Unit 2
Epidemiological Issues in Health

Learning Outcomes

Prerequisites

Key words

Epidemic, Mortality, Surveillance, Prevalence, Health, Diagnosis, Risk factor, Population

Discussion

3.2.1 Epidemiology and health issues

How does epidemiology affect health care? What are the important issues epidemiology can address?

Epidemiology is concerned with the frequency and pattern of health events in a population. Frequency refers not only to the number of health events, for example, the number of cases of meningitis or diabetes in a population, but also to the relationship of that number to the size of the population. The term “epidemiology” is now frequently used to refer to the study of disease in general, including associated ailments, as well as infectious and epidemic diseases. Obesity, mental illness, and high blood pressure are a few examples of areas that epidemiology researches. In or-der to understand how political, social, and scientific factors interact to increase the risk of contracting a disease, epidemiologists must understand the factors that underlie disease distribution, their sources and causes, and methods for controlling it. This makes epidemiology a special science.

The major areas of epidemiological study include disease causation, transmission, out-break investigation, disease surveillance, environmental epidemiology, forensic epidemiology, occupational epidemiology, screening, bio monitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.

In the past epidemiology has helped to explain the transmission of diseases, such as cholera and measles, by discovering factors shared by individuals who became sick. Modern epidemiologists have contributed to an under-standing of factors that influence the risk of heart disease and cancer, which account for most deaths in developed countries today. The fundamental influences on health and disease include natural changes in the environment; environmental changes arising from hu-man invention, discovery, and manipulation; changes in the interaction between humans, microbes, and animals usually for cultural reasons; changes in human circumstances, cultures, and behaviours; and the genetic evolution of microbes, animals, and humans. Among a range of core epidemiologic func-tions recognised (CDC, 2012), monitoring and surveillance as well as outbreak investigation are most immediately relevant in identifying and stopping the spread of infectious disease in a population. Epidemiologists study out-breaks of diseases, the causes, locations, and how various communities are affected, utilizing relative information to aid in the prevention of future outbreaks. Epidemiologists help to keep the public informed of methods to maintain and improve public health. The areas of specialization of epidemiologists include cardiovascular disease, genetics, infectious disease, environmental causes and ageing..

A traditional model of infectious disease causation, known as the Epidemiologic Triad is depicted in the figure below. The triad consists of an external agent, a host, and an environment in which host and agent are brought together, causing the disease to occur in the host. A vector, an organism that transmits infection by conveying the pathogen from one host to another without causing the disease itself, could be part of the infectious process.

The mosquito, or Anopheles, is a well-known illustration of a vector. The parasite plasmodium is ingested by the mosquito along with the blood of a host that has been affected. The mosquito is unaffected by the plasmodium. However, the plasmodium can cause malaria in the affected person once it has been stored in the salivary glands and then injected into the next human the mosquito feeds on. Thus, malaria is transmitted by the Anopheles mosquito.

Another well-known example of a vector is the Ixodes genus of ticks, which can transmit Lyme disease.

3.2.2 Water related diseases and their public health importance

Diseases associated with water exposure include those brought on by both direct and in-direct skin contact with water while bathing or engaging in other water-related activities, as well as illnesses brought on by ingestion of water. Waterborne or water-associated micro-organisms, as well as hazardous compounds, can cause disease. A broader definition includes diseases connected to vectors that spend part of their life cycle in water habitats, diseases related to inhaling contaminated water aerosols, and diseases related to illness due to water shortage or contamination during adverse climate events, such as floods and droughts. Water may contain the infectious organisms (pathogens) that cause contagious diseases in people or may offer the ideal conditions for the reproduction and spread of the vectors that transport those pathogens. Environmental changes brought on by such projects may have a significant impact on the epidemiology of disease through their effect on vector ecology whenever a parasite or another disease-causing organism is present and there is a susceptible human population. In densely inhabited areas near irrigated lands, disease transmission may spread especially quickly. Good engineering practices and effective water management can prevent or lessen the effects of diseases related to water.

3.2.3 Categories of Water-related Disease and Water-borne diseases

Water related and water borne diseases are different. Microorganisms, parasites, poisons, and chemical contamination of water are some of the causes of diseases that are associated to water. Water borne diseases indicate direct transmission and is typically used to refer to illness brought on by pathogens that are microbiological or chemical pollutants in water. Seven categories of water-related dis-ease can thus be identified such as waterborne microbiological disease; waterborne chemical disease; water hygiene disease; water contact disease; water vector habitat disease; excretal disposal disease and water aerosol disease.

The majority of water-borne diseases world-wide mainly affect children due to poor hygiene and weak immunity. Most of these dis-eases are life-threatening. The knowledge of the different types of water-borne diseases has come to the forefront with the advent of globalization over the past few decades. Several pathogenic microorganisms which were previously unknown, have become the focus of major research in this field. About 844 million people lack even a basic drinking-water service. In fact, approximately 159 million individuals are dependent on surface water, the World Health Organization (WHO) reports. Additionally, at least 2 billion people utilize a drinking water source that is contaminated with feces. These water sources can transmit water-borne diseases, which have been linked to about 502,000 diarrheal deaths every year. The pathogenic microorganisms, their toxic exudates, and other contaminants together, cause serious conditions such as cholera, di-arrhea, typhoid, amebiasis, hepatitis, gastro-enteritis, giardiasis, campylobacteriosis, sca-bies, worm infections, etc.

