Viral Disease Emergence and Spread in Humans - Article Example

VIRAL DISEASE EMERGENCE AND SPREAD IN HUMANS Micro organisms are very tiny organisms that can be seen only through the microscope. These include Bacteria, Virus, Fungi, parasites and algae. These micro organisms are both useful and harmful to the man kind. There are many type of viruses that are harmful to the human beings and cause a variety of infections. These viruses are very small micro organisms that obtain its life only inside the living cells. The processes such as replication and other viral protein synthesis occur only inside the cell. These viruses infect all types of organisms from plants, animals to humans and other bacteria. These viruses are about one-hundredth the size of the bacteria. They are obligate parasites. Viruses were studied prior to the development of bacterial knowledge. According to the definitions “viruses are sub-micro scopical entities capable of being introduced into the specific living cells and reproducing inside such cells only”. They are made up of three distinct layers. The genetic material of the virus can be either DNA or RNA, they carry the genetic information, the viral proteins that are present covering these genetic materials and protecting them and a cover of the lipid molecules forming a capsule. The study of the viral replication and the propagation are very important for the analysis of the viral pathogenesis. As a result of this infection, viral disease occurs in the body and they are able to multiply inside the human body. The symptoms of this viral disease vary from coughing to severe stroke. When a virus enters the human body, it gets itself attached to the host cells mechanism and they replicate inside them producing the viral genes and in turn the viral proteins. (Wagner and Hewlett 2004). These Viruses are quite different from other type of cells. They will contain either DNA or RNA and never both together. They depend upon the host to which they get attached for energy and raw materials that are required for viral replication. They are of various shapes and sizes ranging from 20 nanometers to 0.3 micrometers and the shape may vary from bullet shaped, helical, polygonal, spherical, pyramidal to very complex structures. These viruses are classified based on the similarity of the viral components, their size of the nucleic acid. Based on this category Baltemore has classified the viruses into several types such as dsDNA Viruses, ssDNA viruses, dsRNA Viruses, +ve RNA Viruses, - ve RNA Viruses, RNA reverse transcribing viruses and DNA reverse transcribing viruses. These classification helps to identify the virus families. (Vinken, Bruyn and Klawans 1989) These viruses cause many diseases varying from common cold to AIDS. These viral diseases cause varying levels of severity in the humans. Common cold, Chicken pox, influenza, cold sores, Avian flu, SARS are some of the important viruses. The viruses follow different mechanisms for causing the diseases. They vary from lytic infections, latent and persistent infections. Lytic infections result in the destruction of the host cells, they are caused by the virulent viruses, which result in the death of the cells which they affect. Persistent infections are due to the infections that take place after a long time gap after entering in to the host cell. These infections are mainly caused by the enveloped viruses. Many viral diseases are caused after a certain period of time. There exists a gap between the infection and the appearance of the first symptoms. The cold sores are caused by the herpes simplex viruses due to the latent infection. These viruses enter into the host body and reside for some time and the appearance of the first symptom is due to the stresses caused by the environmental factors. Historical - Important Events: 1880 – Tobacco mosaic virus was reported first by Adolf Mayer. 1892- The existence of viruses was scientifically proved by a Russian Botanist, Bomitrii Ivanowski. 1898 – Ivanowski’s observations were confirmed later by Beijernick, Loeffler and Frosch reported foot and mouth disease. 1917 – Bacteriophages were discovered by the French scientist D’Herelle. 1933 –Schelsinger was the first to determine the chemical composition of virus. 1935 – Stanely crystallized the virus. He showed that these crystals have the ability to cause diseases. 1952 – Hershey and Chase demonstrated that DNA is the genetic material in T4 bacteriophage. 1954 – John F Enders won the Nobel Prize for his studies on the mechanism of viral infection of cell culture. 1957 – Successful vaccination against polio by J.Salk and Sabin. 1969 – Max Delbruck got Nobel Prize for his studies on the mechanism of viral infection of cells. 1971 – The first viriod was discovered by T.O.Diener. 1975 – Rodney R Porter and David Balitmore got Nobel Prize for their studies on the transformation of cells by tumor viruses. 1981 – The first virusoid was discovered by J.W.Randles. 