Clinical characteristics of respiratory syncytial virus infection in neonates and young infants

Introduction/Aim. Infection with respiratory syncytial virus (RSV) occurs during the first year of life in 50% of children and 20%–40% of them have signs of lower respiratory tract infection (bronchiolitis or pneumonia). There is an increased risk for complicated course and death from RSV infection in premature infants, especially those with bronchopulmonary dysplasia (BPD) or congenital heart disease. The aim of our study was to analyze clinical characteristics of laboratory confirmed RSV infection in order to evaluate the need for preventive measures in neonates and young infants. Methods. The prospective study included children under age of 12 months admitted to our hospital in the period November 2008–March 2009 who were positive for RSV by enzyme immunoassay membrane test. The course of disease was assessed by clinical score and radiographic findings. Results. Infection with RSV was confirmed in 91 patients: 21 (23.0%) were under the age of 30 days, 37 (40.7%) were between 31–60 days, and 33 patients (36.3%) were older than 60 days (p > 0.05). The highest hospitalization rate was in January – 33 patients (36.3%; p < 0.01). Disease severity score in these age groups (AG) were: 8.4 ± 0.4 (AG 0–30 days); 9.0 ± 0.3 (AG 31–60 days) and 8.3 ± 0.3 (AG > 60 days), without statistically significant difference among the groups (p > 0.05). Clinical scores in patients with and without risk factors were 10.5 ± 0.5 and 8.3 ± 0.2, respectively (p < 0.01). Pathological radiographic findings were observed in 72 (79.1%) and complications (apnea, significant atelectasis, encephalopathy) occured in 15 (16.5%) patients. The average length of hospital stay in complicated and uncomplicated course of the disease was 9 days and 6 days, respectively (p < 0.01). Therapy in 85 (93.4%) patients included bronchodilators, while systemic glucocorticoids and oxygen therapy were used in 51 (56.0%) and 44 (48.4%) patients, respectively. Death occured in 2 (2.2%) patients, both from a high risk group (the patient with BPD and the other one with congenital heart disease and Down syndrome). Conclusion. Infection with RSV in our settings showed marked seasonal characteristics with highest hospitalization rate in January. Although the course and outcome of the disease were favorable in the majority of our patients, the need for hospitalization and administration of therapy with possible side effects warrants that general measures for prevention of respiratory infections are followed especially in the first year of life. Severe disease and death are more probable in neonates and infants with risk factors. In these children passive immunisation with specific monoclonal antibody (e.g. palivizumab) during RSV season should be considered.

Pathological radiographic findings were observed in 72 (79.1%) and complications (apnea, significant atelectasis, encephalopathy) occured in 15 (16.5%) patients.The average length of hospital stay in complicated and uncomplicated course of the disease was 9 days and 6 days, respectively (p < 0.01).Therapy in 85 (93.4%) patients included bronchodilators, while systemic glucocorticoids and oxygen therapy were used in 51 (56.0%) and 44 (48.4%) patients, respectively.Death occured in 2 (2.2%) patients, both from a high risk group (the patient with BPD and the other one with congenital heart disease and Down syndrome).Conclusion.Infection with RSV in our settings showed marked seasonal characteristics with highest hospitalization rate in January.Although the course and outcome of the disease were favorable in the majority of our patients, the need for hospitalization and administration of therapy with possible side effects warrants that general measures for prevention of respiratory infections are followed especially in the first year of life.Severe disease and death are more probable in neonates and infants with risk factors.In these children passive immunisation with specific monoclonal antibody (e.g.palivizumab) during RSV season should be considered.

Introduction
Respiratory syncytial virus (RSV) is the one of the most common causes of acute respiratory tract infections in children.Infection with this virus occurs during the first year of life in 50% of children and 20%-40% of them have signs of lower respiratory tract infection (bronchiolitis or pneumonia).There is an increased risk for complicated course and lethal outcome in premature infants, especially those mechanically ventilated due to the respiratory distress syndrome (RDS), infants with chronic lung disease, in particular bronchopulmonary dysplasia (BPD) and infants with hemodynamically significant congenital heart diseases 1 .
Infection with RSV is transmitted by droplets or direct contact.This virus preserves contagiousness for a few hours on objects as far as 6.6 m from the patient 2, 3 .Immunity after primary infection is short-term and reinfections occur frequently during childhood 4 .In temperate climate RSV infection shows marked seasonability with peak incidence during winter and early spring 5 .Every year between 75,000 to 125,000 hospitalizations in the USA and more than 600,000 deaths worldwide are connected to the RSV infection 6,7 .
According to our knowledge there aren't sufficient data regarding RSV infection in neonates and young infants in Serbia.The aim of our study was to analyze clinical characteristics of laboratory confirmed RSV infection in order to evaluate the need for preventive measures in neonates and young infants.

