Module 7: Novel and Emerging Diseases Originating in Developing Countries

Introduction

In the past, there have been many examples of new and virulent pathogens invading an immunologically naïve population and causing a devastating epidemic. Historic literature dating back hundreds of years demonstrates the terror that unknown pathogens can create. In the cases of the Plague or the Spanish flu of 1918, fear can spread as quickly as the disease.(1) In an age in which new technology helps us understand and investigate new and emerging diseases, disease surveillance is imperative in preventing future pandemics. Surveillance of novel diseases improves our knowledge of the disease, facilitating prompt implementation of control measures. Emerging infections are defined as those that have not previously occurred in human hosts, have occurred in humans before but only infected small/isolated populations, or have occurred throughout time but have only recently been recognized as distinct diseases.(2) Emerging diseases are often borne of environmental changes, natural evolution of the pathogen, antibiotic resistance, vector resistance to pesticides, and decreasing vaccination rates that can lead to a resurgence of previously controlled or eliminated diseases.(3) Antibiotic resistance is a huge problem in the global fight against tuberculosis, and has become an issue when treating infections in developed countries as well. The figure below demonstrates how antibiotic resistance occurs.

Categories of Priority Pathogens

The CDC has created a system for classifying emerging pathogens. Priority pathogens and biological agents have been divided into three categories – Category A, Category B, and Category C – based on their potential to cause widespread public health emergencies.(4)

Category A pathogens are the highest priority pathogens. According to the CDC, their surveillance is considered the most important due to the following characteristics:(5)

• They can be easily introduced into a population and then transmitted between people

• They have high case fatality rates and the potential for a high impact on the public health system

• They have certain traits that may cause heightened panic and disruption in the community

• They require special measures for public health preparedness

Category A Agents/Diseases include:(6)

• Anthrax

• Botulism

• Plague

• Smallpox

• Tularemia

• Viral Hemorrhagic Fevers (e.g. Ebola, Marburg, Lassa, and Machupo)

Category B pathogens are the second highest priority for public health agencies. The CDC says that Category B pathogens:(7)

• are moderately easy to introduce into a population and then transmit between people

• have moderate morbidity and low mortality rates

• require specific enhancements for diagnostic and surveillance systems

Category B agents and pathogens include:(8)

• Brucellosis

• Epsilon toxin of Clostridium perfringens

• Food safety threats (salmonella, E. coli)

• Glanders

• Melioidosis

• Psittacosis

• Q fever

• Ricin toxin

• Staphylococcal enterotoxin B

• Viral encephalitis

• Water safety threats (cholera, cryptosporidium)

Category C agents are emerging pathogens that can be altered in the laboratory to be effective biological weapons in the future because they possess the following traits:(9)

• Easily available

• Easily produced and spread

• Have the potential for high morbidity and mortality rates in human populations

Category C agents include:(10)

• Nipah virus

• Hantavirus

• Influenza

• Prions

• Yellow fever

• Rabies

• Chikungunya virus

• SARS-CoV

• Other novel, emerging infections

This module presents three novel or emerging diseases that are of particular public health importance and require continued surveillance. All three of these examples are zoonotic infections. According to the CDC, approximately 75% of emerging infectious diseases and 60% of all pathogens that infect humans have originated in animals.(11)

Influenza (Category C Pathogen)

Influenza is a virus that requires constant surveillance because it is forever evolving and creating new strains. The RNA virus evolves in two ways – antigenic drift and antigenic shift – that change the virus’ primary surface antigens (hemagglutinin [HA] and neuraminidase [NA]). HA and NA surface antigens are important because they allow the influenza virus to both infiltrate and leave host cells.(12) Influenza often goes by different names, such as H5N1 or H1N1, as these correspond to the type of HA and NA antigens expressed by the virus.

An influenza virus molecule (http://micro.magnet.fsu.edu/cells/viruses/influenzavirus.html)

The changing of antigens helps the virus evade an immune system that has learned (naturally or artificially via vaccination) how to respond to prior strains.(13) Antigenic drift (in Influenza types A and B) occurs during the virus’ replication process in host cells, when translation and transcription errors result in minor alterations in the surface antigens of the virus.(14) Antigenic shift, on the other hand, which occurs in Influenza A, takes place when gene segments mix (gene re-assortment) because multiple strains of different origins infect the same host cell, causing major changes in antigens.(15) Antigenic shift has the potential to create entirely new viruses that humans are underprepared to deal with. Genetic re-assortment can occur when multiple types of the influenza virus infect one animal, which serves as a “mixing vessel” that churns out novel strains of influenza.(16) Charts that explain antigenic shift and drift are included below. Surveillance of novel strains of influenza is critical in creating vaccinations to prevent future pandemics. Influenza’s main reservoirs are birds (avian type), pigs (swine type), and humans. Data collection about influenza strains including their ancestors, virulence, and ability to transmit between people is pertinent to global health security.(17)

Ebola (Category A Pathogen)

