Clearing up the confusion with aerosol disease transmission

Droplet nuclei are transmitted through respiratory secretions, which can be expelled through yawning, sneezing, panting, coughing, vocalizing, or other similar means. Courtesy Elizabeth Anderson Lopez Understanding disease transmission is paramount to our ability to combat disease, prevent its spread, and treat illness when it arises. The role of aerosol disease transmission, including droplet nuclei travel, is often a misunderstood and underestimated mode of disease transmission. As a result, veterinarians and veterinary infection control plans frequently fail to recognize measures aimed at this modality of spread. Thus, within the veterinary environment, if we take steps to limit aerosol disease transmission, we can protect our patients, humans, and the environment, protecting the human-animal bond and ensuring a One Health infection control plan. In today’s One Health climate, we must understand and address health hazards impacting our patients, staff, and environment. With the ever-increasing prevalence of antimicrobial resistance (AMR), we must minimize infections at all costs. This starts in the veterinary setting with our hospitals, kennels, grooming facilities, daycares, and related venues. We must ensure our infection control program protects us to the best of its ability against infectious diseases, including zoonotic ones. We have a duty to not only reduce and prevent disease spread from one patient to another but also from animals to humans and vice versa. We do so with the utmost attention to our infection control plan.1–3 Aerosol disease transmission modalities When considering the routes of disease transmission, we include direct and indirect means. These include:4 Vector borne Mechanical transmission via fomites Vertical (sexual) transmission Direct contact (dermal exposure, g., catheter sites, incisions, bites, or other wounds) Ingestion (Fecal-oral route and including both food and waterborne illnesses Inhalation (droplet transmission and aerosolization) Airborne disease transmission elucidated Let’s focus on airborne transmission, a mode we may be able to further protect against. Most people equate airborne disease transmission as equivalent regardless of the size of the particle-containing infectious material, the organism, or environmental factors such as ventilation and humidity. However, there is a distinction worth understanding between droplet nuclei transmission and true airborne transmission via aerosol. The distinction has to do with the size of the particles, the distance a particle may (or may not) travel, environmental conditions (indoor vs. outdoor and various parameters), and the ability of an organism to spread via aerosolization. However, understanding the distinction, recognizing numerous veterinary diseases fit in the aerosolization category and regular infection control measures such as PPE, routine cleaning, disinfecting, sanitizing, and laundering are not sufficient to prevent aerosol transmission of veterinary and human concern organisms is paramount.5 Thus, when considering measures of disease prevention and identifying hazards to the veterinary environment (e.g., veterinary clinics, grooming/boarding facilities, daycares), we can develop a more multi-modal and comprehensive plan when we fully understand the role aerosol disease transmission could play and how we may control/prevent this means of disease spread. Acknowledging airborne transmission includes close contact exposures and surface contamination with fomite hazards (droplet nuclei transmission) and encompasses particles that can travel long distances and remain suspended in the air (aerosol transmission) provides key information. Understanding this is crucial to permit a comprehensive means to cover and investigate all possible means of protection. Aerosol vs. droplet nuclei transmission Aerosol transmission occurs when particles with a size of less than five to 10 µm remain suspended in the air for a significant period of time. This transmission type also depends on maintaining a distance of more than three to six feet (one to two meters) from the source.4,6 This means close contact (nose-nose) isn’t necessary for disease transmission. Patients sharing the same ventilation system, yet numerous exam rooms away, can still be exposed to the same pathogen. Compare this to droplet nuclei transmission, which pertains to particles with sufficient weight to prevent them from being suspended in the air for extended durations. Typically, these agents have a limited range of only one to two meters, remain in the air for no more than five seconds, and have a size of > five to 10 µm, which allows them to be transmitted over short distances. We refer to this type of transmission as droplet nuclei. The infected doses are transmitted through respiratory secretions, which can be expelled through sneezing, panting, coughing, vocalizing, or other similar means. When considering droplet nuclei transmission, our attention is directed toward the closeness and length of time spent in close proximity to the source of the droplets.4,5,7,8 Despite this distinction between truly aerosolized