Methods
The prospective study included children under the age of 12 months admitted to the Mother and Child Health Institute "Dr Vukan Čupić" in the period 1 st November 2008 -31 st March 2009 with laboratory confirmed RSV infection.Informed consents were obtained from parents or guardians for all children enrolled.The enzyme immunoassay membrane test (BD Directigen™ RSV Test Kit, Becton Dickinson, USA) was used for confirmation of RSV infection by qualitative detection of RSV antigen in nasopharyngeal aspirates within 24 h of hospital admission.The participation in the study did not affect routine management or the length of hospital stay.
The children enrolled were assessed for gender, age, gestational age, month of admission and course of disease in the presence of risk factors (prematurity, neonatal RDS and/or mechanical ventilation, BPD, congenital heart disease).The course of disease was assessed by clinical score with six criteria including transcutaneous oxygen saturation measured by a pulse oxymeter (SaO2) (Table 1) 8 .Some other evaluated parameters included patient's axillary temperature, radiographic findings, white blood cell (WBC) count and C-reactive protein (CRP) level measured immunoturbdimetrically (Turbox ® , Orion Diagnostica), type of complication, therapy administered and length of hospital stay.
Statistical analysis was performed by SPSS v.12.0 for Windows.The Student's t-test, ANOVA and χ 2 test were used and the difference was considered statistically significant when p < 0.05.

Results
Respiratory syncytial virus infection was confirmed in 91 patients, with even distribution by gender and predominance of term neonates and infants born by vaginal delivery with average body mass of 3.140 ± 75 g (Table 2).The monthly distribution of RSV infection was the following: in November and March there were 6 (6.6%) and 8 (8.8%) patients, respectively; in December and February, 25 (27.5%) and 19 (20.9%) patients, respectively (p > 0.05); while in January the hospitalization rate was the highest with 33 (36.3%) patients, which was statistically highly more significant than in the previous months (p < 0.01) (Figure 1).Body temperature was normal on admission and/or during hospitalization in 63 patients (69.2%), while fever with average value of 38.3 ± 0.1 o C was noted in 28 (30.8%) of the patients.Pathological radiographic findings were present in 72 (79.1%) of the patients with hyperinflation seen in 50 (54.9%) of the patients, infiltrates in 13 (14.3%),consolidation in 5 (5.5%) and atelectasis in 4 (4.4%) of the cases.The median of WBC count was 11.3 × 10 9 /L, and the median of CRP level was 2.5 mg/L.
The clinical score for the whole group had the average value of 8.6 ± 0.2 without statistically significant difference between age groups, or between patients born before and after 36 weeks of gestation.The clinical score for those born before 32 weeks of gestation was 10.6 ± 0.7 and was highly statistically greater than for the patients born after this gestational age (Table 3).Risk factors for more severe disease were present in 12 (13.2%) of the patients with clinical score of 10.5 ± 0.5, while in the patients without these risk factors clinical score was 8.3 ± 0.2.This difference was highly statistically significant (p < 0.01).
The median length of hospital stay in the presence of risk factors or in complicated disease was 10 and 9 days, respectively, while in the absence of risk factors the average length of hospital stay was 6 days.This difference was highly statistically significant (p < 0.01).
The prevalence of various therapeutic procedures (the use of bronchodilators, systemic glucocorticoids, oxygen therapy) is shown in Table 4.

Table 4 Therapeutic procedures in patients with respiratory syncytial virus (RSV) infection (n = 91)
Therapeutic procedure n (%) Brochodilators 85 (93.4) Systemic glucocorticoids 51 (56.0)Oxygen therapy 40 (44.0) The course of the disease was complicated by apnea, significant atelectasis and/or encephalopathy in 15 (16.5%) patients, while death occurred in 2 infants, so that mortality rate in our patients was 2.2%.
The discharge diagnosis was bronchiolitis in 77 (84.6%) of the patients.Ten (11.1%) patients were discharged with the diagnosis of pneumonia and 4 (4.4%) with the diagnosis of nonspecific respiratory infection.

