Understanding immunity – lifting the bonnet on livestock health

The livestock in most efficient farming systems have been selected to be elite athletes or finely tuned racing cars. However, to keep them running at their peak, researchers need to discover and optimise the metabolic and physiological processes that promote optimal function, and confer that often referred to but poorly defined quality of ‘resilience’ which keeps livestock productive for longer in modern production systems. Sustainability is another often-used term representing the Holy Grail for agriculture, and when we consider that 20 per cent of global animal production is lost annually to livestock diseases, we appreciate that there remains a lot of work to do.
Immune system

The health of any individual is determined by the immune system, a trait which we could argue is one of the most poorly understood traits in animal science. The immune system has been present since the earliest life forms emerged on earth as its function is a prerequisite to survival. However, over the course of evolution, the immune system has changed significantly across various livestock species leading to a general common conformity amidst relevant species-specific differences. In addition, individual animals have variants which can make their own immune system function differently. These variations developed in response to different pathogen challenges faced by these species and represent critical weapons employed by Mother Nature to overcome infections. Since domestication however, we have changed many behavioural, environmental and management aspects of livestock and, as such, need a much more comprehensive understanding of the impact of these changes on the immune system to optimally support immune function and reduce the occurrence of diseases.

Adaptive and innate immunity

All livestock have a limited supply of (both quantity and quality) nutrients for body maintenance, production (milk, meat) and any other additional functions like pregnancy. When a stressor is introduced, like a low environmental temperature, the animal must reapportion its nutrient and energy allocation to maintain its internal physiological stability – a state referred to as homeostasis. If the stressor is large or long enough, then the physiological reserve of the animal will be depleted, and deficits will result in other traits such as decreased production or suboptimal immunity. 
We readily appreciate the importance of colostrum for a newborn animal, as it is full of protective antibodies known as immunoglobulins. This ability of the dam to pass protective passive immunity across the placenta in some species or in colostrum in ruminants resulted from one of the greatest changes in immunity which occurred about 500 million years ago – the evolution of the adaptive immune system. As a result, all livestock species can produce a diverse array of antibodies, some of which are large and circulate in the blood to provide protection and others which are smaller and can squeeze into tissues to target bacteria – like in the calf intestine. But there are many additional aspects of the immune system, each of which hold valuable keys to the arsenal of weaponry that the animal can employ to target infection. When a cow has a high somatic cell count (SCC), for example, what is happening is that neutrophils (a type of white blood cell) are recruited from her bloodstream to the mammary gland to fight infection – often used as an early sign of subclinical mastitis. These cells exude their DNA to trap the bacteria causing clots, which farmers use to detect infection. Similarly, when a cow reacts to the bovine TB skin test (and is, therefore, called a ‘reactor’), cells in her neck which have been previously sensitised to mycobacteria (the bacteria which causes TB) are recruited, activated and causes a swelling that the vet uses to diagnose the disease on farm. These are all examples of another arm of the immune system in action – the innate immune system.
Both the innate and adaptive immune systems are deployed around the clock in all livestock species to fight infection, repair damage and restore homeostasis. Over the course of evolution, the immune system of some species has learned to tolerate certain bacteria – and these ‘good’ bacteria are now referred to as commensals. However, a commensal in one species is not necessarily a commensal in another. The most common cause of food poisoning in humans for example is caused by a bacteria called campylobacter. This bacterium is carried in the intestines of chickens, but the chicken immune system has learned to tolerate the bacteria whereas in contrast small numbers of the bugs send the human immune system into overdrive leading to pathology and sometimes severe illness.

Research

In studying the immune system, we are casting our scientific eye back over millions of years of evolution and opening a toolbox that has a Swiss-army knife of options for disease protection. It is not simply a theoretical exercise either – enhanced knowledge is critical to supporting optimal animal immunity, and generating resilient livestock. This has knock-on consequences for a reduction in antibiotics, anthelmintics and improving animal welfare. Improved understanding of immunity is also imperative to design vaccines which work well in all farmed animal species – from fish to chickens and cattle. 
At a recent veterinary immunology conference (www.eviw2024.org) that I organised, Professor Alison Van Eenennaam from the US described how a natural mechanism that bacteria use to defend themselves against viruses is now being used to edit the DNA of livestock species to make them disease resistant. Called CRISPR, this

protein is like a mini-scissors and is being used in agriculture to improve disease resistance in fish, poultry and cattle amongst others¹. So edited and resilient livestock might actually be the key to future sustainable livestock production systems.

The immune system has a lot to teach us and understanding these differences across livestock species is a critical focus of the research performed in UCD. Research is then used to inform teaching – and has shaped the design of new modules available as part of the new MSc in Animal Science in UCD as well as short, industry-aligned and accredited courses known as micro-credentials. Further details are available on UCD’s website. 

¹ DOI: 10.1016/j.tvjl.2024.106142