and droplet nuclei particles, even droplet nuclei can still transform into aerosols. This can occur if conditions such as humidity and evaporation, among other environmental factors, permit them to stay suspended for a sufficient duration while retaining their ability to cause infection. In addition, larger droplets can be created and subsequently aerosolized as a result of procedures performed for the patient’s well-being, such as lancing an abscess, dental procedures, bronchoscopy, airway suctioning, high-pressure spraying (which should be strictly avoided), sweeping, and even ordinary vacuuming.6 Therefore, our objective is to reduce the occurrence of any aerosolization. If we produce these larger particles, we must strive to hinder their dispersion and eradicate them wherever possible. Variables affecting airborne disease transmission Numerous variables affect an agent’s ability to travel via air. Whether a pathogen may sustain travel within an aerosol (< five μm) and be carried to an uninfected individual will vary with various environmental factors beyond its ability to endure travel before falling to the ground. Additional factors include: Distance of travel for specific pathogens Particle size HVAC system efficiency Conditions in the environment, including ventilation, airflow, humidity, and temperature Further, a pathogen’s virulence, infectious dose, and the location of infection (e.g., upper vs. lower airway) will also contribute to its disease-causing potential. The number of pathogens necessary to infect the lower respiratory tract is often lower than that for the upper respiratory tract. A host’s genetics and immunological defenses and responses also play a role in disease transmission, with those immunocompromised being more vulnerable.5,7,10 Considering germicidal UV (GUV) radiation in your infection control plan Understanding disease transmission modalities helps ensure a complete and comprehensive infection control plan for your veterinary environment. Within any infection control plan, we need to recognize hazards and understand where risks lie. Gone are the days when simple personal protective equipment (PPE) and surface cleaning were considered sufficient to minimize disease spread. If the COVID-19 pandemic taught us anything, it is aerosolization and droplet spread are much better thwarted with a combination of measures from distancing to preventing the spread of droplets/aerosolization of infectious particles (face masks) to air purifiers and other modalities, such as germicidal ultraviolet (GUV) used to clean the air of microbes.1–3,5,16–18 The U.S.’s Environmental Protection Agency (EPA), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), and the Centers for Disease Control and Prevention (CDC) all support the use of GUV for assisting in preventing respiratory diseases, including those spread via aerosol and droplets.19–22 While standard air purifiers capture large droplets, preventing exposure, they do not actively kill or inactivate pathogens; they simply trap them. While this can be effective for the space the air purifier works, it doesn’t actually disinfect or kill the germs of concern. Still, air purifiers may help reduce pathogen load.23However, newer technologies, such as GUV, disinfect the environment and are capable of killing pathogens of interest. Thus, adding a GUV unit (Figure 1) to your facility may help round out your current infection control plan. Figure 1: A GUV unit. Courtesy Aerapy Potential for aerosol transmission in veterinary medicine Research in veterinary medicine is currently being conducted to better understand which illnesses can be transmitted through aerosols. Conditions previously believed to spread solely through droplets are now thought to also have the capability to spread through aerosols. Preliminary studies classified numerous organisms responsible for upper respiratory tract infections (URIs) in cats and those in the canine infectious respiratory disease complex (CIRDC) solely as being spread through droplets. Nevertheless, this approach does not consider the findings of recent studies, such as the one conducted by Jaynes et al. in 2023, which demonstrated outbreaks can be effectively controlled or averted by implementing comprehensive infection control strategies that include the use of upper-air germicidal ultraviolet (GUV) devices. Therefore, this indicates the organisms were indeed dispersed as aerosols.1,11,12,24,25 Although we lack sufficient studies on aerosol disease transmission in small animals, ample evidence-based research shows that certain zoonotic diseases, such as M. tuberculosis, Yersinia pestis, and Francisella tularensis, can be spread through airborne aerosol transmission.1,4 This research must be continued to further accurately quantify the agents that can be transmitted through aerosols. GUV radiation prevents disease transmission If you feel your infection disease plan is everything it needs to be but haven’t considered an air purification system, UV disinfection, or related product, then perhaps it is time to rethink your control measures. Your everyday surface cleaning and scrubbing will get the droplet particles of most