Packaged Curd and Food Poisoning in Kutch: An Outbreak Across Logistically Challenged Stations

Abstract

Background:

Packaged curd is a rarely reported source of food poisoning. We analyzed a simultaneous outbreak across two stations located 95 km apart in the Kutch region of Gujarat.

Materials and Methods:

A descriptive observational study was conducted using interviews and clinical data from affected individuals. Case identification, symptom analysis, food consumption history, and laboratory investigations were used to trace the source and cause of the outbreak.

Results:

A total of 280 individuals—including personnel and family members—reported gastrointestinal illness over a four-day period following the consumption of curd or curd-based food items at both affected locations. The majority of cases were male (71%) and within the age group of 15–45 years (77%). All reported cases were epidemiologically linked to curd procured from a common dairy source serving the Kutch region. The highest food-specific attack rate was observed for curd, at 33.4%. The median incubation period ranged from 18 to 23.5 hours across the two stations. The predominant clinical manifestations included loose stools (78.2%), abdominal pain (65%), and fever (59.6%). Microbiological analysis confirmed the presence of Salmonella enterica serovar Chester in stool samples from affected individuals. Although Escherichia coli was detected in the curd samples, the clinical profile of the cases and the epidemiological pattern were strongly indicative of Salmonella as the primary causative pathogen.

Conclusions:

This large Kutch outbreak has an unusual source of contamination with rare Salmonella enterica serovar Chester in packaged curd, thereby contributing to a rare identification in foodborne outbreaks. Collaborative efforts in a unique way across public health, food safety, and microbiology were important for the timely identification of the agent and source. The study accentuates the importance of keeping suspicion high and strong interagency protocols in place to effectively manage an outbreak.

INTRODUCTION

A food-poisoning outbreak is typically defined as the occurrence of two or more cases of a similar clinical illness attributable to the consumption of a common food item, or when the number of foodborne disease cases exceeds the expected baseline.[1] Clinical presentation commonly includes diarrhea, nausea, vomiting, abdominal pain, fever, or any combination of these symptoms. Although many cases—particularly mild ones—remain under-reported, foodborne illnesses can occasionally lead to severe complications.[2] Consequently, all suspected outbreaks warrant comprehensive epidemiological investigation to identify the source, the causative pathogen, and contributory risk factors, thereby facilitating timely control measures and the implementation of appropriate preventive strategies.

Food-poisoning outbreaks are most frequently caused by microbial agents—including bacteria, viruses, and parasites—although chemical contaminants and natural toxins may also be involved. Meat and dairy products are among the most frequently implicated food categories. Outbreaks linked to milk or milk-derived products are commonly associated with pathogens such as Campylobacter spp., Escherichia coli, Salmonella spp., and Staphylococcus aureus.[3] Dairy products susceptible to microbial contamination include raw milk, butter, cheese, cream, ice cream, yogurt, and curd.[4] The microbial load in milk is influenced by several factors, including animal health, equipment sanitation, seasonal variations, environmental temperature, storage conditions, and the hygiene practices of personnel involved in production and handling.[5]

Salmonella is among the most frequently identified pathogens responsible for dairy-associated foodborne outbreaks.[6] Milk and milk products such as yogurt, sweets, and curd may harbor both typhoidal and nontyphoidal Salmonella serovars, including S. Enteritidis and S. Typhimurium. Contamination with nontyphoidal Salmonella typically arises from the use of raw, unpasteurized milk or through fecal contamination introduced via improperly cleaned teats and udders.[5] Notably, foodborne outbreaks associated with packaged curd from reputable dairies are exceedingly rare.

The present study investigated a simultaneous outbreak involving a rare nontyphoidal Salmonella serovar that occurred at two stations located approximately 95 km apart in the Kutch district of Gujarat. This analysis provides epidemiological, clinical, and microbiological insights into an unusual outbreak linked to a widely distributed packaged dairy product.

