Brucellosis, one of the world's most common bacterial zoonosis, is an ancient disease that dates as far back as the 1800s (1–3). The discovery of the disease was by David Bruce, a military physician stationed in the island of Malta in the 1880s. During this period, Bruce noted the increased manifestation of a disease characterized by undulant fever and joint pains that debilitated many British soldiers. Autopsy of the deceased soldiers led to the recovery of the causative organism from the spleens, livers, and kidneys. To confirm that the recovered organism was the cause of the disease, Sir Bruce reproduced the infection in monkeys using bacterial cultures from the spleen of infected soldiers. A common practice at that time in Malta was the consumption of fresh raw goat milk (4). Raw goat milk was later confirmed to be the source of the bacteria, and the first consideration of brucellosis as a zoonosis arose from the isolation of its causative agent, Brucella melitensis, from goat milk (2, 3, 5). Subsequently, the prohibition of goat milk and cheese in military establishments led to a significant reduction of the disease incidence among the soldiers of Malta (2).

In the United States, brucellosis was reported as early as the 1900s where 29 cases of brucellosis (B. melitensis) were reported in Houston among Mexican immigrants that had consumed goat cheese before the onset of their symptoms. Additionally, between 1965 and 1978 in the US, over 3,000 cases of brucellosis were reported, and 4% of these cases were attributed to raw dairy products from Mexico, predominantly from the consumption of fresh cheese from unpasteurized goat milk (2, 6).

Although, brucellosis incidence has been attributed to varying factors, the consumption of unpasteurized dairy products accounts for a large number of cases, particularly in endemic countries such as Asia, Middle East, Africa, Central and South America (7–9). Further, in non-endemic countries, brucellosis has also been reported to occur after travel to, and consumption of raw dairy products in endemic countries (10).

In developed countries, the emerging interests in natural foods and products have led to the increased preference for raw milk consumption due to its acclaimed health benefits that are believed to be destroyed upon pasteurization (11, 12). Pasteurization, a process which dates back to the 1800s, involves the heating of raw milk to a defined temperature for a specific period of time to inactivate live, disease-causing organisms such as Brucella, Salmonella, Listeria, Campylobacter, E. coli, amongst others that pose significant health risk to consumers (13, 14). The process has been invaluable in the improvement of the safety of milk and other food products for human consumption. Another added advantage of pasteurization is that it destroys organisms that cause food spoilage, thereby increasing shelf-life and enhancing food security in low to middle income countries (11, 14–16). The presence of harmful pathogens in milk or dairy products can occur from either a direct passage from the animal, contamination of the expressed milk by animal excreta, or unsanitary handling of the milking process (13). For many years, several outbreaks of diseases resulting from the consumption of raw dairy products have been reported to the Centers for Disease Control and Prevention (CDC) (6, 15, 17–19). In recent times, at least three cases of human brucellosis have been confirmed by the CDC resulting from an exposure to the live-attenuated vaccine strain Brucella abortus RB51 following the consumption of raw milk (18, 19). In the most recent outbreak, it is believed that hundreds of persons in approximately 19 states may have been exposed in connection to the consumption of raw milk from a farm in Pennsylvania (19).

Interestingly, despite the significant public health risk that raw milk presents to consumers, the sale of raw milk for human consumption is not prohibited in all states in the US (11, 14, 20). Currently, 13 states allow raw milk to be sold in retail stores, 17 states allow raw milk to be sold only on farms where the milk is produced, 8 states allow raw milk to be obtained via the cow-share program (which involves the leasing of cows to obtain a percentage of a cow's milk production), while 21 states prohibit the sale of raw milk for human consumption (6). Interestingly, outbreaks of raw milk-related diseases including brucellosis have been reported mostly in states that legalize the sale of raw milk (11, 17, 19, 21, 22).

Previous studies have highlighted the varying motivations that drive raw milk consumers including consuming food items in their pure natural forms, better tastes and flavors, the belief that pasteurization destroys the natural components of milk, support of local farmers, and lack of trust of the state government as regards regulation of safe foods for consumption (16, 21, 23).

