During the study period we recorded a total of 566 outbreaks from 339 care homes. This equates to an incidence rate of 38.14 outbreaks per 100 homes per year. Of the 339 care homes reporting outbreaks, 194 (57.2%) reported only one outbreak during the study period, with the maximum being 7 outbreaks reported by one care home. Of the 566 outbreaks, at least one stool sample was submitted for laboratory testing for 362 (64.0%) outbreaks. Of those 362 laboratory-tested outbreaks, a pathogen was detected in 284 (78.5%) outbreaks.
A breakdown of the number of care homes, number and incidence rate of outbreaks, and number and percentage of outbreaks with a faecal sample submitted for pathogen testing is shown in Table 1. The area with the lowest incidence rate of outbreaks was Redcar and Cleveland (25.5 outbreaks per 100 care homes per year) and the highest was North Tyneside (64.1 outbreaks per 100 care homes per year). The area with the highest percentage of outbreaks with a stool sample submitted was County Durham (72%), which was substantially higher than the percentage from Newcastle upon Tyne (51.3%), the lowest of the 12 areas.
The temporal distribution of outbreaks is shown in Figure 1A. In the 2016/17 season the month with the largest number of outbreaks was April 2017 (n = 33). During the 2017/18 season there were more outbreaks than the previous season, with the number of outbreaks peaking in March 2018 (n = 46). The percentage of outbreaks with a sample submitted is shown over time in Figure 1B. There was variation in the percentage submitted by month, with the lowest in September 2016 (37.5%) and highest in February 2018 (83.7%), however there was no notable trend or periodicity.
Figure 1 – Care home gastroenteritis outbreaks by month and year, North East England, 2016-2018 A) Total number of outbreaks B) Percentage of outbreaks with a faecal sample submitted, with loess regression smoothed line (blue line) and 95% Confidence Interval (grey) C) Percentage of outbreaks with a positive norovirus sample, with loess regression smoothed line (blue line) and 95% Confidence Interval (grey)
There were 284 outbreaks with a stool sample submitted and a pathogen detected, of these 263 (92.6%) had a viral pathogen identified. Six viruses were detected, in order of frequency: 181 norovirus (64%), 37 sapovirus (13%), 32 rotavirus (11%), six astrovirus (2%) and one adenovirus (0.4%). Norovirus and sapovirus were detected together in 2 outbreaks; norovirus and rotavirus were also detected together in 2 outbreaks. Clostridium difficile was detected in 4 outbreaks and Campylobacter in 2 outbreaks. Norovirus and C. difficile were detected together in 2 outbreaks. There were only 38 outbreaks for which samples were tested for C. perfringens and B. cereus. C. perfringens was identified in 15 (39%) of these outbreaks, but the toxin gene was only detected in three outbreaks. B. cereus was not identified in any outbreaks.
Overall, 50% of outbreaks with a sample submitted had a positive norovirus result. The percentage of outbreaks with a sample submitted that was positive for norovirus is shown by month in Figure 1C. Norovirus was detected in every month, with the lowest proportion of outbreaks being caused by norovirus in June 2018 (11%). There was a seasonal change in this relationship, with a higher percentage of samples positive for norovirus during the winter months in both seasons.
The median population (residents and staff) of care homes in this dataset was 96 people (Interquartile range (IQR) 70 - 121); the median number of residents was 44 (IQR 34 – 57). The median ratio of staff to residents was 1.16:1 (IQR 0.99:1 – 1.38:1). Of the 284 outbreaks where a stool sample was submitted, the median number of cases tested was 3 (IQR 2 – 4). The median attack rate in residents was 27.3% (IQR 15.7% - 41.7%). For those 263 outbreaks of a viral cause, the distribution of resident attack rates is shown by pathogen in Figure 2. The attack rate was highest in norovirus outbreak (39.1%), followed by astrovirus outbreaks (35.4%), rotavirus outbreaks (33.3%) and sapovirus outbreaks (27.6%). However, these differences in AR are not statistically significant (Figure 2).
Figure 2 – Boxplot showing the distribution of attack rates in residents during care home gastroenteritis outbreaks of a confirmed viral cause (n = 263), North East England, 2016-2018
For the 263 outbreaks of a single viral cause, the association between various outbreak characteristics and norovirus detection is shown in Table 2. These are compared with outbreaks where sapovirus, rotavirus, astrovirus or adenovirus were identified. In the univariable analysis norovirus outbreaks had a significantly higher attack rate; the Odds Ratio (OR) for an attack rate of 25-50% was 1.76 (95% CI 0.97 – 3.21) and for an attack rate over 50% was 2.74 (95% CI 1.24 – 6.49). None of the other outbreak characteristics such as care home population size, outbreak duration, number of cases tested, number of positive samples, outbreak during winter or the staff to resident ratio were significantly associated with norovirus in the univariable analysis. In the multivariable analysis, when simultaneously adjusted for other variables, higher attack rates in residents were significantly associated with norovirus; attack rate of 25-50% (aOR 2.82, 95%CI 1.44 – 5.66), attack rate over 50% (aOR 5.88, 95% CI 2.26 – 16.5). Larger care homes with 35-55 residents (aOR 2.65, 95% CI 1.28 – 5.59) and over 55 residents (aOR 2.35, 95% CI 1.05 – 5.38) were significantly associated with norovirus. Norovirus was also significantly associated with shorter outbreak duration (aOR 0.46, 95% CI 0.24 – 0.85) and fewer cases being sampled (aOR 0.43, 95% CI 0.23 – 0.79).