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April 18, 2003

Current Understanding of the Causes and Factors in MRLS

Jimmy C. Henning
Assistant Director for Cooperative Extension
Agriculture and Natural Resources
Former Extension Forage Specialist
University of Kentucky


The University of Kentucky has studied Mare Reproductive Loss Syndrome (MRLS) intensively since the spring of 2001. Many factors have been investigated to better understand their relationship, IF ANY, to MRLS. To date, the only proven way to induce MRLS is to expose pregnant mares to eastern tent caterpillars through their diet or environment. While other factors (e.g. weather, tall fescue) may play an as yet undefined role, reducing exposure to the eastern tent caterpillar is the key to minimizing the chance of MRLS.

The following represent the current management recommendations to minimize the risk of MRLS. Also, the current status and some background information on selected factors that had been previously thought as having been related to MRLS are discussed.

Current Management Recommendations to Minimize MRLS.

1. Minimize or eliminate exposure of pregnant mares to the eastern tent caterpillar (ETC).

Research and field observations implicate the eastern tent caterpillar as the primary cause of MRLS But the mechanism or causal agent has not been identified. Farms where exposure of pregnant mares to ETC was minimized or eliminated in 2002 had little or no MRLS.

2. Keep pregnant horses out of proximity to wild cherry trees.

The wild or black cherry tree is the preferred host for ETC. ETC can travel long distances (up to 0.25 miles) from the cherry trees, so look outside of field boundaries for host trees. The caterpillar will also infest other trees, including ornamental pear, crabapples, and sometimes oak.

3. Reduce exposure of pregnant mares to endophyte infected tall fescue and perennial ryegrass.

Endophyte-infected tall fescue can also cause reproductive problems in mares but has not specifically caused MRLS. Infected tall fescue may accentuate or aggravate MRLS symptoms by interacting with the unknown toxic principle of the eastern tent caterpillar. Farms should minimize exposure of mares to endophyte-infected tall fescue fields, especially late in gestation. In addition, perennial ryegrass can have an endophyte infection that may also result in reproductive upsets in the pregnant mare. Any perennial ryegrass used should be endophyte-free, which is confirmed by the presence of the green endophyte tag stating a zero or low (<5%) endophyte infection level.

Discussion of other factors potentially involved with MRLS but subsequently eliminated, minimized or of secondary importance.

1. Weather.

Why suspected: Weather, specifically an early rise in temperatures in the spring followed by late frosts, was very similar in the MRLS years of 1981 and 2001.

Status: Still under study.

Discussion: Several of the early potential MRLS factors were weather dependent. Cyanide release and mycotoxin generation were dependent on the stresses caused by the large temperature swings observed in 2001. The hemlock theory indicated the reason mares ate the hemlock (not a very palatable plant) was due to snow cover in 2001. It was theorized that frosts lead to a spike in potassium content in pasture plants, therefore raising the K/Ca ratio. All of these theories have been minimized or eliminated in importance. Weather is still being investigated for any potential effect on pregnancy failure in mares.

The complexity of weather continues to be analyzed for any potential effect on pregnancy failure in mares. Preliminary weather analysis indicated several components of weather displayed little or no correlation with MRLS. This included environmental factors such as soil temperature, snow, dew point temperature, relative humidity or solar radiation. Three environmental variables, however, showed a degree of correlation, indicating the need for further study.

Those environmental factors were:

2. Ergot alkaloids (from tall fescue or perennial ryegrass).

Why suspected: Tall fescue and ergot alkaloids were investigated for an MRLS correlation in 2001 and 2002 because these alkaloids are known to cause reproductive problems in mares.

Status: Minimized but still under study.

Discussion: Ergot alkaloids are toxins produced in tall fescue in late spring due to the presence of an internal fungus called the endophyte. Perennial ryegrass can also be endophyte-infected and can produce ergot alkaloids, as well. Ergot alkaloids can cause reproductive problems in pregnant mares, primarily prolonged gestation, foaling difficulty and lack of milk. These symptoms were not observed in MRLS cases in 2001 or 2002. The presence of endophyte infected tall fescue or perennial ryegrass had little to no correlation with MRLS in 2001 or 2002. In addition, mares contracted MRLS that were being treated with domperidone, a very effective remedy for fescue endophyte toxicosis. However, pastures are still being monitored for ergot alkaloid content because of its known ability to affect the reproductive process. In addition, there is some question as to whether a spike in ergot alkaloids coincidental with the flush of tall fescue pasture growth in the fall of 2002 could have played a role in the elevated number of abortions during this same period.