To date, scientific validation of the health benefits of consuming raw milk is very limited, and it has been extensively demonstrated that the health risks associated with the consumption of raw milk significantly outweigh the unfounded proclaimed health benefits (6, 21, 24, 25). Additionally, information exchanged via social media and networks have been shown to influence the attitudes and decisions of consumers (16, 21, 23, 26). Currently, there is a gap in knowledge about the variables by which consumers evaluate the information exchanged in their food safety and preference conversations, or how consumers perceive the varying recommendations regarding raw milk.

Despite the significant health risks posed by the consumption of unpasteurized milk and dairy products, there is still an increased trend in the purchase and consumption of raw milk (21), which may consequently lead to an increased prevalence of raw-milk acquired brucellosis as well as other diseases in the US. In order to design a more effective approach to educate consumers on the public health risks associated with this practice, the significance of the sources of information related to the purchase and consumption of raw milk and milk products must be critically evaluated to enhance or come up with an effective strategy in the control and prevention of raw milk-acquired brucellosis. Therefore, the objectives of this report are to identify the evidence gaps for future investigations that will facilitate informed policy decision about the sale and consumption of raw milk and milk products in the US, and to systematically review the sources of information that contribute to the increased trend of raw milk sale and consumption in the US, and associate the findings with the rising prevalence of raw milk-acquired brucellosis cases in the country. Results from this current study will facilitate efforts that are necessary to enhance research into the development of innovative approaches to disseminate information about the dangers of raw milk consumption; intensify the surveillance of human brucellosis as a differential diagnosis to enable physicians to better control the disease; establish a quality control of the sales process; and highlight the significance of collecting and analyzing data about nation-wide raw milk sales, which will help to frame food safety policies for the benefit of the human population.

The scientific evidence of the health risks associated with the consumption of raw milk and products has been known for a long time (30, 31). However, less clear is the impact of acquiring infectious diseases including brucellosis from raw milk consumption in the US due to incomplete reporting of cases and the complex factors associated with the sale and consumption of raw milk, including inconsistent policies that range from total prohibition to legal sales in retail stores. One of the aims of this current study was to determine if increase in sales and consumption of raw milk in the US is directly associated with increased media advocacy and public opinions about the benefits of consuming raw milk. Interestingly, we found that the majority of public opinion published by newspapers and magazines in the Northeastern, Midwestern, Southern, and Western regions of the US were against the sale and consumption of raw milk due to the associated health risks. Hence, the rise in the trend of raw milk consumption may be a result of other factors such as the dissemination of misleading information that are neither evidence nor science-based on other social media networks like Facebook, Twitter, and other social interactive platforms. Previous studies have shown that discussions on these types of social platforms have severe implications in influencing consumer behaviors (32).

Another important finding in this current study was that majority of the media advocacy and public opinion in favor of raw milk consumption were published in the Northeast compared to other US regions. Why the Northeast had more favorable public opinion is not known, but it may be due to a stronger commitment of individuals in this US region to healthy local foods as indicated by Lovacore Index which ranks states based on the support of natural products or food (Figure 6). Additionally, the favorable raw milk regulations in the Northeast also facilitates the ease of access to raw milk via various means including availability in retail stores and farms where raw milk is produced (Figure 6). Moreover, media advocacy and public opinion were also accessible via both printed and online, making it possible to reach a larger audience. Therefore, efforts should be intensified for the adoption of media advocacy as well as social networks to increase awareness and educate the public about the disease risks associated with raw milk consumption. It is important to bear in mind that the more favorable public opinion in the Northeast does not directly correlate with increased incidence of brucellosis when compared to other regions like the southern states. A possible explanation for this might be the interplay of factors that contribute to disease incidence and prevalence in different regions including immigration and interaction with wild animals that serve as reservoir hosts.

