Can Infectious Agents Cause Non-Infectious Diseases?

Infectious Agents
The question can infectious agents cause noninfectious diseases? may sound weird to many. However, emerging evidences suggest new host-pathogen interactions that may cause pathogens to contribute to the development of medical conditions, which are not classified as infectious diseases.

Infectious diseases are defined as diseases caused by infectious agents such as bacteria, virus, fungi, parasites. Development of infectious diseases involves the presence of the infectious agent in the host and its propagation therein. Another characteristic feature of infectious diseases is that it can be transmitted from the infected host to uninfected hosts through physical contact either with the infected persons or with their body fluids and excreta such as blood, saliva, stool. Infectious diseases can also be transmitted through airborne pathogens or pathogens carried by vectors such as mosquito.

Noninfectious diseases, by contrast, are caused by impaired physiological functions caused by environmental factors, genetic aberrations, nutritional deficiencies or life style. Moreover, noninfectious diseases are not transmitted from one host to other. More and more evidences suggest that infectious agents, particularly, bacteria and viruses may be involved in causing noninfectious diseases such as cancers, atherosclerosis as well as neurodegenerative and inflammatory diseases.

One example of such unconventional host-pathogen interaction is the involvement of Epstein-Barr virus (EBS) in development of Burkitt's lymphoma, a form of B lymphocyte carcinoma. Infections with EBS, which invade B lymphocytes, is common in many parts of the world. In most cases, particularly in children, EBS infections do not produce any symptom. However, in young adults, firs time infection with EBS may cause flue like symptoms including swelling of lymph nodes. Once the symptoms subside, DNA of EBS, in many cases, is maintained in the nucleus of B cells in the form of plasmids, which extrachromosomal genetic materials. EBS DNAs maintained in B cells produce proteins, which enable the host cells to escape apoptosis or programmed cell death. Apoptosis is the mechanism by which body gets rid of cancer cells and other abnormal cells. Thus, when a B lymphocyte containing EBS DNA acquires a mutation that may lead to cancer, the natural mechanism of eliminating precancerous cells fail to eliminate it and Burkitt's lymphoma develops.

Other examples of cancers in which viruses have been linked to the development of the diseases include liver cancer and cervical cancer. Hepatitis viruses and Human papilloma virus are implicated in the etiology of these cancers respectively.

Pathogens have also been implicated in causing atherosclerosis, a condition which leads to blockage in blood vessels due to deposition of lipids, particularly, cholesterol. In the early stage of atherosclerotic plaque formation, macrophages containing engulfed fatty globules deposit on the inner wall of the blood vessels. Cytokines secreted by these macrophages, known as foam cells, initiate migration of other white blood cells to the site of plaque formation. The plaque also collects extracellular matrix. As the accumulated cells and extracellular matrix increase over time, the plaque gradually grows in size, eventually completely clogging the blood vessel and impairing blood circulation to vital organs. Chlamydia pneumoniae, a bacterial pathogen that caused pneumonia, have been isolated from the foam cells in atherosclerotic plaques, suggesting a role of the pathogen in causing atherosclerosis. In fact, previous studies have suggested strong link between pneumonia and atherosclerosis.

Mycoplasma, small bacteria without cell wall, has been implicated in arthritis and chronic asthma, which have no known infectious origin. Studies suggested that activation of the immune system by mycoplasma is associated with arthritis and chronic asthma. Association of infections with obesity and metabolic disease has also been established by epidemiological studies. For example, epidemiological studies have established the link between obesity and adenovirus 36, a pathogen that causes upper respiratory tract infection. Further studies suggested that adenovirus 36 infection altered expression of enzymes and transcription factors in adipocytes leading to increase deposition of lipid in adipose tissues.

Association of neurodegenerative disorders such as Alzheimer's disease and multiple sclerosis with Chlamydia pneumoniae have also been established by epidemiological as well as clinical studies. It has been observed that C. pneumoniae can cross the blood-brain barrier and a large number of postmortem brain samples of late onset Alzheimer's disease patients have been found to contain C. pneumoniae. In case of multiple sclerosis, studies suggested involvement of genetic predisposition and C. pneumonia infection in the etiology of the disease.

Written by Musharraf Ashraf (Ph.D. (Health Chemistry). Medical & life science)

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Edited by: Rajesh Bihani ( Find me on Google+ )

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