Streptococcus spp. in Dairy Foods? Is it a New Problem?

A recent article (Bosica et al., 2023) described a foodborne illness outbreak due to Streptococcus in unpasteurized milk. The title caught my attention because while some species of Streptococcus are known to cause mastitis in dairy cattle, we rarely hear about foodborne illnesses being attributed to Streptococcus. The causative agent of the outbreak was Streptococcus equi subspecies zooepidemicus. If this doesn’t sound familiar to dairy producers, this species of Strep is not known for causing bovine mastitis. The Streptococcus species typically associated with bovine mastitis are the contagious pathogen, S. agalactiae, and “non-ag” Streps, including S. uberis and S. dysgalactiae, which are primarily environmental.

How did S. equi subsp. zooepidemicus get into milk, and is it a pathogen of concern in dairy products? The previously mentioned outbreak occurred in Italy between November 2021 and May 2022. The outbreak resulted in 37 cases in humans, including five deaths (Bosica et al., 2023). Patients reported a wide range of symptoms such as fever, diarrhea, vomiting, and headaches. The age group of 31 of the 37 cases was 60 years old or older. Whole-genome sequencing results indicated a strong correlation between milk from a cow with mastitis on the farm from where positive products were recovered, the unpasteurized dairy products from the operation, and ill patients who consumed the products.

In the United States, an outbreak in the 1980s was attributed to group C Streptococcus. An isolate from one of the cases in this outbreak was S. zooepidemicus (CDC, 1983). The implicated source of the outbreak was "queso blanco", a raw cheese made from unpasteurized milk, including milk from cows with mammary infections. Of the 16 cases in the outbreak, two fatalities occurred in patients with underlying health conditions. Another outbreak of "Strep zoo" associated with the consumption of unpasteurized bovine milk occurred in England in 1984 and resulted in 7 deaths of patients all over 70 years old (Edwards & Roulson, 1988).

S. equi subsp. zooepidemicus is an opportunistic pathogen, meaning it will infect a host when the host's resistance is compromised by another condition. This pathogen affects several species, including humans, as a zoonosis, a disease acquired from animals or through the consumption of unpasteurized dairy products. Other species that can serve as a host for the pathogen include horses, swine, cattle, goats, sheep, guinea pigs, and monkeys (Spickler, 2020; Wainwright, 2019). Streptococcus species, which includes S. equi subsp. zoo, were listed as a moderate hazard in developing countries due to its association with raw dairy products and zoonotic transmission (Grace et al., 2020).

In the U.S., infections stemming from species other than dairy cattle or dairy products have been recorded. Two people in Washington state were diagnosed with illnesses resulting from S. equi subsp. zooepidemicus due to close contact with an infected horse (CDC, 2016). One fatality resulted. The swine industry has a heightened awareness of this pathogen as outbreaks have led to swine mortality and economic losses (Wainwright, 2019). Potential zoonotic and foodborne transmission of this pathogen increases its relevance to public health.  

Producers can protect themselves by practicing good biosecurity. Proper handwashing and glove usage are always important. The elderly, young children, and those with compromised immune systems should be aware of an increased risk of infection. The median age of cases associated with several outbreaks was over 60 years.

In summary, the most common streptococci species implicated in bovine mastitis are unlikely to sicken people; however, a less common streptococcus species has been associated with both bovine mastitis and foodborne illnesses. While not a new pathogen, Streptococcus equi subsp. zooepidemicus is one of the lesser-known pathogens that poses a risk to both humans as a zoonosis or through the consumption of contaminated raw milk and products manufactured from contaminated milk, and to the health of cattle from mastitis.

References

Bosica S, Chiaverini A, De Angelis ME, Petrini A, Averaimo D, Martino M, Rulli M, Saletti MA, Cantelmi MC, Ruggeri F, Lodi F, Calistri P, Cito F, Cammà, Di Domenico M, Rinaldi A, Fazii P, Cedrone F, Di Martino G, Accorsi P, Morelli D, De Luca N, Pomilio F, Parruti G, and G Savini. 2023. Severe Streptococcus equi Subspecies zooepidemicus outbreak from unpasteurized dairy product consumption, Italy. Emerg Infect Dis 29(5):1020-1024.

Centers for Disease Control (CDC). 1983. Group C streptococcal infections associated with eating homemade cheese -New Mexico. MMWR 32(39):510-516.  

Centers for Disease Control (CDC). 2016. Fatal infection associated with equine exposure -King County, Washington, 2016. MMWR 65(30):788.

Edwards AT, and M Roulson. 1988. A milk-borne outbreak of serious infection due to Streptococcus zooepidemicus (Lancefield Group C). Epidem Inf 101:43-51.

Grace D, Wu F, and AH Havelaar. 2020. MILK Symposium review: foodborne diseases from milk and milk products in developing countries -review of causes and health and economic implications. J Dairy Sci 103:9715-9729.

Spickler AR. 2020. Zoonotic Streptococcosis

Wainwright S. 2019. Emerging risk notice -Streptococcus equi subspecies zooepidemicus.