influenza is caused by eating eggs which are produced by infected chickens present on highly pathogenic avian influenza virus found on surface of eggs.
It always been a question that what came first the egg or chicken! Likewise our question is that what came first, the egg or virus? In biological systems there is presence of single pass-transmembrane protein mediates gamete formation in unicellular eukaryotes and plants like flowers. Class II fusion proteins are the proteins found in viruses like Zika, dengue, West Nile and Yellow fever. There is HAP2 fusion protein that plays important role in fertilization in specific step of gamete post binding.
It is noticed that HAP2 genetic ablation or mutagenesis of its ectodomain ceases fertilization but not binding of gametes. It remains consistent with gene as a fusion protein. Research highlights that using the sequence alignment and template based structural model program cysteine rich region is found in HAP2 ectodomain in eukaryotes which have same homology of structure with class II fusion proteins of virus.
Most strains of avian influenza virus are mainly found in the respiratory and gastrointestinal tracts of infected birds, and not in meat.
Although phylogenetic analysis shows that it does not show any homology like fusion proteins but it do originates from one common ancestor. One of the important fusion reaction steps is when conformational changes occur in fusion protein a fusion loop (amino acids stretch made disulphide bond with base) exposed and inserted into host cell.
It was found that antibodies raised to fusion loop can inhibit fusion of gametes, as HAP2 is expressed by number of human pathogens like malaria caused by Plasmodium species. Another question is that did eukaryotes capture HAP2 from viral gene or did virus obtain these proteins from eukaryotes? This says that HAP2 is present in major eukaryotic taxa like plant, fungi, and amoebae.
So, HAP2 consider as gene of ancestor that carries gamete formation though that was re-proposed by early eukaryotic virus for entry in host cell (same case seen in virus as it is also ancient). Gamete formation from fusion proteins membrane was possible by virus by enhancing genetic diversity accelerated evolution and it ponders us what came first Virus or Egg?
Proof of influenza virus from egg
Coming towards a virus caused by egg in human is called influenza virus. It is a major threat to human health. It kills many people worldwide and is a respiratory disease. This virus is resistant to many antiviral drugs. It is caused by eating eggs which are produced by infected chickens present on highly pathogenic avian influenza virus found on surface of eggs.
Most strains of avian influenza virus are mainly found in the respiratory and gastrointestinal tracts of infected birds, and not in meat. It happens due to re-assortment and mutation complicates control on virus and necessitates the permanent development of novel vaccines and drugs.
For the propagation of virus, lab based influenza study needs a cost effective and reliable method. High titre of viral stock is achieved by using chicken eggs (fertile) for propagation of Influenza A virus. Incubation was done of the serum pathogens free (SPF) fertilized eggs of chicken at 37˚C for 10-11 days at 55-60% humidity. Egg Candler used to monitor development of embryo. Inoculation of virus is done in allantoic cavity using needle carried out by virus stock injection.
After few days (1-2 days) of incubation at 37˚C eggs are cooled at 4˚C for 4 hours. Above the air sac and chorioallantoic membrane the egg shell are monitored and opened the allantoic fluid have virus growth later it can be harvested. By centrifugation from debris the fluid is cleared aliquots are made and for temporary storage transferred to -80˚C. Large amount of virus titre is achieved in this way and has made utilization of eggs for preparation of virus.
Influenza vaccines
Vaccines to treat influenza have been developed and many companies currently working to develop subunit egg derived vaccines. Production is by cell culture technologies which is advantageous in various ways like cell culturing is faster and easier as compared to egg based vaccine production. In cell culturing non-adaptation of virus phenomenon is avoided. Cell lines used are Vero (African Green Monkey) cell line and MDCK (Madin Darby Canine Kidney) cell line to produce Influenza vaccines.
Cell line usage has disadvantage also, that is pre-existing facilities require to be adapted or rebuild completely, contaminants could also be produced like oncogenic viruses or original clinically isolate virus. Vaccines are made without influenza virus inactivation in production process. In this pattern desired protein is extracted rather than glycoprotein nucleotide sequence, for example hemagglutinin (vaccine is completely independent of virus).
DNA vaccines can be future Influenza virus vaccine candidates. Egg came or virus came first needs to be worked a lot more, though the egg virus and influenza virus arises from egg producing birds (chickens) are the major causative agent of Influenza virus disease in human which is ultimately treated by virus derived or egg derived vaccines and drugs.
This article is jointly written by Beenish William and Sara Asad Malik.