Guide Communicable Disease: Epidemiology and Control

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Footnote 1. This paper presents the results for eight selected diseases: measles, mumps, rubella, varicella, pertussis, meningococcal disease, hepatitis A and seasonal influenza. Most evidence was found for measles, mumps, rubella, and varicella in terms of all the searched parameters, most likely due to the very specific symptom of onset of cutaneous eruption, considered as a key component to the clinical diagnosis of viral exanthemas.

We identified seven eligible articles for measles.

Most estimates arose from outbreak investigations carried out in different settings including schools [ 6 , 7 , 8 ], hospitals [ 9 ] and the community [ 10 , 11 ]. With respect to vaccination status, the subjects were vaccinated or had unknown vaccination status or had not been protected by previous exposure [ 6 ]. Five of the articles captured laboratory-confirmed cases using serology [ 7 ], PCR [ 8 , 10 ], positive reverse-transcriptase PCR [ 11 ] or virus isolation in culture [ 9 , 11 ] from serum, urine, nasopharyngeal exudate and respiratory secretion; laboratory methods were not described in two articles [ 6 , 12 ].

We also included results from the search of other data sources see above. Summary measures for the incubation period, infectiousness and shedding period for measles by source. In these four studies, the incubation period was defined as time from exposure to onset of rash or fever. No peer-reviewed articles on infectiousness were found. Information on exclusion was available mainly in the grey literature. We identified two eligible peer-reviewed articles for mumps.

Summary measures for the incubation period, infectiousness and shedding period for mumps by source. Both peer-reviewed articles defined the duration of shedding as the period when mumps virus could be isolated both before and after onset of symptoms. We identified two eligible peer-reviewed articles.

The cases were defined based on clinical symptoms including fever, rash [ 21 ] and rubella-characteristic enlarged posterior auricular or sub-occipital lymph nodes [ 20 ]. Enterovirus interference method was used to detect viral RNA from throat swabs [ 20 ]. Summary measures for the incubation period, infectiousness and shedding period for rubella by source.

Infectiousness was not reported in peer-reviewed articles. We identified six eligible peer-reviewed articles on varicella. Data came from three outbreak investigations, two household studies and one case series analysis in children of different ages. Incubation period was not investigated in these studies.

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Summary measures for the incubation period, infectiousness and shedding period for varicella by source. In Ozaki et al. The exclusion seemed not to have been effective since most transmission already occurred after exposure to prodromal cases. No eligible peer-reviewed articles were identified for meningococcal disease. Summary measures for the incubation period, infectiousness and shedding period for meningitis by source. Two eligible peer-reviewed articles, one outbreak investigation and one descriptive study, were identified [ 27 , 28 ].

The samples used for isolation of Bordetella pertussis were nasal swabs and sputum, using culture and identification by neutralisation or immunofluorescence test from notified cases. Summary measures for the incubation period, infectiousness and shedding period for pertussis by source. The disease is described as most contagious in the first two weeks after cough onset [ 15 ].

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When deciding on disease control measures, more attention should be paid to pre-school age contacts for whom the disease has more harmful consequences. We identified three peer-reviewed studies, two outbreak investigations and a study comparing epidemiological, clinical and immunological hepatitis A, conducted among school-age children in different school settings with one or more statements on the searched parameters.

Summary measures for the incubation period, infectiousness and shedding period for hepatitis A by source. In Krugman et al. It is important to note that this study was conducted among institutionalised children with low hygienic standards. Because of the laboratory value in the incubation period definition, asymptomatic cases were also included in the study.

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No data were available in the peer-reviewed articles on infectiousness and shedding periods. Exclusion from school until severe symptoms persist combined with application of hygienic measure was found useful [ 31 ] while the Red Book recommends one week of exclusion after onset of jaundice [ 13 ]. The search identified eight eligible peer-reviewed studies for influenza A or B. No peer-reviewed publications reported on the incubation period or period of infectiousness for influenza. Summary measures for the incubation period, infectiousness and shedding period for influenza by source.

Five peer-reviewed articles presented data on the duration of influenza A shedding, measured from onset of illness and or admission to hospital [ 32 , 33 , 34 , 35 , 36 ]. Three of these were case series analyses, one an outbreak investigation and one a retrospective follow-up study. A randomised controlled trial for measuring the efficacy of oseltamivir presented data on mean values but not on the entire period of shedding. No studies reporting on the exclusion period were identified. According to one source, there is no need for exclusion unless the child is unable to participate in lessons [ 14 ].

In this review, we searched for and assessed available evidence on an important but neglected area of public health. Specifically, the review focused on the incubation period, period of infectiousness or shedding and exclusion period for eight infectious diseases that account for the majority of disease outbreaks and absenteeism among children and adolescents. The key parameters originated from a comprehensive search in PubMed and Embase to identify published literature, complemented with estimates from other sources including WHO, CDC and clinical guidelines.

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We found that estimates obtained from these two types of sources did not differ substantially, although there were some discrepancies and outliers. Understanding the underlying disease mechanisms and the determinants of the incubation period are likely to be critical when interpreting these differences. The review highlighted some of the challenges in drawing conclusions from a diverse range of studies and difficulties in making comparisons based on diverse definitions and methods, as discussed below.

