WHEN a stray is brought in to the practice, vets will usually hazard a guess as to age.
From the clues – say, bad teeth, that slightly gingery fur and a rather dazed look – we can tell a cat is pushing 20 years of age rather than 10. And we know that these signs are not just on the outside; the blood profile of a geriatric animal often looks very different from that of a young, healthy adult.
But have you ever wondered what exactly is happening when an animal ages? How are the body systems changing? What is going on at the level of the cell? At a recent, unusual, comparative science conference, vets and doctors met to address these questions. The Merial European Vaccinology Symposium was held in Prague during May, and saw a panel of immunologists, virologists, molecular biologists and epidemiologists gather to share the secrets of ageing and the immune system.
Gradual decrease
When we look at the older pet, many of the changes that we observe – such as loss of sight and hearing – result from a gradual decrease in normal bodily function. Cells die and are not replaced.
When the immune system starts to undergo changes and become impaired with age, it’s called “immunosenesence” and this leaves older pets prone to infectious and auto-immune diseases.
A part of these age-related changes is a process which some experts refer to as “inflamm-ageing”. This describes how the body is exposed relentlessly to antigenic challenge over the course of its life. This creates a chronic inflammatory state in the body and is a factor in the pathogenesis of many age-related diseases that we see in pets and people, including osteoarthritis, diabetes, cancer and infections.
Although the agerelated changes to the immune system happen throughout the body, one of the key organs of importance is the thymus. At the conference we saw that the canine thymus involutes with age, i.e. functional thymic tissue is replaced with fatty, non-functional tissue, in much the same way as it does in man.
Donald Palmer, senior lecturer in immunology at the Royal Veterinary College, explained how the decrease in thymic output affects an animal’s immune system. “In the young animal there is a very high T-cell output and there are many naïve cells. At the middle-aged stage the thymus starts to decrease and an increased number of memory phenotypes have been acquired.
Memory phenotype “By old age there is very little T-cell output and most of the T-cells in the periphery have memory phenotype whilst some have become defective.”
This means that although the levels of T-cells in circulation remain the same – the older animal is not cytopaenic – there are relatively more memory and effector T-cells, so the animal is less able to respond to novel antigens. This emphasises one of the main take-home messages for vets from the conference: that companion animals need to receive an effective vaccination scheme at a young age to be properly protected in old age.
Doctors at the conference, including Richard Aspinall of Cranfield University and Beatrix Grubeck-Loebenstein of the Austrian Academy of Sciences in Innsbruck, discussed a number of methods to support the immune system in old age that are under investigation, from the cutting-edge science of stimulating thymic output by using cytokines, to advances of a more tried and tested route, vaccine adjuvants.
Human cytomegalovirus
Another area of interest for vets at the conference was that of human cytomegalovirus (CMV). CMV is a common virus that infects a large proportion of people worldwide but rarely causes illness. It is thought that CMV infection becomes more relevant as a person ages.
Current research suggests that infection with CMV can alter a person’s T-cell profile and, in theory, “tie up” T-cells so they can’t be used to fight pathogens. There is no knowledge of whether a type of CMV, or a CMV-like virus, exists in animals, but it’s an obvious area for further investigation in veterinary medicine.
From the interaction between vets and doctors at the event, there were a number of important take-home messages for both groups.
Of most importance was a common consensus that elderly populations, whether human or animal, need specialised attention with regards to immune support and vaccination.
The research shows that elderly animals do not respond to novel antigenic stimuli as well as younger ones. In veterinary medicine this has relevance for situations such as rabies vaccination for the pet passport scheme.
According to the science, any animal that is likely to need a rabies blood test later in life could probably benefit from a first anti-rabies vaccination before its senior years. This is because older pets, in a small number of cases, may not be able to mount a sufficient antibody response to the vaccination to pass the test.
Not one-sided
The swapping of information at the symposium was not one-sided and doctors also learned from vets. A take-home message for those from the human side was that, for some reason, dogs seemed to be less likely to die from vaccination-preventable disease than humans.
The human medics also heard how rinderpest had been eradicated, something which has not yet been attained with polio or measles. So, as we both face ageing populations, there can only be increased relevance for comparative science.
Conferences such as the Merial European Vaccinology Symposium, which provided a forum for discussion and idea-sharing, are the continuing education events of the future.
