Re fl exiones fi nales

Estimates of STEC prevalence in cattle have been determined by field studies in slaughterhouses and farms in different countries across the globe. The reported rates vary considerably, depending on study design, sensitivity of detection and isolation methods, age of animals, and season [172]. Some studies assessed prevalence using culture isolation with or without the use of immunomagnetic separation (IMS), while others applied PCR screening methods to detect stx genes in faecal samples. Prevalence estimates can also be affected by different sampling methods applied. For example, Meichtri et al. [173] compared STEC prevalence in young beef cattle in a slaughterhouse study in Argentina, using two different sampling methods (caecum stool sample vs. rectal swab sample). They reported a significantly lower prevalence of STEC in stool samples than rectal swab samples (34.3% vs. 87.7%), although this could have been due to different enrichment media they have used for each sampling method and the season when the samples were collected. Similarly, Cobbold et al. [171] noted differences in prevalence estimates for STEC O157 among feedlot cattle using two different sampling methods. The STEC O157 prevalence in recto-anal mucosal swab samples was 11.0% (95% CI 9.9% – 12.1%) and significantly higher compared to prevalence in freshly passed manure (6.6%; 95% CI 5.6% – 7.6%).

Acknowledging differences in methodologies, prevalence estimates of STEC in cattle can also be affected by animal or environmental factors. For example, intermittent shedding of STEC

(as described above) can lead to an underestimation of the true STEC prevalence in a cattle population, while the phenomenon of seasonal prevalence of STEC should also be considered. Seasonality of STEC prevalence in cattle is well documented, with greater faecal shedding of STEC in warmer seasons (late spring and summer) having been reported [174, 175]. A 12- month survey of 118 slaughterhouses was conducted in Great Britain assessing the faecal carriage of STEC O157 in cattle (3,939 rectal samples) and sheep [176]. The study reported an annual prevalence of 4.7% (95% CI 4.1% – 5.4%) for cattle, with highest prevalence observed in summer (5.5%; 95% CI 4.2% – 7.1%). Similarly, in another 12-month slaughterhouse survey in the UK, the risk of faecal carriage of STEC O157 in cattle was associated with the summer season [177]. The phenomenon of seasonality was also observed in the southern hemisphere as reported in a study on five dairy farms in Argentina [178]. A significant difference in faecal prevalence (rectal swab) of stx-positive cows was observed between warm and cold seasons, with a prevalence of 44% and 56% in spring and summer, respectively, compared to 22% and 28% in autumn and winter, respectively. In contrast to this, Alam and Zurek [179] did not observe a seasonal difference in prevalence of STEC O157 in 891 faecal samples collected from feedlot cattle during a 6-month study in Kansas (US) and reported the highest monthly prevalence of 18.1% in February (later winter). Similar contradictory evidence was provided in a Scottish study by Ogden et al. [180], reporting a higher faecal prevalence of STEC O157 in slaughter cattle in cooler months (11.2%; 95% CI 8.4% – 13.9%) compared to warmer months (7.5%; 95% CI 5.4% – 9.6%)

(χ2

test, p = 0.035). Interestingly though, they observed a six fold increase in average concentration of STEC O157 shed in the warmer season (1,932 CFU/g faeces) compared to the cooler season (330 CFU/g faeces), and suggested that this could explain to some extent the increased incidence of STEC infections in humans over the summer period. The reason for seasonality of STEC in cattle is still not understood, but it is hypothesised that other environmental factors (e.g. weather, climate, length of day-light) could play a role.

Considering animal, methodological and environmental factors as discussed above, longitudinal studies provide generally more accurate prevalence estimates in cattle populations compared to cross-sectional studies as the latter are equivalent to ‘snap-shot’ assessments of prevalence distributions.

STEC O157

Meyer-Broseta et al. [181] evaluated previously published surveys in North American cattle (conducted between 1986 and 1994) and reported that herd prevalence rates for STEC O157 ranged from 0% to 22% for dairy farms, and up to 61% for feedlot farms, whereas herd prevalence rates in European cattle (surveys conducted between 1992 and 1998) were 0% to 3%. However, other European studies showed higher herd prevalence estimates. A study conducted among 75 dairy herds in England and Wales in 1999 reported a herd prevalence of 38.5% (95% CI 28.1% – 50.4%) [182]. Similarly, Bonardi et al. [174] detected STEC O157 in 16.6% (37/223) of feedlot cattle and in 16.1% (22/137) of dairy cull cows sampled at three slaughter plants in northern Italy. They estimated a herd prevalence for STEC O157 of 28.3% (15/53) and 21.7% (13/60) in feedlot and dairy farms, respectively.

Several studies have also assessed within-herd prevalence rates of STEC O157, observing higher prevalence rates in animals from weaning (2 months-old) to two years of age as compared to younger calves or older cattle [165, 183, 184]. In the US, for example, Hancock et al. [185] described a within-herd prevalence for STEC O157 of less than 1.5% in calves <8-weeks-old, while animals aged eight weeks to four months showed a prevalence from 1.8% [186] to 5% [187]. A study on heifers of four to 24 months of age observed a mean within-herd prevalence for STEC O157 of 2.3% [188]. In contrast to this, a within-herd prevalence estimate of <1% was described for adult dairy cows [185].

STEC O26

A number of studies have also reported the prevalence of STEC O26 in cattle. Chase- Topping et al. [189] described in a national survey of 338 Scottish cattle farms a herd prevalence of 18.6% (63/338) and a prevalence of 4.0% in 6,086 faecal pats. A similarly low intestinal prevalence for STEC O26 was reported in a slaughterhouse study in northern Italy including 182 slaughter cattle (dairy cows and feedlot cattle) [190]. Compared to an overall STEC prevalence of 4.9% (9/182 animals), STEC O26 was detected in the caecal contents of only 1 (0.5%) animal. In contrast to this, Lee et al. [191] assessed the faecal prevalence of STEC O26 (and STEC O111) in 442 beef and dairy calves of <16 weeks of age from 115 farms in Korea. They observed a prevalence of 14.4% (37/257) and 7.6% (14/185) in diarrhoeic and non-diarrhoeic calves, respectively, providing evidence for cattle being a potential source of STEC O26 infections in humans.