Discussion
According to the available literature this is the first prospective study in our country including patients under the age of 12 months with laboratory confirmed RSV infection.The enzyme immunoassay test with two monoclonal antibodies (BD Directigen™ RSV Test Kit, Becton Dickinson, USA) was used for confirmation of RSV infection.This test is simple, reliable, has high specificity of 97.5% and preva- lence of false-positive results of only 0.7% 9 .The results of testing were available in a very short period of time (< 1 h).Besides, RSV and bacterial co-infection occur in less than 2% of previously healthy infants.Therefore, the confirmation of RSV infection by this test excludes with high probability the suspicion of systemic bacterial infection, which enhances the safety and rationality of diagnostic and therapeutic procedures especially in children of the youngest age 10 .
Our results also confirm the seasonal character of RSV infection with the increase in hospitalization rate from November until January when the highest prevalence was reached.Afterwards, there was slow decline in hospitalization rate, so there were no confirmed cases in these age groups after the end of March.Large epidemiological studies have shown that the regional season pattern of RSV infection distribution does not change significantly over time 11,12 .
Our youngest patient was 10 days old on admission, and RSV infection in the first week of life was also described 13 .Since the incubation period lasts 2-8 days, from the epidemiological point of view it is important that the infection with RSV can occur in, or shortly after discharge from maternity ward 14 .The most important preventive measure against the spread of nosocomial infection is the appropriate hand hygiene of the staff.It should be also bared in mind that infected persons eliminate virus in 3 to 8 days, and in infants it takes up to four weeks 15 .
Similarly to the previously published results there was a slight predominance of male gender in our group as well, while the rate of 13.2% of patients born before 36 weeks of gestation was twice as high as the general rate of premature delivery in our country 16 .This result is in agreement with the finding that premature infants are more often hospitalized due to RSV infection than term infants.The need for hospitalization in these infants is explained by higher prevalence of severe forms of bronchiolitis due to the slow and incomplete increase of small airways diameter during the first year of life 14,15 .Also, in our patients hyperinflation was the most common radiographic finding, while infiltrations and consolidations were less prevalent.Although there is an evidence that the presence of atelectasis does not complicate the course of disease, it has been described that in more than 5% of infants there is a whole lobe consolidation with respiratory failure and the need for mechanical ventilation 17 .
The majority of our patients did not have fever on admission or during hospital stay which supports the view that RSV infection is an "afebrile illness" 4 .There was not a significant rise in WBC count nor in CRP level which is in agreement with previous studies and indicates that RSV infection is rarely complicated by bacterial infections 18 .
Since usually there is involvement of small airways in RSV infection leading to bronchoobstruction, we used clinical score for assessment of disease severity that was originally designated for use in acute asthma attacks 8,11 .The advantage of this score lies not only in quantification of respiratory distress severity and early recognition of hypoxemia but also in possibility of comparison with results from different authors.For example, the consistency of our criteria for hospital admission is confirmed by the fact that the average clinical score in our group (8.6 ± 0.2) was very similar to that of Constantopoulos et al. (8.7 ± 1.7) 12 .The lack of a significant difference in our age groups probably comes from standardized criteria for hospital admission.Opposite to this, the higher score was obtained in premature infants born before 32 weeks of gestation and is most probably caused by before mentioned morphological characteristics of airways.The higher clinical score was present in high-risk group of infants for severe disease and unfavorable outcome of RSV infection due to the chronic lung disease (BPD) and hemodynamically significant congenital heart disease 18 .In these situations, according to our results as well, there is higher rate of complications and greater length of hospital stay, but one can also expect unfavorable course with lethal outcome 7 .In a group of serious, early complications of disease common are central apneas which can be seen in up to 10%-26% of patients 19 .The risk for their occurrence is higher in premature, young infants and in infants with apneas of prematurity 20 , but they can be the first sign of disease in previously healthy children.Taking this into account current recommendations for hospitalization include all infants under the age of 3 months with RSV lower respiratory tract infection 21 .The signs of encephalopathy were present in one of our patients, but this rare, previously described complication most probably is not related to the structural changes in the CNS 22 .
The average length of hospital stay in our group was 6 days in case of uncomplicated disease which is consistent with previously published results 23 , while in the case of complicated disease the length of hospital stay was significantly increased (on average for 3 days).
When compared to other respiratory viral infections, RSV infection treatment in neonates and infants is based on more extensive approach and the cost of treatment per patient in USA goes up to 3,000 US dollars 24 .Symptomatic, mostly empiric therapy is used.The exception is the need for oxygen therapy when oxygen saturation of hemoglobin is lower than 90% 15 .Inhalations with beta-2 agonists are used in up to 85% of patients with positive effect on clinical score, but they do not affect the length of hospital stay 25 .Similarly to our experience, hospitalized patients with RSV bronchiolitis receive glucocorticoids as strong anti-inflammatory medications in up to 60% of cases 26 .But recent systematic analysis has shown that treatment with systemic glucocorticoids does not have impact on clinical score or the length of hospital stay 27 .Although certain authors suggest the use of nebulized epinephrine as more effective treatment, new studies show that isotonic and hypertonic (3%) NaCl solution are equally effective 28 .
Death has occurred in two of our patients at the age of 3 months.Both patients had known risk factors for unfavorable outcome of RSV infection.The first patient was born at 28th week of gestation with body mass of 1,100 g, and as a result of mechanical ventilation due to the RDS suffered from severe form of BPD.The second patient had congenital heart disease (ventricular septal defect) and Down syndrome.
Lethal outcome of RSV infection is a problem in a group of children with risk factors for severe disease form.Therefore, in most countries with developed healthcare system passive immunization with palivizumab, recombinant monoclonal antibody, is performed in specially defined groups.Recom- mendations for the use of palivizumab in high-risk groups of children were recently published in our country, as well 29 .

Conclusion
According to our study, the first of this kind in our country, we can conclude that the course and outcome of RSV infection in hospitalized neonates and young infants are favorable in most cases.The average length of hospital stay in uncomplicated disease course is six days.Serious complications (apnea, significant atelectasis and encephalopathy) prolong hospital stay for three days on average.Severe disease and unfavorable outcome are more probable in children with risk factors.In these children passive immunisation with specific monoclonal antibody (palivizumab) during RSV season should be considered.