I shall consider several forms of laminitis which are not the typical classical type: static laminitis, transport laminitis, road founder, other anoxic situations, and road founder. There are considerable overlaps of these categories as will be apparent.
This condition was first described, I believe, by clinicians at Colorado State University. Animals with severe lameness of one foreleg such as a broken bone routinely developed typical clinical and gross pathological laminitis in the other foreleg. It became apparent that this was associated with confinement to a stall. When the lame animal was allowed to move freely about a large paddock, laminitis did not develop. It appears quite clear, to me at least, that the pathogenesis was failure of adequate venous return from the foot with stagnant anoxia as the immediate cause of the laminitis.
Other Anoxic Situations
The extensive and beautiful work of Chris Pollitt on the pathophysiology of classical laminitis indicated that the basic or primary step in the pathogenesis may not be vascular. I am not completely convinced that is always the case as indicated above and elaborated below. That is not to denigrate Dr. Pollitts work; it is simply and without question the best work done to date on classical laminitis.
Routine gross and/or histological examination of the feet of horses dying with gastrointestinal disease (colic), postparturient metritis, severe renal disease, etc. sometimes reveals early changes in the sensitive laminae of the foot and the dermoepidermal junction of the chestnut (Rooney 1984). While these changes appear to be consistent with Pollitts hypothesis, they also appear to be consistent with anoxia (the inadequate perfusion of the circulatory system in these conditions).
Occasionally, mares late in pregnancy, particularly older mares, appear to develop signs of laminitis. These signs usually disappear after foaling but may recur in a later pregnancy. I know of no pathological study of such cases (they dont die!) and suggest that this is an anoxic situation very much like both static and transport laminitis and road founder. That is, the older mare late in pregnancy moves around less than usual is more static. When she does move, the feet experience greater resistance to breakover because of increased weight and, usually, because the feet have not been trimmed during the later stages of pregnancy and the toes have grown out.
Road founder is tearing of the tissues between the distal phalanx and the hoof wall. [This is discussed, and an illustration provided, in the essay on white line disease on this web site.] The lesion is most severe and obvious at the apex of the third phalanx, extending a variable distance toward the quarters. Histologically, there is disruption and tearing of collagen fibers with hemorrhage and edema. With time the damaged area is filled with granulation and, eventually, scar tissue.
In the old literature this was described in carriage horses, animals with heavy bodies, relatively light legs, and long toes for flashy, high action. These animals characteristically moved at speed on hard surfaces, for considerable distances, and pulling considerable loads. This combination of conditions is not often met with in modern times, but road founder does occur in heavier horses with long toes. It is, also, not uncommon in Shetland ponies in the Spring of the year. These animals bred for and adapted to the harsh conditions of the Shetland Islands grow long toes and put on weight with astonishing rapidity in the flush grass of early Spring. The toes are not worn down on grass pastures as they would be on the stony island ground. I do not wish to imply that every Shetland pony with clinical signs of laminitis has road founder, but it is a not insignificant cause of acute lameness in these animals.
One might think of Standardbred racehorses as candidates for road found since they move at high speed on hard surfaces. While there are undoubtedly some cases in these animals, the care given their trimming and shoeing and absence of excessive weight of animal or load, is protective.
The clinical signs with road founder can vary from slight to severe lameness of one or both front feet and in the acute stage are not readily differentiated from classical laminitis. The subsequent clinical course is less catastrophic, of course.
For those who might be interested there is information on the normal wearing of the foot on this web site.
Rooney, J R and Robertson, J L (1996) Equine Pathology. Iowa State University Press, Ames.
Rooney, J R (1984) Arteriovenous anastomoses in the digit of the horse. Journal Equine Veterinary Science 4:182-3.