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Discussion
Rates of foodborne and waterborne infections among LTCF residents were lower than or similar to rates among community residents, except for salmonellosis, which was higher. In particular, rates of campylobacteriosis in LTCF residents were consistently lower throughout the entire study period, which was unexpected because incidence of this infection is universally high. Despite the high incidence of campylobacteriosis, outbreaks are rare in Australia, possibly because of the high dose required to cause infection and because foods causing infection are thought to become contaminated through cross-contamination. In Australia, Campylobacter infections are the most common cause of bacterial foodborne disease, with contaminated chicken meat causing ≈30% of all infections each year.
The lower incidence of campylobacteriosis might result from the highly regulated food hygiene system for LTCFs. In 1998, Victoria was the first Australian state to implement mandatory food safety programs for food service settings, and those serving vulnerable populations require independent auditing. These programs might have resulted in better understanding and practices by LTCF staff about food storage, cooking, and cross-contamination than by elderly persons in their own homes. Campylobacter infections in elderly community residents have been associated with risk possibly from cross-contamination during food preparation. Even though Campylobacter infections are associated with travel, a case–control study in Australia found that only 23 (2.8%) of 833 persons >5 years of age with campylobacteriosis had traveled overseas during the week before illness. When we accounted for known travel history and outbreak-associated cases, the IRR for Campylobacter spp. infections in LTCF residents was lowered.
S. enterica is a common cause of foodborne and waterborne outbreaks in LTCFs, a finding that our study confirmed. We found that outbreaks of S. enterica serotype Typhimurium infections accounted for the higher incidence of these infections, but not for non-Typhimurium serotype infections, in LTCF residents. Sources of outbreaks in LTCFs often are not identified, although eggs are commonly suspected as the cause in S. enterica serotype Typhimurium–associated outbreaks. Although residents are at higher risk for outbreak-associated disease, ascertainment of cases is biased in the institutional setting because of the common living environment, centralized access to health care, and collection of specimens by public health staff. During outbreaks, public health investigators often collect fecal specimens from LTCF residents with diarrhea, which would not occur for elderly persons in the community. Because surveillance is well established in Victoria, LTCFs are more likely than persons in the community to report outbreaks.
We did not find any evidence to suggest that living in an LTCF increased a person's risk for legionellosis, despite the occasional occurrence of outbreaks and sporadic cases in this setting. LTCF residents reported with legionellosis were more likely to be infected with Legionella species other than with L. longbeachae. L. longbeachae is associated with gardening and potting mix, so we expected the incidence of this infection to be low in LTCF residents.The incidence of listeriosis was similar in LTCF and community residents. Given the food safety program requirements in facilities in Victoria, LTCF residents plausibly could be exposed to lower concentrations of L. monocytogenes in food, compared with community residents who may keep food longer, have poorer food preparation practices, and eat foods considered higher risk for transmitting foodborne pathogens.
Different clinical investigative approaches for LTCF and community residents with potential foodborne and waterborne disease might account for some of our findings. Although clinicians might elect not to collect specimens when LTCF residents have diarrheal illness, we think it more likely that reporting is more complete in LTCFs. Most of the diseases in our study are serious illnesses, and infected persons would have severe gastrointestinal and extraintestinal symptoms lasting for several days or weeks. In a case–control study of campylobacteriosis in Australia, 41% of case-patients had bloody diarrhea, and 75% had fever; both of these symptoms are strong predictors for physicians ordering laboratory tests.
We were unable to control for potential confounding factors, such as concurrent conditions and factors that might predispose for infection. Many elderly persons with concurrent conditions live in the community, but the health status of LTCF residents is likely to be lower, and they are likely to be more frail. We would have partly controlled for frailty through inclusion of age in our multivariable model because elderly persons in institutions are the oldest and the most frail in society. In addition, our study was underpowered to detect an effect for diseases where notification rates were very low. The potential bias in the final estimates from lack of control of confounding would be more likely to result in increased incidence rates in LTCF residents. However, except for salmonellosis, infection rates were higher in community residents.
Our findings should not be overinterpreted because our study was a retrospective record–based study in which we manually coded surveillance data and were unable to validate case-patients' addresses. It is possible that we were unable to correctly identify residential status of case patients from addresses. In some instances, residents were recorded as living at addresses where an LTCF and retirement village were on the same grounds, making determining whether a person lived in the facility difficult. Similarly, some persons might have been infected after moving into an LTCF, but the address on a pathology report still recorded their residential address in the community where they had previously lived. However, in Victoria, physicians and laboratories were required to report these infections, making it unlikely that both sources of notification would incorrectly report the residential address. For Campylobacter infections, however, physicians report only 50% of notifications, with the remainder coming from laboratories.
Elderly community residents might receive meals from organizations that provide community support. In addition, elderly residents of LTCF might eat food that has been prepared outside the facility during excursions or brought in by visitors, which could result in exposure to foodborne pathogens. For both groups, these alternative routes of exposure would modify the risk for infection so that it did not truly reflect the risk in their place of residence.
The strength of our approach was that we consistently coded addresses without regard to disease-causing agent, yet we observed distinct differences in reported incidence from disease to disease. Our findings were consistent with what we know about these diseases, such as increasing incidence in older persons for diseases such as listeriosis. The CFRs were consistent with reports in the literature for elderly persons, although we assessed deaths only short term (i.e., in the weeks after infection). In general, elderly persons have more severe outcomes from foodborne infections than do younger persons. Large-scale studies that used population-based registers have demonstrated that enteric diseases contribute to more deaths than recognized from short-term follow-up, even when controlling for concurrent conditions.
In our study, rates of surveillance reports for most infections in persons >65 years of age were similar to or lower than for persons <65 years of age, a finding that contradicts the common statement that elderly persons are at higher risk for foodborne disease. However, we did find that the CFRs were high for some infections and that LTCF residents were affected more severely. We believe that our findings can be generalized to other Australian states and territories with similar rates of infection and methods of surveillance. Other investigators could repeat this study by using record-linkage to compare their findings with our findings.
We observed a lower incidence of reported Campylobacter spp. infection in LTCF residents, which provides some reassurance for food safety regulators and the aged care industry. Our study highlights that most foodborne and waterborne infections are rare in elderly residents of LTCF and the community, but that these infections do cause occasional deaths. Primary research is needed into the specific causes of foodborne and waterborne infections in elderly persons in the community and in institutional settings that particularly accounts for the effect of concurrent conditions. In our study, elderly LTCF residents had an incidence of foodborne and waterborne infections that was similar to or lower than that that for elderly persons living in the community, except for S. enterica infections.
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