- The first major difference is in the number of lymph nodes. Humans have ~460 while horses have ~8000 (1).
- Horse lymph nodes tend to be smaller in size (2).
- Similar to dogs, horse’s collecting, i.e., afferent, lymphatics collecting lymph from interstitial spaces tend to be thinner than those of humans and goats (3).
- Unlike humans, horse cisterna chyli, the starting point of the thoracic duct, have valves (4).
- Horse collecting lymphatic walls have substantially fewer smooth muscle cells (5). While human lymph vessels have well-developed smooth muscle fibers, horse thoracic duct, the main lymph vessel that empties lymph fluid into the blood circulation, appears to completely lack musculature at certain points (6). Similar lack of lymph vessel musculature has also been observed in dogs (6). The human’s erect gait may have contributed to greater musculature of human lymph vessels (7).
- Skin-associated collecting lymphatic walls are ~40% elastic fibers. According to Rebecka Blenntoft, horse skin is akin to a ‘compressive bandage‘. Absence of lymphatic musculature, especially in their limb lymphatics, means that horses depend far more than humans on physical movement to maintain limb lymphatic flow towards the thoracic duct.
- Another major difference between horse and human lymphatic system stems from different eating habits. Again, according to Rebecka Blenntoft, horses evolved to be ‘trickle feeders‘, i.e., small amounts of forage taken in over prolonged grazing periods. Thus, horses need forage to maintain continuous peristalsis through their GI tract. In turn, major lymph movement in the horse’s GI tract requires peristalsis. This is emphasized by the fact that ~4000 of the horse’s ~8000 lymph nodes are present in its ascending colon area.
- Baum, H., Trautmann (1933), Das Lymphgefäßsystem der Säugetiere. In Bock —Göppert —Kallius —Lubosch, Handbuch der vergleichenden Anatomie der Wirbeltiere. VI. Leipzig.
- Lymphatics and Lymph Circulation. Physiology and Pathology. By Istvan Rusznyak, Mihaly Foldi, Gyorgy Szabo. 1967. Pergamon Press. p.66.
- C. C. C. O’MORCHOE. Anatomy of lymphatic system. p.126-134. Blood Vessels and Lymphatics in Organ Systems. Edited by David I. Abramson and Philip B. Dobrin. 1984. Academic Press.
- Baum, H. (1928), Lymphgefäßsystem des Pferdes. Berlin. p.77.
- Blenntoft, Rebecka. “The importance of the lymphatic system.” equine Health 2012.6 (2012): 36-41.
- Richter, Η. (1907), Eine Untersuchung über den histologischen Bau des Ductus thoracicus des Pferdes. Berl. tierärztl. Wschr., 213.
- Ranvier, L. (1873), Du S y s t e m e l y m p h a t i q u e . Lemons recueillies par le dr. Weber. Progrés Med. 1 , 25, 51, 73, 99, 145, 181, 206.