Mare Reproductive Loss Sydrome - Equine Breeding Centre and Veterinary Services

MARE REPRODUCTIVE LOSS SYDROME (MRLS)

Those bloody caterpillars are back again & thicker than ever!!! I don't know if they cause abortion but I can tell you I wouldn't let a pregnant mare of mine anywhere near them!! Time to recap on what we know about them & some new control measures.

Mare Reproductive Loss Sydrome (MRLS) Hypothesis:

That some of the abortions seen in Australia are caused by ingestion of setal fragments of the Processionary Caterpillar (Ochrogaster lunifer)

Mare Reproductive Loss Sydrome (MRLS) Evidence:

There are marked similarities between the abortions seen in Australia and those seen in the "Red Bag" abortions in Kentucky in 2002. The abortions in the U.S. are suspected of being caused by the Eastern Tent Caterpillar (Malacosoma americanum).

What did they find in Kentucky?

  1. Horses do eat the caterpillars & their larvae.
    Visit www.ca.uky.edu/gluck/mrls/index.htm where you can see a video of them eating.
  2. Many aborting horses had access to the American Tent Caterpillar and their host the wild cherry tree.
  3. Septic barbed setae fragments from the caterpillars have been found to penetrate the mouth and intestinal tract on ingestion.
  4. The setae fragments and their hitchhiker bacteria migrate through the tissues and enter the blood stream to spread via the blood stream to all tissues including the foetal fluids. Evidence suggests that the setae do not carry their own bacteria but simply pick up bacteria from the mares tongue, throat & oesophagus & carry them to the blood stream.
  5. Most tissues mount an immune response but the foetal fluids have a very poor immune response and are highly susceptible to infection.
  6. As few as 10 setae fragments ingested per day may cause abortion.
  7. Muzzling horses in infected areas appears to prevent the syndrome.
  8. Very slight bacterial contamination of the amniotic fluid causes rapid bacterial growth and subsequent foetal death and abortion, sometimes in as little as 36 hours.
  9. The caterpillars may migrate up to a kilometer from their host and trees.

What have we found in Australia?

    1. Many aborting mares have had access to the Processionary Caterpillars.
    2. Processionary Caterpillars have been seen in their highest numbers in recent history over the last two years.
    3. The natural host species of the caterpillars is white box and acacias (wattles) in particular.
    4. An individual colony can consume foliage equivalent to that carried by a tree 2 meters tall. Defoliated trees usually recover when the caterpillars move on.
    5. The caterpillars are grey & hairy with a brown head.
    6. Hairs can cause sensitivity if come in contact with the skin.
    7. Moths have grey or brown wings & a yellow banded abdomen which ends in a white tuft of hairs which can also cause sensitivity. Some have white lines across their wings.
    8. There may be 2 species of caterpillars; one nesting in the branches & laying eggs there & the other nesting at the base of their trees & laying eggs there. The ground - laying species seems to be more common on the costal belts.

    WHAT DO WE KNOW ABOUT THE LIFE CYCLE ???

    1. Eggs hatch in Spring to form larvae (caterpillars)

    2. Newly hatched caterpillars spin a silken nest high in the tree branches or in dead leaves & humus at the base of the trees.

    3. Caterpillars emerge from their nests in the evenings & at night, provided it is not too hot or too wet, to feed on the leaves of the host tree. They return to the nest 2 to 3 hours later.

    4. During the heat of the day the caterpillars stay in their nests.

    5. Caterpillars take 4 to 6 weeks to mature & reach a size of up to 4 cm.

    6. Mature caterpillars leave their nests & go looking for somewhere to pupate (spin a cocoon in ground debris.)

    7. Mature caterpillars may also migrate in search of new host trees have defoliated their host tree. Migrating caterpillars leave a silken trail which will be followed by nest mates or by other migrating caterpillars forming the familiar "procession". Some processions may be of up to 135 caterpillars.

    8. Adult moths emerge from the cocoon 3 weeks after pupation begins.

    9. Female moths mate & lay several hundred eggs in a mass on tree branches. The eggs are covered by a thin layer of slime from the moth's tail.

    10. The female moth then dies.

    11. Eggs over-winter in the tree branches & hatch in spring.

    Suggested Control Methods

    1. Removal of access to caterpillars by removal of trees or movement to areas without trees. Larvae (caterpillar) nests & eggs can also be removed from trees.
    2. Nests removed from trees must be smothered in oil immediately. (Caterpillars will burrow into the ground almost immediately if allowed.)
    3. Mature caterpillars are resistant to most insecticides whether in their nests or out in the open.
    4. Biological control is available using the biological insecticide DIPEL DF
    5. The drug contains the bacteria Bacillus Thuringiensis which attacks the caterpillars when they eat the leaves.
    6. Must spray early in the morning or preferably late in the evening.
    7. Drug kills moths & caterpillars only; not natural predators.
    8. Dipel does not kill immediately but caterpillars eating sprayed leaves will stop eating immediately
    9. Dead caterpillars may hang in trees for 3 to 4 days before dropping to the ground.
    10. The dead caterpillars may persist in the pasture for 6 to 8 weeks.
    11. Must keep horses off pasture for 8 weeks after spraying.
    12. Require at least 2 sprays 3 days apart & preferably follow up with weekly sprays.
    13. Must use a wetting agent at 0.01% e.g. Agral or Amino Feed or milk powder. (Ask your supplier)
    14. Must thoroughly wet the foliage if possible.
    15. Use at the high dose level ( 100gm per 100 liters)
    16. Systemic insecticides e.g. Malathion or Rogor ( Dimethoate) may also help. Drill holes every 5 cm around the tree at 45 degrees & 3cm deep.
    17. Systemic insecticides will only work when the sap is running i.e. not in autumn.
    18. Natural predators especially wasps should be encouraged & not killed.
    19. High levels of alternative vegetation (vegetation other than the host plants) will also assist by interfering with caterpillar & moth host-seeking behavior.
    20. Parental antibiotics to attempt to control the rapid bacterial growth seen in foetal fluids when contaminated with caterpillar setae.
    21. This requires almost continuous bacterial therapy because of the rapid onset of bacteraemia within the uterus (36 hours) after contact.
    22. Progesterone (Regumate) supplementation.
    23. Avoidance of all other stresses.

    Mare Reproductive Loss Sydrome (MRLS) References:

    Henning, J.C. (2003)
    http://www.uky.edu/Agriculture/VetScience/2003/041803currentUnderstanding.html

    Tobin, T et al (2004) Intern J Appl Res Vet Med Vol. 2, No.2 pp142-157

    Naomi Strong, Zadco. Naomi is a client from Allora.