organisms. However, those particles that remain suspended in the air are the unseen and forgotten threats. Using UV light can help disrupt the DNA of the microorganism of concern, “cleaning” the air. This will help lessen the chance of respiratory disease spread of infectious and zoonotic diseases from COVID-19, RSV, and influenza in people to B. bronchiseptica, feline calicivirus, and canine influenza, among others in our animals. While we think of aerosolization as most commonly associated with viral and bacterial pathogens, we could prevent a myriad of diseases by adding to our infection control plans and considering human, animal, and environmental factors when updating these plans. The COVID-19 pandemic reinforced what many already knew: measures besides those under human control (distancing, face mask-wearing, cleaning surfaces, hand washing) is necessary to prevent the spread of aerosolized organisms. Throughout COVID-19, UV disinfection emerged as a viable option to aid in preventing disease spread in clinical settings and non-healthcare-related scenarios. The veterinary community can learn from this and apply it to our needs.9,16–18,26 A few studies in veterinary medicine evaluate the use of GUV in the air, including a 2020 study25 that assessed the effect of URI transmission in kittens in a nursery setting. Researchers demonstrated URI incidence decreased, emphasizing more of an aerosol spread role exists than we realize. However, additional research highlighting UV’s role in preventing animal diseases would be ideal. Based on the human literature (moderate to high-grade evidence), the benefits are significant, and early studies in animals concur. We need more robust studies and systematic reviews to round out the evidence. Additional research is warranted, reviewing specifics in veterinary medicine in more detail, such as the specific duration that animal pathogens can remain in the air (before dropping/evaporating) and their travel distances. While it would be nice to know these specifics and more about common threats we face in small animal practice, e.g., B. bronchispetica, canine influenza, and feline calicivirus, not to mention human concerns such as COVD-19 and influenza A/B, we have sufficient evidence demonstrating the effectiveness of UV disinfection in the prevention of aerosol transmission in human diseases. Consider a GUV unit for your practice, but ensure you do your due diligence. Not all products are created equal. GUV products will vary in efficacy from one unit to another and from one company to another. Choosing products with independent laboratory testing and scientific research that backs up the product’s ability to kill pathogens of concern and reduce the risk of overall infections is crucial. Ensuring products meet your practice’s size requirements and layout also is key. In addition to minimizing disease in our animal population, implementing a GUV system can help reduce the illness of staff and clients. Further, we can then have a more beneficial impact on the overall well-being and health of staff, clients, pets, and pet parents, helping us to maintain the human-animal bond and preserve our work sanctity while minimizing the spread of infectious (zoonotic and otherwise) diseases and protecting our environment, a true One Health outlook on infection control in the veterinary setting. Time to update your control plans Given what we know about COVID-19 and many other diseases spread via the aerosol route, revisiting your current infection control plans may demonstrate gaps in protection. While vital, vaccination, surface cleaning and disinfection, and PPE alone are insufficient to protect humans and animals alike from various disease transmission.27 Consider an air cleaning system and attention to air quality and the irradiation of aerosolized organisms, taking prevention to the veterinary practice, boarding facility, and daycare to the next level, not just at the individual level. Recognizing the role aerosol disease transmission plays in numerous infectious diseases (zoonotic and otherwise) helps one evaluate and update infection control plans, helping to minimize the risk of infection with one more layer of protection. Erica Tramuta-Drobnis, VMD, MPH, CPH, is the CEO and founder of ELTD One Health Consulting, LLC (https://eltdonehealth.com/). Dr. Tramuta-Drobnis works as a public health professional, emergency veterinarian, freelance writer, consultant, and researcher. She is passionate about One Health issues and believes pet health, food safety, agricultural health, and more can address the interconnection of human, animal, and environmental health. References Sykes JE, Weese JS. Infection Control Programs for Dogs and Cats. Canine and Feline Infectious Diseases. Published online 2014:105-118. doi:10.1016/B978-1-4377-0795-3.00011-9 Morss H. Preventing Transmission of Infectious Disease Among Patients. Today’s Veterinary Practice. 2011;November/December. Accessed May 23, 2024. https://todaysveterinarypractice.com/infectious-disease/preventing-transmission-of-infectious-disease-among-patients/ Jordan R. Surveillance of hospital acquired infections in veterinary practices. The Veterinary Nurse. 