Occurrence of Listeria species on cattle farms and abattoirs
Overall, 14.6% (48/328) of the samples collected from the cattle farms were positive for the genus Listeria, while for the beef abattoirs, it was 11.1% (29/262). The difference was not statistically significant (P = 0.201).
Of a total of 590 samples processed from the cattle farms and beef abattoirs, 77 (13.1%), 23 (3.9%), 52 (8.8%), and 2 (0.3%) were positive for Listeria species, L. monocytogenes, L. innocua, and L. welshimeri, respectively (P < 0.001).
Prevalence of L. monocytogenes on cattle farms and beef abattoirs
The prevalence of L. monocytogenes on cattle farms and the univariate analysis of associated factors are shown in Table 1. The prevalence of L. monocytogenes was 3.4% (11/328). The three variables assessed (area, type of farm, and type of samples) had no statistically significant (P > 0.05) on the prevalence of L. monocytogenes.
Table 1
Prevalence of Listeria monocytogenes and Listeria innocua in samples collected from farms and univariate analysis of associated factors
| | | Prevalence (%) of: |
Variable | Level | No. of samples testeda | L.monocytogenesa | L. innocuac |
Area | Winterveld | 48 | 4.2 | 12.5 |
| Soshanguve | 63 | 1.6 | 4.8 |
| DiepslootNature Reserve | 16 | 6.3 | 6.3 |
| Acacia | 15 | .0 | 13.3 |
| Doornrandjies | 15 | 13.3 | 6.7 |
| Haakdooii-Gboom | 13 | 0.0 | 15.4 |
| Hammanskrasl | 15 | 6.7 | 13.3 |
| Moretele | 45 | 2.2 | 13.3 |
| Cullinan | 36 | 5.6 | 11.1 |
| Onderstepoort | 31 | 3.2 | 12.9 |
| Bronkhorstspruit | 31 | 0.0 | 19.4 |
| 95%Cl | | 1.3–5.1 | 7.4–14.4 |
| p-value | | 0.50 | 0.811 |
Farm type | Communal | 83 | 3.6 | 10.8 |
| Cow-calf | 147 | 3.4 | 9.5 |
| Feedlot | 98 | 3.1 | 14.3 |
| 95%Cl | | 2.5–7.2 | 7.8–14.2 |
| p-value | | 0.196 | 0.508 |
Type of samples | Individual faeces | 190 | 2.6 | 12.6 |
| Pooled faeces | 75 | 2.7 | 6.7 |
| Feed-Grains/grass | 27 | 11.1 | 3.7 |
| Silage | 6 | 0.0 | 33.3 |
| Water | 30 | 3.3 | 16.7 |
| 95Cl | | 1.4–5.3 | 8.1–15.1 |
| p-value | | 0.228 | 0.116 |
a328 No. of samples tested; b3.4% of L. monocytogenes; c11.3 of L. innocua |
Table 2 shows the prevalence of L. monocytogenes in abattoirs and the univariate analysis of associated factors. The prevalence of L. monocytogenes on beef abattoirs was 4.6%(12/262). The location of the abattoirs, type of abattoir, and type of samples tested did not significantly (P > 0.05) affect the prevalence of L. monocytogenes.
Table 2
Prevalence of L. monocytogenes, L. innocua, and L. welshimeri in samples collected from abattoirs and univariate analysis of associated factors
| | | Prevalence (%) of: |
Variable | Level | No. of samples testeda | L. monocytogenes a | L. innocua b | L. welshineric |
Area | Merafong | 39 | 7.7 | 5.1 | 5.1 |
| Ekurhuleni | 39 | 7.7 | 0.0 | 0.0 |
| Tswane | 50 | 4.0 | 6.0 | 0.0 |
| Benoni | 39 | 10.4 | 2.6 | 0.0 |
| Holfontein | 26 | 0.0 | 3.8 | 0.0 |
| Cullinan | 30 | 0.0 | 13.3 | 0.0 |
| Heidelberg | 39 | 0.0 | 10.3 | 0.0 |
| 95%Cl | | 2.0-7.1 | 2.9–8.6 | -0.3-1.8 |
| p-value | | 0.163 | 0.239 | 0.073 |
Abattoir type | High Throughput (HT) | 216 | 5.6 | 5.6 | 0.9 |
| Low Throughput (LT) | 46 | 0.0 | 6.5 | 0.0 |
| 95%Cl | | 2.0-7.1 | 2.9–8.6 | -0.3-1.8 |
| p-value | | 0.102 | 0.07 | 0.429 |
Type of samples | Faecal Swab | 66 | 1.5 | 7.6 | 0.0 |
| Pre-Evisceration Swab | 66 | 7.6 | 4.5 | 0.0 |
| Post-Evisceration swab | 66 | 4.5 | 0.0 | 0.0 |
| Chilled Swab | 42 | 2.4 | 0.0 | 0.0 |
| Environmental Sample | 22 | 9.1 | 31.8 | 9.1 |
| 95%Cl | | 2.0-7.2 | 2.9–8.6 | -0.3-1.8 |
| p-value | | 0.355 | < 0.001 | < 0.001 |
a4.6% Prevalence of L. monocytogenes; b5.7% Prevalence of L. innocua; c0.8% Prevalence of L. welshineri. |
In comparison, although the prevalence of L. monocytogenes in samples collected from cattle farms, 3.4%, was lower than found in beef abattoirs, 4.6%, the difference was not statistically significant (P = 0.444).
