When Asad stepped of the trailer last September, he had rubbed most of the hair of off his dock. He still had long tail hairs from the nottom third or so, but the top two-thirds was rubbed to shiny bare skin. Since he did not rub his tail once he got here, I assumed that he had rubbed the hair of his dock during the 16 hour trailer ride it took to get him here. I also assumed that pinworms were susceptible to moxidectrin and ivermectin.
When Asad began to rub his backside bloody as well as bald, I learned differently. It turns out that pinworms may never have been as susceptible to any of the moxid-iverm-ectin etc class of wormers as veterinarians thought. Which in turn means that the pinworms that survived are now completely unphased by and resistant to that class of wormers. In fact, vets are now advising that pinworms are a problem in mature horses on a once appropriate worming schedule that can cause depression and loss of appetite and even colic as they mostly inhabit the right dorsal colon.
The benzimidazole class of wormers is recommended for killing pinworms. So I gave Asad a normal dose of Oxibendazole. That did give him relief from the itching, for about three days. It also gave him diarrhea.
So I went back to searching the internet for what might kill off the little buggers. I wasn’t happy to learn that vets were blaming owners underdosing their horses for the drug’s failure to kill worms. There is a very fine line between a therapeutic dose and a toxic dose of anything that is intended to kill worms. Since the results of over-dosing any of the benzimidazole class of wormers is ‘idiosyncratic hepatotoxicity’, meaning the liver is so overwhelmed by the medication that it can no longer function, I was not about to randomly increase the dose until some pinworms decided to quit wriggling.
Pyrantel pamoate, usually sold as Strongid, is generally one of the least toxic wormers. Double the usual dose does not harm the animal and will kill tapeworms as well as pinworms. The problem is that the small print will now tell you that pyrantal pamoate does have toxic side effects when combined with organophosphates. What organophosphate actually means in this context is fly-spray. Apparently actually coming out and saying you should not give Strongid at the same time as you are spraying for flies is just not the done thing.
Asad hates fly spray on his body, so I put it on the side wall of his shelter where he can choose to stand in a cloud of pesticide funk or not as he pleases. But I still decided to wait to use Strongid until after the first frost kills of most of the flies. For now, I decided to use Panacur’s 5 day larvicidal mebendazole protocol. Mebendazole can and has been used with the organophosphate trichlorfon, sometime sold as dichlorvos, without signs of clinical toxicity like liver failure.
For the last five days, I have given Asad 10mg/100 pounds of mebendazole in hte morning. The first day, he stopped rubbing his tail and his diarrhea eased off. On the third day, his diarrhea got worse again, his eyes began running yellowish green mucus and he coughed every time he trotted about or rolled.
Today is the fifth and last day on Panacur and while Asad’s eyes still have some slightly cloudy mucus, it is not green or yellow any longer. He is not coughing when he trots or rolls. He is not rubbing his tail and his diarrhea is easing up. He has also quit ripping his mane out scratching his neck and shoulders.
Asad’s appetite is good. And while I definitely feel lethargic when it is 95+ degrees, he is pretty cheerful and willing to bounce about, even more so when the temperature cools off. So I am pretty sure that his liver function is OK.
In my mind I know it takes about a year for a rescue horse to completely work through the problems they show up with. I am hoping that I have finally managed to kill off enough of his parasites that his immune system can kick in and kill off any re-infestations. I am also really glad that I’ve managed to do it without causing him even more health issues.
But every time I have to actually live through that first year with a rescue horse, problem solving every issue that appears, I swear I won’t be doing it again.