SELECTED EPIDEMICS & EMERGING PATHOGENS - RESPIRATORY ILLNESSES - AN OVERVIEW.

As discussed earlier, infections remain a leading cause of death worldwide, as well as in the United States [1]. Of concern, some of these infections are associated with vaccine preventable pathogens, such as S. pneumoniae, and influenza [1], [2], the morbidity and mortality of which could thus be significantly reduced or prevented were immunization rates higher, in both the developed and developing countries. In addition to well studied pathogens, novel and more virulent ones continue to be identified, that are capable of causing human and animal illness. These include various avian influenza strains [2], [3], [4], [5], [6], metapneumovirus [7], multidrug resistant tuberculosis [8], more aggressive coronaviruses [9], [10], [11], [12], and others. Respiratory Syncytial Virus (RSV) is an underappreciated respiratory pathogen. While it is well known as the most common etiology of lower respiratory tract infections in children, resulting in nearly 2.1 million outpatient visits among those less than 5 years of age, and over 57,000 hospitalizations in the same cohort, it is estimated that RSV causes 177,000 hospitalizations, and 14,000 deaths among adults over 65 years of age [13], [14], [15]. RSV clinically is variable in presentation, referable to age, patient health and comorbidities. Young healthy individuals typically experience mild, cold-like symptoms, with recovery expected in one to two weeks. Infants present with bronchiolitis, adults mild upper respiratory infections (URI). Severe pneumonia may occur, especially the elderly who have comorbidities, and/or impaired cellular immunity. Aerosolized Ribavirin can be used for RSV in infants. Risk benefit must be balanced when considering the use of Ribavirin in adults.

New pathogens are being discovered – some through unknown means, and others through natural adaptation. Globalization, population shifts and the changing ecology, including encroachment of previously unexplored regions has altered the longstanding epidemiology of infectious diseases – causing spread where once continents and oceans contained the pathogen.

Influenza viruses are ubiquitous in the animal population, with a wide array of natural hosts, and possess the capacity through the phenomenon of reassortment to infect an expanded range of hosts, including humans, as well as acquire greater pathogenicity [16], [17], [18], [19], [20], [21], [22]. Pandemic influenza viruses can thus emerge [16], [18], [19], [20], [21], [22].

It has long been recognized that influenza viruses exchange genetic material, (reassortment) either emerging as a new strain, as we continue to see with H5N1 [16], [23], [24], [25], H1N1 [26], [27], [28], and now the latest H7N9 [3], [4], [29], [30]. But this likely holds true for other viruses, as recently demonstrated with a novel coronavirus, most recently referred to as Middle East Respiratory Syndrome (MERS CoV) [11], [12].

Given the proximity of people to animals, through occupation and avocation, the human-animal interface becomes a significant risk for human illness from influenza viruses, as has been seen in several outbreaks, including H5N1 since 1997 [16].

There are a multitude of respiratory pathogens worth describing. However, with the recent emergence of yet another highly pathogenic avian influenza - H7N9 [29], [30] and novel coronavirus (MERS CoV) which appears more deadly than SARS CoV [11], [12], it seems worthwhile to discuss these infectious agents. It is hoped that lessons learned from these latest outbreaks can be applied towards preparedness against a wide range of pulmonary threats, and enhance our infection control capacity.

Regardless which emerging pathogen we discuss – Avian Influenza H5N1, H7N9, MERS CoV, swine flu or other viruses, it is important to recognize that preparedness efforts as a response to a potential pandemic caused by swine or avian influenza, SARS or MERS, can also enhance awareness, promote vaccine use, advances in diagnostic and treatment capabilities toward other significant infectious disease worldwide.

Controlling infectious diseases can be challenging given the large number of disease causing pathogens, their capacity for adaptation to environmental changes and antimicrobial therapies, and opportunities for spread. Respiratory contagions remain an issue of enormous concern in the containment of infections, especially with overcrowding and other population, social, and travel determinants.

The spread of pulmonary infections occurs readily from both the upper and lower respiratory tracts (Figure 1)

Fig. 1 – Respiratory infections (Upper and Lower) can readily be spread airborne [6], [26], [27], [28], [29], [30].

How respiratory illnesses spread [6], [26], [27], [28]

Droplets

Proximity (Less than 6 feet social distancing especially)

Environment (overcrowding for example)

Fomites

Mucosa

Prior immunity (or lack thereof)

Poor hygiene

Inherent transmissibility of pathogen (Ro)

In the next section we will discuss influenza viruses, including the newest pathogenic one H7N9, and coronaviruses. Although traditionally they caused mild respiratory illness, from 2003 there are more highly pathogenic CoV illness causing ones, as seen with SARS coronavirus, and MERS coronavirus.