Boots-on-the-ground epidemiology — including interviews, disease surveillance, and traceback — was key in helping health officials solve and control an 8-year salmonella outbreak, the longest in U.S. history, which was ultimately tied to mail order chicks.

Between 2004 and 2011, 316 reported illnesses from 43 states were linked to the same outbreak strain. A new paper published in the New England Journal of Medicine chronicles just how investigators were able to crack the case. Researchers say it is likely that thousands of additional infections occurred in association with the outbreak, but were not reported.

In April 2005, the Colorado Department of Public Health and Environment identified three Salmonella serotype Montevideo isolates with the same genetic patterns. After interviewing the patients, local health officials learned that all three had been exposed to chicks or ducklings bought at feed stores the week before they got sick.

Officials then checked PulseNet, the national network for foodborne disease surveillance, and found that the same rare outbreak strain had been isolated from five other people in four states: Kansas, New York, Oklahoma, and Texas.

By March 2006, health officials had zeroed in on New Mexico agricultural feed stores that sold young poultry during 2005.

“New Mexico was chosen because it had a large number of cases as well as resources available to support investigation activities,” reported researchers in the NEJM paper. “Stores identified in an Internet search were randomly selected for an in-person or a telephone interview. The standardized questionnaire focused on the source of the live poultry, volume of live poultry sales, handling and hygienic conditions of poultry in the store, knowledge about the risk of transmission of Salmonella from poultry to humans, and education of customers about this risk.”

Using information from patients, investigators were able to trace young poultry back to where it had been purchased at the retail level, and back to mail-order hatcheries.  

According to the paper, over the duration of the outbreak, cases peaked annually during the spring, but the greatest number of reported cases came in 2006. Those sickened ranged from age 1 to 86 years old with a median age of 4. Of those with information available, 143 (54 percent) were 5 years of age or younger and 149 patients (53 percent) were female.

Perhaps most concerning — especially in light of the growing interest in raising backyard chickens across America — is that most of the parents whose children were involved in the outbreak told investigators they did not know that Salmonella could be spread through contact with poultry.

“When interviewees were asked if they had known that Salmonella infection could be acquired from contact with poultry, 16 of 76 respondents (21%) said yes, and 5 respondents (7%) reported receiving an oral or written warning at the time of purchase on the risk of acquiring salmonella infection from live poultry,” wrote researchers.

Once investigators pinned down live poultry as the likely culprit for the illnesses, they faced the difficult task of tracing the birds back to where they came from.

“In 2006, we surveyed representatives from 54 agricultural feed stores identified as selling chicks in New Mexico; half were surveyed in the store and half by telephone,” read the paper. “The median number of chicks sold per store was 600 per year (range, 50 to 5500). Of the 54 respondents, 21 (39%) reported that chicks from different shipments were comingled at the store, 31 (57%) reported that the bird housing area was cleaned daily, 11 (20%) reported that it was disinfected between shipments and 19 (35%) reported that it was disinfected only at the end of the season. Artificially colored chicks were sold by 32 of the stores (59%).”

Between 2005 and 2011, investigators traced live poultry from 59 case patients to determine where the birds had originated. For 48 of these households (81 percent), the origin was what officials have dubbed “Hatchery C.”

They also identified seven other mail-order hatcheries during the investigation.

“Hatchery C is known to drop-ship birds for other mail-order hatcheries,” the researchers reported. Drop-shipping is what happens when a hatchery can’t fill a customer order and sends the request to another facility, which then proceeds to send the birds to the customer under the first hatchery’s name. The drop-shipping further complicated the traceback. As the researchers noted, “investigators were unable to determine whether implicated birds were drop-shipped from Hatchery C.”

According to the paper, “Hatchery C” ships birds to all 50 states and raises 4 million birds each year. Researchers reported that in 2006, veterinary consultants and poultry experts started helping the company prevent and control salmonella. “Specific recommendations were provided for increased biosecurity, enhanced rodent control, feed decontamination, routine surveillance of the hatchery environment through microbiologic sampling at the premises, and the use of autogenous poultry vaccine.”

In August 2006, the company started vaccinating all adult chickens, but illnesses continued to be connected with live poultry from the hatchery.

“It is likely that a temporary lapse in the hatchery’s standard operating procedures in 2010 resulted in increased bacterial loads in the hatchery environment,” read the paper. “Findings from epidemiologic and environmental investigations and the fact that Hatchery C has a closed breeding facility that does not introduce birds from outside flocks suggest that this outbreak strain persisted in the hatchery’s environment throughout the 8-year investigation.”

The paper’s authors said the long-running outbreak showed that control interventions at mail-order hatcheries can play a role in reducing the transmission of Salmonella, but it also shows the difficulty in making that happen, especially considering the length of the outbreak.

“Between January 1 and April 30, 2012, only one human case of infection with the outbreak strain was reported to PulseNet,” the paper noted. “The trend of decreasing numbers of cases reported after the interventions at the mail-order hatchery and the low number reported in the first 4 months of 2012 support the conclusion that the interventions are helping to reduce human infections.”

The paper suggests that consumers looking to reduce their risks practice good hand hygiene and encourage this behavior for children and young children, and points to CDC recommendations.

“High-risk groups, including children younger than 5 years of age, elderly persons, and immunocompromised persons, should not handle or touch chicks, ducklings, or other live poultry. Live poultry should not be allowed inside a residence, in bathrooms, or in areas where food or drink is prepared, served, stored, or consumed. Any equipment or materials associated with raising or caring for live poultry should be cleaned outside the residence.”

Researchers noted that there are no hygienic or biosecurity standards for hatcheries selling live birds to the public, but argue that they should be put in place: “We recommend that public health and animal health officials and industry partners develop guidelines to reduce Salmonella transmission.”

The full paper is available for purchase at NEJM here.