Outbreak investigation shows importance of effective surveillance system

The work in Eurosurveillance said there were gaps between generating hypotheses on the contamination routes, the sampling programme and interviews of cases.

They said it confirms the importance of having an effective epidemiological surveillance system, to early identify all possible suspected clusters of infection and to quickly mobilise the human and diagnostic resources for a rapid identification of sources and vehicles of infection.

Waterborne source probable​

The results suggested a waterborne mechanism for the outbreak, and might point to a possible relationship between heavy precipitations and the presence of Salmonella in surface waters, maybe via the flooding of sewage treatment plants, leading to contamination of irrigation waters.

“Here, the contamination of surface and irrigation waters by the outbreak strain could also have been due to problems with the treatment plants or to persisting damage inflicted to water pipelines of the sanitary sewer system of the city of L’Aquila and surrounding villages by the devastating earthquake in 2009​,” said the researchers.

However, given the time elapsed between the possible environmental contamination and collection of samples, the results cannot definitely rule out pigs or vegetables as sources of infection.

“Rainfalls have also been proven to play a role in Salmonella dispersal and contamination of tomato plants in the field, especially during concentrated and relatively intense rain events and when plastic mulch was used,” ​they added.

From June 2014 the local health authorities banned the use of surface water for crop irrigation on a larger area. Sanitization interventions were performed in water cleaning plants and no more human cases were observed after September 2014.

A total of 206 cases (98 confirmed, 101 probable, seven possible) were identified in Italy between June 2013 and September 2014.

Among symptomatic cases, 46 (51%) were hospitalised and a man aged >70 died following the S. Typhimurium infection.

Information on patient’s age was collected for 192 cases and the median value was four years.

“The observed higher incidence in patients aged between one and four years-old is not surprising in Salmonella infections, given the increased vulnerability of children and the elderly to this infection. On the other hand, this finding could suggest the implication of food vehicles normally consumed also by children or the contamination of drinking water.”​

An international alert was launched through the European Epidemic Intelligence Information System (EPIS) in April 2014. No cases were reported from the 14 countries (including the US) reacting to the alert.

Monophasic variant of Salmonella enterica subspecies enterica serovar Typhimurium (monophasic S. Typhimurium), with antigenic structure 1,4 [5],12:i:-, accounted for 4.6%, 7.2% and 8.6% of salmonellosis cases reported to the European Surveillance System (TESSy) in 2011, 2012 and 2013.

It also ranked third among the Salmonella serovars identified in humans in the European Union (EU) during this period.

Investigation to find source​

Among those with available information, only eight were detected outside the Abruzzo region.

A total of 106 took part in the questionnaires but not all questions were properly answered by all interviewed cases.

In the week prior to onset of disease, 38 of 49 cases ate food outside their home: in school canteens (20/41), restaurants (10/49), bars (13/44), fast foods (7/45), or in other places (8/23).

The investigation on food eating habits did not reveal any particular food considered at risk for Salmonella infection, apart from eggs and vegetables.

Most reported drinking tap or bottled water, respectively 41/66 (62%) and 56/76 (74%).

All samples taken among the pig herds (n=123), the fruit and vegetables produced by farms (37), and in the slaughterhouses (32) from April to October 2014 tested negative for Salmonella.

Monophasic S. Typhimurium isolates, fitting the definition of outbreak strain, were detected in 11 of 191 samples of surface water, in one of four of irrigation water taken on farms, and in nine of 111 of sewage treatment plants taken in several locations of L’Aquila province.

A cross-correlation analysis between rain and human cases showed that an increased level of rainfalls (averaged over a 10 days period) preceded the onset of salmonellosis cases by ca 20 days.

In Italy, the laboratory-based surveillance network for enteric pathogens EnterNet Italia provides information on the microbiological characteristics of Salmonella spp. strains isolated from humans.

The data are gathered through regional reference laboratories which characterise the strains isolated from peripheral diagnostic laboratories. The EnterNet Italia network is coordinated by the National Reference Laboratory for Salmonella infection in humans of the Istituto Superiore di Sanità (ISS).

Biochemical identification of isolates was done using the automated Vitek 2 system (Biomerieux). The isolates were serotyped with commercial antisera (Statens Serum Institut, Denmark) according to the Kauffmann–White scheme by slide agglutination.

Source: Eurosurveillance, Volume 21, Issue 15, 14 April 2016​

“Outbreak of unusual Salmonella Enterica serovar typhimurium monophasic variant 1,4 [5],12:i:-, Italy, June 2013 to September 2014”​

Authors: F Cito, F Baldinelli, P Calistri, E Di Giannatale, G Scavia, M Orsini, S Iannetti, L Sacchini, I Mangone, L Candeloro, A Conte, C Ippoliti, D Morelli, G Migliorati, NB Barile, C Marfoglia, S Salucci, C Cammà, M Marcacci, M Ancora, AM Dionisi, S Owczartek, I Luzzi, et al