or grain by-products (Table 1). More research needs to be done to determine the effects of probiotics as unfortunately nutritional advantages do not always transfer from one species to the next, and the results so far are variable.
TABLE 1.
Ingredients that flag caution NSC > 10%
Is Considered Unsafe
Barley average starch 60% sugar 3% = 63% NSC
Bran/pollard (wheat) average starch 23% sugar 7% = 30% NSC
Cereal byproducts include bran, pollard, wheat middlings and millrun = >30% NSC
Cereal coproducts generally corn or wheat meal, hominy feed, bran and pollard = NSC>30%
Corn approximately 73% starch and 2% sugar = 75% NSC
Millmix contains wheat pollard and bran = >30% NSC
Millrun contains bran and pollard = >30% NSC
Molasses average 65% sugar = 65% NSC
Wheat average starch 70% sugar 3% = 73% NSC
Rice bran (fibre < 4%) average starch 42% sugar 4% = 46% NSC
* Feedipaedia, Dairy One
Feeds high in starch and sugar reduce microbial diversity and stability, inducing changes linked with behavioural stress, known as alimentary stress. Understanding the words on feed labels helps select a low starch/ sugar feed (Table 2).
WHATS IN THE FEED - what the words actually mean
Grain-free No official definition, generally means no whole grains but could still include grain-byproducts such as bran and pollard, millrun etc which are high in NSC
Hominy meal Ground corn meal ~ 38-45% NSC
Low GI Glycaemic index (GI) is determined by measuring insulin, glucose and ACTH before and again 1, 2 and 3 hours after feeding to be labelled ‘low GI’ - check if the maunufacturer has done this.
https://www.ncbi.nlm.nih.gov/pmc/articles PMC9889680/pdf/JVIM-37- 302.pdf https://web. archive.org/web/20190222221947id_/http://pdfs. semanticscholar.org/3d7d/5697df47bdbc435d0deab6699143877bb1c5.pdf
Low sugar It is the sugar + starch that is important so if the % starch or total NSC are not also given, it could be unsafe.
Low starch It is the starch + sugar that is important so if the % sugar or total NSC are not also given, it could be unsafe.
Micronised The feed is ‘cooked’ making it more digestible and dramatically increasing the availability of glucose - which can cause a profound increase in glucose and the glycaemic response.
Extruded
Extruded feeds may also not be a good choice for horses with metabolic syndrome. Extrusion increases starch digestibility making it easily converted to glucose and absorbed. Easy-keepers often gain too much weight on an extruded feed. Horses with metabolic issues may not be able to handle the starch and sugar content of extruded feeds. https://ivcjournal.com/extrudedhorse-feeds-proscons/#:~: text=What%20is%20 extrusion%3F,causes%20the%20feed%20t o%20 expand
Nutrition analysis - Needs to list sugar - which may be listed as WSC or ESC AND starch or else the total NSC <10% or it may be unsafe
Cool feed - Cool feeds are described as formulated to avoid spikes of rapid energy - unless the blood glucose levels have been measured after feeding, this term is misleading - contact the manufacturer
Starch - Recommended feed analysis <10%
Meeting nutritional needs in times of stress
During times of stress (as exercise increases, during lactation, weaning and growth, travel, changes in routine, stabling at showgrounds and racetracks where they’re at increased risk of exposure to pathogens) dietary needs change. In addition to a good vaccination program, a balanced diet is one of the best defences a horse has against disease. The higher the demand for nutrients, the lower the margin for error. The impacts of an unbalanced or deficient diet are more likely to be seen in all areas of performance, including immune function.
Mares and foals
Newborn foals are protected by antibodies they get from the colostrum – a super rich immunity drink – produced by the mare before she starts milk production. Mares produce antibodies to infectious agents in the environment and to vaccinations they receive in the last month of pregnancy. These antibodies concentrate in the udder colostrum. Because the newborn foal relies on the colostrum to protect it, proper vaccination of mares is important to protect the foal. Antibodies are quite large structures and the foal’s gut can only absorb them for 18 hours after birth. After 24 hours the gut is closed and no antibodies can be absorbed.
