Epidemiology of Airborne Diseases Dr. Yeşim YASİN Fall-2013
Outline • Basics of epidemiology • Basics of airborne infections • Epidemiology of “tuberculosis” • Epidemiology of “measles” • Epidemiology of “influenza”
Introduction • “Epidemiology: The study of the distribution and determinants of a health-related events and application of this study to control of health problems” (John Last, 1988). • Epidemiology of air-borne diseases provides an overview of airborne disease burden and its likely future evolution.
Basics of Epidemiology • INFECTIVITY:The ability of an agent to invade and multiply (produce infection) in a susceptible host. • How to measure (Infectivity);ease & spread of infection? Secondary Attack Rate The proportion of exposed susceptible persons who become infected. Measles has high infectivity whereasleprosy has low infectivity.
Basics of Epidemiology (cont.) • PATHOGENICITY: It is the ability of the organisms to produce specific clinical reaction after infection. It refers to the proportion of infected persons who develop clinical disease. • How to measure pathogenicity? • By the ratio of clinical to sub-clinical cases. Measles has high pathogenicity whereasTB has low pathogenicity.
Basics of Epidemiology (cont.) • VIRAL SHEDDING Multiplication of a virus in an infected person with subsequent release of the virus from that infected person, such that others who come into contact with the person may become infected. A state of being contagious. • VIRULENCE The degree of pathogenicity of an infectious agent. i.e. the ability of the agent to invade and damage tissues of the host causing severe manifestations or death.
Airborne transmission Airborne transmission occurs by particles that are suspended in air. There are two types of these particles: - dust - droplet nuclei • Dust particles: • result from re-suspension of particles that have settled on floor or bedding, • infectious particles blown from the soil by the wind. Example: Fungal spores.
Airborne transmission (cont.) • Droplet nuclei • They represent the dried residue of droplets that have been coughed or sneezed into the air. • They are very tiny particles less than 5 µ (microns) in size and may remain suspended in the air for long periods. Examples: • Tuberculosis is transmitted more often indirectly, through droplet nuclei, than directly, through direct droplet spread. • Legionnaires’ disease and histoplasmosis also spread through airborne transmission.
Airborne infection requirements • Pathogen must be dispersed as fine particles (1-5 μm size) • Respiratory tract-cough aerosol • TB wound • Remain suspended in air • Reach the alveolar level (TB) • Resistant upper respiratory tract • Minute infectious dose (droplet nucleus)
Particle size and suspension in air • Particle size & deposition site • 100 μ • 20 μ • 10 μ-upper airway • 1-5 μ-alveolar deposition • Time to fall the height of a room • 10 sec • 4 min • 17 min • Suspended indefinitely by room air currents
Droplet vs. airborne spread • Transmission within a meter of the source • Relatively large numbers of organisms in inoculum (small inoculum may be tolerated) • Access to vulnerable site (mucosal membranes of eye, nose, mouth, trachea, etc.) • Hand washing may be effective • Transmission beyond a meter-shared breathing volume • Relatively small numbers of organisms in inoculum-virulence required • Access to vulnerable site (alveoli in the case of TB) • Hand washing not effective.
More than 1m! Mycobacterium tuberculosis?
Model airborne infections • Focus on TB (MDR, XDR-TB) and measles but implications for other infections that are partially, opportunistically, or conditionally airborne. • Rhinovirus, influenza, adenovirus, SARS, Bioterrorist agents (smallpox, anthrax), environmental agents (M. bovis, coccidiomycosis, Q-fever, Hanta – not necessarily person to person)
Agent and transmission • Agent: A virus of the paramyxovirus(RNA) family causes measles. The measles virus normally grows in the cells that line the back of the throat and lungs. • Modes of transmission: • Direct: Droplet • Indirect: Airborne The virus spreads by the respiratory route via aerosol droplets and respiratory secretions which can remain infectious for several hours. The infection is acquired through the upper respiratory tract or conjunctiva.
Reservoir • Reservoir:Humans in the form of: • Carriers (sub-clinical, during the incubation period) • Cases (through-out the course of the clinical syndrome) • In contrast to the influenza virus, measles does not have an animal reservoir, which makes it candidate for “elimination” if we manage to successfully prevent infection among human reservoirs.
