Not all mares that inadvertently eat caterpillars will be likely to abort. Even under experimental conditions when mares were given large doses of caterpillar material, not every mare aborted.
The time period from exposure (when a mare eats caterpillar material) to abortion is variable and depends on individual mare factors and on the dose of material ingested.
If a mare ingests a large dose or is particularly susceptible, then it is possible that abortion can occur within days or weeks. When experimental mares were given large doses of caterpillar material, abortions began in this sort of time frame.
In most natural cases, the sorts of changes that are seen in aborted foetuses and placental tissue, are likely to take months to develop. Our best guess is that the most common intervals from exposure to abortion will range from about 1 to 4 months.
The appearance of the aborted foetus can provide important clues to this time interval. In acute abortions, there may be relatively little gross change in the appearance of the foetus and placenta, and yet the laboratory tests may confirm EAFL. Under these conditions, it would be more likely to have a larger exposure that occurred more recently (days to weeks). In chronic cases, there may be very obvious changes to the appearance of the placenta (thickening, necrosis, purulent discharge etc) and the foetus may be wasted and poorly developed. These changes suggest a chronic disease process and exposure is likely to have happened months prior to the abortion. In the field, there appears to be relatively few acute cases and this has led us to believe that the most common time intervals from exposure to abortion are between 1 to 4 months.
This has important practical ramifications. The first is when an abortion occurs or if a case of EAFL is diagnosed, you will need to look at the paddocks where the mare was between 1 and 4 months prior to the abortion and not necessarily at the paddock where she was when the abortion occurred.