Prevalence of L. innocua on cattle farms and beef abattoirs
The prevalence of L. innocua in cattle farms and the univariate analysis of associated factors are shown in Table 1. The overall prevalence of L. innocua on cattle farms was 11.3% (37/328). The three variables (area, type of abattoir, and type of samples) did not have a statistically significant (P > 0.05) effect on the prevalence of L. innocua.
At the abattoirs, L. innocua was detected in 5.7% (15/262) of the samples processed (Table 2). Statistically significant (P < 0.001) differences were detected only among the type of samples tested, with a range from 0.0% (chilled carcass swabs) to 31.8% (environmental samples).
Comparatively, the prevalence of L. innocua in the samples collected from cattle farms (11.5%) was statistically significantly (P = 0.018) higher than that detected in abattoir samples (5.7%).
Prevalence of L. welshimeri in cattle farms and beef abattoirs
All the 328 samples collected from cattle farms were negative for L. welshimeri, with a prevalence of 0.0%
The prevalence of L. welshimeri in abattoirs, along with the univariate analysis of associated factors, are shown in Table 2., where the overall prevalence of the organism was 0.76% (2/262). The prevalence of L. welshimeri varied significantly (P < 0.001) across sample types only (environmental samples, 9.1% versus other types of samples, 0.0%).
The difference between the prevalence of L. welshimeri on cattle farms (0.0%) and beef abattoirs (0.76%) was not statistically significant (P = 0.197).
Prevalence of other species of Listeria in cattle farms and abattoirs
All the 328 and 262 samples collected from various sources in cattle farms and abattoirs, respectively, were negative for L. ivanovii, L. grayi, and L. seeligeri.
Characteristics of L. monocytogenes isolates from cattle farms and beef abattoirs
Frequency of the serogroups of L. monocytogenes isolated from cattle farms
The frequency distribution of the serogroups of L. monocytogenes isolated from farms was 72.7% (8/11) and 27.3% (3/11) for 1/2a-3a and 4b-4d-4e, respectively. The difference was statistically significant (P = 0.033). The distribution of the serogroups among isolates of L. monocytogenes recovered from cattle farms by area, type of farms, and type of samples is shown in Table 3. Statistically significant differences were detected in the frequency of L. monocytogenes serogroup 1/2a-3a according to the variables assessed as follows, area (P = 0.001), type of farm (P = 0.030) and type of sample (P = 0.030).
Table 3
Frequency of detection of L. monocytogenes serogroups by the area, farm type/size, and type of samples
| No. of isolates of L. monocytogenes | No (%) of isolates belonging to | No (%) of isolates belonging to |
Variables | | Serogroup 1/2a-3a | Serogroup 4b-4d-4e |
Area | | | |
Winterveld | 2 | 2 (100.0) | 0 (0.0) |
Soshanguve | 1 | 1 (100.0) | 0 (0.0) |
Diepsloot Nature Reserve | 1 | 1 (100.0) | 0 (0.0) |
Doornrandjies | 2 | 1 (50.0) | 1 (50.0) |
Hammanskrasl | 1 | 1 (100.0) | 1 (0.0) |
Moretele | 1 | 0 (0.0) | 1 (100.0) |
Cullinan | 2 | 1 (50.0) | 1 (50.0) |
Onderstepoort | 1 | 1 (100.0) | 0 (0.0) |
p-value | | 0.001 | 0.104 |
Farm type | | | |
Communal | 3 | 3 (100.0) | 0 (0.0) |
Cow-calf | 5 | 3 (60.0) | 2 (40.0) |
Feedlot | 3 | 2 (66.7) | 1 (33.3) |
p-value | | 0.030 | 0.1869 |
Type of samples | | | |
Individual faeces | 5 | 3 (60.0) | 2 (40.0) |
Pooled faeces | 2 | 2 (100.0) | 0 (0.0) |
Feed-Grains/grass | 3 | 2 (66.7) | 1 (33.3) |
Water | 1 | 1 (100.0) | 0 (0.0) |
p-value | | 0.020 | 0.180 |
For beef abattoir samples, the frequency of distribution of the serogroups among the L. monocytogenes isolates was 25% (3/12), 8.3% (1/12), 50% (6/12), and 16.7% (2/12) for the serogroup 1/2a-3a, 1/2b-3b, 1/2c-3c, and 4b-4d-4e respectively (Table 4). The area, type of farms, and type of samples did not significantly affect the serogroups of L. monocytogenes (P > 0.05).