Failure of the foal to receive good protection can occur if the foal didn’t nurse strongly in the first hours after birth or became separated from the mare; if the mare died or didn’t produce good quality colostrum. Without enough good quality colostrum, foals can become sick within two to five days. Veterinarians commonly take a blood sample for testing when the foal is 1 – 2 days old and will advise treatment if the foal hasn’t received enough antibodies. Because foals are an easy target for infections, it’s important that mares are vaccinated and live for at least a month in the place where they will foal – this allows her to respond to infectious agents in the local environment. Make sure mares don’t become fat because overfed and overweight mares have poorer quality colostrum – thought to be due to excess fat taking up space in the udder. Foals usually nibble at mares’ manure during the first weeks of life, colonizing the gastrointestinal tract with good bacteria and this shouldn’t be prevented.
Antibodies from the colostrum last for around 6 months in the foal’s body which means they are declining when weaning occurs, Weaning is a stressful time that renders a foal more susceptible to infection because increased cortisol (the stress hormone) diminishes the ability of white blood cells to react to invaders. Weaning stress affects appetite and immune competence. It’s often thought that foals are less stressed when weaned in pairs – but in fact even if they have spent a lot of time together before weaning, they show little care-giving and more aggressive behaviour towards each other when weaned together. Placing foals singly in stalls for weaning rather than in pairs has been shown to be beneficial because aggressive behaviour was moderated. Vitamin C requirements are increased with weaning stress and access to pasture will allow the foal to increase vitamin C levels.
Aging Horses
Older horses : Immune function declines with age and this increases susceptibility to diseases. Around 30% of older horses are affected by Cushings disease (PPID) which adds another layer of immunosuppression. PPID-affected horses have diminished immune function and often suffer chronic infections, such as dental disease, sinusitis, and parasitic disease. Horses with EMS also have lowered immunity, especially of overweight.
Supplements: A balanced diet is fundamental. Feeding beyond requirements can lead to imbalances, excesses and still deficiencies can occur. Oversupplementation with selenium and iodine (in seaweed meal) are the most common causes of toxicity. Recently the European Union imposed legal restrictions limiting the amount of zinc fed to horses after it was found to be toxic to the gut biome and increase antibiotic resistance. Given the importance of a balanced microbial composition and metabolic activity, especially for hindgut fermenters like horses and ponies, the depressing effects of high dietary zinc concentrations might be critical with regard to gut health. Many hoof supplements contain added zinc, high intakes of which have been found to reduce the richness and activity of the microbiota of horses and ponies.
As simple as it sounds, the key to maintaining a healthy immune system is a balanced diet. Diet is the foundation of health. Nothing can replace a healthy diet for promoting optimal health, and in warding off both acute and chronic disease. Processed commercial pet foods can contain both excesses and deficiencies of many minerals, making supplementation potentially hazardous if certain minerals, such as zinc, become excessive. It’s safer (and often cheaper!) to feed a correctly balanced, low starch/sugar, vitamin, mineral and amino acid formulation than to add supplements.
© Dr J H Stewart My 2023
All content provided in this article is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian.
ABOUT THE AUTHOR –
Dr Jennifer Stewart
CEO BVSc BSc PhD Dip BEP
Equine Veterinarian and Consultant Nutritionist
Dr Jen Stewart has been an equine veterinarian for more than 40 years and an equine nutritionist for more than 10 years. Jen has been developing premium formulas for studs, trainers and feed companies in Australia and around the world and regularly consults to leading international studs and trainers in various countries.
Jen has spent a fair bit of time researching and being involved in nutritional management of developmental orthopaedic diseases, colic, tying-up, laminitis, performance problems, post-surgery and other conditions. And is currently the only practicing equine veterinarian and clinical nutritionist in Australia. Jen’s promise is to continue to BRING SCIENCE TO YOUR FEED BIN www.jenquine.com