2022;13(2). Accessed May 23, 2024. https://www.theveterinarynurse.com/content/education/surveillance-of-hospital-acquired-infections-in-veterinary-practices/ National Association of State Public Health Veterinarians (NASPHV). National Association of State Public Health Veterinarians Compendium of Veterinary Standard Precautions for Zoonotic Disease Prevention in Veterinary Personnel. Published online 2015. Accessed May 23, 2024. https://www.avma.org/resources-tools/avma-policies/compendium-veterinary-standard-precautions-zoonotic-disease-prevention-veterinary Tellier R, Li Y, Cowling BJ, Tang JW. Recognition of aerosol transmission of infectious agents: a commentary. BMC Infectious Diseases. 2019;19(1):101. doi:10.1186/s12879-019-3707-y National Association of State Public Health Veterinarians (NASPHV). A Review of Zoonotic Disease Threats to Pet Owners: A Compendium of Measures to Prevent Zoonotic Diseases Associated with Non-Traditional Pets Such as Rodents and Other Small Mammals, Reptiles, Amphibians, Backyard Poultry, and Other Selected Animals. Wang CC, Prather KA, Sznitman J, et al. Airborne transmission of respiratory viruses. Science. 373(6558):eabd9149. doi:10.1126/science.abd9149 Tramuta-Drobnis EL. Veterinary Infection Control Plans: Don’t Forget Aerosol Disease Transmission Prevention. Aerapy Animal Health. July 29, 2024. Accessed August 8, 2024. https://aerapyanimalhealth.com/veterinary-infection-control-plans-dont-forget-aerosol-disease-transmission-prevention/ Thornton GM, Fleck BA, Fleck N, et al. The impact of heating, ventilation, and air conditioning design features on the transmission of viruses, including the 2019 novel coronavirus: A systematic review of ultraviolet radiation. PLOS ONE. 2022;17(4):e0266487. doi:10.1371/journal.pone.0266487 Fennelly KP. Particle sizes of infectious aerosols: implications for infection control. Lancet Respir Med. 2020;8(9):914-924. doi:10.1016/S2213-2600(20)30323-4 Dear J. Canine Infectious Respiratory Disease Complex. Clinician’s Brief. December 2020. Accessed November 1, 2021. https://www.cliniciansbrief.com/article/canine-infectious-respiratory-disease-complex Reagan KL, Sykes JE. Canine Infectious Respiratory Disease. Vet Clin North Am Small Anim Pract. 2020;50(2):405-418. doi:10.1016/j.cvsm.2019.10.009 Cole EC, Cook CE. Characterization of infectious aerosols in health care facilities: An aid to effective engineering controls and preventive strategies. Am J Infect Control. 1998;26(4):453-464. doi:10.1016/S0196-6553(98)70046-X Thomas RJ. Particle size and pathogenicity in the respiratory tract. Virulence. 2013;4(8):847-858. doi:10.4161/viru.27172 Tang JW, Li Y, Eames I, Chan PKS, Ridgway GL. Factors involved in the aerosol transmission of infection and control of ventilation in healthcare premises. J Hosp Infect. 2006;64(2):100-114. doi:10.1016/j.jhin.2006.05.022 Chiappa F, Frascella B, Vigezzi GP, et al. The efficacy of ultraviolet light-emitting technology against coronaviruses: a systematic review. Journal of Hospital Infection. 2021;114:63-78. doi:10.1016/j.jhin.2021.05.005 Ragan I, Perez J, Davenport W, Hartson L, Doyle B. UV-C Light Intervention as a Barrier against Airborne Transmission of SARS-CoV-2. Viruses. 2024;16(1):89. doi:10.3390/v16010089 Fischer RJ, Port JR, Holbrook MG, et al. UV-C Light Completely Blocks Aerosol Transmission of Highly Contagious SARS-CoV-2 Variants WA1 and Delta in Hamsters. Environ Sci Technol. 2022;56(17):12424-12430. doi:10.1021/acs.est.2c02822 Environmental Protection Agency (EPA). What is Upper-Room Ultraviolet Germicidal Irradiation (UVGI)? What is HVAC UVGI? Can either be used to disinfect the air and help protect myself from COVID? EPA. March 21, 2022. Accessed May 30, 2024. https://www.epa.gov/indoor-air-quality-iaq/what-upper-room-ultraviolet-germicidal-irradiation-uvgi-what-hvac-uvgi-can Harvard Medical School: Center for Global Health Delivery. Germicidal Ultraviolet Light. Harvard Medical School: Center for Global Health Delivery. Accessed May 30, 2024. https://ghdcenter.hms.harvard.edu/guv-lighting Beggs CB, Avital EJ. Upper-room ultraviolet air disinfection might help to reduce COVID-19 transmission in buildings: a feasibility study. PeerJ. 2020;8:e10196. doi:10.7717/peerj.10196 Centers for Disease Control & Prevention (CDC). Ventilation in Buildings. Centers for Disease Control and Prevention. February 11, 2020. Accessed May 30, 2024. https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html Salmonsmith J, Ducci A, Balachandran R, et al. Use of portable air purifiers to reduce aerosols in hospital settings and cut down the clinical backlog. Epidemiol Infect. 2023;151:e21. doi:10.1017/S0950268823000092 Spiri AM, Meli ML, Riond B, Herbert I, Hosie MJ, Hofmann-Lehmann R. Environmental Contamination and Hygienic Measures After Feline Calicivirus Field Strain Infections of Cats in a Research Facility. Viruses. 2019;11(10):958. doi:10.3390/v11100958 Jaynes RA, Thompson MC, Kennedy MA. 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Clearing up the confusion with aerosol disease transmission

A look at One Health prevention measures that make a difference

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