First recognized in 1976, Ebola hemorrhagic fever is a virulent RNA virus that infects humans and non-human primates. Ebola virus can cause fever, headache, sore throat, weakness, diarrhea, stomach pain, and internal and external bleeding.(18) Ebola is in need of further surveillance because it is thought to be zoonotic, yet its natural reservoir is unknown.(19) This is troubling because without an idea of how the disease enters the population, we cannot efficiently prevent outbreaks. Transmission, which occurs mostly in hospitals, is possible between humans via blood or secretions and contaminated fomites.(20) Surveillance and correct diagnosis are important in preventing transmission. Prevention, however, remains challenging because some symptoms manifest in ways that do not distinguish Ebola from common infections. Because of its very high case fatality rate (50-90%) and the rarity with which it is found, Ebola is difficult to diagnose, and is often considered a potential biological weapon, making its surveillance an even greater priority for developed nations.(21)

Nipah Virus (Category C Pathogen)

The Nipah virus is an emerging virus that causes the rapid onset of acute encephalitis and (in some cases) respiratory illness. Its case fatality rate can be as high as 33% to 75%.(22) Symptoms include fever, headache, dizziness, and loss of consciousness. Some survivors (20%) have long-lasting neurological problems.(23) The first reported outbreaks in 1998 were in Malaysia and Singapore. The virus primarily infected men in the pig/pork industry, indicating the possibility of pig to human transmission of the virus.(24) The virus has been implicated in many outbreaks since that time in Bangladesh and India.(25)

The natural reservoir of the Nipah virus was recently discovered to be the Pteropus genus fruit bat. Transmission between bat and humans often occurs when the bat contaminates date palm syrup that is not boiled before it is consumed by villagers.(26) Surveillance of the Nipah virus is important for implementation of control efforts, and in tracking changes in the disease. Some countries are watching changing behavior of fruit bats in hopes of predicting future epidemics.(27) Recent investigations into other novel diseases have focused on bats as potential reservoirs, as they were in the case of Nipah virus.

Footnotes

(1) Hellerman, C. (April 30, 2009). "Scientists dig for lessons from past pandemics." History News Network. http://hnn.us/roundup/entries/80880.html.

(2) National Institutes of Health. (2007). "Understanding Emerging and Re-emerging Infectious Diseases." Biological Sciences Curriculum Study. https://www.ncbi.nlm.nih.gov/books/NBK20370/.

(3) Ibid.

(4) National Institute of Allergy and Infectious Diseases. Biodefense and Emerging Infectious Diseases. (February 27, 2012). Category A, B, and C Priority Pathogens.

(5) Centers for Disease Control and Prevention. Emergency Preparedness and Response. (n.d.). Bioterrorism Agents/Diseases.

(6) Ibid.

(7) Ibid.

(8) National Institute of Allergy and Infectious Diseases. Biodefense and Emerging Infectious Diseases. (February 27, 2012). Category A, B, and C Priority Pathogens.

(9) Centers for Disease Control and Prevention. Emergency Preparedness and Response. (n.d.). Bioterrorism Agents/Diseases.

(10) National Institute of Allergy and Infectious Diseases. Biodefense and Emerging Infectious Diseases. (February 27, 2012). Category A, B, and C Priority Pathogens.

(11) National Center for Emerging and Zoonotic Infectious Diseases. (May 17, 2012). Centers for Disease Control and Prevention.

(12) Webster, R. G. (1998). Influenza: an emerging disease. Emerging infectious diseases, 4(3), 436.

(13) Centers for Disease Control and Prevention. (July 10, 2019) "Understanding Influenza Viruses." https://www.cdc.gov/flu/about/viruses/index.htm.

(14) Webster, R. G. (1998). Influenza: an emerging disease. Emerging infectious diseases, 4(3), 436.

(15) Centers for Disease Control and Prevention. (July 10, 2019) "Understanding Influenza Viruses." https://www.cdc.gov/flu/about/viruses/index.htm.

(16) Webster, R. G. (1998). Influenza: an emerging disease. Emerging infectious diseases, 4(3), 436.

(17) Centers for Disease Control and Prevention. (July 10, 2019) "Understanding Influenza Viruses." https://www.cdc.gov/flu/about/viruses/index.htm.

(18) Centers for Disease Control and Prevention. (2009) "Ebola Hemorrhagic Fever Information Packet." http://pop.h-cdn.co/assets/cm/15/06/54d153fbede3c_-_Ebola_Fact_Booklet.pdf.

(19) Ibid.

(20) Ibid.

(21) Globalsecurity.org. (Oct. 10, 2014). "Ebola Haemorrhagic Fever." https://www.globalsecurity.org/wmd/intro/bio_ebola.htm.

(22) The Center for Food Security and Public Health, Iowa State University. (Jan. 2016). "Nipah Virus Infection." http://www.cfsph.iastate.edu/Factsheets/pdfs/nipah.pdf.

(23) World Health Organization. "Nipah Virus." https://www.who.int/news-room/fact-sheets/detail/nipah-virus.

(24)The Center for Food Security and Public Health, Iowa State University. (Jan. 2016). "Nipah Virus Infection." http://www.cfsph.iastate.edu/Factsheets/pdfs/nipah.pdf.

(25) Ibid.

(26) World Health Organization. "Nipah Virus." https://www.who.int/news-room/fact-sheets/detail/nipah-virus.

(27) Ibid.