Effect of pig population density on methicillin resistant Staphylococcus aureus prevalence in bulk tank of Italian dairy farms

MRSA contamination of pig holdings in Europe with ST398 and other lineages is a fact. Dairy cows can also be colonized and suffer mastitis by MRSA. Professional categories linked to farming resulted colonized at higher rates than general population due to exposure along all the production chains. The proximity to livestock itself is at risk. A Dutch recent study showed that people without direct contact with pigs or cows and living in a rural area were more likely to be LA-MRSA positive than people in urban settings. The mutual risk between different species in livestock has been not completely investigated yet. We used bulk tank milk (BTM) as a screening sample to detect MRSA in dairy herds located within a high- ensity pig farming area. Applying Geographic Information Systems (GIS) to the area, the association between density of finishing pigs and MRSA status of dairy herds was evaluated. Bulk tank milk (BTM) samples were collected in March 2011 from 225 dairy farms resulted positive for Staphylococcus aureus in the bulk tank at a previous control. Each milk sample underwent a double enrichment, firstly in Mueller Hinton broth with 6.5 % NaCl and then in Tryptic Soy broth with 3.5 mg/l cefoxitin and 75 mg/l aztreonam. Each enrichment step required an incubation for 18-20 h at 37°C. Finally, 100 μl were plated onto MRSA Chromogenic Agar (Pronadisa, Spain). After incubation of 48 h at 37°C, putative colonies were confirmed MRSA by a duplex PCR targeting nuc and mecA genes. Dairy herds with at least a MRSA isolate were classified MRSA-positive. Isolates were typed by Multi Locus Sequence Typing and spa-typing. Each milk sample underwent also DNA extraction. Dairy herds in whose milk the detection of mecA by PCR occurred were classified mecA-positive. All the other dairy herds were classified as negative. Geographic Information System ArcVIEW (ArcGis, 9.3.1, Esri, Redlands, CA) computed number of finishing pig holdings and pig heads within 3 km range around each dairy farm. Statistical analyses by SPSS 19.0 (IBM, SPSS Inc., Chicago IL, USA) and test U Mann-Whitney were applied to compare pigs consistency based on the MRSA status of dairy herds. Statistical significance was accepted when P<0.05. A total of 53 herds (23.6%) were mecA-positive and 9 herds (4%) were effectively MRSA positive. Two STs were isolated (ST398 and ST97) and five spa-types (t899, t001, t108, t4795 and t9305). The number of finishing pig holdings was significantly higher around both mecA-positive (P=0.004) and MRSA-positive herds (P=0.000) than those surrounding negative herds. Also the number of finishing pigs was significantly higher around mecA-positive and MRSA-positive (P=0.019 and P=0.001, respectively). The present work give evidence to the significant effect of pig population density on dairy herds status regarding MRSA. STs and spa-types confirmed this link too, as they are most common in Italian finishing pigs.