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Practitioners at the Royal Veterinary College have led an investigation into the cause of a 2021 outbreak of feline pancytopenia in the U.K.

Researchers have zeroed in on toxins produced by fungi as a likely culprit behind a severe outbreak in 2021 of pancytopenia that killed at least 365 cats in the United Kingdom and sickened hundreds more.

The researchers identified potato flakes in three brands of recalled pet food as a possible source of the mycotoxins.

Naturally produced by fungi, mycotoxins can contaminate a variety of food ingredients. Exposure to high concentrations may cause numerous adverse health effects in animals, including humans, such as acute poisoning and immune deficiency.

One of the problems associated with mycotoxin exposure is pancytopenia, a rare bone-marrow condition that causes a rapid decrease in the patient's number of blood cells. The pancytopenia outbreak in British cats made international news headlines because of the number of documented cases — at least 580, with a mortality rate of about 63%.

Investigations early on appeared to rule out a range of suspects, including common feline infectious diseases, common toxicants such as heavy metals and estrogen, and abnormal levels of vitamins or minerals.

Early suspicions that mycotoxins were involved have been firmed up by two new investigatory research studies led by practitioners at England's Royal Veterinary College and published Jan. 7 in the Journal of Internal Veterinary Medicine.

In one study, the researchers analyzed data collected through an online registration forum available to all veterinarians in the U.K. The data included patient demographics, clinicopathological findings and dietary history, among other criteria.

A total of 580 cats with pancytopenia seen at 378 practices were included for analysis. Dietary history was available for 544 cats, of which 500 were fed one of three dry cat-food products marketed by Fold Hill Foods that were recalled during the outbreak.

In five of seven samples of the recalled diets, the researchers found the sum concentration of two types of trichothecene mycotoxins, T-2 and HT-2, exceeded European Commission recommended values. One of three control samples also was found to have the same mycotoxins, but at a level within the EC guidance values.

A third type of trichothecene mycotoxin, diacetoxyscirpenol, was found in all seven samples of the recalled diets, and none in the control samples.

The scientists concluded that it is "reasonable to propose" that mycotoxin contamination caused the outbreak. They also called for standardized testing of cat foods for mycotoxin contamination across the pet food industry.

The second study was a deeper analysis of clinical signs in 50 patients based on retrospective case studies and its authors drew a similar inference about the role of mycotoxins. They found various factors that "make a causative link likely," including similarities in clinical presentation, similar clinicopathological findings — especially bone marrow findings — and the exclusion of other previously documented causes.

It is interesting to note, the researchers said, that all three affected brands were marketed as grain-free diets. Trichothecene mycotoxins generally are associated with grain products but have been found to occur in starchy root vegetables, too.

"The diets associated with the pancytopenia outbreak in cats in the U.K. all contained potato flakes which, in this scenario, are thought to have been the likely source of mycotoxin," the researchers wrote.

The outbreak and related food recall prompted the likes of supermarket chain Sainsbury's and pet store and veterinary practice owner Pets at Home to pull the diets from their shelves.

Pets at Home has since launched a £100,000 (US$122,000) grant to support researchers "seeking to better understand the condition, identify causes and improve treatment options." Applications for the grant closed at the end of May 2022, and at least one study is in the works.