Distribution trends & antibiogram pattern of Salmonella enterica serovar Newport in India

Genus Salmonella is comprised of two species and 2,579 serovars1. Salmonella enterica subsp. enterica comprises 1,531 serovars which are largely associated with human infections2, commonly acquired by consumption of contaminated food. Although typhoidal Salmonellae remains to be the important cause of morbidity and mortality, non-typhoidal Salmonellae are a leading cause of food poisoning and enteric infections and emerged as an important public health problem worldwide3. It is estimated that non-typhoidal serovars of Salmonella cause 93.8 million human infections and 1,55,000 deaths annually4. These serovars account for 38 per cent of foodborne illnesses5, and are responsible for 35 per cent of hospitalizations and 28 per cent of deaths due to foodborne illnesses6. Moreover, non-typhoidal Salmonellae have a wide range of reservoirs and are widely distributed7.

Salmonella Newport is one of the most important serovars and it is ranked in top three Salmonella serovars associated with foodborne outbreaks in the United States5. S. Newport has been reported to be responsible for several major outbreaks associated with tomatoes, ground beef, alfalfa sprouts, mung beans, cantaloupe and many other food products8. S. Newport has been reported to cause a wide spectrum of clinical diseases in humans, such as diarrhoea, ileocecal lymphadenitis, chest wall abscess, pyosalpinx, osteomyelitis, endocarditis, meningitis, splenic abscess, septicaemia9 and bacteraemia10.

Although S. Newport has emerged as an important human pathogen in the last few decades and initiation of enhanced surveillance of this serovar is advocated in several countries, there is scarcity of data about the prevalence and distribution of S. Newport in India. Therefore, an attempt was made in the present study to generate data on the distribution trends of S. Newport throughout the country.

Material & Methods

Ninety seven S. Newport isolates, received at the National Salmonella and Escherichia Centre, Central Research Institute, Kasauli, India, from various medical, veterinary and research institutes throughout the country, during January 2010 to December 2013 constituted the material for the study. Bacterial isolates were identified on the basis of culture characteristics, Gram staining and conventional biochemical tests11. Isolates identified as salmonellae were further subjected to serotyping1 using an array of various Salmonella antisera (Statens Serum Institute, Copenhagen, Denmark; Denka Seiken Co. Ltd., Tokyo Japan).

Antimicrobial susceptibility testing: All serologically confirmed S. Newport isolates were tested for antimicrobial susceptibility by disc diffusion method12 using the following 12 antimicrobials (Hi Media Laboratories, Pvt. Ltd., Mumbai, India): ampicillin (10 μg), ceftazidime (30 μg), cefotaxime (30 μg), cefuroxime (30 μg), chloramphenicol (30 μg), trimethoprim-sulphamethoxazole (co-trimoxazole) (1.25/23.75 μg), kanamycin (30 μg), norfloxacin (10 μg), ciprofloxacin (5 μg), nalidixic acid (30 μg), tetracycline (30 μg), and nitrofurantoin (300 μg), according to the Clinical and Laboratory Standards Institute (CLSI) guidelines and interpretative criteria12. Escherichia coli ATCC 25922 was used as standard strain.

Results

A total of 3,924 suspected Salmonella isolates were received during the study period. Of these, 1,850 (47.1%) isolates were identified as Salmonella. Among serotyped isolates, 97 (5.24%) were serologically identified as S. Newport which were received from eight States and one union territory viz. Tamil Nadu (56.7%), West Bengal (14.4%), Uttrakhand (7.2%) Bihar (6.2%), Haryana (6.2%), Maharashtra (4.1%) Delhi (NCT) (3.1%), Himachal Pradesh (1%) and Punjab (1%) (Table I). Among isolates from humans, highest proportion was found from stool (86.5%) followed by blood (11.5%) and urine (1.9%) (Table II). S. Newport isolates from animals were found to be of poultry origin (77.7%) and porcine origin (22.2%) whereas milk (66.6%) and meat (33.3%) were found to be the main sources of S. Newport amongst food items. S. Newport isolates were found to be resistant to ampicillin (87.62%), cefotaxime (30.9%), ceftazidime (30.9%), kanamycin (30.9%), co-trimoxazole (25.7%), tetracycline (24.7%), nalidixic acid (23.7%), nitrofurantoin (23.7%) and norfloxacin (22.7%) whereas, all the isolates were found to be susceptible to chloramphenicol, ciprofloxacin and cefuroxime.

Table I Place-wise distribution of Salmonella enterica serovar Newport

Table II Source wise distribution of Salmonella enterica serovar Newport

Discussion

Of the 97 S. Newport isolates analysed in the present study, maximum number of isolates was found to be from humans followed by animals and food indicating its importance as human pathogen. Among human isolates, maximum number of S. Newport isolates was from stool. Inappropriate treatment and disposal of sewage continues to be the major hurdle in attaining good hygiene and sanitary conditions and therefore, may contribute to dissemination of S. Newport into the environment. Presence of S. Newport in human faeces may result in possible transmission of S. Newport through contaminated water bodies and vegetables irrigated using contaminated water sources apart from the other animal reservoirs. S. Newport outbreaks due to the consumption of contaminated mung beans8, cantaloupe13, ready to eat salad vegetables14, watermelon15 and mango16 have been reported earlier from various countries. Isolation of S. Newport from blood and urine shows the potential of this serovar to cause invasive infections as has been reported earlier9.

S. Newport isolates obtained from animals and food products of animal origin have caused septicaemic illness in both animals and humans17. Amongst S. Newport isolates from animals, 77.8 and 22.2 per cent were found to be of poultry and swine origin, respectively. Both of these reservoirs may act as an important source of S. Newport infections in humans due to consumption of undercooked poultry18, eggs19 and pork20. Live poultry may also act as an important source of S. Newport infection21. Swine can be asymptomatic reservoirs of salmonellae22 and may become colonized by ingesting contaminated faeces or through snout to snout contact23. Salmonellae can be found commonly in the environment of pig farms which helps in the maintenance of the bacteria in the herds24.

Both vegetarian and non-vegetarian populations are susceptible to S. Newport infections due to the consumption of contaminated meat and milk25. Therefore, high standards of hygiene are required to be maintained in dairy farms and food processing industries. Administration of S. Newport vaccine in cattle may further help in controlling infections in dairy herds. It will also check environmental dissemination of S. Newport by controlling faecal shedding into the environment26. No specific trends were observed in the number of the S. Newport isolates from various parts of the country.

Highest resistance was observed against ampicillin and third generation cephalosporins followed by co-trimoxazole, tetracycline, and nalidixic acid. However, all isolates were found to be susceptible to chloramphenicol and ciprofloxacin. Third-generation cephalosporins are the drugs of choice for treatment of persons with non-typhoidal Salmonella infections that require chemotherapy or when fluoroquinolones are contraindicated.

Although an attempt has been made in the present study to generate data on the distribution of S. Newport in India, the scenario may actually be much complicated and, therefore, necessitate formulation and execution of efficient surveillance programmes which will result in generation of enough epidemiological data, further facilitating public health authorities to understand the epidemiological trends of S. Newport in the country. Moreover, shifts in the prevalence of specific strain types and serovars in human and animal populations is a consequence of its introduction through international travel, human migration, food, animal feed and livestock trade. Therefore, national level surveillance programmes should be collaborated with global monitoring programmes.