Mice Contaminated Hay – What’s the Risk?

- District Veterinarian Jillian Kelly

The current heavy mice burdens mean that our precious hay stores are fast being eaten and destroyed by the rodents.  Mice not only do damage, they carry disease that can pose a risk to our livestock, so please be cautious if you are planning on using the remaining hay.

A major risk is that mice urine can spread Leptospirosis, commonly known as “Lepto”.  There are more than 250 different serovars (or “strains”) of Lepto. Two of these strains (L.hardjo and L.pomona) are known to cause abortion in cattle, and there is a vaccine available to prevent this. Hence, I would strongly recommend if you are planning on feeding out hay that has been contaminated by mice, that you fully vaccinate your cattle for Lepto well in advance of feeding (this means giving them two shots, 4-6 weeks apart).  There is no vaccine available for sheep, but Lepto is not thought to cause clinical disease in sheep.

Mice carry many more strains of Lepto than just two though, and these other strains may infect humans that are handling the hay.  Lepto is a debilitating zoonotic disease and there is no vaccine for people, therefore the best way to reduce the risk is to limit contact with mice urine.  You may choose to do this by electing not to use the hay, using mechanical methods to move the hay so that people aren’t handling it, or wear appropriate PPE (long clothing, gloves, eye protection and a mask) when touching the hay.

Another real risk that can come from mice contamination of hay is Botulism.  Botulism comes from a bacterial toxin that lives in the dead mice carcasses.  When livestock eat the hay and pieces of the carcasses, they may become infected.  It causes flaccid paralysis which results in animals that go down, cannot eat and drink and eventually die. Large numbers of stock may be affected and there is no treatment.  Vaccination is available for cattle and sheep for botulism, but two shots are needed for most products available on the market, so again, plan ahead.

It may be prudent to vaccinate for botulism, regardless of whether you are planning to feed hay, due to the extreme contamination of paddocks with mice carcasses.  Stock that are phosphorus or protein deficient will actively select to chew on carcasses and bones and are at a higher risk.

If you’ve been poisoning mice using either anti-coagulants or zinc phosphide, the actual bait itself is a poisoning risk to livestock if present in the hay when it’s fed out, however secondary poisonings of livestock from eating the carcasses is low.

I think it is a questionable decision to feed hay now, just to use it up before the mice totally destroy it, if that means the stock are going to forego the dry standing feed in the paddock that will then blow away or deteriorate in quality as the season progresses.  However, if your paddocks need a grazing spell, confinement feeding with hay and other grains might be a suitable option.

In summary, I think every haystack and farm situation will need to be assessed individually. Break open some of the bales and see how much damage and contamination there has been. If there is a high number of carcasses and contamination, or if your farm business has a high aversion to risk, it might be best discarded.  If there is less contamination and you can take reasonable steps to manage the risks via vaccination and safe handling of the hay, then it may be useful.

If you’d like to discuss this topic or any other livestock health issue give your District Vet or Livestock Officer a call.

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