MATERIALS AND METHODS

Study design and setting

The research was a descriptive (observational) study, with descriptive analysis used both retrospectively and prospectively to conduct an epidemiological investigation of food poisoning outbreaks, and was initiated in August 2023, as a food poisoning outbreak was developing in two different stations in the Kutch region of Gujarat state, India. Users self-identified curd and its preparations as the earliest during the outbreak. Local health, food safety and dairy officials, were contacted to assess the outbreak. Food safety officers with the Food and Drug Control Administration of the Bhuj Circle were contacted, and food samples identified as sealed packaged curd and buttermilk of differing batches were collected at both stations and submitted to the Food and Drugs Laboratory at Vadodara, Gujarat, for analysis.

Notification and initial response

First reports of a cluster of gastrointestinal illnesses were received, and the Chief District Health Officer (CDHO) of Kutch was informed within a few hours. Rapid response teams were formed, and the epidemiological assessment began. Early in the outbreak, curd and its preparations were implicated by the users themselves. Local health, food safety, and dairy authorities attempted to collaborate to determine the nature of the outbreak.

Establishment and verification of outbreak

The first step in our investigation was to check the diagnosis per the usual case definition and confirm that there was an outbreak by collecting food, blood, and stool samples. In addition, cases were stratified into mild, moderate, and severe cases, dependent on the nature of symptoms and hydration status.

Case definition

Any person with one or more of the following symptoms: loose stools, vomiting, abdominal pain, or fever, onsets of symptoms occurring 6–72 hours after consuming suspected contaminated packaged curd or curd products, was deemed a suspected case of food poisoning. Mild cases defined by no dehydration were treated on an Outpatient Department (OPD) basis using oral medications and oral rehydration therapy. Moderate cases, defined by some dehydration, received intravenous antibiotics and IV fluids—these cases had a short hold for a few hours in the health care center. Severe cases characterized by signs of severe dehydration, with or without high-grade fever, were sent to the nearest hospital for in-patient monitoring and specialist management.

Active surveillance and data collection

Active surveillance involved a multi-tiered strategy combining rapid notification, active case searching, syndromic assessment, and laboratory confirmation of the cases. Immediate reporting from both stations triggered deployment of rapid response teams, followed by door-to-door case identification of all persons who consumed the implicated curd or attended the social gatherings with daily symptom monitoring for 72 hours (covering one to three incubation cycles). The active surveillance also involved review of health care facility records and structured recall-based interviews, ensuring comprehensive case ascertainment. Syndromic surveillance enabled rapid triage of mild, moderate, and severe cases based on the case definition. Laboratory-based surveillance, including stool culture, serotyping, antimicrobial susceptibility testing, and genomic sequencing, was undertaken. Concurrent environmental and food-surveillance efforts traced the contaminated batches of packaged curd to a common dairy source. The gathered information was entered onto the epidemiological case sheet, specifically formatted for food poisoning outbreaks. The clinical case sheet recorded relevant information such as demographic information, symptom classification, date and time, and place of food consumption and types of food consumed, along with the date and time of the onset of symptoms for each case. In total, 280 (201 + 79) cases were interviewed from both affected stations. Food-specific attack rates and risk ratios were calculated to identify the incriminating food item.

Environmental investigations

An environmental assessment was undertaken at the impacted sites, including gathering locations and food preparation facilities. This assessment involved comprehensive engagement with all relevant key stakeholders involved in the food handling process, and also with the dairy suppliers, to identify possible sources of cross-contamination and to look back as far as possible to trace the implicated food item(s) back to the original source. Communicated with food safety authorities to appraise the on-site investigation of the common dairy farm, in particular, the associated hygiene practices applied to the distribution chain. Engaged with the Food Safety Officer with the Food and Drug Control Administration of the Bhuj Circle, and contacted the cooperative dairy linked to the observed outbreak and requested them to perform an internal quality check on specific batches of packaged curd.