1982 – “Prion” was announced from the University of California. Viral disease emergence: It was until the Neolithic Period , humans did not had the viral diseases such as influenza, measles, dengue etc., it was also found that the viruses are able to establish the long term persistent infections with the humans and they were able to spread among the human beings. Some important viruses include papilomaviruses, herpes viruses and Hepatitis B and some other retro viruses have these long terms persistent inside the body. The molecular phylogenics determined by Bernard has proved that HPV has come out from African continent and has spread through out the world through the human invasion of different lands. (Leal. and Zanotto 2000). The cytomegalovirus was found to be associated with the primitive societies of Amazon basin. The molecular phylogenetics have identified that the primate T Lymph tropic virus from the Old World Apes and Monkeys in Africa. Similarly some of the Human T-cell Leukemia Virus are found among the Indian populations in America and some at Africa indicating that they must have originated from the same species and have gone changes in their DNA over time. Recently it is found that the viruses have crossed the species specific barrier and are found have different degrees of morbidity and mortality. The endogenous retro viruses which were initially called as having “Junk DNA” are found to play a major role in the evolution. Studies have found that the viruses are related to humans in many ways. The HERV- K10 and the Murine ERV transcripts are found to be responsible for the production of the steroid hormones. The products of these genes are found at the primate placental and embryonic cells. (Carter and Saunders 2007). Interaction between virus and the host: There are many hypotheses for the interaction of the virus with the host. Whenever the viruses infect the humans, they create genetic variation and the genetic mutation among themselves to become resistant to the responses. The increase in the human population and agricultural population has resulted in the increase of new viral diseases. One important break through in the viral diseases is the ongoing epidemic Acquired Immuno Deficiency Syndrome ( AIDS) and some sporadic diseases such as hemorrhagic fever such in many countries of Africa, Asia and Some states of the United States. The causative agent of AIDS is the Human Deficiency Virus (HIV), that is found to have originated from the African Continent from the monkeys and the transmission of these virus from them to the animal inhabitants and then to the rest of the world. Many social factors have led to the distribution of AIDS to all parts of the world. This disease is found among the heterosexual persons more frequently in the women. (Zuckerman 2004), (White and Fenner 1994), (Dulbecco and Ginsberg 1988). Damages caused by the viral diseases: Viral Diseases of Plants: Plant viruses cause disease in the parts of the plant by relocating the photosynthetic pigment and the normal cellular processes. Sometimes the viruses infect the plants with no significant symptoms. (Field, Knipe and Howley 2007). As many human communities evolved and migrated from one region to another, the pathogens started to look for the correct conditions to continue their establishment with the humans. Due to the changes in the ecology and the invent of new tools , the increase in the usage of chemicals for the land, the emergence of new diseases occurred. The new pests and other massive epidemics are the best examples for this evolution. (Leal and Zanotto 2000) Adolf Mayer, a German scientist first started to experiment the infectious nature of the viral diseases in plants. Though he failed to isolate the viruses, the era began on the study of the plant diseases. More than 1700 viruses have been visualized till today. Most of the plant viruses are RNA viruses. They are spherical or rod shaped. The plant viruses are generally named after its host plant with symptoms of that disease. The plant viruses have been classified into 19 groups. Except the last two groups all the groups have ssRNA. Some of the important plant viruses include Tobacco mosaic virus Turnip Yellow Mosaic virus, tobacco necrosis virus, Satelite virus etc. Animal Viruses: The animal viruses are covered by envelope around the capsid. So the capsid may be enveloped capsid or naked capsid. Animal viruses are grouped into 6 classes and they are discussed already. Some of the important viral diseases include 1. Small pox: Small pox is caused by the pox virus variola. The virus is brick- shaped with a length of 270 nm. 2. Chicken pox: This is caused by the virus Varicella zoster. It is a generalized infection. It is a mild self-limited illness of young children. 3. Rabies: This virus looks like a bullet and it is an RNA virus. It is fatal to man and animals like dogs, cats, rats, horses and bats. This virus is a neurotrophic virus which attacks the central nervous system that results in mental disorder and paralysis of the muscles of face and tongue. 4. Influenza: it is caused by a helical virion with a diameter of about 100nm. The virion has ssRNA. The Envelope has a series of projections called spikes with enzymes for contamination of respiratory mucus and saliva. 5. Common cold: This infection is produced by many viruses. The common cold is characterized by running nose, secretion of tears, irritation of naso pharynx and sneezing. 6. Measles: it is caused by a helical RNA virion with a diameter of 250 nm. This is a highly contagious disease of children characterized by fever, coughing, sneezing etc., The stages of viral infections: 1. Entry into the host 2. Primary Replication 3. Dissemination stage. 