To compare the key parameters, it is critical to have clear definitions of the measurements. The exact time of exposure, the description of the symptoms used to define disease onset, the characteristics of the exposure, serotype, infective dose, the population characteristics, the study design and the diagnostic tools used for measurement can all have an impact on the value of these parameters [ 37 ]. In this context, outliers are most likely due to the lack of standardised methods and missing or varying definitions for measuring the estimates. The incubation period is defined as the time from infection to clinical onset.

Therefore, when describing the incubation period, it is important that authors accurately define the symptom of reference and the date of onset of the symptom. This is because some diseases might have more than one symptom of onset e. Inconsistent description of the symptoms used as the onset reference would also result in a different duration of the incubation period. It can also be challenging to define the incubation period when the exact time of exposure is unknown or where the accurate recording of symptoms is difficult e.

The inclusion of asymptomatic cases or in some contexts, low levels of exposure with poor hygiene standards could have prolonged the length of the incubation period [ 29 ]. When the incubation period was not available for diseases, serial intervals were retrieved [ 28 , 38 ]. For highly infectious diseases such as measles or diseases with asymptomatic onset such as hepatitis A in settings with frequent contact between subjects, serial interval is likely to be a good approximation for the incubation period [ 30 ].

Differences might also exist for other key parameters. It was difficult to compare periods of infectiousness for the diseases of interest because we found very limited evidence on this in the peer-reviewed literature. Pathogen shedding and infectiousness are closely related — mostly the period of infectiousness is based on shedding or viral excretion data [ 39 ] — so, for some diseases, infectiousness could perhaps be determined from data on shedding.

However, the results are highly influenced by the sampling methods, the frequency of sampling, the specimen and the laboratory method used as well as by the definition of the parameters for the period of shedding. For instance, the value of the duration of shedding depends on the point at which measuring starts, e.

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We identified articles where the measurement of the duration of shedding only started at the time of hospital admission [ 33 , 40 , 41 , 42 , 43 ], although it is important to note that these studies mainly focused on the effect of treatment. With respect to the impact of laboratory methods used, one example of the consequences for estimating the period of infectiousness is the shorter duration of virus excretion as measured by viral culture for influenza as compared to measurement by reverse transcription polymerase chain reaction RT-PCR.

When interpreting the estimation for infectiousness or shedding, the diagnostic methods used need to be taken into account. Shedding before symptoms appear seems to be independent of sub-type, age or antiviral therapy [ 44 ]. For instance, oseltamivir treatment was not associated with statistically significant reduction in the duration of viral shedding in influenza patients [ 45 ].

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Further, excretion may occur after recovery or in asymptomatic carriers. In terms of underlying diseases, prolonged shedding could be found in children, immuno-compromised individuals, and patients with underlying diseases including those receiving corticosteroid or other immunotherapy agents [ 1 ]. Presenting conclusive data on exclusion is difficult because measures may be influenced by a range of factors, such as the age of the affected child, the setting and staff availability [ 46 ]. The decision to exclude a child largely depends on the perceived severity of the condition and its potential impact on the health of the affected child, other children and adolescents, and the wider community, and cannot therefore be completely evidence-based.

Such decisions also need to consider the fitness of affected child to attend lessons and the ability of staff to care for the child and for other children. Decisions about the length of the exclusion period should be based on data on infectiousness if they exist or, if not, on data on shedding.

The availability of immunological and molecular methods has brought new perspectives to this area of research because of the high speed and quantity of data generation [ 47 ]. RT-PCR detection of viruses present in immune complexes can happen, but it does not necessarily mean the presence of infectious virion. Thus, when taking decisions about exclusion based on the period of shedding, the impact of different laboratory methods used to detect the shedding of virus or bacteria should always be considered.

The need for exclusion should be considered carefully. For some infectious diseases, even where there is evidence of shedding, the risk of transmission could be relatively low. In the case of viral skin exanthema, this can be infectious before children develop a clinical illness [ 9 , 17 , 23 ] and exclusion might, therefore, be not fully effective. For immuno-compromised patients exclusion should be considered for the whole duration of illness. Another important aspect to be considered is high-risk close contacts, such as pregnant mothers, younger siblings or immuno-compromised relatives.

For some diseases, available recommendations on exclusion practice differ. For this reason, the exclusion is usually recommended until clinical recovery. Some authors deem exclusion to not be necessary, due to the mildness of symptoms, and recommend that standard hygienic measures should be applied during the whole course of the infections [ 48 , 49 ]. However, CDC recommends one week of exclusion after onset of symptoms, when this is defined as jaundice. Each disease is summarised to include aspects of the causative organism, clinical features, diagnosis, transmission, occurrence, distribution, incubation period, period of communicability, control, prevention, treatment, and surveillance.

Easily understood tables, line-diagrams, maps and graphs are presented to cover additional aspects ranging from life-cycles to temporal variations in disease incidence. Throughout the book emphasis is placed on practical approaches with regard to control, rather than the details of each disease. One should approach Webber's classification with the foreknowledge that, while transmission is among the most important determinants of control, the dynamics of the host, agent and environment are complex and must also be considered in the development of comprehensive control interventions.

Epidemiologists are spoilt for choice with regard to reference manuals on communicable diseases and the competing wealth of information available on the internet via disease control agencies. Webber's experience as a public health practitioner in developing countries is what sets this book apart. He understands the burden faced in these regions, and the limited time, knowledge and resources available for intervention.


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