Laboratory methods

Food samples were collected, including sealed packaged curd and buttermilk from different batches at both sampling locations, and were sent for analysis to the regional Food and Drugs Laboratory in Vadodara, Gujarat. At the same time, single stool specimens were collected from symptomatic individuals as soon as feasible at both Stations 1 and 2. All samples were transferred under stringent cold chain to the microbiological laboratory for culture and sensitivity; stool samples from Station 1 (which had only basic medical facilities without microbiology support) were sent to a medical college at Station 2, to do an initial culture and sensitivity, sub-culture plates were sent to Byramjee Jeejeebhoy Medical College (BJMC), Ahmedabad for retesting of organism and for genomic sequencing at Gujarat Biotechnology Research Centre (GBRC) at Gandhinagar, because Station 2 was not having sufficient resources to type the organisms. Drinking water samples from source and consumer locations were sent for bacteriological and chemical analysis at an accredited laboratory.

Control and prevention measures

Remedial measures were simultaneously applied to control the outbreak as the investigation progressed. There was an immediate ban on the consumption and sale of packaged curd and butter milk by the single dairy vendor inside both stations. The residents were made aware of the outbreak by making an announcement over the loudspeaker near the residential area and were asked to report sick if any symptoms arose. Preventive measures such as proper storage of food items, food hygiene, protection of the water supply, and proper solid waste disposal were ensured. Early warning systems have been established for the prevention of future outbreaks.

RESULTS

Between August 19 and 23, 2023, a total of 226 and 82 suspected gastroenteritis (suspected food-poisoning) cases were reported from the healthcare facilities of Station 1 and Station 2, respectively. Following detailed line listing and removal of duplicate entries, the final case counts were 201 for Station 1 and 79 for Station 2.

The index case at Station 1 developed symptoms at 0600 hours on August 19, 2023, and reported consumption of packaged curd beginning at 2100 hours on August 18, 2023. At Station 2, the first symptomatic individual presented at 1200 hours on August 19, 2023 with a history of consuming a home-cooked meal accompanied by the same brand of packaged curd.

The mean age of affected individuals was 24.5 years (SD = 11.7) in Station 1 and 27.9 years (SD = 10.5) in Station 2. A marked male predominance was observed, with males constituting 69.2% of cases at Station 1 and 72.2% at Station 2. The most frequently reported symptoms across both locations were loose stools (n = 219; 78.2%), abdominal pain (n = 182; 65%), and moderate-to-high-grade fever (n = 167; 59.6%).

At Station 1, 3.5% of cases were classified as severe and required referral to the nearest hospital for specialist care. Moderate cases accounted for 39.8% and were managed at the local healthcare establishment, while 56.7% presented with mild symptoms and were treated on an outpatient basis. At Station 2, severe, moderate, and mild cases comprised 7.6%, 32.9%, and 59.5%, respectively. Management followed a syndromic approach, including oral or intravenous rehydration therapy (ORS/IV fluids) and empirical antibiotic treatment. All affected individuals recovered fully, and no fatalities were reported.

The median incubation period (IP) among cases at Station 1 was 18 hours, with a minimum of 6 hours and a maximum of 68.5 hours. Determination of an exact IP at Station 2 was not feasible due to the absence of a single organized exposure event and the variability in the timing and location of consumption of the suspected food item. However, based on available histories, the estimated median IP at Station 2 was 23.5 hours, with incubation ranging from 5.75 to 37 hours. Box-and-whisker plots depicting the median IP and the corresponding 95% confidence intervals for Stations 1 and 2 are presented in Figure 1a and b, respectively. Epidemic curves for both stations demonstrate that the peak number of cases occurred approximately 48 hours after symptom onset in the index case at each location; Figure 2a and b. A spot map of cases with clustering (indicating place distribution) in Stations 1 and 2 is depicted in Figure 3a and b, respectively.

Figure 1.

(a) Box plot of the incubation period of the food poisoning cases: Station 1. (b) Box plot of the incubation period of the food poisoning cases: Station 2

Figure 2.

(a) Epidemic Curve of onset of symptoms in cases at Station 1. (b) Epidemic Curve of onset of symptoms in cases at Station 2

Figure 3.

