Respiratory Diseases: Understand the types of diseases, viruses, and ways to prevent them

Emerging infectious diseases are frequently caused by respiratory viruses, which play significant roles in respiratory tract infections, ranging from the common cold to severe respiratory illnesses (1). 

 

In the past 15 years, molecular detection and sequencing have enabled the increased identification of pathogens responsible for common respiratory diseases, as well as the identification of pathogens during pandemics (2). 

 

What Are Respiratory Diseases?

Respiratory diseases encompass a range of conditions that affect the respiratory system, from the nose and throat to the lungs.  

 

These diseases can be caused by various factors, including viral and bacterial infections, exposure to environmental pollutants, tobacco smoke, and other irritants. The severity of symptoms can range from mild to severe, and may include cough, difficulty breathing, chest pain, wheezing, and extreme fatigue. 

 

Main Respiratory Diseases

According to the International Forum of Respiratory Societies, at least two billion people worldwide are exposed to toxic smoke from biomass fuels; and over two billion inhale pollutants and are exposed to tobacco smoke, resulting in a significant global health burden, with five respiratory diseases being among the most common causes of death worldwide (3):  

 

• Chronic Obstructive Pulmonary Disease (COPD): These are respiratory diseases that block airflow and make breathing difficult, such as emphysema and chronic bronchitis. Approximately 65 million people suffer from the disease, and around three million die each year, making it the third leading cause of death worldwide (4); 

• Lung Cancer: This is the most common fatal neoplasm in the world. In 2018, there were around 2.09 million cases and 1.76 million deaths, according to WHO data (5); 

• Tuberculosis: This infectious respiratory disease is transmitted from person to person through the air and primarily affects the lungs. According to the World Health Organization (WHO), there are about 9 million cases of the disease and 2 million deaths each year (6); 

• Asthma: This is inflammation of the bronchial tubes affecting approximately 334 million people worldwide. Although it manifests at all ages, it is the most common chronic disease in childhood, affecting 14% of children (7); 

• Acute Respiratory Infections: Respiratory tract infections are a major cause of morbidity and mortality worldwide, with approximately 3 to 5 million severe cases each year, being more common at the beginning of autumn and during winter (8).

 

Several new respiratory viruses have emerged, including Influenza A virus (also known as H1N1), avian influenza viruses A(H7N9) and A(H5N6), the coronavirus responsible for Middle East Respiratory Syndrome (MERS), and the novel coronavirus (SARS-CoV-2) causing the COVID-19 pandemic, which has led to over 14.9 million direct and indirect deaths worldwide between January 1, 2020, and December 31, 2021 (9-12). 

 

Also check out the article “COVID-19 tests: Everything You Need to Know.” 

 

What Are Respiratory Viruses? 

Respiratory viruses are pathogens that infect the human respiratory tract, causing a range of illnesses from the common cold to more severe conditions like pneumonia and Severe Acute Respiratory Syndrome (SARS).  

 

These viruses spread primarily through respiratory droplets expelled when coughing, sneezing, or talking, and can be transmitted both through direct contact with infected individuals and through contaminated surfaces. 

 

Main Respiratory Viruses 

The viruses most frequently involved in respiratory infections are rhinoviruses, respiratory syncytial virus (RSV), coronaviruses, adenoviruses, parainfluenza viruses, and influenza viruses (13). 

 

All of these viruses share the ability to transmit from person to person, and their transmissibility is influenced by the environment in which the pathogen and host are located. 

 

Understand the viruses associated with respiratory tract infections and the related clinical conditions: 

 

 

Read the details about each of them. If you prefer, click on the name to go to the section related to the virus: 

• Influenza Virus (flu); 
• Parainfluenza; 
• Respiratory Syncytial Virus (RSV); 
• Metapneumovirus; 
• Adenovirus; 
• Rhinovirus; 
• Enterovirus; 
• Bocavirus; 
• Coronavirus.

 

Influenza Virus (Flu) 

Influenza is a respiratory infection caused by the influenza virus (Myxovirus influenzae) with significant morbidity and mortality rates worldwide. Influenza viruses are classified into types A, B, and C based on their nucleoproteins and matrix proteins (14). 

 

What Are the Symptoms of Influenza? 

Influenza, or the flu, typically causes mild respiratory problems such as: 

• Cough;
• Fever;
• Myalgia (muscle pain);
• Chills or sweats;
• Malaise.

 

Symptoms may persist for two to eight days. 

