*Dr. Gil Patterson is with the Swine Vet Center in St. Peter, Minn.
CLINICAL cases involving acute toxicities are rare in modern swine production systems; however, they do occur frequently enough that swine professionals cannot forget about or rule them out when working up health problems.
In a case of ionophore toxicity, it is my experience that the effects can be both very sudden and devastating to a population of pigs.
Ionophores are used most commonly in ruminant and poultry diets to control disease and promote animal growth. Toxicity can occur when these products are improperly mixed, mistakenly fed to a non-target species or combined with other antimicrobial products that amplify their effects.
In the following case study, a population of nursery pigs was accidentally fed a diet containing an ionophore. At the time of this particular incident, there was no legally approved ionophore for swine in the U.S.
The following case took place over the course of approximately two weeks on one single-site farrow-to-finish farm in the Midwest.
The farm had 500 sows in a batch farrowing system that produced weaned pig batches every four weeks. Nursery and finishing barns were run in an all-in/all-out manner, and pigs were fed a complete diet consisting of a corn/soybean meal base.
Starter nursery diets were made at a regional feed mill and trucked to the farm site. All other feed milling, including grower, finisher and sow diets, was completed on site with stored bulk ingredients. The sow herd was porcine reproductive and respiratory syndrome virus positive and would intermittently produce positive weaned pigs (confirmed by polymerase chain reaction testing of weaned pigs).
When clinical signs in this case began, the approximately 400 nursery pigs were 13 days postweaning.
Day 1. The grower reported that when he entered the barn to complete morning chores, he found several dead pigs throughout the nursery room. In addition, approximately 30 other pigs were down, weak and unable to rise.
Throughout the general herd population, the remaining pigs were beginning to show signs of anorexia, dullness and weight loss. The recumbent pigs showed varying severities of weakness and lack of coordination. Neurological symptoms such as dizziness, down/paddling and nystagmus were not observed. Diarrhea and respiratory signs were not present, and rectal temperatures did not reveal fever among the pigs.
Water and feed availabilities were appropriate, and the room environment was comfortable.
Postmortem exams of dead pigs were visually unremarkable, but tissues were submitted to the University of Minnesota Veterinary Diagnostic Laboratory for additional workup.
A further discussion of feed rations indicated that the pigs had been transitioned to the phase 2 starter ration two days earlier. Based on the severity of signs and the bacterial pathogen history of this farm, it was recommended that all pigs be individually treated with a broad-spectrum injectable antibiotic to provide coverage against any potential primary or secondary bacterial pathogens.
Day 3. The pigs' clinical signs were not improving. More pigs continued to become recumbent, were euthanized due to failure to improve or acutely died. Morbidity of the general population was approaching 100%.
No significant findings were observed from the initial tissue samples submitted for diagnostic workup. At this time, all remaining pigs were retreated with a different injectable antibiotic.
Feed and water samples were taken for analysis. Ten whole pigs were also taken to the diagnostic laboratory for additional testing.
Day 4. Clinical signs continued to worsen. At this point, all of the feeders were dumped and refilled with a commercial starter pig ration. Contact was made with the feed mill to inquire about any potential feed mixing errors that could have occurred.
Days 5-6. Many of the chronically affected or previously recumbent pigs continued to die or were euthanized. No new cases of pigs going down or acutely dying were reported. The pigs' appetite improved gradually as time progressed following the change in feed.
Day 7. Histopathology analyses from submitted pigs showed all samples having consistent lesions in the skeletal muscles. The pathologist reported that the lesions were characteristic of ionophore toxicity. Feed samples that had been collected were sent in for a toxicology panel.
The remaining pigs on the farm seemed to be improving each day following the switch in diet.
Day 14. Quantitative results of the feed sample showed that monensin, an ionophore, was present in the feed at 472 parts per million. Monensin is toxic to pigs at 200-220 ppm. This confirmed a diagnosis of ionophore toxicity for this nursery group.
The case described here draws attention to the potential risks for feed toxicities to occur in swine diets.
After inquiring with the feed mill mentioned in this case, it was realized that a cattle ration containing monensin was hauled just prior to the mixing and hauling of the swine ration that went to the affected swine farm. The routine flushing procedure between the manufacture and hauling of the ruminant and swine rations had not been completed.
In total, 145 pigs out of the 400 placed died or were humanely euthanized due to complications brought on by the toxicity. Nearly all pigs that were affected to the point of being recumbent were not able to recover following the feed change.
It was estimated that the surviving pigs were set back at least two weeks in terms of weight gain. It is unknown what, if any, future growth and efficiency complications resulted from this initial insult.
It is my hope that the lessons learned in this case may help others recognize and diagnose ionophore toxicities so that corrective measures can be taken as soon as possible.
In this case, it was not until the contaminated feed was removed that the rapid deterioration of pig health started to reverse, indicating that early recognition of clinical signs may help minimize losses.
The clinical signs of weak and recumbent pigs in the absence of neurologic signs, along with the high percentage of anorexic pigs among the general population, are consistent with other reported cases of ionophore toxicity.
It should also be noted that, in this case, the veterinary pathologist was able to pinpoint the diagnosis because of specific lesions observed in the skeletal muscles of submitted pig samples. This is important to remember because skeletal muscles are not often included in routine tissue submissions from the field.
Just recently, an ionophore has been licensed for swine in the U.S., so this is a good reminder of the dangers of toxicity if mixed incorrectly.
Finally, it is known that certain medications -- tiamulin in particular -- can slow the metabolism of ionophores in the body, which can increase the severity of toxicosis when they occur. For this reason, manufacturer label recommendations should be followed.