3. White clover.

Why suspected: High levels of white clover were reported for MRLS pastures in the late 1970's and also in 2001. White clover can potentially contain phytoestrogens and cyanide. Cyanide potential in white clover is based on the amount of cyanide-containing sugars. These cyano-sugars are genetically determined and moderated by factors such as weather or stress. Frosts will raise the level of these compounds and, if severe, could result in the release of free cyanide.

Status: Eliminated

Discussion: Tests for phytoestrogens in 2001 were too low to cause reproductive symptoms. The UK pasture monitoring program in 2002 found generally low levels of cyanide potential in white clover. When cyano-sugars were detected in the pastures of pregnant mares, they were not correlated to the incidence of MRLS. Research at UK specifically found that cyanide would not induce MRLS.

4. Fungal mycotoxins.

Why suspected: Fungal mycotoxins were suspected because certain ones were known to be able to produce abortions in livestock and because they could have been produced as a result of the unusual weather in 2001 (premature high temperatures followed by late frosts).

Status: Eliminated after intensive study in 2002.

Discussion: Mycotoxins are produced by fungi that are commonly present in soil and on plants as a result of stress. Mycotoxins are active at very low levels (parts per billion) and are water soluble. Because of rain on the Sunday after the 2001 Kentucky Derby, no pasture samples were available for mycotoxin analysis from fields that contained MRLS mares. Therefore, there was much early speculation that mycotoxins were the cause of MRLS. A limited number of pre-derby hay samples were analyzed for mycotoxins in 2001; all were negative. In 2002, the UK monitoring program screened hundreds of pasture samples for over 30 mycotoxins. Mycotoxin levels were low in general and did not correlate with occurrences of MRLS in 2002.

5. Mineral imbalance in pasture.

Why suspected: It was theorized by some that high levels of fertilization or other farm practices led to abnormally high ratios of potassium to calcium (K/Ca)in pastures. In addition, it was theorized that the late frosts of 2001 led to temporary spikes in potassium (K) content of pasture plants, leading to abnormally high K/Ca ratios. High K/Ca ratios supposedly led to gastric disturbances allowing bacteria from the GI tract to enter the blood stream and attack the fetus.

Status: Eliminated after intensive study in 2002.

Discussion: Data supplied by equine consultants comparing K/Ca ratios from 2001 pasture samples to previous years found years where K/Ca ratios were higher than those found in 2001 but which did not lead to MRLS. Also, the UK pasture monitoring program found that K/Ca ratios in 2002 were not correlated to MRLS. In addition, MRLS was found on pastures that had received no commercial fertilizer in the past 15 years.

6. Poison Hemlock.

Why suspected: Poison hemlock was present in many MRLS pastures in 2001. However, the main reason that poison hemlock was listed as a potential risk factor in some accounts was a controversial but unequivocal report from an equine consultant that stated there was a high probability that poison hemlock caused MRLS.

Status: Eliminated.

Discussion: UK pasture monitoring in 2002 found the presence of poison hemlock was not correlated to MRLS. While the toxins in poison hemlock can cause abortions, they also result in some very dramatic and obvious symptoms in horses. None of these were observed on MRLS positive farms in 2001 or 2002. In addition, research in 2002 on the effects of hemlock ingestion by pregnant mares produced symptoms markedly different from those of MRLS.

7. Feeding hay on pasture.

Why related: Feeding hay on pasture was common on many low MRLS farms in 2001.

Status: Eliminated as preventative for MRLS.

Discussion: It is unclear why hay feeding was found to be related to low levels of MRLS, unless providing hay made horses less likely to come into contact with eastern tent caterpillars in the pasture. Feeding hay on pasture is not considered to lower the risk of MRLS.



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