of Respiratory Infections

 

Disease or injury


DALYs (millions)


Total burden (%)


1


Lower respiratory tract infection


94.5


6.2


2


Diarrhea


72.8


4.8


3


Unipolar depression


65.5


4.3


4


Cardiac ischemia


62.6


4.1


5


HIV/AIDS


58.5


3.8


6


Encephalic vascular disease


46.6


3.1


7


Prematurity


44.3


2.9


8


Asphyxia and neonatal trauma


41.7


2.7


9


Traffic accidents


41.2


2.7


10


Neonatal infections


40.4


2.7



AIDS acquired immunodeficiency syndrome, DALYs disability-adjusted life years, HIV human immunodeficiency virus




The clinical manifestations of viral respiratory diseases vary from asymptomatic cases to fatal infections, with several intermediate scenarios. Some viruses tend to produce infections mainly in the upper respiratory tract (rhinovirus, coronavirus, adenovirus), while others can similarly compromise the lower respiratory airways, with variable severity (adenovirus, RSV, metapneumovirus, influenza, and parainfluenza). In general, it is an accepted fact that any respiratory virus can compromise one or several levels of the respiratory system and cause clinical and subclinical infections, but there is a certain preference of viruses for compromising specific levels of the respiratory system (Table 28.2). In this way, during an epidemic of a virus such as RSV or influenza, the major proportion of upper and lower respiratory tract infections will be caused by the prevalent virus. In addition, there will be an important incidence of subclinical infections, which act as efficient sources of transmission. As a result of herd immunity, important epidemics of multiple different viruses do not usually coexist in a community; instead, they alternate in terms of their presence in the community. For example, in Chile, the most commonly observed pattern involves parainfluenza outbreaks, followed by influenza outbreaks, and then RSV outbreaks; later, during winter–spring, metapneumovirus appears. Thus, the apexes of the epidemics follow one another and rarely overlap unless they affect populations of different ages. This phenomenon of viral interference may be explained by generation of interferon in infected patients, which “interferes” with the development of an infection of other viruses circulating at the same time.


Table 28.2

Predominant viruses in the respiratory system






































Virus

Varieties: types, serotypes, genotypes, and other

Rhinovirus

Species A, B, and C: >101 serotypes


Coronavirus

Alpha: 229E, NL63; beta: OC43, HKU1, SARS, MERS


Respiratory syncytial virus

A and B groups; genotypes and lineage


Metapneumovirus

A and B groups; genotypes


Adenovirus

55 serotypes


Influenza

Types A, B, and C; subtypes A H1–3, N1–2; several strains


Parainfluenza

4 serotypes


Bocavirus

? 1 serotype


Others

Hantavirus, enterovirus, measles, chickenpox, cytomegalovirus



MERS Middle East respiratory syndrome, SARS severe acute respiratory syndrome


These characteristics allow us to suspect specific causes of outbreaks or epidemics with reasonable certainty, considering associated clinical cases and detection of viruses at sentinel sites. Thus, a winter outbreak that involves infants under 1 year old and causes cases of obstructive bronchial illness will be due to RSV. A characteristic trait of influenza is an epidemic of disease that is marked by fever, cough, headache, and musculoskeletal aches, and that compromises preschool children, schoolchildren, and young adults. Parainfluenza virus outbreaks are associated with symptoms of hoarseness, gruffness, and cough, besides lower respiratory tract infections, in infants. Adenoviruses are feared because they sometimes cause intense feverish conditions and severe nosocomial infections.


Epidemiology contributes a lot to clarifying these situations, because alongside defining “suspicious cases” it implements diagnostic systems for circulating viruses (Fig. 28.1). The high infectivity of viruses makes epidemiological information vitally important in etiological clinical diagnosis, especially in terms of contact between sick patients and their relatives or close people in the community. The two kinds of respiratory virus that have the greatest impact on global health are RSV, which affects infants and elderly patients, and influenzas A and B, which compromise all of the population. Vaccines and antivirals have been developed just for influenza virus.

../images/441522_1_En_28_Chapter/441522_1_En_28_Fig1_HTML.jpg

Fig. 28.1

Viral monitoring at Hospital Roberto del Río (Santiago, Chile). Detection of respiratory viruses (adenovirus, RSV, influenza, and parainfluenza) by immunofluorescence in infants hospitalized for acute lower respiratory tract infection in 1989–2004


Viral respiratory infections are a very good example of a model of acute viral infection, in which viruses affect the individual, with or without symptoms, and then they leave him or her within a period of days or weeks. The course of infection depends on the interaction of various factors depending on: (1) the human host: age, immune status (based on previous infections and vaccinations), activity, tobacco use, etc.; (2) the virus: infecting dose, type, serotype, and viral strain; and (3) the environment: season, weather, humidity, contamination, geographical location, rural/urban setting, hospital/community, etc. From the host standpoint, acute respiratory infections are more frequent in childhood, particularly in infants and children under 2 years old, who represent the group with the biggest serious risk.


Diagnosis


Specific diagnosis of respiratory viruses is quite attainable nowadays because there are several immunodiagnostic techniques (immunofluorescence, enzyme-linked immunosorbent assay (ELISA), immunochromatography) available at public and private centers. They are easy to implement, with reasonably acceptable sensitivity and specificity, allowing appropriate study of cases that need it. Nowadays, molecular techniques (polymerase chain reaction (PCR) and reverse transcription PCR) are also available in many laboratories; they have high sensitivity and specificity, and provide results in less than 24 hours. The great sensitivity of these techniques enables detection of many agents, even in the same sample, raising questions about their interpretation. In severe cases or in deceased patients, they should always be implemented to establish the causes.


Emergent Infections


Among respiratory viruses, very good examples of “viral emergencies” are found, the most classic being the occurrence of global epidemics of influenza A virus derived from birds or pigs in 1918, 1957, 1968, and 2009; on these occasions, the virus broke through the species barrier and, thanks to multiple mutations, was able to transmit itself efficiently and establish itself as a “human virus.” A pandemic of severe acute respiratory syndrome (SARS) due to a coronavirus is another example of a phenomenon that has threatened the world and was able to be controlled. During August 2014, an outbreak of lower respiratory tract infections in children, associated with enterovirus D68, started in the USA, and its evolution is ongoing. Likewise, an emergency involving the Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV) started in Saudi Arabia in 2012. Probably of animal origin (camels and bats) and transmissible through air, it has caused over 900 cases (30% lethal); luckily, it has not expanded efficiently in other territories.


Pathogeny of Viral Infections


Some viruses have certain particularities that require special consideration (Table 28.3). This process is described considering a population as a host. There are over 200 respiratory viruses capable of infecting humans, with diverse structures—RNA or DNA, naked or enveloped—and although they can be grouped into a few families, the great variety of serotypes, genotypes, and strains that can be identified entails the existence of many different potential pathogens.


Table 28.3

Respiratory syndromes

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Nov 7, 2020 | Posted by in Uncategorized | Comments Off on of Respiratory Infections

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