 

Gastrointestinal symptoms such as vomiting and diarrhea can occur in children. A minority of patients, especially the elderly, may experience severe illness due to viral or bacterial pneumonia (15). 

 

Influenza Type A and B 

Influenza type A and B viruses are responsible for seasonal epidemics, with the infection by influenza A virus (H1N1pdm09 and H3N2) characterized by the abrupt onset of: 

• Fever;
• Cough;
• Chills or sweats;
• Myalgia and malaise.

 

Subtypes: 

The H1N1 subtype emerged from a quadruple combination of two swine viruses, an avian virus, and a human virus, causing the 2009 pandemic (16). Symptoms are usually mild, including nausea, vomiting, and diarrhea, but can worsen, leading to pneumonia or respiratory failure. 

 

The incidence and mortality of swine flu or H1N1 infection are higher among young adults and lower in elderly patients compared to seasonal flu, most likely because younger individuals have not been previously exposed to similar influenza viruses. 

 

The virus was standardized as influenza A (H1N1)pdm09 to denote the pandemic and to distinguish it from seasonal H1N1 strains and the pandemic H1N1 strain. 

 

Since 2011, outbreaks of the H3N2 subtype of swine origin have been predominantly reported in children. Additionally, the avian influenza or bird flu virus H5N1 has become a global concern (17). 

 

Prevention: 

As a preventive measure against influenza viruses, annual vaccination against the flu is recommended for everyone. Vaccination efforts should primarily target those at higher risk of complicated or severe influenza (the elderly and immunocompromised) and those who care for or live with high-risk individuals, including healthcare professionals. Current seasonal flu vaccines are also effective against the A (H1N1)pdm09 virus. 

 

Parainfluenza 

The parainfluenza virus (PIV) is an RNA virus from the Paramyxoviridae family, classified into four serotypes (PIV-1, PIV-2, PIV-3, and PIV-4), which causes various respiratory illnesses ranging from the common cold to flu-like syndrome or pneumonia. It is a known cause of infection in pediatric and immunocompromised patients, and is increasingly recognized as a relevant pathogen in hospitalized adults, with infection rates between 2% and 11% (18).  

 

It is the second most common cause of lower respiratory tract infection in children. 

 

What Are the Symptoms of Parainfluenza? 

Symptoms include: 

• Cough;
• Rhinorrhea (runny nose);
• Sore throat;
• Asthma;
• Chronic obstructive pulmonary disease (COPD);
• Heart failure;
• Pneumonia;
• Croup (acute laryngeal obstruction) – the most common severe manifestation.

 

There is evidence suggesting that PIV can result in recurrent and mildly asymptomatic infections in adult populations, with long periods of asymptomatic viral shedding (> 8 months) (19,20). 

 

Serotypes 1 and 2 tend to cause epidemics in the fall, with each serotype occurring in alternating years. Serotype 3 is usually endemic and infects most children under one year of age, potentially causing pneumonia and bronchiolitis; while serotype 4 has cross-reactive antigenicity with the mumps virus. 

 

Respiratory Syncytial Virus (RSV)

Respiratory syncytial virus (RSV) is an enveloped RNA pneumovirus belonging to the Paramyxoviridae family. It is the most important viral agent causing severe respiratory illnesses in infants and children worldwide (21). 

 

RSV infections are responsible for one-third of deaths related to acute respiratory infections in infants under one year of age and are particularly problematic in premature infants, as well as in children with cardiac and respiratory issues. The virus also causes severe illness in the elderly. 

 

Approximately 40 to 60% of children are infected with RSV in their first year of life, and more than 95% of children by the age of 2 will have had or will have at least one RSV infection (22).  

 

Additionally, RSV infection is a major cause of morbidity in adults, particularly in the elderly and immunocompromised (23). 

 

Transmission: 

RSV is transmitted through contact with oral or nasal secretions of an infected person when they cough, sneeze, or talk, and indirectly through contact with contaminated surfaces and objects, causing recurrent infections throughout life. RSV infection triggers both innate and adaptive immune responses; however, immunity against the virus is not long-lasting. The transmission period begins two days before symptoms appear and only ends when the infection is controlled. 

 

RSV has seasonal circulation, with higher detection rates in late fall and early spring. In tropical regions, it can be detected year-round. 

 

What Are the Symptoms of Respiratory Syncytial Virus (RSV)? 

