Infectious disease transmission as a forensic problem: who infected whom?

Observations on infectious diseases often consist of a sample of cases, distinguished by symptoms, and other characteristics, such as onset dates, spatial locations, genetic sequence of the pathogen and/or physiological and clinical data. Cases are often clustered, in space and time, suggesting that they are connected. By defining kernel functions for pairwise analysis of cases, a matrix of transmission probabilities can be estimated. We set up a Bayesian framework to integrate various sources of information to estimate the transmission network. The method is illustrated by analysing data from a multi-year study (2002-2007) of nosocomial outbreaks of norovirus in a large university hospital in the Netherlands. The study included 264 cases, the norovirus genotype was known in approximately 60 per cent of the patients. Combining all the available data allowed likely identification of individual transmission links between most of the cases (72%). This illustrates that the proposed method can be used to accurately reconstruct transmission networks, enhancing our understanding of outbreak dynamics and possibly leading to new insights into how to prevent outbreaks.

P2 domain profiles and shedding dynamics in prospectively monitored norovirus outbreaks.

BACKGROUND: Norovirus P2 domain is commonly used to extrapolate transmission within an outbreak (OB) setting. The current definition is that transmission among cases is considered to be proven when no sequence variation is found. OBJECTIVES: Previous studies have shown a high mutation rate and errors during replication of the norovirus genome, therefore the validity of this criterion must be evaluated. STUDY DESIGN: Sequences of the P2 domain were obtained from patients and health care workers sampled during 4 prospectively GII.4 outbreaks. Fecal samples were tested by RT-PCR for presence of norovirus RNA against a standard control preparation to allow quantification. Estimated time of onset of shedding was derived from shedding kinetics modeled on data from sequential sampling. Thereby P2 sequence variation could be linked to estimated total virus excretion in individual subjects. RESULTS: In all the outbreaks, P2 domain variation was found that resulted in unique codon changes in some patients. Mutations were found in 14% of initial samples and >50% of follow-up samples taken from patients involved in an outbreak. In three patients, aa mutations was observed in or near sites involved in host or antigen binding. CONCLUSIONS: We concluded that P2 domain variation increases with duration of virus shedding, but was unrelated to total amounts of virus shed. Therefore, we propose that cluster identification based on identical sequences should be relaxed to accommodate minor sequence variation. When using sequence data to support outbreak investigations, sequence diversity should be interpreted in relation to timing of sampling since onset of illness.

Unrecognized norovirus infections in health care institutions and their clinical impact.

Noroviruses (NoVs) have emerged as the leading cause of acute viral gastroenteritis (GE) in humans. Although diagnostic facilities have greatly improved, significant underdiagnosis of NoV in hospitals may still occur, thereby increasing clinical burden and nosocomial spread. We evaluated the underdiagnosis of sporadic NoV infections in a tertiary care hospital and estimated its clinical impact. From December 2008 until July 2009, fecal samples specifically referred for bacterial but not viral examination were retrospectively tested for NoV by real-time PCR. The clinical and virological data from patients with undiagnosed NoV infection (missed patients) were evaluated and compared with those from patients with recognized NoV. During the study period, 45 patients with undiagnosed NoV were detected, whereas 50 patients were regularly diagnosed. The missed NoV cases were more frequently adults than children (80% versus 46%; P < 0.001). The viral load levels did not differ between the diagnosed and missed patients, but missed patients more frequently presented without diarrhea (20% versus 4%; P < 0.07). The newly admitted missed NoV cases with GE underwent more diagnostic imaging (24% versus 4%; P < 0.01) and tended to be hospitalized longer. When missed-NoV patients were included, the number of nosocomial clusters doubled and missed patients were index cases in 5 of the 6 clusters. These data indicate that NoV infections are frequently missed despite routine laboratory testing and demonstrate that underdiagnosis of NoV patients is associated with costly abdominal imaging and nosocomial clustering. Awareness of NoV infection in adult patients and education about the importance of viral GE should be increased.

Nosocomial transmission of norovirus is mainly caused by symptomatic cases.

BACKGROUND: Nosocomial norovirus (NoV) infection is common and may increase the burden of disease in healthcare settings, particularly in vulnerable hospitalized patients. Implementing effective infection control during and after admission may limit further spread, but evidence-based measures are lacking. METHODS: In this study, we performed a systematic evaluation of sources and modes of transmission during NoV outbreaks within 2 types of healthcare facilities. An outbreak protocol was developed to sample all patients and healthcare workers (HCWs) with and without symptoms on wards involved in outbreaks. Data on clinical history and possible high-risk exposures were collected. Five outbreaks were investigated, involving 28 patients with recognized symptomatic NoV infection. RESULTS: Enhanced sampling, however, yielded 65 additional cases, of whom 14% (n = 9) were asymptomatic patients, 57% (n = 37) were symptomatic HCWs, and 17% (n = 11) were asymptomatic HCWs. For 12% (n = 8), clinical data were not provided (2 HCWs and 6 patients). On the basis of the shedding kinetics, the onset of infection was estimated for each case. The generation interval was then used to construct plausible transmission pathways and reproduction numbers for symptomatic and asymptomatic patients and HCWs. CONCLUSIONS: We found that symptomatic patients and HCWs were more often involved in transmission events than asymptomatic shedders. Asymptomatic HCWs rarely contributed to transmission, despite high levels of fecal virus shedding.

Using molecular epidemiology to trace transmission of nosocomial norovirus infection.

Nosocomial norovirus (NoV) infection is common and may lead to complications in vulnerable hospitalized patients. Understanding sources and modes of transmission of noroviruses within health care settings will support the design of evidence-based strategies for reducing introduction and further spread. We sequenced a highly variable segment of the genome to identify possible clusters in patients with and without acute gastroenteritis who were hospitalized in the period 2002-2007. Admission and sampling dates were used to separate patients with nosocomial infection from those without nosocomial infection. Epidemiological clustering retrieved 22 clusters, defined as ≥ 2 patients with nosocomial infection on the same ward within 5 days. In total, 264 patients (of 2,458 tested) were diagnosed with NoV infection, and 61% of the patient strains could be genotyped. Of those, 51% (n = 82) belonged to GII.4, 34% (n = 54) belonged to GII.3, and 15% (n = 24) belonged to other genotypes (GI.6B, GII.17, GII.7, and GII.2). In children's wards, GII.3 strains were associated with nosocomial spread more often than other viruses were, whereas in adults this was the case for GII.4 strains. Sequence alignment recognized 11 new clusters based on identical P2 domains (4 GII.3 and 7 GII.4 clusters), involving patients in different wards. This increased the total number of recognized clusters by 50%. Five of these clusters involved at least one outpatient, providing a possible target for improvement of infection control. We concluded that the use of sequence-based typing should be considered for identifying hidden nosocomial clusters of NoV infections within health care settings.

Chronic shedders as reservoir for nosocomial transmission of norovirus.

Norovirus (NoV) infection in immunocompromised patients may lead to prolonged norovirus shedding. Here, we demonstrate the involvement of three chronic shedders in hospital outbreaks. Combined epidemiological and molecular evidence suggests that in one case, NoV transmission occurred at least 17 days after the first diagnosis.