Bird flu persists in raw milk cheese

Raw milk cheese products contained infectious avian influenza virus when made with contaminated raw milk, creating potential health risks for consumers, according to a new study.

At the same time, no virus was detected in test samples of highly acidic raw milk cheese. Feta cheese is an example of a more acidic variety.

The studywas published Oct. 8 in Nature Medicine.

"In this study, we were specifically looking at the stability or persistence of highly pathogenic avian influenza H5N1 in raw milk cheese products," said senior author Diego Diel, professor of virology in the Department of Population Medicine and Diagnostic Sciences and director of the Virology Laboratory at the Animal Health Diagnostic Center (AHDC), all in the College of Veterinary Medicine (CVM).

"This research was initiated due to previous work demonstrating high levels of virus shedding in milk from infected cows and the fact that we have previously shown that the virus survives in refrigerated raw milk for extended periods of time," he said.

While the Food and Drug Administration requires that raw milk cheese must be aged a minimum of 60 days at or above 35 degrees Fahrenheit, the study's authors detected infectious virus after 120 days of aging at a temperature of 39 Fahrenheit. The required 60-day aging process, which can reduce moisture, is intended to eliminate pathogenic bacterial contaminants potentially present in raw milk cheeses.

A pH of less than 7 is considered acidic, with the pH of most cheeses falling between 5.4, such as with cheddar, and 7, as in camembert. In a few types, such as feta, the pH is around 4.6 or even lower.

In the study, raw milk cheese products contained viable avian influenza virus when made with contaminated raw milk and the pH of the cheese was between 5.8 and 6.6. At the same time, no virus was detected in acidic cheeses with a pH of 5 or less.

Possible steps to avoid contamination of raw milk cheese products could include testing milk prior to cheese making and only using virus-free milk. Another possibility is heating milk at sub-pasteurization temperatures before using it, which could retain the qualities desired by the raw milk cheese industry and effectively inactivate the virus, Diel said.

"The work we've done on H5N1 is critical to providing practical, timely, data-driven knowledge and recommendations to the dairy industry in the face of this outbreak that has affected a large proportion of the milk supply in the U.S., and it allows raw milk cheesemakers to reduce risk," said study co-author Nicole Martin, assistant research professor in dairy foods microbiology and director of the Milk Quality Improvement Program in the Department of Food Science, in the College of Agriculture and Life Sciences.

The research also included an animal study where ferrets, which are highly susceptible to H5N1, were fed contaminated raw milk and contaminated raw milk cheese from the study. The researchers observed that some of the animals became infected after drinking the raw milk, but the animals that ingested the raw milk cheese remained uninfected.

While more study is needed, Diel said the fluid matrix of the milk might have allowed the virus to have higher contact with mucous membranes in the pharynx, while the cheese products could have a lower contact time with cells in this area where the virus could initiate an infection.

In the study, the team developed an experimental model in the lab where they made and tested 5-gram mini cheeses made with raw milk spiked with H5N1 virus. They also tested samples of commercial milk cheeses sent to Cornell by FDA regulators who suspected a batch of cheddar cheeses were made with contaminated raw milk.

"All four samples of company-made cheddar that we received tested positive for H5N1," Diel said.

Acidity in cheese is primarily achieved by direct acidification or by adding lactic acid bacteria cultures, which make use of lactose in the milk to form lactic acid. "This acid drops the pH of the milk and depending on how far this fermentation is allowed to proceed determines how low the pH goes," Martin said.

The team made their mini cheeses by direct acidification where they added lactic acid to H5N1-spiked raw milk to create samples with varying pH.

Mohammed Nooruzzaman, an assistant research professor in the Department of Population Medicine and Diagnostic Sciences (CVM), is the paper's first author. Co-authors include postdoctoral associate Pablo Sebastian Britto de Oliveira; research associate Salman Butt; Samuel D. Alcaine, associate professor of food science (CALS); and Stephen Walker at the FDA's Office of Dairy and Seafood Safety.

The study was supported by the FDA and the New York State Department of Agriculture and Markets.