Italian researchers investigating the kidneys of horses have found a curious anomaly between the left and right organs that they cannot yet explain.
Francesca Freccero and her colleagues conducted a series of pulsed-wave Doppler ultrasound examinations of healthy horses and donkeys to learn more about their kidney function.
They used the ultrasound device to gather data on the renal arcuate arteries which enabled them to calculate the renal resistance index (RRI) — the resistance to blood flow through the tiny blood vessels of the kidneys, where waste and excess water is removed.
In human medicine, it has been long considered a good tool to evaluate kidney microcirculation. Resistance that is too high points to health problems.
However, there are few papers on the application of RRI in equine medicine.
For their study, the researchers performed ultrasounds on 33 conscious horses, aged 3 to 24, comprising trotters, Warmbloods and Thoroughbreds; and nine donkeys.
In horses, the median RRI values for the right kidney were found to be statistically higher than for the left, whereas in the donkeys, which have fewer arcuate arteries, the values were comparable.
The difference in RRI between kidneys in horses is difficult to explain, the researchers wrote in the open-access journal PLOS ONE.
Unlike other species, anatomical differences between the right and left kidney in equines are marked. However, the kidneys are of similar weight, density and volume. Even the volume fractions — the proportions of cortex, medulla, pelvis — are similar.
“Whether there are any microscopical differences in functional subunits is not known,” they said.
The study team noted that the right and left renal arteries in horses merge at slightly different levels and angulation with respect to the aorta, and the branching and distribution of the principal renal vessels is not completely symmetric between sides.
The geometry of blood vessels can affect blood hemodynamics, which might have played a part in the difference found.
“In contrast, we found no differences between the right and left kidney in donkeys.”
The researchers said the RRI values they obtained for horses fell in the lower reference ranges for small animals and humans. The RRIs in donkeys were similar and within the ranges obtained in horses.
They found no significant difference in RRI between horses younger or older than 15 years, nor between the breeds of horses used in the study.
There was no correlation between RRI and age, body weight or body condition score in either horses or donkeys.
“As in other animals, in horses body fat is the most variable body tissue, and it increases with body weight and body condition score.
Interestingly, in humans, excess weight and/or central body fat distribution are associated with an unfavorable renal hemodynamic profile. This may play a role in chronic renal damage and is characterized by higher resistance in the microvessels of the kidneys.
“The relationship between body fat distribution and intra-renal resistance in equines demand further investigations,” they said.
The authors said pulsed-wave Doppler evaluation of RRI in horses and donkeys is feasible but time-consuming, with challenges including movement from the likes of breathing and gas contents. It requires meticulousness and technical ultrasonographic skills.
They said the median RRI values obtained for the right and left kidney in horses in a larger population of subjects confirmed the differences previously reported.
They said further studies are needed to clarify the relationship between RRI and the shape and function of equine kidneys, as well as age, body weight and body condition score.
The study team comprised Freccero and Mario Cipone, who are with the Department of Veterinary Medical Sciences at Ozzano dell’Emilia in Bologna; Marina Petrucelli, who is with the School of Biosciences and Veterinary Medicine, part of the Veterinary Teaching Hospital in Matelica; and Irene Nocera and Micaela Sgorbini, who are with the Department of Veterinary Sciences, part of the Veterinary Teaching Hospital “Mario Modenato”, in San Piero a Grado.
Freccero F, Petrucelli M, Cipone M, Nocera I, Sgorbini M (2020) Doppler evaluation of renal resistivity index in healthy conscious horses and donkeys. PLoS ONE 15(2): e0228741. https://doi.org/10.1371/journal.pone.0228741