What prompted the investigation of this current study were the recent increase in the number of confirmed human brucellosis cases resulting from the consumption of raw milk and products. Interestingly, human brucellosis is an almost nonexistent disease in the US, but endemic in countries where the consumption of raw milk is greatest and unregulated. Symptoms in infected individuals are non-specific and can include fever, sweats, arthralgia, myalgia, and in complicated cases, miscarriage or spontaneous abortion (1, 33–35). In this study, an attempt to directly correlate confirmed human brucellosis cases with the consumption of raw milk and products was impossible, which demonstrates a gap in evidence of unavailable data reporting human brucellosis acquired from raw milk consumption. Therefore, we cannot prove that raw milk consumption contributed to increased prevalence of brucellosis in the US. It is notable that apart from raw milk consumption, there are several other factors that increase the risk of acquisition of human brucellosis including occupations that allow direct contact with animals (e.g., veterinarians, butchers, ranchers, animal care givers, etc.), laboratory personnel or research scientists that have direct exposure to animal samples, immigration, as well as feral swine hunting that results in species-specific infections. An important issue for future research is to unravel how raw milk consumption specifically contributes to the prevalence of brucellosis in the US. However, in endemic regions, raw milk consumption accounts for the most common cause of human brucellosis (36, 37). For this study, the number of brucellosis cases publicly provided by the CDC did not delineate the cases based on the source of infection. Therefore, the ease of access to cases of raw milk-acquired brucellosis is paramount to further generate effective brucellosis control and prevention strategies so as to avoid a backward trend of disease outbreaks due to raw milk consumption that occurred in the 1900s. Moreover, in order to reduce the incidence of raw milk-acquired brucellosis, several measures must be implemented including but not limited to promoting pasteurization, restricting the sale and access to raw milk, establishing rigorous quality control of the raw milk sale process, and the development of diagnostic tests for validating the safety of raw milk. This is crucial due to the fact that no diagnostic tests are available or approved for raw milk and milk products in the US or globally (US—Food and Drug Administration). Because of the current difficulty in restricting the sale of and access to raw milk, a better approach to limiting the associated disease risks may be an effective regulation of the quality of raw milk provided for human consumption, which will involve integrated efforts from veterinary services, regulations from the Federal Department of Agriculture (FDA), as well as effective training of physicians, farmers, ranchers, and consumers.

To facilitate informed policy decisions about restricting the sale and consumption of raw milk and products in the US, and ultimately reduce the risk of raw milk acquired diseases including brucellosis, an analytical framework was proposed (Figure 7). One of the research priority areas identified was the lack of regional or nation-wide data reporting the sales and purchases of raw milk in the US. This lack of raw milk sales data precluded the probable analyses and conclusions that may demonstrate the direct association of the magnitude of sales and purchases of raw milk with an increased brucellosis prevalence. We argue that data reporting the US regional or state-wide raw milk sales will help to establish the influence of the sales process on disease prevalence in the country. In addition, information about the purchases and demographics of raw milk consumers will help to further understand consumer attitude and behavior toward the consumption of raw milk, and also help to increase awareness about the potential risks of raw-milk acquired diseases. In other words, to formulate policies on the reduction of raw milk sales and distribution, the impact of raw milk production, sales promotion, consumer attitudes, and behavioral patterns must all be critically evaluated (12).

Additionally, we propose that raw milk sales promotion may influence consumer behavior and motivation by contributing to increased awareness of the availability of raw milk for purchases. Previous studies have shown that the purchase and consumption of raw milk are not restricted to a particular age group, income, education level of consumers, or distance to the place of purchase. In fact, in these studies, there was no association between these factors and raw milk consumption (21). Therefore, sales promotion, possibly through social media networks, likely presents a huge influence on consumer attitude and motivation for raw milk consumption.

The findings in this current study have demonstrated that media advocacy and public opinion possibly contribute to increased trend of raw milk consumption in different US regions, and that several factors might be involved in the prevalence of raw milk-acquired diseases like brucellosis. Additionally, the evidence gaps identified in this study have provided a strong basis for future investigations and the development of effective strategies to alleviate the risks associated with raw milk-acquired infectious diseases including brucellosis. This will help to prevent outbreaks of human brucellosis in the US, which can have both direct and indirect implications including increased healthcare costs and potential threats to food safety and security due to the loss of livestock production. Evidence-informed health policies are most effective when guided by science, consumer preferences, and political reality. Hence, we strongly recommend an interdisciplinary approach and effort toward the building of the raw milk consumption evidence base.

All datasets generated for this study are included in the article/Supplementary Material.

SA, AF, GR, and AA-G: conceptualization. SA, MF, and GR: data curation. SA, GR, and LP: formal analysis. SA, GR, and AA-G: investigation. SA, GR, MF, and LP: methodology. SA, AF, MF, LP, and AA-G: resources. AA-G: supervision. SA, GR, and AA-G: writing—original draft. SA, AF, MF, LP, and AA-G: writing—review & editing.