(a) Spot map of cases with clustering (place distribution): Station 1. (b) Spot map of cases with clustering (place distribution): Station 2. Note: 1. Numbers mentioned inside red circles indicate the number of cases in each location. 2. Exact location details have been partly masked, considering confidentiality and sensitivity aspects of the place

Preliminary epidemiological inquiry revealed that several social events involving food service were held over a 2-day weekend period at Station 1. Nonetheless, a portion of affected individuals from this station reported no participation in any of these events, indicating an alternative route of exposure. Notably, no such events took place at Station 2, which is located approximately 95 km away. Food-specific attack rates and relative risks were calculated for both curd-based and non-curd-based food items to assess the association between consumption and illness. The maximum attack rate (AR) of 33.4% and maximum risk ratio (RR) of 5.9 (95% CI 2.8–12.2 and P value 0.00) was seen for packaged curd. Analysis of the outbreak data revealed that curd-based food items exhibited notably higher AR and relative risks (RR) when compared to food items that did not contain curd. These differences were found to be statistically significant, highlighting a marked association between consumption of curd-based products and the incidence of gastroenteritis in the affected population. The comparative data for AR and RR between curd and noncurd food items are presented in Table 1.

Table 1.

Food-specific attack rates (AR) and risk ratio (RR)

Food items	Ate specified food			Did not eat specified food			AR (%)	RR	CI (95%)	P
	Ill n (%)	Well n (%)	Total	Ill n (%)	Well n (%)	Total
Curd	81 (40.3)	120 (59.7)	201	7 (6.9)	95 (93.1)	102	33.4	5.9	2.8–12.2	0.00
Dahi puri	27 (25.2)	80 (74.8)	107	5 (6.2)	76 (93.8)	81	19.1	4.1	1.6–10.1	0.00
Dahi vada	25 (27.8)	65 (72.2)	90	4 (6.2)	60 (93.8)	64	21.5	4.4	1.6–12.1	0.00
Raita	21 (26.9)	57 (73.1)	78	8 (11.1)	64 (88.9)	72	15.8	2.4	1.1–5.1	0.02
Butter milk	19 (23.5)	62 (76.5)	81	6 (6.8)	8293.2)	88	16.6	3.4	1.4–8.2	0.00
Pav bhaji	12 (17.1)	58 (82.9)	70	8 (12.5)	56 (87.5)	61	4.1	1.3	0.6–3.1	0.61
Chole bhature	10 (13.3)	65 (86.7)	75	6 (11.1)	48 (88.9)	54	2.2	1.2	0.5–3.1	0.92
Vada pav	8 (15.1)	45 (84.9)	53	5 (9.6)	47 (90.4)	51	5.3	1.5	0.5–3.1	0.58
Idli sambhar	6 (10.2)	53 (89.8)	59	3 (6.7)	42 (93.3)	45	3.5	1.5	0.4–5.8	0.78
Veg Sandwich	4 (8.7)	42 (91.3)	46	4 (7.1)	52 (92.9)	56	1.6	1.2	0.3–4.6	1.00

Three (1.07%) out of 280 stool samples (1 sample from Station 1 and 2 samples from Station 2) grew Salmonella spp. at the microbiology lab in the medical college at Station 2. The sub-culture samples were processed by the Microbiology Department, BJMC, Ahmedabad. They carried out manual biochemical testing, serotyping, automated identification by Vitek-2 compact, antimicrobial susceptibility testing by the modified Kirby-Bauer disk diffusion method, and automated antimicrobial susceptibility by Vitek-2 compact. The genomic analysis by multilocus sequencing at GBRC revealed the organism to be nontyphoidal Salmonella: Salmonella enterica subsp. enterica serovar Chester. The food samples (sealed packaged curd and buttermilk) were analyzed at the regional Food and Drugs Laboratory at Vadodara, Gujarat, and the packaged curd samples from three different batch numbers were found to be positive for Escherichia coli.

