A one health approach to high blood cholesterol
As the past 18 months have shown us, the health of humans, animals and ecosystems are interconnected. Here, Dr Simon Spiro, Wildlife Veterinary Pathologist at the Zoological Society of London, explains how studying disease in one species can further our understanding of pathogenesis in humans.
The great joy of being the veterinary pathologist for the Zoological Society of London (ZSL), the international conservation charity behind ZSL London and Whipsnade Zoos, is getting to work with the widest possible range of species, from the smallest corals to the biggest whales. Every species presents its own challenges at post mortem. For an elephant, these are mostly solvable with a team of helpers and a JCB, while a starfish may require long hours of reading and Googling just to familiarise myself with the anatomy. Against this background, a post mortem of a medium-sized, non-venomous, soft-bodied animal like a meerkat may seem like an opportunity to relax into familiarity. However, as this case will demonstrate, nothing can be taken for granted in the world of wildlife.
The great joy of being the veterinary pathologist for the Zoological Society of London … is getting to work with the widest possible range of species, from the smallest corals to the biggest whales.
Meerkats are a common species at zoos around the world and their high levels of activity and anthropomorphic qualities make them a favourite of visitors and keepers alike. We were all very saddened when one of our older meerkats, Hari, was found collapsed and semi-responsive in his enclosure at ZSL Whipsnade Zoo last December. Hari was immediately rushed to our veterinary hospital, where radiology revealed changes to the meninges (the surface coverings of the brain) that blurred the distinction between brain and skull. Because of the poor prognosis and lack of response to supportive therapy, the sad decision was taken to put Hari down. Post-mortem examination quickly revealed the true nature of the brain lesion − a gelatinous mass fusing the parietal lobes of the brain and the overlying skull into a single, indivisible unit.
Meerkats are most closely related to cats, of all the domesticated species. If I had seen a similar lesion in a domestic cat, I would have expected the symptoms to have been caused by tumours, such as a meningioma or osteosarcoma, or a chronic inflammatory process such as osteomyelitis. Histopathology revealed that the true diagnosis was one almost unique to meerkats − a meningeal cholesterol granuloma. Cholesterol granulomas are an inflammatory lesion formed of needle-shaped cholesterol crystals surrounded by foreign-body type inflammation. In humans, they are most commonly found in the petrosal bone around the ear, while in veterinary medicine they are sometimes found within the lateral ventricles of horses’ brains. In both cases, they are thought to be the result of chronic haemorrhage, with the cholesterol-rich membranes of the blood cells slowly breaking down and inducing a granulomatous response. In meerkats, no association with haemorrhage has been shown. Instead cholesterol granulomas seem to be directly related to high blood cholesterol (hypercholesterolaemia).
In human medicine, hypercholesterolaemia leads to cholesterol plaques being deposited in the walls of arteries (atherosclerosis), potentially leading to coronary heart disease. In meerkats, these plaques are deposited in the brain causing neurological disease. In both humans and meerkats, hypercholesterolaemia is, at least in part, the consequence of poor diet choice. In the wild, meerkats eat a wide variety of arthropods, such as beetles, termites and scorpions. In captivity, however, such variety is hard to replicate, and zoos have traditionally fed meerkats high calorie, high fat foods like mealworms, mice and day-old chicks. These are the meerkat equivalent of burgers and chips.
By thoroughly investigating … we hope to not only improve the health of captive and wild meerkats around the world, but also understand more about the fundamental pathology of disease in animals and in humans.
At ZSL, we carefully monitor the blood cholesterol of our meerkats and feed a high protein, low fat diet including crickets and mixed vegetables to minimise and manage the risk of hypercholesterolaemia, although cases such as Hari’s do still present. We know that a minority of visitors to our zoos ignore our signage and will attempt to feed the meerkats with inappropriate treats. When we were closed during the COVID-19 lockdowns we saw the meerkats start to lose weight, suggesting that they were receiving significant amounts of unsuitable food in this way.
ZSL takes a rounded approach to animal health, and employs specialists in veterinary medicine, pathology, behaviour, welfare and nutrition to tackle these knotty problems. By thoroughly investigating and publishing these cases, we hope to not only improve the health of captive and wild meerkats around the world, but also understand more about the fundamental pathology of disease in animals and in humans.