Prevalence and Detection of Virulence Genes of Shigatoxigenic Escherichia coli Serotypes Isolated From Cattle Carcasses in Slaughter Houses in Nairobi, Kenya

Abstract

Food borne infections continue to exert a significant toll on human health globally and affect the ability to provide safe food. Shigatoxigenic Esherichia coli (STEC) typically causes an afebrile bloody colitis, and may be followed by Haemolytic Uraemic Syndrome. Most outbreaks of STEC infections have been linked to serotype E. coli O157:H7 strains. The pathogen excretes potent toxins called verotoxins or Shiga toxins so called because of their close resemblance and identical function to the Shiga toxin of Shigella dysenteriae. Human infection is documented to be associated with the consumption of contaminated meat (especially ground meat) or produce contaminated with animal manure that has not been cooked sufficiently to kill dysenteric E. coli. Contamination of carcasses and environment with E. coli from intestinal contents of cattle during slaughter is one of the most important roots of transmission to humans. The aim of this study was to determine contamination by shiga toxin producing E. coli of cattle carcass in licensed public abattoirs, and to assess the prevalence of virulence genes associated with these organisms. The abattoirs were selected from various regions of Nairobi which included Njiru area, Dagoretti market, Kiserian and private abattoirs including Hurlingham and Kenya Meat Commission. This was a cross-sectional laboratory based study, where a rectal swab was collected from every 10th cow just before slaughter and a carcass swab sample collected immediately after removal of the hide from the hind quarters. Microbiological procedures included pre-enrichment and immunomagnectic separation, culture, biotyping and detection of virulence genes by PCR. From the 162 carcasses sampled, a total of twenty one (6.48%) Non Sorbitol Fermenting (NSF) colonies of E. coli O157 were isolated from cattle carcasses and a total of nineteen (5.86%) NSF colonies of E. coli O157 isolated from rectal content of the same cattle after Immunomagnetic separation, enrichment culturing, selective plating and biochemical tests. The majority (97.5%) of the forty E. coli O157 isolates were susceptible to Gentamycin, Nalidixic acid, Ceftriaxone, Ceftazidime, Ciprofloxacin and Cotrimoxazole. Resistance to Ampicilin at 47.5% was the highest prevalance while, resistance to co-amoxiclav, streptomycin and tetracycline was moderate at 25%, 12.5% xix and 27.5% respectively.Using multiple PCR assays with specific primers for rfbO157, flicH7, eae, stx1 and stx2, thirty one out of 40 E. coli isolates studied were positive for various virulence genes. These genes were widely distributed among the isolates. Virulence genes were detected in 13 rectal swab isolates and remaining 18 were from carcass swab isolates. Only two isolatesfrom carcass swab samples contained all the five genes while only one rectal swab isolate had all five genes. Nine isolates contained the gene rfbO157 only and 3 contained the eae gene only. Two isolates contained stx2 gene only. Virulence genes were detected in four carcass isolates, no virulence genes was detected in the rectal isolates of these same cattle suggesting- that STEC strains contaminate carcass yet they were not necessarily present in fecal matter of the same cattle. This confirms cross contamination of carcasses during the dressing of carcasses during slaughter.Though prevalence appears low; it’s of significance as only very low doses are needed to cause infection. Finding of dispersed virulence genes in E. coli O157:H7 from cattle carcasses for slaughter strongly suggests their virulence potential as well as the fact cattle may act as reservoirs for transmission to humans. This finding is of major public health concern because when the environment and conditions are suitable, the virulence genes may link to form an entire complement capable of causing an outbreak. Necessary measures should be urgently put in place to minimize contamination of carcasses during slaughter in abattoirs and other measures including possible consideration for a vaccine development.