DISCUSSION

In India, surveillance of foodborne disease outbreaks is predominantly informed by media reports, reflecting a critical gap in systematic reporting mechanisms. Analysis of Integrated Disease Surveillance Programme (IDSP) data from 1980 to 2016 indicates that foodborne outbreaks constitute nearly 50% of all documented disease outbreaks in the country.[1] Among the implicated food categories, milk and milk products, meat, poultry, seafood, cooked and uncooked rice, and both raw and cooked vegetables are most frequently associated with foodborne illnesses.[7]

Nontyphoidal Salmonella species—particularly S. Enteritidis and S. Typhimurium—represent one of the leading etiological agents of food poisoning in India. These pathogens are commonly isolated from poultry, meat, and dairy products, reflecting widespread contamination risks across multiple food supply chains. Globally, an estimated 93.8 million cases of nontyphoidal Salmonella infections occur annually, of which nearly 80 million are attributed to contaminated food sources. This substantial burden underscores the need for enhanced food safety surveillance, improved hygiene practices, and stronger regulatory frameworks to mitigate the incidence of foodborne Salmonella infections.[8]

In the present investigation, a food-poisoning outbreak occurred concurrently at two geographically distinct stations situated approximately 95 km apart in the Kutch region of Gujarat. While several social gatherings had taken place at Station 1 prior to the onset of illness, no such events were reported at Station 2, suggesting differing potential exposure contexts. Across both locations, the predominant clinical manifestations included diarrhea, abdominal pain, and fever. The dominance of lower gastrointestinal symptoms, coupled with a median IP of 6–96 hours and the identification of a dairy product as the suspected vehicle of transmission, strongly indicated Salmonella spp. as the likely etiological agent.[9,10]

Nontyphoidal Salmonella gastroenteritis is frequently underdiagnosed in developing countries due to limited diagnostic infrastructure.[11] A comprehensive review of foodborne outbreaks reported in India over a 27-year period (1980–2016) identified Salmonella species as the most common causative pathogen across diverse outbreak settings.[12] More recently, an epidemiological investigation of a localized food-poisoning incident within a military establishment also suggested Salmonella Typhimurium as the most probable causative organism; however, microbiological confirmation could not be obtained due to the unavailability of food samples.[13]

Curd, a fermented dairy product derived from boiled milk, is produced through the action of specific bacterial cultures. Its quality is influenced by the type of starter culture used, the initial quality of milk, and any additional ingredients incorporated during preparation. As with other milk-based products, loose or unpackaged curd is highly susceptible to microbiological contamination. Coliform bacteria—particularly enteropathogenic Escherichia coli—as well as yeast and molds, can readily contaminate curd that is loosely prepared, handled, or stored under suboptimal hygienic conditions, contributing to foodborne illnesses.[14,15,16,17,18] In contrast, packaged curd is rarely implicated in food-poisoning outbreaks due to stringent quality control measures implemented across its production and distribution chain.

In the present dual outbreaks, food samples were available and were rapidly transported under adequate cold-chain conditions to accredited laboratories for microbiological analysis. Curd samples tested positive for E. coli. Moreover, nontyphoidal Salmonella was isolated from the stool specimens of three affected individuals across the two stations, providing strong evidence toward the etiological agent responsible for the outbreak. Although the detection of E. coli in the curd and nontyphoidal Salmonella in stool samples may suggest the possibility of polymicrobial contamination, the clinical symptomatology and the IP observed among cases were more consistent with infection due to Salmonella spp., thereby supporting it as the likely causative organism.

The twin outbreaks observed within our area of responsibility were distinctive in several respects. In both affected stations, the implicated food item was packaged curd distributed from a single source—namely, a cooperative dairy unit belonging to a well-established and reputable dairy product chain. The scale and geographical spread of the outbreak were notably extensive. Two stations separated by approximately 95 km experienced cases linked to the same batch of packaged curd originating from this dairy facility. During the same timeframe, similar foodborne illness cases were reported in the Mandvi, Gandhidham, Mundra, Anjar, and Rapar taluks, as documented by local media. Importantly, the entire Kutch region receives its dairy supply from the centrally located dairy plant at Sarhad,[19] suggesting a common-point contamination event affecting a wide distribution network.

