Does a past outbreak give us a clue to the cause of a present one?

As of today 14 are ill across 6 states with E. coli O145 – Alabama (2), California (1), Florida (1), Georgia (5), Louisiana (4) and Tennessee (1). The dates when those people became ill range from April 15 to May 12, 2012.

Screen Shot 2012-06-09 at 7.12.13 PM.pngTwo years ago, according to the CDC, as of May 20, 2010, a total of 26 confirmed and 7 probable cases related to this outbreak have been reported from 5 states since March 1, 2010. The number of ill persons identified in each state with this strain is: MI (11 confirmed and 2 probable), NY (5 confirmed and 2 probable), OH (8 confirmed and 3 probable), PA (1 confirmed), and TN (1 confirmed). Among the 30 patients with available information, 12 (40%) were hospitalized. Three patients have developed a type of kidney failure known as hemolytic uremic syndrome, or HUS. No deaths have been reported.

The bacteria responsible for this outbreak are referred to as Shiga toxin-producing E. coli, or STEC. STECs have been associated with human illness, including bloody diarrhea and HUS. STEC bacteria are grouped by serogroups (e.g., O157 or O145). The STEC serogroup found most commonly in U.S. patients is E. coli O157. Other E. coli serogroups in the STEC group, including O145, are sometimes called “non-O157 STECs.” Currently, there are limited public health surveillance data on the occurrence of non-O157 STECs, including E. coli O145; therefore, E. coli O145 may go unreported. Because it is more difficult to identify than E. coli O157, many clinical laboratories do not test for non-O157 STEC infection.

Freshway_home_Where.gifThe multistate outbreak of E. coli O145 infections was everntually linked to shredded romaine lettuce from a single processing facility that the CDC did not name, but we learned in discovery to be Ohio based Freshway Foods.

According to the FDA, the suspect romaine lettuce was grown in four fields of a farm in Wellton, AZ. An initial investigation of the growing fields resulted in three soil samples from two growing fields testing positive for non-O157 Shiga toxin- producing strains of E. coli (STEC); but, the outbreak strain, an O145 shiga toxin 2-producing E. coli, was not detected. Significant potential sources of STEC were not identified in the initial on-farm investigation of the growing fields.

The subsequent environmental assessment initially identified six potential sources of STEC in the Wellton, AZ area; three Concentrated Animal Feeding Operations (CAFOs), one housing development with a co-located sewage treatment facility, one recreational vehicle (R.V.) park with multiple septic leach systems, and the seasonal grazing of sheep on harvested wheat and alfalfa fields.

The three CAFOs are located within approximately eight to eleven miles from the suspect growing fields. With the exception of birds and wind, no route of pathogen transmission from the CAFOs to the growing fields was found. Based upon the fact that the outbreak was not associated with produce from any other farm, we determined that birds and wind are not reasonably likely to be the transportation mechanism of the outbreak pathogen. It was determined that the CAFOs are not reasonably likely to be the source of the outbreak pathogen.

Screen Shot 2012-06-09 at 7.13.11 PM.pngNo information on the location and duration of seasonal sheep grazing was available other than the grower’s statement that no sheep were observed in the immediate vicinity of the Grower’s farm this year. According to the grower, the closest sighting of sheep this year was approximately five miles west of the farm. We found no route of pathogen transmission from westerly growing fields to the suspect fields. However, due to the lack of information available to us, seasonal sheep grazing cannot be completely ruled out as a potential source of the outbreak pathogen.

A housing development is located along an elevated mesa on both sides of the main Wellton irrigation canal. The canal is separated from the development on both sides by canal maintenance roads and berms. The development is serviced by an on-site wastewater treatment plant utilizing subterranean activated sludge tanks. Treated wastewater is held in two plastic-lined ponds approximately 300 yards from the main Wellton canal. Water samples collected from one of these ponds tested negative for STEC. Although the new housing development cannot be completely ruled out as a pathogen source for the outbreak, no evidence of drainage from the development to the irrigation canal was observed.

An R.V. park is located on a knoll directly above the lateral irrigation canal that supplies water to the suspect fields. The R.V. park is serviced by eight on-site septic leach systems. During this investigation we found evidence of drainage from the R.V. park property directly into the lateral irrigation canal. Of particular concern was an area that exhibited evidence of drainage into the irrigation canal in which the soil was moist; no surface source of the moisture was observed and there had not been any recent rains.

Soil samples from these moist drainage areas tested negative for STEC. Non–O157 STEC Shiga toxin 2 (Stx2)-producing E. coli was detected in two drag swabs and one mud sample collected from the irrigation canal adjacent to the R.V. park; but none were the O145 shiga toxin 2-producing outbreak strain. We determined that the R.V. park is a reasonably likely potential source of the outbreak pathogen based upon the evidence of direct drainage into the lateral irrigation canal; the moist soil in this drainage area; the multiple sewage leach systems on the property; the presence of other STEC found in the lateral irrigation canal and in the growing fields of the suspect farm; and the fact that the section of the lateral canal downstream from the R.V. park supplies water to only one other farm in addition to the suspect farm.

Two pumps are located on the main Wellton canal near the lateral canal split that supplies water to fields of the suspect farm; one gasoline powered pump on a trailer and one permanent electric pump with an attached hose. The electric pump supplies canal water to an attached open-end hose. The site is not secured from vehicles and the hose pump is also unsecured. At the time of this investigation there were people living in recreational vehicles on undeveloped land within one mile of the hose pump. The fact that this area is open to vehicles and the pump and hose are unsecured make it possible for an R.V. owner to dump and rinse out their R.V. septic system into the main Wellton canal at the lateral canal split that supplies the farm. The ground near the hose pump shows erosion evidence of drainage into the Wellton canal. Soil collected from this erosion site tested positive for other Stx2-producing STEC, but did not match the outbreak strain.