Hemlock, bacteria spark new MRLS dialogue

Hemlock, bacteria spark new MRLS dialogue

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Mar 01, 2002

Cleveland-Nearly a year has passed and the 2001 Mare Reproductive Loss Syndrome (MRLS) still evokes impassioned discussions of its cause, which was never conclusively determined.

New talks at meetings in central Kentucky and Clemson University pit an alpha streptococcus organism against a hemlock theory, the latter of which had been swiftly denied.

About 350 veterinarians and equine professionals met Feb. 4 in Lexington at a meeting hosted by the University of Kentucky College of Agriculture and the Equine Maintenance Managers Association to probe further the circumstantial evidence of MRLS.

Dr. Stuart Brown of Hagyard-Davidson-McGee Associates, presented data on 3,000 broodmares that lost foals early in their pregnancies.

Broodmare findings

"What led me to investigate the broodmare population was all the fetuses that were falling out in front of me," he explains. "Mares within the 60-70 day window presented with expelling fetal membranes and a mucoidic discharge."

In early May, at the syndrome's peak, Brown took uterine cultures on mares that had early fetal loss and discovered two bacterial species: Alpha Streptococcus and Actinibacillus sp. organisms. About 65 percent of samples displayed alpha bacteria; 20 percent showed actinobacillus.

Although Brown says he is not implying these bacteria are the cause, he thinks they relate to MRLS.

"Finding these two organisms is a striking occurrence. Alpha strep organism is not something you'd commonly expect to find," says Brown.

Similar data

Brown soon learned his findings matched those of Dr. Mike Donahue, supervisor, bacteriology laboratory at the University of Kentucky Livestock Disease Center. Donahue took samples of 500 late-term fetuses in 2001- 70 percent of tissues from the lung or umbilical cords contained one or both of the same bacteria. Donahue then froze 380 samples for further study.

Two ongoing goals for Donahue: 1) Determine the exact bacterial species involved. "It doesn't look like they fit taxonomy-wise any of the known streps and actinobacillus," he says, and 2) Find the source - whether it is environmental or internally existing in the horse. "The bacteria are probably normally found in the upper reproductive tract or the intestinal tract, but we haven't looked for them."

Donahue was in Kentucky the last time a similar mare reproductive catastrophe occurred in the early 80s. "If I can get ahold of some of the strains of bacteria that occurred 20 years ago, I would compare them. The outlook is not very great."

Adds Brown, "In 1981, people thought these organisms were secondary invaders. That may prove true, but we better make damn sure that's what we really think is occurring, so we don't miss a key ingredient that might steer us to figure it out. Finding out the origin of the organisms might carve out the pathophysiology of the whole syndrome."

Revisited theory

In a separate meeting at Clemson, Feb. 8, Dr. Dee Cross, professor of animal and veterinary sciences, presents telling evidence. Remember the poison hemlock idea?

In the aftermath of MRLS, Cross and other researchers collaborated to determine the differences between control pastures in Kentucky with no abortions and affected pastures. They created a checklist of potential MRLS triggers: external ergots (fungus with high levels of toxic alkaloids); pasture/forage plants; Zerealonone (fungal mycotoxin); internal ergots; cherry trees; feed; and hay. All items turned up negative, with little exception.

Researchers then began checking for plants near fencerows or tree enclosures when they found poison hemlock. "Not only was hemlock present on affected farms, but there was evidence of consumption," says Cross. "Correlation showed an extremely high relationship." (T < 0.0001: less than 1 in 10,000 chance that poison hemlock was not related to the problem, according to Cross.)

In all, 27 of 28 MRLS-affected farms, had hemlock presence and consumption. In 10 pastures that did not exhibit problems, there was no sign of hemlock consumption.

Cross then teamed with an expert in poison hemlock, Dr. Kip Panter, USDA-ARS poisonous plant lab, Utah. They sent samples from foals and plant tissue for assay.

"Plant tissue came back with toxic piperidenic alkaloids as suspected. But samples from foals and some serum samples from mares detected no levels of the alkaloids," he says.

These alkaloids have a nicotine-like effect, says Cross, who adds they are known to have caused abortions in several species, but have not been researched in horses.

Researchers then dosed two non-pregnant horses with poison hemlock to determine depletion rate (how soon after consumption it can be detected in the blood.) Hemlock reportedly cleared the horse's system in 50 hours.

"Unless you took a blood sample or had a foal exposed within the last 50 hours, you would never pick it up," says Cross. "There was no chance for us to have picked up the alkaloids."

If a future study proposal receives funds, Cross says 14 pregnant mares will be dosed at toxic levels to see if it causes abortion.

Piperidenic alkaloids

Research indicates that young, tender, as well as stressed poison hemlock has a very high level of piperidenic alkaloids, says Cross. In early spring, hemlock has high sugar content and is more palatable.

He suggests horses consumed hemlock following the unusual weather.

The reaction to the hemlock theory from scientists at the Gluck Center - "I'd rather not comment on how it was received by ... the University of Kentucky, which walked way out on a cherry tree limb," he says.

Although Dr. Roy Smith, of the UK diagnostic laboratory, saw the number of farms that showed signs of hemlock consumption as brow-raising, he quickly added that hemlock is "highly improbable as a cause," according to a Bloodhorse.com report. He based it on a sampling of 50 fetuses examined in 2001 that found no evidence of hemlock consumption.

"Smith based his comment on the fact that samples didn't show any of the residues," answers Cross. "That's why we did the research to show him you would not have found any residues unless we had taken a sample within 50 hours of consumption."

Cross stands by the hemlock theory. "We think this is the total cause. It is likely exacerbation from other things(the freeze), but this can cause those abortions. Also, literature is clear that this plant is very addictive. Reports show animals have been addicted and want to consume it again."