Table 4
Frequency of detection of L. monocytogenes serogroups by the area, abattoirs type/size, and type of samples
| No. of isolates of L. monocytogenes | No (%) of isolates belonging to | No (%) of isolates belonging to | No (%) of isolates belonging to | No (%) of isolates belonging to |
Variables | | Serogroup 1/2a-3a | Serogroup 1/2b-3b | Serogroup 1/2c-3c | Serogroup 4b-4d-4e |
Area | | | | | |
Merafong | 3 | 1 (33.3) | 1 (33.3) | 1 (33.3) | 0 (0.0) |
Ekurhuleni | 3 | 2 (66.7) | 0 (0.0) | 0 (0.0) | 1 (33.3) |
Tswane | 2 | 0 (0.0) | 0 (0.0) | 2 (100.0) | 0 (0.0) |
Benoni | 4 | 0 (0.0) | 0 (0.0) | 3 (75.0) | 1 (25.0) |
p-value | | 0.215 | 0.391 | 0.100 | 0.188 |
Abattoir type | | | | | |
High Throughput (HT) | 12 | 3 (25.0) | 1 (8.3) | 6 (50.0) | 2 (16.7) |
p-value | | | | | |
Type of samples | | | | | |
Faecal Swab | 1 | 0 (0.0) | 1 (100.0) | 0 (0.0) | 0 (0.0) |
Pre-Evisceration Swab | 5 | 1 (20.0) | 0 (0.0) | 4 (80.0) | 0 (0.0) |
Post-Evisceration Swab | 3 | 1 (33.3) | 0 (0.0) | 1 (33.3) | 1 (33.3) |
Chilled Swab | 1 | 0 (0.0) | 0 (0.0) | 1 (100.0) | 0 (0.0) |
Environmental Sample | 2 | 1 (50.0) | 0 (0.0) | 0 (0.0) | 1 (50.0) |
p-value | | 0.099 | 0.373 | 0.106 | 0.189 |
Frequency of detection of virulence-associated genes in L. monocytogenes isolates
For cattle farms, among the L. monocytogenes isolates, the frequency of detection of the virulence-associated genes assessed was 100.0% (11/11) each for seven virulence genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ) but 81.8% (9/11) for actA.
Similarly, in the abattoir isolates of L. monocytogenes, the frequency of detection of the virulence-associated genes was 100.0% (12/12) each for seven virulence genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ) but 83.3% for virulence-associated actA.
Overall, the differences in the frequencies of detection of virulence-associated genes among isolates of L. monocytogenes recovered from cattle farms and beef abattoirs were not statistically significant (P > 0.05).
Among the serogroups of L. monocytogenes, statistically significant differences were detected in their frequencies between farm and abattoir isolates only for 1/2a-1/3a (P = 0.022) (72.7% versus 25.0%) and 1/2c-3c (P = 0.014) (0.0% versus 50.0%).
Frequency of detection of virulence-associated genes in L. monocytogenes isolates according to demography and serogroups
Among cattle farms isolates, all (100.0%) the 1 isolates of L. monocytogenes from the eight areas were positive for seven genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ) except for virulence-associated gene, actA, detected in 9 (81.8%) isolates.
Similarly, all the isolates of L. monocytogenes from the three types of farms (communal, cow-calf, and feedlot) were carriers of the seven virulence-associated genes, but one isolate each from communal farms, and cow-calf operation was negative for gene actA. A total of seven virulence genes were detected in the two serogroups (1/2a-3a and 4b-4d-4e), but the actA gene was detected in 75% (6/8) of the isolates in serogroup 1/2a-3a.
For the L. monocytogenes isolates from beef abattoirs, all (100.0%) the isolates of L. monocytogenes were positive for seven genes (hlyA, inlB, plcA, iap, inlA, nlC, and inlJ) except for virulence gene, actA, detected in 10 (83.3%). The area, type of abattoir, and type of samples did not significantly (P > 0.05) affect the detection frequency of virulence-associated genes.
A total of seven virulence genes were detected in the four serogroups (1/2a-3a, 1/2b-3b, 1/2c-3c, and 4b-4d-4e), but the actA gene was detected in 66.7% (4/6) of the isolates in serogroup 1/2c-3c.
Overall, there were no statistically significant differences (P > 0.05) in the frequencies of detection of virulence-associated genes in farm and abattoir isolates of L. monocytogenes.