4. Tissue cell/Tropism 5. Host immune responses 6. Secondary Replication 7. Direct Cell and Tissue Damage 8. Persistence versus Clearance 1. Entry into the host: This is the first stage of viral infection. The pathogenic and non pathogenic viruses enter into the host cell by various portals. Some of them are Skin, Respiratory tract, gastrointestinal tract, genitourinary tract, Conjunctiva. Skin – Most of the viruses enters into the body by breaking the barriers through cuts and abrasions. Even some viruses will employ the vectors for the infection such as mosquitoes to puncture the skin and infect the host. Respiratory tract - The respiratory tract has well established immune defense mechanisms and also non specific inhibitory mechanisms like ciliated epithelial cells, lower temperature and the mucus secretions. In spite of these preventory mechanisms this port is the most common mode of entry for the pathogens. Gastrointestinal tract – The entero viruses and the polio viruses use this mode of entry, though they have a very good mucosal surface. Genitourinary tract – though this tract is less protected than the other routes, this is very less frequently attacked by the viruses. Conjunctiva – This is the most unprotected site and it receives a wide exposure to viruses. (Joklik 1985), (Nathanson 1997). 2. Primary Replication: After entering into the host cell, the viruses reach the susceptible cells and initiate the infection. Some viruses will remain at the site of infection and some will move to the secondary site and spread their infection. The Rhinoviruses, Rota viruses and Papillloma viruses create a localized infection at the primary replication site. The Rhino viruses affect the upper respiratory tract and the Rota viruses affect the intestinal epithelium. The systemic infections are caused by the Enteroviruses (Primary Replication – Intestinal Epithelium, Secondary replication – Lymphoid Tissues), Herpes virus (Primary Replication –Oropharyngeal or Urogenital tract, Secondary replication- Lymphoid tissues). The systemic infection viruses are very dangerous to the humans. They cause severe damage to the host cells. 3. Dissemination Stage: The virus spreads inside the host through two ways- the first one is through the blood stream and the second one is through the nervous system. The viruses travel in the blood stream either as free cells or found attached to the red blood cells, platelets, lymphocytes, monocytes etc. This condition is termed as viremia. This mode of transfer of the viruses into the blood stream is termed as primary viremia and is followed by the secondary viremia. The viruses thus reach the target site directly through the blood stream. The transfer of the infection though the cells occurs through the central nervous system also. The neurons are the primary source of infection and they transfer the viruses by passing them to the near by cells. 4. Tissue/Cell Tropism: This is the ability of the cells to replicate in the particular tissues or cells. The specific viral receptors play an important role in the pathogenesis effects. 5. Host Immune responses: There are a lot of host immune responses for the viral pathology. The cell mediated immunity kills the cells that are infected with the virus. The antibody mediated immunity is also possible but they don’t prove good for all viruses. Autoimmune pathology also occurs. (Levy 1994). 6. Secondary replication: This sort of infection occurs when the virus reaches the other sites of infection. The best example is the polio virus infections. Polio viruses initially affect the GI Tract and later moves into the brain and the spinal cord through the neuron and cause serious paralytic complications. (Dimmock, Easton, and Leppard, 2007). 7. Direct cell and Tissue damage: The viruses replicate inside the host cells without causing any significant damage. Sometimes the viruses undergo budding and cause the persistent infections and pause them to the offspring also. Sometimes the virulent viruses cause damage to the host cell by many different mechanisms. 8. Persistence versus Clearance: The ability of the virus to remain inside the body and the ability of the host to eliminate the virus completely from the body. This is very important. This occurs by two mechanisms: 1. The regulation of the lytic potential. 2. Immune surveillance evasion. (Shors 2009). Current Developments: The recent advances in the virology period include the Production of vaccines for Yellow fever, Salk and Sabin vaccine for poliovirus, various drugs developed for the viruses such as Hepatitis B and C, influenza virus and Herpes Simplex Virus 1 and 2, identification of the virulence genes in the viruses, identification of the molecular bases of the viral diseases in cellular defenses, antiviral immune defenses, identification of the viruses that cause the specific cancers, Elucidating the viral mechanisms of infection and Understanding of the viral tropism are some of the landmarks in the history of virology. (Pelczar, Chan and Krieg 2005), (Cann, 2005), References: Cann, A 2005, Principles of molecular virology, Elsevier Academic Press. Carter, JB and Saunders, VA 2007, Virology : principles and applications , John Wiley and sons. Dimmock, NJ, Easton, AJ and Leppard, K 2007, Introduction to modern virology, 6th Edition, Blackwell Publishing. Dulbecco, R and Ginsberg, HS 1988, Virology, 4th Edition, Lippincott. Field, BN, Knipe, DM and Howley, PM 2007, Field’s virology, Lippincott Williams and Wilkins. Joklik, WJ 1985, Virology, Appleton-century crofts. Leal, ES and Zanotto, PMA 2000, Viral Diseases and Human Evolution, Memórias do Instituto Oswaldo Cruz, vol. 95, no.1, p.193-200 Levy, JA 1994, Virology, Prentice Hall. Shors, T 2009, Understanding Viruses, Academic Press. Nathanson, N 1997, Viral Pathogenesis, Lippincott- Raven. Pelczar, MJ, Chan, E and Krieg, NR 2005, Microbiology, 5th Edition, Tata Mc-Graw Hill publication. Vinken, PJ., Bruyn, GW and Klawans, HL 1989, Viral Disease, Elsevier Health Sciences. Wagner EK and Hewlett MJ 2004, Basic Virology, Wiley Blackwell. White, DO and Fenner, F, 1994, Medical Virology, Gulf Professional Publishing. Zuckerman, AJ, 2004, Principles and practice of clinical virology, John Wiley and Sons. Read More