The first foal was born March 18 and died March 20. The University of Florida herd of research mares lives near Ocala, Fla., until about 30 days prior to foaling, when they are brought to the university's farm in Alachua County.
The Quarter Horse foal was born on a private farm March 25 and died March 26.
Zimmel said there is still one suspect case that was a late-term abortion from a Thoroughbred mare from the University of Florida breeding herd.
Eastern tent caterpillars (ETC) were found in fields with the dams of all foals.
While no definitive cause was found for the MRLS outbreak in Kentucky in 2001-2002, a strong correlation was found between the losses and the ETC. Subsequent research showed that feeding ETC to pregnant mares caused MRLS-type abortions. Specifically, the research showed that feeding the outside of the caterpillar (as opposed to the inside of the caterpillar) caused abortions.
University of Florida pathologist Dr. John Roberts, who is handling pathology on the foals, performed necropsies at the University of Kentucky's Livestock Disease Diagnostic Center during the 2001-2002 MRLS outbreak.
Bruce Web, PhD, a University of Kentucky entomologist, said because research showed a strong correlation between MRLS and Eastern tent caterpillars, his advice to Florida horse breeders is the same as to Kentucky horse breeders: Avoid exposure of pregnant mares to Eastern tent caterpillars. "It's mostly an awareness and preventive issue," he said.
Webb said he is gearing up this spring for some virus work associated with MRSL. "Around here (Central Kentucky), the tent caterpillars hatched in the last couple of days with warm weather." The hatch is nothing like the excess seen in 2001-2002.
Jimmy Henning, PhD, a forage specialist with the University of Kentucky who did extensive research during and after the 2001 MRLS outbreak, said Florida horse breeders are benefiting from that work. "The thing they need to take some comfort in is that we analyzed everything that we could -- mycotoxins, cyanide in white clover, fescue ... We spent tens of thousands of dollars on assays. It all came up negative. The only correlatives are cherry trees (a favorite food of ETC) and a high number of caterpillars around mares.
"We analyzed many, many, many things," Henning continued. "We never found any evidence of toxins in grasses. Tall fescue has some toxins that affect horses, but those symptoms weren't the same (as seen with MRLS)."
Lee Townsend, PhD, an extension entomologist at the University of Kentucky, said much has been learned about control of ETC.
He said insecticides can be used to control ETC. Insecticides made from Bacillus thuringiensis (BT), a toxin from a soil bacterium, are specific for caterpillars and have a good margin of safety for beneficial insects, humans, and animals, and they can be used for application around pastures.
The BT insecticides have to be eaten by caterpillars. "Spray the foliage around the nests in order to control caterpillars," said Townsend. "Don't spray nests or worms; treat the foliage they are going out and feeding on." He said the insecticide works slowly. Caterpillars stop eating very quickly because the gut becomes paralyzed, but they don't die right away.
BT insecticides are broken down quickly by sunlight, so farm owners need to come back and look and see what kind of effect they're getting in terms of control. "Look at nests and see if caterpillars are there and active," advised Townsend.
Pyrethroid insecticides are also effective against ETC. Townsend said there are a number of products registered for control of a large number of insects on shade trees and ornamentals. Pyrethroids offer quick "knock-down" of the caterpillars. Townsend said the insecticide can be sprayed directly on the caterpillars or can be used for treating foliage on which the caterpillars feed.
"It has a longer residual," said Townsend, meaning it says in the environment longer before breaking down and becoming ineffective.
Townsend warned that even dead caterpillars can be a threat to pregnant mares. "If caterpillars are falling to the ground (in the pastures), you need to keep mares away from the dead caterpillars."
Another approach to kill ETC is with insecticides injected into the trunk of trees that ETC nest in (cherry, apple, or other Prunis-type plants). Townsend said this requires drilling small holes in the tree and injecting insecticide that is taken up into the tree by the sap flow.
"That probably works best early in season on small caterpillars, but it could be effective on some of the larger size ones as well," said Townsend. "Caterpillars rain out of trees pretty quickly when they eat the foliage."
He said that with any of those insecticide approaches, horse breeders still have the problem of exposure. He said insecticides limit movement away from trees, "which happens naturally as caterpillars crawl away for a pupation site."
Henning summarized by saying, "With pastures, don't do anything out of the ordinary except do what it takes to fence off or segregate mares away from cherry trees (or whatever host trees are in Florida) and the caterpillars. Mowing excessively or muzzling mares has not helped (prevent MRLS).
"Control is not sophisticated, but that should be comforting," he said.