Author: Ashley Teasley, Equine Thermal Imaging Specialist
Managing equine wellness not only involves having the ability to diagnose and treat medical conditions, but also having the ability to identify potential breakdowns in the equine athlete as soon as possible. Thermography is a valuable imaging modality that enables humans to visualize surface temperature through a pictorial representation. Dr. Tracy Turner, DVM, stated “Infrared emission can be directly correlated to blood flow” (Turner, DVM). That simple yet profound statement provides validity for the use of thermography in monitoring equine athletes and identifying areas of inflammation that may lead to injury. In order to further understand this relationship, a basic understanding of inflammation is warranted.
Inflammation is a complex, physiological defense mechanism of the body that is triggered by a multitude of stimuli both internal and external including, but not limited to, injury and pathogens. Upon visual examination, inflammation can be characterized by five cardinal signs “namely redness (rubor), swelling (tumour), heat (calor; only applicable to the body’s extremities), pain (dolor) and loss of function (function laesa)” (Punchard, Wheland and Adcock). The purpose of the inflammatory process is to remove the causative stimuli and initiate the repair of damaged or dead tissue.
The inflammatory process can be identified in three subphases: acute, subacute and chronic (or proliferative) (Scott, merckvetmanual.com). The acute phase has a sudden onset and the cardinal characteristics of inflammation are produced. One of the most essential cells responsible for activating the inflammatory response is the mast cell. These cells are widely dispersed in tissues throughout the body and at locations of contact with the external environment such as the skin, the lungs and the gastrointestinal system.
Once these cells have been activated by offensive stimuli, they release inflammatory mediators. Among these mediators are histamine and bradykinin, which are responsible for vasodilation and increasing vascular permeability. Blood flow is increased to the compromised tissue and allows for the transport of white blood cells called phagocytes, to the area and into the interstitial space. The most abundant phagocytotic cell in the body is the neutrophil and this cell is the main phagocyte of the acute phase. This increase in blood flow creates the redness, increased heat and pain associated with inflammation. Upon arrival, phagocytes eliminate pathogens, foreign particles, and damaged or dead cells via ingestion, destruction and elimination, a process known as phagocytosis.
At this point, the body is in the subacute phase of inflammation and the monocyte becomes the dominant cell responsible for removing damaged or dead tissues and cells. Once a monocyte leaves the bloodstream, it can differentiate into a macrophage and “phagocytose pathogens or toxins, secrete chemokines to recruit other immune cells, and migrate to local lymph node beds via lymphatics where they present processed antigens” (Chiu, Stephen, and Ankit Bharat). Chemokines are a type of cytokine secreted by the macrophage, the latter being essential for inducing the formation of new tissue and blood vessels. Damaged tissues are replaced with fibroblasts, endothelial cells or collagen and there are several mediators in the form of growth factors that are active depending on the type of damage experienced.
Cytokines also exhibit the ability to be anti-inflammatory as well as inflammatory. As new tissues are formed, the same cytokines are responsible for tissue destruction because of continued recruitment of more macrophages and T-cells, causes the body to enter into a state of chronic inflammation. As chronic inflammation persists, the downregulation of mediators responsible for vasodilation means that the blood flow and temperature of the injured areas decreases yet is still higher than that of tissue not experiencing inflammation.
Due to the delicate balance and incredibly intricate relationship of acute and chronic inflammation in regard to healing or disease development, it is crucial to have the ability to identify inflammation in the equine as soon as possible. Thermograms, the pictorial representations of surface temperature, create thermal patterns and are directly correlated to blood flow as stated earlier. In a clinical setting, thermography has been used to evaluate several different clinical syndromes not only in the diagnosis of inflammation, but also to monitor the progression of healing (Eddy, A.L.). It is this correlation of thermal patterns that represent blood flow and identifying early inflammation that thermography is useful along with other diagnostic tools available in equine medicine.
Sources:
Turner, DVM, Tracy A. “Diagnostic Thermography.” Veterinary Clinics of North America: Equine Practice. Vol 17. No.1. April 2001, pp 95-144. Science Direct, doi.org/10.1016/S0749-0739(17)30077-9. Accessed 3 November 2019.
Punchard, N.A., Whelan, C.J. & Adcock, I. The Journal of Inflammation. J Inflamm 1, 1 (2004). https://doi.org/10.1186/1476-9255-1-1. Accessed 6 November 2019.
Edward, BSc, BVMS, PHD, MANZCVSC, Scott H. “Pathophysiology of Inflammation.” Merk Manual, Veterinary Manual. https://www.merckvetmanual.com/pharmacology/anti-inflammatory-agents/pathophysiology-of-inflammation. Accessed 6 November 2019.
Chiu, Stephen, and Ankit Bharat. “Role of monocytes and macrophages in regulating immune response following lung transplantation.” Current opinion in organ transplantation vol. 21,3 (2016): 239-45. doi:10.1097/MOT.0000000000000313. Accessed 25 April 2020.
Eddy, A.L., Van Hoogmoed,L.M., Synder, J.R. “The Role of Thermography in the Management of Equine Lameness.” The Veterinary Journal. Vol 162. No.3. 2001, pp172-181. Science Direct, doi.org/10.1053/tvjl.2001.0618. Accessed 25 April 2020.