Time and portals • Temporal pattern: Peak in late winter-spring • Portals of exit/entry Respiratory system • Exit: exhalation • Entry: inhalation
Incubation and clinical features • The incubation period: 7 to 21 days with an average of 14 days. • After incubation period, the patient enters the prodromal stage with fever, coryza, malaise, sneezing, rhinitis, congestion, conjunctivitis and cough followed by a maculopapular rash that usually appears first on the face and then spreads distally. A case of measles is infectious for a period of 4 days prior to the onset of rash until 4 days after the onset of rash.
Clinical features • Koplik's spots, which are pathognomonic for measles, appear on the buccal and lower labial mucosa opposite the lower molars. • The distinctive maculo-papular rash appears about 4 days after exposure and starts behind the ears and on the forehead. From here the rash spreads to involve the whole body.
Complications and risk groups • Measles can cause complications such as otitis media, pneumonia, severe diarrhea, and encephalitis leading to hospitalization and death in severe cases. The rates of hospitalization due to complications can be as high as 40% even in developed countries. • Due to its high communicability, even a minor decrease in immunization coverage can result in rapidly spreading outbreaks and re-establishment of endemic transmission, as noted in the United Kingdom in the recent past. • Unvaccinated children and young adults are at a higher risk of developing measles and they place vulnerable groups such as infants and persons with contraindications to immunization at risk.
Spread • Airborne spread through aerosolized droplet nuclei has been documented in closed environments (e.g., clinics or waiting rooms) for up to 2 hours after the infected person has left the area. • The R0 (expected number of secondary cases resulting from a primary case in the absence of community immunity) for measles is approximately 15, more than 10 times higher than that of the swine-origin H1N1, and three times higher than smallpox.
Treatment • No specific antiviral treatment exists. • Severe complications due to measles can be avoided through supportive care that ensures good nutrition, adequate fluid intake and treatment of dehydration. • This solution replaces fluids and other essential elements that are lost through diarrhea or vomiting. Antibiotics should be prescribed to treat eye and ear infections, and pneumonia. • All children in developing countries diagnosed with measles should receive two doses of vitamin A supplements, given 24 hours apart.
Control • Controlling the spread of such a contagious disease that has an 8-9 day-long period of infectiousness remains a major public health challenge. • In addition to the isolation of all laboratory-confirmed cases, post-exposure immunization of susceptible contacts with a single dose of measles-containing vaccine within 72 hours of exposure has been demonstrated to decrease transmission and is a standard recommendation. • Both serologic and epidemiologic evidence suggest that the immunity induced by the vaccine remains effective long term and possibly for life, in most individuals.
Control (cont.) • In spite of the progress achieved over the past few decades in eliminating and controlling the disease from many parts of the world through immunization, regions of high measles transmission still exist. • Global migration and international travel to and from such regions pose a constant threat of re-introduction of virus transmission in regions that have eliminated measles.
Measles in Turkey • 20509 cases in 2001 (9 deaths), 1119 cases in 2005 and 34 cases in 2006 whereas 349 cases in 2012 • As of September 2013: 6983 cases • Measles case burden: Third country in the world • Notification is mandatory • Elimination program prevails
Prevention • Routine measles vaccination for children, combined with mass immunization campaigns in countries with high case and death rates, are key public health strategies to reduce global measles deaths. • The measles vaccine has been in use for over 40 years. It is safe, effective and inexpensive. It costs less than one US dollar to immunize a child against measles.
Prevention (cont.) • The measles vaccine is often incorporated with rubella and/or mumps vaccines in countries where these illnesses are problems. It is equally effective in the single or combined form. • In 2011, about 84% of the world's children received one dose of measles vaccine by their first birthday through routine health services – up from 72% in 2000. • Two doses of the vaccine are recommended to ensure immunity and prevent outbreaks, as about 15% of vaccinated children fail to develop immunity from the first dose.
Measles vaccine • Composition Live virus • Efficacy 95% (range, 90%-98%) The seroconversion rate is 95% and the immunity lasts lifelong. • Duration ofImmunity Lifelong • Schedule 2 doses • Should be administered with mumps and rubella as MMR or with mumps, rubella and varicella as MMRV.
