The final theory was presented by Dr. Lenn Harrison, head of the Kentucky Livestock Disease Diagnostic Center, and private practitioner Dr. Stuart J. Burns. Their hypothesis is that a plant cyanogen called mandelonitrile might have caused the spring syndromes.The theory contends that cherry trees produce cyanogens, including mandelonitrile, in response to drought, freezing, and insect stress. The weather pattern caused the cherry trees to have increased concentrations of cyanogens, making the leaves an "enriched bomass" on which the Eastern tent caterpillars fed. Once the caterpillars had eaten the leaves of cherry trees, they traveled to look for other food sources and water. On their travels across horse fields, they regurgitated the mandelonitrile."In MRLS, the Eastern tent caterpillar served as both a bioconcentrator and biodistributor of mandelonitrile and became the conduit for this molecule to pass from the plant kingdom to the animal kingdom with dramatic results," they said. But how did this chemical supposedly get into horses?"The contamination of the pastures by mandelonitrile, and/or hydrogen cyanide plus benzaldehyde (all of which are produced rom the breakdown of prunasin in the cherry tree) occurred on or about the 20th, 21st, and 22nd of April when the caterpillars left the defoliated trees. On these dates, there was little or no wind, no rain, and the ambient temperature was 15 to 20 degrees above seasonal norms. Most cyanogens will vaporize at room temperature, and mandelonitrile vapor is heavier than air, so it would be expected to hover at ground level in low or confined areas."They propose that because this toxin would be diluted in the environment (pastures), that is why horses either got a low dose or did not display any clinical signs of toxicity; only about 5% had general weakness and tremors."The route of entry of mandelonitrile was most probably by inhalation or ingestion," they noted. "If exposure to this tissue irritant was by ingestion, one would expect to observe intestinal irritation, diarrhea, protein loss through the bowel wall, followed by some degree of inflammation in the liver if ingested in sufficient amounts. None of these lesions was part of MRLS. In contrast, if the agent was inhaled while grazing, it would cause inflammation in the lungs. The insult was not fatal, and no adult lungs were available for necropsy. Leaving the lungs, the irritant would next be carried to the heart, where sometimes epicarditis was observed. The next tissue with a high demand for blood would be the brain and eyes, where uveitis was sometimes observed."As mandelonitrile moved through the mare to affect the foal, it would first pass through the placenta. Its irritation would cause placental inflammation with premature placental separation (red bag). Next the irritant would pass through the umbilicus, causing inflammation. This lesion was observed consistently. "Once in the foal, the blood would follow a similar path as it did in the mare. The fetal lung should be inflamed, which it was. The irritation from mandelonitrile would damage the capillary walls and cause protein leakage and hemorrhage. This was observed in the eyes of the affected (live) foals. Term foals that were born alive were born in respiratory distress from the induced pulmonary inflammation and premature placental separation. These foals had the appearance of being septic, but blood cultures were almost always negative."The foals had low circulating protein levels from the leakage from damaged capillaries. The white blood cell counts were extremely low because the inflammatory cascade initiated by the mandelonitrile and/or behzaldehyde 'activated' the white blood cells, causing them to adhere to the capillary walls, which removed them from the general circulation."One might consider this a form of secondary immunosupression because the white blood cells were not available to fight the opportunistic invader. The adhering white blood cells in the capillaries eventually occluded blood flow through the capillary, causing a secondary tissue hypoxia (lack of oxygen) in the cells supplied by that section of capillary. These were all consistent findings in the clinical cases observed and in the necropsied fatalities. (Burns also mentioned the unusual laminitis cases and the possible connection to ischemia or lack of blood flow.)"The clinical appearance, laboratory findings, and necropsy results are all compatible with what one might expect from a low level dose of a systemic irritant such as mandelonitrile and/or benaldehyde."In looking beyond horses and to other species present while the insult was occurring last spring, Harrison and Burns noted that veterinarians doing bovine embryo transfer reported an unusually high frequency of early pregnancy failures during the time of MRLS. Necropsy reports at the Diagnostic Center recorded bovine cases similar to MRLS, and some goat flocks also appeared to have been affected.They will be conducting a project to attempt to reproduce the clinical signs of MRLS using the toxin mandelonitrile.Cyanide, Cyanogens, and MRLS
Dr. Tom Tobin, a well-known toxicologist and chemist with the Gluck Equine Research Center, had to start at square one when the industry wanted to study cyanide in horses. He noted that cyanide kills by blocking oxygen utilization in cells. Cyanide is "a small molecule that is highly volatile and difficult to analyze that is rapidly lethal at small doses," noted Tobin.Tobin found in a recent study on horses that mandelonitrile as an oral dose has a nearly immediate reaction (within three minutes) and was extremely toxic, resulting in weakness and incoordination. When trying to determine how cyanide and/or mandelonitrile caused fetotoxicity, Tobin said he thinks the cyanide likely enters the fetus rapidly and blocks "oxidative bursts." These are when white blood cells take up oxygen rapidly and produce oxygen radicals that are used to kill bacteria. If the white blood cells are "sludged," they can't function to kill bacteria, and you see low white cell counts. This also leaves the animal open to invasion by secondary bacterial invaders that don't like oxygen (anerobic).Research is continuing to rule in or rule out cyanide in MRLS.Forage-Induced Electrolyte Imbalance
Dr. Tom Swerczek was performing necropsies in the late 1970s and early 1980s when there was a limited number of fetal losses occurring that appears to be similar to what was experienced in 2001. He said in 2001, a cattle farm with sudden deaths after the freezes helped him focus on studying frost and freeze damage causing a mineral imbalance in plants.Swerczek theorized that potassium, which is the natural "antifreeze" in plants, spikes when there is a freeze and the potassium is "caught" in the plant. Drought also can cause a rise in potassium in pasture grasses. Then when animals graze the grasses, it causes a mineral imbalance in their bodies. He noted a "massive freeze" that occurred in Florida in 2000 was followed by one farm reporting 100% of the foals lost with something similar to MRLS.