The primary symptoms associated with RSV can range from mild symptoms (in healthy individuals) such as: 

• Rhinorrhea (runny nose);
• Sneezing;
• Dry cough;
• Fever;
• Sore throat;
• Headache;
• Otitis (ear infection);
• Lower respiratory tract infection.

 

To more severe conditions such as acute bronchiolitis (inflammation of the bronchioles) and pneumonia (25). 

 

Lower respiratory tract involvement occurs in approximately 15–50% of infants and children with primary infection, and hospitalization is necessary in 1–3% of cases in infants aged 2 to 6 months, who are at higher risk. 

 

The acute phase of this infection is often followed by episodes of wheezing that recur over months or years and generally lead to a diagnosis of asthma. 

 

Treatment includes bronchodilators and mucolytic agents, while high-risk young pediatric patients also receive prophylactic treatment with monoclonal antibodies (Palivizumab) (26). 

 

Metapneumovirus

Human metapneumovirus (HMPV) is a significant cause of upper and lower respiratory tract illnesses in children and adults. It is an RNA virus and a member of the Paramyxoviridae family, which also includes the respiratory syncytial virus (RSV) and parainfluenza viruses (27). 

 

Metapneumovirus generally causes upper respiratory tract infections and flu-like illnesses, but it is also associated with lower respiratory tract infections such as: wheezing bronchitis, bronchitis, bronchiolitis, and pneumonia, particularly in very young children, the elderly, and immunocompromised patients (28, 29). 

 

What Are the Symptoms of HMPV? 

Symptoms generally include: 

• Rhinorrhea (runny nose);
• Fever;
• Productive cough with wheezing;
• Nasal discharge;
• Sore throat;
• Shortness of breath;
• Vomiting;
• Diarrhea;
• Headache;
• Exacerbation of asthma may also occur.

 

In infants under six months, the initial symptom may be a period of breathing cessation. Some small infants develop severe respiratory distress. In healthy adults and older children, the illness is usually mild and may only present as a common cold. Most children do not require hospitalization (30). 

 

Diagnosis and Transmission: 

When necessary, nasal secretions are tested with a rapid antigen test, and polymerase chain reaction (PCR) technique helps identify the virus. 

 

Transmission occurs through direct contact with the infected person or through infected secretions. Home treatment primarily involves symptomatic relief. 

 

Seroprevalence studies show that primary infection occurs before the age of 5, and individuals are reinfected throughout their lives; the four subgroups of HMPV vary from year to year (31). 

 

Humoral immunity plays an important role in HMPV infection, and the study of HMPV antibodies provides valuable information, including HMPV seroprevalence, cross-serological protection between HMPV subgroups, and strategies for prophylaxis and therapy using monoclonal antibodies (mAbs) (32). 

 

Widely neutralizing monoclonal antibodies have significant clinical implications for the prophylaxis and treatment of high-risk patients. 

 

Adenovirus 

Adenoviruses (AdVs) are DNA viruses that typically cause mild infections affecting the upper or lower respiratory tract and the gastrointestinal tract. Adenovirus infections are more common in young children due to the lack of humoral immunity (33). 

 

There are seven species of human adenoviruses (A – G) and approximately 57 different serotypes. The predominant serotypes associated with diseases vary by country or region and may change over time (34). 

 

What Are the Symptoms of Adenovirus? 

Typical symptoms include: 

• Fever;
• Pharyngitis; 
• Tonsillitis; 
• Cough; 
• Throat ulcers; 
• Hemorrhagic cystitis; 
• Hepatitis; 
• Hemorrhagic colitis; 
• Pancreatitis; 
• Nephritis; 
• Meningoencephalitis.

 

Gastrointestinal symptoms may be present, particularly in children, and pneumonia occurs in up to 20% of newborns and infants (35). 

 

Severe clinical manifestations are more likely in immunocompromised patients (transplant recipients, HIV infection) and occur in 10 to 30% of cases (36). 

 

Transmission: 

Adenovirus infections are increasingly recognized as causes of severe respiratory diseases and can result from exposure to infected individuals (inhalation of droplets, conjunctival inoculation) and contaminated objects. 

 

The incubation period ranges from two to 14 days, and latent AdV can reside in lymphoid tissue, renal parenchyma, or other tissues for years; reactivation may occur in severely immunosuppressed patients (37). 

 

Rhinovirus 

Human rhinoviruses (RVs) are responsible for more than half of cold-like illnesses, being the most common cause of upper respiratory tract infections and costing billions of dollars annually in medical consultations and lost work and school days (38). 

