Phosphine Poisoning in Horses

Horse owners hold high expectations when it comes to feed quality, not only in terms of nutrient composition and ingredient quality but also purity, especially when it comes to insects. One way to control insects in feed ingredients is through the use of fumigants. Aluminum phosphide is a common pesticide used to fumigate feed ingredients.

When combined with atmospheric moisture, aluminum phosphide releases phosphine gas, the active form of the pesticide. “The use of phosphine is reasonably widespread across the world as a treatment for live insects in grain and finished feeds that are stored in airtight bins or silos,” said David Nash, Director of Nutrition Technology at Kentucky Equine Research. “Cereal grains are the main raw materials treated with this pesticide, but other ingredients can be treated if insects are a problem.”

For maximal effectiveness, grain should be exposed to phosphine for a set number of days and then followed by ventilation and withholding phases, the length of which depends on the individual product used. When aluminum phosphide is applied as directed by the manufacturer and the appropriate withholding periods are followed, feeding of treated grains is considered safe. When the manufacturer’s instructions are disregarded, however, phosphine poisoning can occur.

Although phosphine poisoning has been reported in horses in the United States, the first documented case in Australia was recently published.^* In this instance, 14 Thoroughbred yearlings were affected, found dead in their paddocks the day after being given feed from a silo that had been treated with aluminum phosphide two days earlier by an unlicensed pesticide technician.

Because the yearlings were found dead, clinical signs were not observed. In other cases of phosphine toxicity, though, clinical signs involved profuse sweating, elevated heart rate, fever, incoordination, muscle tremors, recumbency, and seizures. Two of the 14 yearlings were submitted for necropsy. Pathologists noted profound vascular changes, including hemorrhage, congestion, and edema of multiple organs. Phosphine toxicity was confirmed through the identification of phosphine gas of stomach contents.

Most horse owners do not worry about using pesticides because they purchase textured or pelleted feeds in bags, so the onus of pesticide use falls on the feed manufacturer. The same might not hold true for commercial farms, such as breeding farms, that purchase feed in bulk for storage in silos.

“Pesticides can be useful as part of manufacturing high-quality feeds. The use of pesticides in a mill situation should be treated with extreme caution with tightly controlled procedures,” explained Nash.  “Hygiene of the mill and storage facilities is extremely important to reduce the risk of insect infestation. For farm managers that fill silos with feed, cleaning of silos between loads greatly reduces the risk of contamination.

“In situations where feeds are placed in shipping containers and exported, the feed may spend weeks inside a container, where conditions can become quite warm and humid, which is a perfect environment for insects to breed and mature, if feed is not treated effectively,” explained Nash.

All fumigation should be performed by a licensed technician with manufacturer guidelines followed to the letter.

How can horse owners have peace of mind when it comes to where their feed comes from? According to Nash, it’s important to choose a trusted manufacturer with a reputation for safety.

“Kentucky Equine Research works with many feed manufacturers around the world. Part of this role may include assisting with the implementation and management of quality programs that address all the control and assurance measures associated with pesticides and other contaminants,”  he said. “In many areas of the world, feed companies must subscribe to a mandated quality program that addresses the issue of fumigant contamination.”

^*Guthrie, C.A., K. Campbell, R. Wallace, and N.S. Stephens. 2020. Phosphine intoxication in fourteen horses following ingestion of contaminated feed. Australian Equine Veterinarian 39(1):44-51.