Control • In the majority of patients, measles is an acute self-limiting disease that will run its course without the need for specific treatment. However, it is far more serious in the immuno-compromised, the undernourished, and children with chronic debilitating diseases. Such patients can be protected by the administration of human anti-measles gamma-globulin if given within the first 3 days after exposure. Alternatively, the exposed individual can simply be vaccinated within 72 hours of exposure. • Pneumonia - antibiotics may be indicated in cases of secondary bacterial pneumonia or otitis media. • Encephalitis- treatment of acute measles encephalitis is only symptomatic and supportive. A wide variety of treatment has been tried for SSPE but no convincing effects have been demonstrated.
Global Plan • In April 2012, the MR Initiative launched a new Global Measles and Rubella Strategic Plan which covers the period 2012-2020. • The Plan includes new global goals for 2015 and 2020: • By the end of 2015 - To reduce global measles deaths by at least 95% compared with 2000 levels. - To achieve regional measles and rubella/congenital rubella syndrome (CRS) elimination goals.
Global Plan (cont.) • By the end of 2020 • To achieve measles and rubella elimination in at least five WHO regions. The strategy focuses on the implementation of five core components: • achieve and maintain high vaccination coverage with two doses of measles- and rubella-containing vaccines; • monitor the disease using effective surveillance, and evaluate programmatic efforts to ensure progress and the positive impact of vaccination activities; • develop and maintain outbreak preparedness, rapid response to outbreaks and the effective treatment of cases; • communicate and engage to build public confidence and demand for immunization; • perform the research and development needed to support cost-effective action and improve vaccination and diagnostic tools.
Overview • Measles is one of the leading causes of death among young children even though a safe and cost-effective vaccine is available. • In 2011, there were 158 000 measles deaths globally • More than 95% of measles deaths occur in low-income countries with weak health infrastructures. • Measles vaccination resulted in a 71% drop in measles deaths between 2000 and 2011 worldwide. • In 2011, about 84% of the world's children received one dose of measles vaccine by their first birthday through routine health services – up from 72% in 2000.
Definitions of Terms • Seasonal influenza: Influenza that occurs every year with gradual variations in the previous year’s virus surface proteins (antigenic drift) • Avian Influenza: a disease of birds that occasionally jumps species and infects humans. Ultimately is the source of new influenza A viruses in humans that can lead to pandemics • Pandemic influenza: a worldwide surge in human influenza cases caused by the introduction of a new type A virus surface protein (antigenic shift)
Influenza Viruses • Classified into types A, B, and C • Types A and B cause significant disease worldwide • Types B and C limited to humans • Type A viruses • More virulent • Wild waterfowl reservoir • Affect many species
Influenza A Viruses Categorized by subtype • Classified according to two surface proteins • Hemagglutinin (HA) – 17 known subtypes • Site of attachment to host cells • Antibody to HA is protective • Neuraminidase (NA) – 10 known subtypes • Helps release virions from cells • Antibody to NA can help modify disease severity
Influenza A HA and NA Subtypes Other Animals Other Animals Other Animals Other Animals Other Animals Other Animals
Importance of Influenza • Global burden largely unknown • Data from temperate climates • 3-5 million severe cases/year • 300,000 - 500,000 deaths/year
Transmission • Influenza is an acute respiratory disease (causative agent is influenza virus from Orthomyxoviridae family) • Signs and symptoms reflect respiratory route • Fever, cough, headache, muscle aches • Sometimes lower respiratory • Transmission of influenza viruses • Person-to-person through droplets from coughing or sneezing • Transmission from objects (fomites) possible • Infectious 1 day before and up to 5 days after becoming sick
Communicability • Viral shedding can begin 1 day before symptom onset • Peak shedding first 3 days of illness • Subsides usually by 5-7th day in adults • Infants, children and the immunosuppressed may shed virus longer
Seasonality Incubation period • Time from exposure to onset of symptoms • Average 2 days (range = 1-4 days) • Peak shedding first 3 days of illness Seasonality • In temperate zones, increases in winter months • Driven by mutations and viral preference for cold, dry weather conditions • In tropical zones, circulates year-round • Fall-winter and rainy season increase has been observed • More international data are needed
Clinical Disease, Human Influenza • Clinical symptoms non-specific • Couple with laboratory data to verify diagnosis. • Abrupt onset • Fever, chills, body aches, sore throat, non-productive cough, runny nose, headache. • GI symptoms and muscle inflammation more common in young children