Etiology

• Preliminary observations in 2001 indicated that reproductive losses occurred simultaneously over a wide area of central Kentucky on many farms commencing in the latter part of April. This observation was more indicative of an environmental toxin as distinct from an infectious agent.
• Studies to identify possible toxins included mycotoxins, fungal endophytes, phytoestrogens and chemical compounds all gave negative results. 
• At the present time the exact etiology of MRLS has not been determined.
• Experimental studies undertaken by scientists at the University of Kentucky during 2003 suggest that the factor from ETC giving rise to abortion is present in or on the exoskeleton. 
• Another study demonstrated that irradiated ETC (which would destroy infectious agents carried by caterpillars) could induce fetal loss in late-term pregnant mares.
• One hypothesis is that MRLS is caused by an ETC-related toxin, with secondary opportunistic bacterial invasion of the fetus.
• Al alternative hypothesis is that MRLS is caused by the hairs of the caterpillar breaching gastrointestinal mucosal integrity, thus allowing opportunistic bacteria access to the bloodstream, and resulting in bacteremia.
• The majority of bacteria found in cases of MRLS are non-hemolytic Streptococcus spp and Actinobacillus spp.

Predisposing factors

General

• Bacterial infection in cases of EFL and LFL represent an ante mortem event and most pathological lesions could be attributable to bacterial infection.
• However, epidemiological findings and bacterial isolation suggest a secondary role for bacteria. Bacteria gain access to the amniotic fluid and sites with direct contact with this fluid but show little tendency to invade.
• The high incidence of red bag syndrome suggests premature placental separation indicative of placental damage.

Timecourse

• Field and experimental studies indicate the onset of both EFL and LFL following exposure to ETC is rapid, a matter of days, whereas the onset of endophthalmitis and pericarditis is more protracted.

Epidemiology

• An extensive epidemiological study undertaken in the summer of 2001 on 133 farms in central Kentucky identified high to medium levels of ETC and cherry trees as strongly associated with a high incidence of MRLS.
• Field and experimental studies in 2002 confirmed the role of the ETC.
• The overwintered eggs of the insect hatch in the early spring over a 3 week period that is influenced by an increase in mean daily temperatures. In Kentucky, this coincides with about 50% bloom of Forsythia occurring in late March and early April.
• The emerging larvae     (caterpillars) build a communal silk shelter (tents)     which are visible in tree branches through the months of April and May. 
• ETC are ravenous feeders and when present in massive population densities     (as occurred in Kentucky in 2001) rapidly defoliate their natal tree before falling to the ground in search of another food source. It was during this period (late April to early May) that horses in Kentucky were exposed to caterpillars in 2001 and 2002.