Virus or viral diseases does not produce symptoms directly in the animals (humans). But it indirectly affects the host. Virus causes different diseases in animal and plants.
- Some virus stimulates the lysosome of host cell to produce hydrolytic enzymes. These enzymes cause bursting of host cell. These hydrolytic enzymes damage other cells.
- Sometimes, envelope of the virus is toxic to the body of host. It causes allergic reactions.
- In some cases, virus DNA incorporates in the DNA of host. It stimulates the genes of host cells. Thus, host cell starts dividing and produce tumor (cancer).
- In some cases virus destroy the immune system of the host like AIDS.
Vaccines have not been introduced for many viral diseases in animals. Their is still search going on and methods are being invented to fight these viral diseases and one of them is AIDS. There are following animal viral diseases:
- AIDS (Acquired Immuno Deficiency Syndrome): AIDS is a viral disease caused by HIV Virus. The HIV virus affects helper T- Lymphocytes (a white blood cell). T- Lymphocytes are major component of the immune system. The increase of infection of the HIV decreases the number of T- Lymphocytes. It results in failure of immune system. So, the infected person is attacked by many diseases. The HIV also infects the cells of the central nervous system (brain and spinal cord). Its symptoms are sudden weight loss, swollen lymph nodes and general loss of immunity.
- Cancer: Some retroviruses have special genes called oncogenes. These viruses are called tumor viruses. These viruses inject these genes into host cells. Thus, divisions start in the host cells. It produces tumor. The cell of this tumor may spread to other part of the body and cause cancer.
- Hepatitis: The inflammation of the liver is called hepatitis. It is caused by viral infection. It has different types like Hepatitis A, B, C, D etc. Its symptoms are: Jaundice, abdominal pain, Liver enlargement, Fatigue and sometimes fever. It may be mild. Sometimes it is acute and liver cancer.
- Polio: This viral disease is caused by polio virus. It is present all over the world. It occurs mostly in childhood. The polio virus is the smallest know virus. It contains RNA in spherical capsid. Polio virus attack on nervous system. It makes the children handicapped.
- Influenza and Cold: These viral disease are caused by influenza and cold virus. It is an enveloped RNA virus. Influenza and cold is a wide spread disease. It occurs in epidemic form. Both these diseases infect the respiratory tract.
- Small Pox: Small pox is caused by pox virus. It is a DNA enveloped virus. Small pox was common disease of the world before early twentieth century. Raised vesicles are formed on the body. These vesicles are fluid – filled. Later, these vesicles change into pustules. These pustules later forms pitted scars. These scars are called pocks.
- Mumps and Measles: Mumps and Measles viruses are belonged to group Paramyxo-viruses. They are large and RNA enveloped viruses.
- Herpes Simplex: It is caused by Herpes virus. It is a DNA virus. It is naturally occurring disease of mankind. It forms vascular lesions in the epithelial layers of the ectodermal tissues. This disease occurs in the mouth, on the lips and at other skin sites.
Viral Diseases in Plants
Viral diseases have caused major destruction in plants. That’s why plant viruses are serious agricultural pests. They stunted the growth of plants. Thus, crop yield is lowered. Once plant virus enters into a plant, it spreads in all its part through plasmodesmata. It is almost impossible to control plant viruses. Therefore, scientists are preparing virus free varieties. There are following viral plant diseases.
- Tobacco Mosaic Virus (TM V): TMV is RNA virus. Its capsid is coiled rod like. It attacks on the leaves and stem of plant. The leaves become wrinkle and molted. Mosaic appears due to molting. These mosaics are present in the form of light green spots. The leaves become narrow and stunted. The plant soon loose vigor and start dying. TM V spread by mechanical rubbing of leaves. It is also s read by infected seeds.
- Mosaic Virus of Cucumber and Lettuce: Virus also causes mosaic in cucumber and lettuce plant. These viruses are mostly spread by pest like aphids and leaf hoppers.
- Yellow Disease of Potato: It infects the leaves, stems and roots of plant. It ultimately destroys the plant.
- Tomato Bunchy Top Disease: It is caused by infectious particles called viroid. The tomato plant fell down wilt and become dry.
- Cotton Leaf Curl Viruses (LCV): This virus attacks on cotton plants. It caused huge cotton losses in Pakistan in previous years. LCV is spread by white fly named Bemisia tabaci. Its symptoms are thickening of sub-veins in young leaves. It causes upward curling of the leaves. The leaves become leathery. Foliar outgrowths come from the veins. It causes lengthening and twisting of internodes. It decreases the yield of cotton. Sometimes, whole crops are destroyed.
- Necrotic Disease: In this disease, virus kills the leaf of plant by necrosis. This disease occurs in potatoes and tomatoes.
- Tumors: Many viruses cause tumors in tomatoes. cucumber and tobacco plants.