7.1 Introduction to Papilloma Viruses
The nature of the warts on the basis of virulence was shown by using a filtrate, which was free of cells. Papilloma viruses stayed out of hand to the studies in standard virology because the main problem lied in the methods of propagating the papillomaviruses in standard conditions of Temperature and pressure and related conditions were difficult.
Even though some advances have been made in the standard methods of propagating Papilloma viruses in a laboratory by using organotypic raft cultures of epithelial cells, the study and research will go on.
7.2. Source of Current Knowledge of Papilloma Viruses
Most of the knowledge currently acquired regarding the biology of papillomaviruses at molecular level and the genetics of the Papilloma viruses has been an outcome from the advances in basic research and also by the application of reverse genetics by using cloned viral DNAs.
7.3. Physical Structural Difference between Papilloma Viruses and Papova Viruses
The Papilloma virus families are icosahedral in shape and are circular. They have a double stranded DNA genome and thus, on these characteristics the Papilloma virus families were previously classified under the papovavirus families.
Now, a clear difference has been noted in the Papilloma virus families and the papovavirus families. On the basis of the different characteristics of biologic and genetic grounds, the Papilloma viruses and the Papova viruses like the Polyoma and the Simian virus 40 (SV40) are noted to be different.
While the Polyomaviruses have 5000 base pairs in their double stranded circular DNA genome, the Papilloma viruses have 8000 base pairs in their double stranded circular DNA genome.
Correspondingly, the capsid diameter of Polyomaviruses is 40 nanometers while the capsid diameter of papillomaviruses is 55 nanometers.
The Polyomaviruses can be propagated in the tissue culture under standard conditions; the Papillomaviruses cannot be propagated in the tissue culture under standard conditions.
7.4. Types of Papilloma Viruses
More than 150 types of HPV are known to exist of which some sources indicate that there are more than 200 subtypes of Human Papilloma Viruses. Of the total of nearly 200 HPVs, 15 papillomaviruses are classified as high-risk types, namely 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68, 73, and 82., Other 3 types which come under probable high-risk are HPVs 26, 53, and 66 while 12 other HPVs are classified as low-risk, namely, 6, 11, 40, 42, 43, 44, 54, 61, 70, 72, 81, and CP6108. Although the low risk do not have strong evidence for causing Cancer of Cervix, but even those may cause cancer. Types 16 and 18 are generally acknowledged to cause about 70% of cervical cancer cases. Together with type 31, they are the prime risk factors for cervical cancer.