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Rodenticide Toxicity

The harrier was just quiet, not putting up any resistance, and thankfully, did not hurt the person who found it by the roadside. They explained it was ‘just sitting there, so I stopped and walked up to it, and just picked it up’.  As you gently start your examination, you roll through the possibilities. Roads and cars are our stock in trade, and we know many of our patients arrive after being damaged in a collision. But this bird does not seem to have been in an accident as we do not see any obvious damage! Maybe internal injuries? Next, we consider what else brings raptors into our rehabilitation facility. Rodenticides are another major cause for admissions of these beautiful birds. So, next we need to explore if this bird’s clotting time is off! Our vet clinic is open, so we prepare the bird for transport as we head there to pull a blood sample with the help of the veterinary nurse on duty.

This scenario plays out both here in NZ and globally as secondary rodenticide toxicity is a global problem. Second-generation rodenticides are readily available and often considered a reasonable solution for the mice and rats invading our homes. Sadly, these animals do not die immediately and often are driven outside or into the open as they become debilitated after having a final, and now fatal, feast of the rodent block. They are then picked off as an easy meal for any raptors that happen along.

This first exposure to the rodenticide may not cause any major impact to a healthy raptor, but subsequent small doses may do so as the rodenticide accumulates in the bird's liver. A large dose can cause an immediate issue, and this bird's behaviour described above may be the result of a hefty dose. Slow incremental doses are additive, and the bird’s Vitamin K reserves and then its clotting factors are depleted over time as the second-generation rodenticides stop the blood from clotting.  This slow loss of clotting ability can often show up in blood spots or petechiae on gums and tissues, bleeding from the mouth, eyes or rectal area, and in severely compromised, anaemic animals. This is so obvious when you see them; what is not obvious, however, is the case of the ingestion of a high dose of a rodenticide. There are no obvious signs except for a quiet, downed bird! Sadly, my experience with these acute cases is mixed, as is the outcome for severely compromised birds. However, that does not mean we should not try to turn these birds around. But first, a presumptive diagnosis! Getting back to the blood test.

You head to your vet clinic, or if you have training and the help, to run a presumptive clotting time by pulling a small blood sample (1ml is sufficient) and placing it in a red top tube (or sample tube with no anticoagulant). Set your timer and check every minute by inverting the tube gently. If the blood has not clotted after 2 minutes, it may be a coagulopathy linked to rodenticide exposure. This is especially likely if the clotting time is over 5 to 10 minutes.

This simple test is simply an indication that there is a clotting time issue and may help your veterinary team decide on the best support needed. The chart attached is also a great guide and gives guidance for all levels of support needed. As you are drawing blood, it is also a good idea to run a packed cell volume (PCV) or hematocrit at the same time. This will give you a baseline percentage of red cells in the bird’s blood and can be very helpful in managing the treatment of your bird. Most birds have a PCV of between 40 and 50%, so if it is lower, it may be an indication of anaemia linked to loss of blood.

So, you have a presumptive anticoagulant rodenticide-impacted bird (phew, that’s a mouthful!), your vet team has prescribed treatment, and you know the level of care needed to aid these birds in recovery. But there is another issue. As I noted, these second-generation anticoagulants are stored in the liver and studies in great horned owls, and red-tailed hawks have shown they may remain there for up to a year. For us as rehabilitators, that means a quick fix is unlikely. 

After the first course of treatment and hopefully a far more stable harrier, you need to check on its PCV to see if it is in the normal range. Yes! Great! Now, you need to wait for 7 to 10 days and repeat it. Is it still in the normal range? Yes! Great, and the bird can be prepared for release. If the answer is no, it is not in the normal range, or the PCV drops down below normal, then further treatment is needed. Remember, these chemicals are persistent in the liver for up to a year. We need to ensure that the bird we release does indeed have a good chance of getting on with its life. Please download and share the chart, as it is a great guide to help manage either a secondary rodenticide case or one of alpha-chloralose.

This article was written by

Dr Lynn Miller of WildWays for WReNNZ 



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