The Centers for Disease Control issued an investigation update describing the epidemiology of the nationwide outbreak involving SE with a PFGE pattern JEGX01.0004.
An increase in the number of reports of infection associated with this serotype commenced in late May 2010 as noted in the epidemic curve. Incident cases increased in July but declined after traceback, recall and diversion of eggs produced by Wright County Eggs and affiliates in Iowa.
Based on the previous five years of reports to PulseNet, 1,369 total illnesses would have been expected during the four-month period contrasted with the 3,182 cases reported from May 1 to October 15, 2010. It is noted that the pulse field gel electrophoresis assay is not definitive with respect to identification of isolates and the assay is not definitive for a common source. The procedure does provide an indication which correlates source and patients especially with extensive outbreaks. Genetic sequencing is required to definitively relate an isolate to a vehicle of infection or source.
An interesting question arises as to the cause of the sharp increase of cases noted during early July. Based on evidence released by FDA, congressional testimony and news reports it is apparent that the affected farms were releasing eggs with potential contamination since at least mid-2007. There must have been some reason for the sharp increase which was detected by FoodNet and verified by PulseNet.
It is hypothesized that thermal abuse occurred as a result of either failure of refrigeration equipment on the affected complex or group of complexes since a large number of the incident cases were noted within a few weeks involving nest run eggs sold to packers for subsequent distribution over 11 states.
A second alternative is that flocks were subjected to stress by either post-Easter induction of molt using starvation which would have increased the prevalence rate of vertical transmission. A combination of these factors could result in the epidemic curve as detected by CDC surveillance.
FDA investigators should pursue the possibility that refrigeration failure occurred allowing eggs to be stored at a temperature higher than the statutory 45F. Plant HACCP records, if accurate and representative of the actual situation, should denote the possibility of storage at high temperature. Thermal abuse would have resulted in proliferation of SE after packing, resulting in high infective doses being present in eggs sold to consumers and the food service industry.
The positive SE status of farms and obvious defects in biosecurity, rodent control and management have been documented and were in all probability of long standing. The specific reason why there were a large number of incident cases in June and July has yet to be explained.