 

Approximately 50% of all colds are caused by one of the more than 100 existing rhinovirus serotypes, which are very common during the fall and spring and less common during the winter months. 

 

What Are the Symptoms of Rhinovirus? 

The most common symptoms are (39): 

• Pharyngitis followed by sneezing;
• Nasal congestion and discomfort;
• Exacerbations of chronic lung disease;
• Development of asthma;
• Severe bronchiolitis (in infants and children);
• Fatal pneumonia (in the elderly and immunocompromised adults).

 

Transmission: 

Rhinoviruses are transmitted from person to person through direct contact with large airborne particles. HRV infection begins with intranasal and conjunctival inoculation, but not via the oral route. Scientific studies have shown that the virus is regularly deposited on hands and introduced into the environment, as it has been detected on 40% of the hands of naturally infected volunteers and on 6% of household objects (40). 

 

Enterovirus

Respiratory enteroviruses (EVs), like rhinoviruses (RVs), are small RNA viruses and are primary causes of upper respiratory tract infections, being among the most common infectious agents in humans worldwide (41). 

 

Both belong to the Picornaviridae family and have been classified into seven distinct species, with three rhinovirus species (RV-A to RV-C) and four enterovirus species (EV-A to EV-D). 

 

Despite being from the same family, these viruses have distinct characteristics; the tropism (the ability of a virus to specifically infect certain cells of an organism) of RVs is restricted to the upper respiratory tract, except in some rare cases, while EVs can infect a wide range of different cells and cause very diverse clinical conditions (42,43). 

 

Diseases caused by EVs range from: 

• Febrile illnesses to myopericarditis;
• Poliomyelitis;
• Hand, foot, and mouth disease;
• Paralysis;
• Encephalitis;
• Viral meningitis.

 

What Are the Symptoms of Enteroviruses? 

While other types of enteroviruses are primarily found in the respiratory tract and cause symptoms similar to those of rhinoviruses, enteroviruses of species C and D, known as respiratory enteroviruses, can also cause a range of symptoms. 

 

Transmission: 

The virus is mainly transmitted through direct contact or via contaminated objects (fomites), usually with inoculation into the eye or nose via the tip of the finger. These viruses are capable of surviving on hands for several hours, allowing for easy person-to-person transmission in the absence of proper hand hygiene, especially in the presence of high viral loads (44). 

 

Bocavirus

Human bocavirus (HBoV) is a parvovirus, isolated about a decade ago, found in respiratory samples, primarily in children aged 6 to 24 months with acute respiratory infections, and in stool samples from patients with gastroenteritis (45). Since then, three additional subtypes of HBoV (HBoV1) have been identified in stool samples and named HBoV2, HBoV3, and HBoV4. The virus has also been detected in other biological samples such as blood, saliva, and urine, as well as in river water and wastewater (46). 

 

What Are the Symptoms of HBoV? 

HBoV primarily affects children aged 6 to 24 months with respiratory symptoms such as: 

• Rhinitis;
• Pharyngitis;
• Cough;
• Dyspnea;
• Pneumonia;
• Acute otitis media;
• Fever;
• Nausea;
• Vomiting;
• Diarrhea;
• Asthma exacerbation.

 

HBoV2, like the other subtypes, is more frequently found in stool samples and is associated with gastroenteritis, as possibly HBoV3. Recent studies show that HBoV can be specifically detected in tissues such as the duodenum, nasal mucosa, and intestinal biopsies (47,48). 

 

Bocavirus enters the body through the respiratory tract, bloodstream, or direct ingestion, reaching the gastrointestinal tract. Cases of HBoV infection show a high rate of co-infections with other respiratory and gastroenteritis pathogens such as human rhinovirus, adenovirus, norovirus, and rotavirus (49). 

 

Coronavirus 

Coronaviruses are a family of enveloped RNA viruses classified under the order Nidovirales. This family includes pathogens that affect various animal and human species, such as Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and the novel coronavirus (SARS-CoV-2) (50). 

 

Human coronaviruses were previously known only to cause the common cold until 2003, when SARS-CoV was responsible for Severe Acute Respiratory Syndrome (SARS). 