Climate change plays a crucial role in the out-breaks of such infections. Major precipitation events such as heavy rains and snowfall in-crease the risk of the water-borne diseases. Several instances of the outbreak of epidemic infections after natural calamities such as flood have been recorded in history. The over-flowing of sewage treatment plants during floods becomes the immediate risk that needs to be curbed. On the other hand, drought-affected areas also become high-risk due to the accumulation of high concentration of pathogens in a limited amount of available water sources. Natural calamities such as earth-quakes or major cyclones often cause drastic changes in the ecosystem of the water bodies. Sometimes, the newly created environment (pH, temperature, etc) is favourable for the growth of a particular type of pathogen. The unprocessed water from such sources be-comes harmful for routine use due to the high microbial burden. The mere negligence of the cleaning staff of water treatment plants can also cause huge damage to the community, especially in small towns where people do not have water purifiers installed in their homes. Also, the persistent usage of contaminated water for agricultural purposes (due to lack of pure water in the area) results in the colo-nization of pathogens in the soil. Consuming crops in that particular area may predispose the residents to disease-causing microorganisms.

In affected individuals, antibacterial, anti-parasitic, or antiviral medications are used for treatment depending on the nature of the disease. Additionally, maintaining personal hygiene also dramatically reduces the occurrence of water-borne diseases. Apart from the precautions at an individual level, several other approaches including mass recycling of water and carbon sequestration are employed to control the water-borne diseases.

3.2.4 Vector-borne diseases

Vectors are living organisms that can transmit infectious diseases between humans or from animals to humans. Many of these vectors are bloodsucking insects that ingest disease- producing micro-organisms during a blood meal from an infected host (human or animal) and later inject them into a new host during their next blood meal. Mosquitoes are the best known disease vector. Others include certain species of ticks, flies, sand flies, fleas, bugs and freshwater snails.

Vector-borne diseases are illnesses caused by pathogens and parasites in human populations. Every year more than one billion people are infected and more than one million people die from vector-borne diseases including malaria, dengue, schistosomiasis, leishmaniasis, chagas disease, yellow fever, lymphatic filariasis and onchocerciasis. These diseases affect urban, peri-urban and rural communities but thrive predominantly among communities with poor living conditions – particularly lack of access to adequate housing, safe drinking water and sanitation. Malnourished people and those with weakened immunity are especially vulnerable. These diseases also exacerbate poverty. Illness and disability prevent people from working and supporting themselves and their family, causing further hardship and impeding economic development. Dengue, for example, imposes a substantial economic burden on families and governments, both in medical costs and in working days lost due to illness. In order to reduce poverty and pro-mote economic growth, vector-borne diseases are crucial.

3.2.5 Air-borne diseases

Airborne diseases are caused by pathogenic microbes small enough to be discharged from an infected person via coughing, sneezing, laughing and close personal contact or aerosolization of the microbe. The discharged microbes remain suspended in the air on dust particles, respiratory and water droplets. The microorganisms may come from a person or animal who has a disease or from soil, garbage, or other sources. There are many types of airborne diseases, and the symptoms, treatment, and outlook will vary according to the disease.

What are the methods of preventing the trans-mission of air-borne diseases?

Ways of preventing transmission include the use of personal protective equipment and effective ventilation systems. Depending on the disease, an individual can help prevent trans-mission by avoiding close contact with others or wearing a face mask. Air currents can disperse the microorganisms, but how far they travel depends partly on the environment. The more the droplets travel from the source, the lower the risk of infection, due to environ-mental factors.

Depending on the pathogens, factors that affect how long they remain active include:

• air temperature
• humidity
• exposure to sunlight or other forms of radiation
• the weight of the particles, which can affect how long they take to settle
• the structure and stability of the pathogen

It is not always possible to prevent the spread of airborne diseases, but individuals and authorities can take measures to reduce the risk by recommending or ensuring adequate ventilation and the use of protective equipment.

Depending on the type of organism, the degree of exposure, and individual factors, air-borne particles may cause illness to develop if exposure occurs. Many diseases can arise after exposure to airborne particles, including

• the common cold, which can develop from a rhinovirus
• chickenpox, caused by the Varicella zoster virus
• mumps, caused by a paramyxovirus
• measles, caused by another paramyxovirus
• whooping cough, a bacterial infection caused by Bordetella pertussis
• COVID-19, caused by the SARS-CoV-2 virus
• aspergillosis, caused by the Aspergillus fungus
• tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis
• anthrax, a bacterial infection resulting from contact with Bacillus anthracis spores
• diphtheria, a bacterial infection caused by Corynebacterium diphtheriae
• meningitis, which can result from exposure to certain bacterial, viral, or fungal particles

Recap

Objective type questions

Answer to Objective type questions

Self Assessment Questions

Assignment

Suggested Reading

Reference