Although loose, unpackaged curd has been implicated in sporadic foodborne outbreaks,[20,21] no documented outbreaks to date have been attributed to pre-packaged curd. Moreover, the causative pathogen in most foodborne outbreaks often remains unidentified or only partially characterized, largely due to limited microbiological and sub-typing capacities in resource-constrained settings. Despite substantial operational, temporal, and logistical challenges, our investigation was able to undertake both typing and sub-typing of stool isolates through accredited reference laboratories employing standardized techniques. The etiological agent identified was nontyphoidal Salmonella, specifically Salmonella Chester.

Historically, S. Chester has been associated with food-poisoning outbreaks in Western countries, predominantly linked to contaminated meat and fish products.[22,23] Reports of S. Chester infection in humans in India are exceedingly rare. A 16-year surveillance study of nontyphoidal Salmonella among pediatric acute gastroenteritis cases in Kolkata analyzed 9,957 rectal swab samples, of which only 1% yielded Salmonella isolates, and S. Chester was isolated from a single sample.[24] Similarly, a study from southern India identified only one S. Chester isolate among 1,436 nontyphoidal Salmonella strains recovered from stool specimens.[25] In contrast, S. Chester has been documented more consistently in animal populations.[26] These findings underscore both the rarity of S. Chester in human infections in India and the epidemiological significance of its identification in the present outbreak.

The present study had several limitations. Both affected stations operated under significant resource constraints, and food samples had to be transported approximately 500 km to an external laboratory for advanced microbiological analysis. Additionally, diagnostic facilities capable of detecting and characterizing nontyphoidal Salmonella were not available at the medical college associated with Station 2. The remote location of Station 1 posed further challenges for clinical management, as the nearest hospital equipped to handle such cases was situated nearly 100 km away.

Despite these limitations, the study exhibited notable strengths. A rapid and coordinated interagency response facilitated timely containment efforts. Importantly, a substantial number of packaged curd and buttermilk samples were preserved under an appropriate cold chain immediately upon suspicion, enabling reliable microbiological testing. Furthermore, Multi Locus Sequence Testing (MLST) was employed to perform genomic characterization of the nontyphoidal Salmonella isolates recovered from stool samples. The use of this advanced molecular approach—rarely feasible in outbreak settings of this nature—constitutes a key strength and novelty of the investigation, providing robust confirmation of the causative organism.

It is important to note that only a limited number of stool specimens tested positive by culture during this outbreak. This low yield of culture-positive results is a known challenge in outbreak investigations. Several factors can contribute to reduced detection rates, such as collecting only a single specimen from each case, delays in specimen collection after symptom onset, or prior administration of empirical antibiotics to affected individuals. Despite this limitation, the low rate of culture positivity alone does not undermine or negate the evidence for causation in the context of the outbreak.

CONCLUSION

This investigation documents a significant foodborne outbreak in the Kutch district linked to contamination of pre-packaged curd with Salmonella enterica subsp. enterica serovar Chester—an exceptionally rare occurrence in India. The study highlights an unusual epidemiological association involving a widely distributed, commercially processed dairy product. Timely communication, active engagement with state public health authorities, and coordinated efforts with food safety officials and reference microbiology laboratories enabled the rapid identification of nontyphoidal Salmonella (S. Chester) as the etiological agent through comprehensive epidemiological assessment and microbiological analysis of both stool and food samples. Overall, the findings underscore the critical importance of maintaining a high index of suspicion for foodborne pathogens, even in packaged and regulated products, and emphasize the need for robust, well-coordinated interagency response systems. Establishing clear communication pathways, standardized operating procedures, and rapid laboratory support is essential to prevent, detect, and mitigate similar large-scale outbreaks in the future.

Practical Recommendations for Packaged Dairy Surveillance include establishing a robust Hazard Analysis and Critical Control Points (HACCP) system, maintenance of strict cold chain management, verifying pasteurization effectiveness, conducting regular testing for microbial contamination and chemical residues:, ensuring high-quality packaging materials, incorporating smart packaging technologies, implementing traceability and record-keeping, training personnel on hygiene and safety, regular audit of suppliers and distributors.

Conflict of interest

There are no conflicts of interest.

Funding Statement

Nil.