 

Coronaviruses cause both acute and chronic respiratory diseases, as well as enteric and central nervous system (CNS) diseases in animals and humans (51). The most common types that infect humans are:

 

• Alpha coronavirus (229E and NL63): The 229E subtype is considered an etiological agent of the common cold, while the NL63 subtype was isolated in children with severe respiratory symptoms, including upper respiratory infections, bronchiolitis, and pneumonia, and in immunocompromised adults with respiratory tract infections (52); 

• Beta coronavirus (OC43 and HKU1): The OC43 subtype is also considered an etiological agent of the common cold. On the other hand, the HKU1 subtype has been associated with elderly patients with pneumonia (53); 

• SARS-CoV (causing Severe Acute Respiratory Syndrome or SARS): Discovered in February 2003 when the World Health Organization (WHO) received reports from China about a new outbreak of respiratory illness with cases of “atypical pneumonia.” By the end of the epidemic, more than 8,000 cases had been reported, with over 800 deaths worldwide (54).

 

SARS infection presents a broad clinical picture, primarily characterized by fever, dyspnea, lymphopenia, and lower respiratory tract infection (55). Gastrointestinal symptoms and diarrhea are also common. Infected individuals may show slightly decreased platelet counts, prolonged coagulation profiles, and mildly elevated serum liver enzymes. Airborne droplets from infected patients are suggested to be the primary mode of transmission. 

 

The discovery of the ACE2 enzyme in human cells as the receptor for SARS-CoV (187) demonstrated how SARS-CoV enters host cells and has allowed the elucidation, at the molecular level, of SARS-CoV cross-transmission (56). 

 

• MERS-CoV (Middle East Respiratory Syndrome Coronavirus): MERS-CoV is similar to SARS-CoV, presenting as a severe lower respiratory tract infection with extrapulmonary involvement and high mortality rates (57). MERS is considered by the global health community as a potential pandemic agent with a high rate of person-to-person transmission and limited effective therapeutic options. Early reports of MERS focused on severe cases with acute pneumonia, rapid respiratory deterioration, and extrapulmonary manifestations. Common symptoms of MERS are nonspecific and include fever, chills, sore throat, dry cough, dyspnea, chest pain, myalgia, headache, and malaise (58).

 

• SARS-CoV-2 (causing COVID-19): The novel SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) is the virus responsible for the disease known as Coronavirus Disease (COVID-19) and the ongoing pandemic from 2020 to 2022.

 

 For more information on SARS-CoV-2, refer to our article. 

 

How to Prevent Respiratory Diseases 

Preventing respiratory viruses is closely linked to basic hygiene practices, such as: 

• Frequent hand washing with water and soap;
• Applying hand sanitizer before and after coming into contact with infected individuals;
• Disinfecting surfaces and objects contaminated by the virus;
• Maintaining a proper diet;
• Keeping environments clean and well-ventilated;
• Avoiding touching the eyes, nose, or mouth with hands after contact with surfaces.

 

Avoiding crowded places and maintaining distance from individuals showing signs of illness are important measures to control the spread of viruses. 

 

Diagnosis of Respiratory Viruses 

Diagnosis of respiratory infections is typically clinical, based on the manifestation of symptoms such as the common cold, bronchiolitis, croup, or pneumonia, and local epidemiology.  

 

However, advancements in molecular methods have facilitated the detection and characterization of various virus groups and strains. These methods are essential, particularly when identifying the specific pathogen alters clinical treatment and for determining the cause of an outbreak. 

 

What Tests Does Synlab Offer for Respiratory Viruses?

Synlab provides a molecular panel for respiratory viruses, covering both DNA and RNA pathogens responsible for respiratory diseases. This panel includes the detection of the following viruses: 

• Influenza A (H1, H3, H1pdm09);
• Influenza B;
• Respiratory Syncytial Virus (RSV A/B);
• Metapneumovirus;
• Adenovirus (AdV A/B/C/D/E/F);
• Coronavirus (CoV 229E/NL63/OC43);
• Enterovirus;
• Rhinovirus (HRV A/B/C);
• Bocavirus (HBoV 1/2/3/4);
• Parainfluenza Virus (PIV 1/2/3/4).

 

Samples can be collected from the upper respiratory tract (nasopharyngeal/oropharyngeal swabs) and the lower respiratory tract (BAL, sputum, and/or BAS) using swabs or scraping from the affected site.  

 

Detection is performed using real-time polymerase chain reaction (qPCR).  

 

Indicated in cases of: 

• Respiratory illness with unknown pathogen;
• Need for pathogen specification to determine appropriate treatment;
• Selection of specific antiviral therapy;
• Epidemiological surveillance and control. 

 

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Contact the SYNLAB team to learn more about the available tests. 

 

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