Test 2- Prevention and control of Viruses Flashcards
Antiviral Drugs
Interfere with the ability of a virus to infiltrate a target cell or target different stages of replication/Synthesis of components required for replication of the Virus.
Immune system stimulation
Interferons, class of proteins that has antiviral effects and modulate functions of the immune system.
Antimicrobial chemotherapy
Antimicrobial chemotherapy is defined as the treatment of infectious diseases by drugs
(chemical compounds) that are inhibitory or lethal to the pathogenic microbe.
Antibiotics
Antibiotics, also known as antibacterials, are types of medications that destroy or slow down the growth of bacteria.
Antimycotics
Antimycotics are medications used to treat fungal diseases.
Antiparasitics
Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases
Antiviral drugs
Antiviral drugs are a class of medication used specifically for treating viral infections
Compared to antibiotics, there are only few effective antivirals available so far.
Have you wondered why?
Viruses are intimately dependent on the metabolic pathways of their host cell for their replication, hence most agents that interfere with virus replication are toxic to the cell.
A logical approach to the development of new antiviral drugs is
A logical approach to the development of new antiviral drugs is to devise strategies that will interfere with virus replication without harming or causing minimal harm to the infected host cell.
Antiviral chemotherapeutic agents are not in common use in veterinary practice because
The high cost of development of new chemical compounds, particularly for use in food species.
Often use restricted to a single virus and a specific animal species .
Difficulties encountered in development of broad-spectrum antivirals with low
cytotoxicity
Absence of rapid diagnostic techniques allowing prompt use of a specific antiviral agent in the course of an acute infection
In the future how will this change?
The successful use of antiviral chemotherapy in some human viral diseases has increased confidence and awareness of the existence of efficient antiviral drugs that can also be used in veterinary medicine.
In addition, veterinary internal medicine has undergone refinement and progress with diagnostics and treatment, allowing the use of sophisticated and expensive protocols.
Vaccination has limitations, attributed to the antigenic diversity of viruses, stressing on development of new antivirals.
Acyclovir
Antiviral activity primarily restricted to herpesviruses.
Administered as a prodrug, inactive form.
Requires virus enzymes in infected host cell to convert itself into active form, which then interferes with virus replication.
Treatment of:
Herpesvirus infections in humans
Feline herpesvirus 1 induced corneal ulcers
Equine herpesvirus-1 induced encephalomyelitis
Mechanism of Antiviral effect of Acyclovir
Acyclovir molecules entering the cell are converted to acyclovir monophosphate by virus induced thymidine kinase enzyme.
Host-cell enzymes add two more phosphates to form acyclovir triphosphate, which is transported to the nucleus.
Cleavage of 2 phosphates from the acyclovir triphosphate by the herpes simplex’s own enzymes to form acyclovir monophosphate.
The herpes simplex’s DNA polymerase enzyme incorporates the acyclovir monophosphate into the growing DNA strand as if it were 2-deoxyguanosine monophosphate (a “G” base).
Stop the growing viral DNA chain: Further elongation of the growing viral DNA chain is impossible because acyclovir monophosphate lacks the attachment point necessary for the insertion of any additional nucleotides.
Acyclovir is________ to the uninfected host cell
Acyclovir is Non-toxic to the uninfected host cell
Since the enzymes herpesvirus thymidine kinase and herpes virus DNA polymerase are viral enzymes and not found in uninfected host cells, acyclovir cannot be phosphorylated and incorporated into the host DNA.
Thus, acyclovir is non-toxic to uninfected host cells.
Acyclovir: Mechanisms for Resistance
Some herpesviruses are resistant to acyclovir because of following:
Absent production of viral thymidine kinase due to mutations in virus genome(TK-
negative mutants)
Partial decrease in the production of viral thymidine kinase (TK-partial mutants)
Altered viral thymidine kinase substrate specificity that results in phosphorylation of thymidine but not acyclovir (TK-altered mutants)
Mutations in viral DNA polymerase that causes a decreased binding of acyclovir- triphosphate to viral DNA polymerase.
Amantadine
Amantadine is a synthetic tricyclic amine of the adamantane family.
Amantadine acts as both antiviral and anti-Parkinson drug.
Amantadine inhibits replication of most strains of influenza A viruses by blocking and uncoating the virus
Uncoating of Influenza viruses in host cell cytoplasm
As the endosomal vesicles that contain the virus particles move towards the cell nucleus, their pH drops.
When the endosomal pH reaches 5.0, the viral HA protein undergoes a conformational rearrangement.
When this occurs, the viral RNAs are released into the cytoplasm. They are then transported into the cell nucleus where viral RNA replication occurs.
In the influenza virion, the viral RNAs are bound to a number of viral proteins, including the M1 protein.
This M1 protein forms the shell that underlies the lipid membrane of the virion.
Unfortunately, if the viral RNAs are bound to M1 protein when they are released from the virion, the viral RNA cannot enter the nucleus.
The viral M2 protein forms a channel in the membrane that actively pumps protons from the endosome into the interior of the virion.
These protons lower the pH in the interior of the virion, releasing the viral RNAs from M1 protein.
Mechanism of Antiviral effect of Amantadine
The M2 ion channel is the target of the antiviral Amantadine.- THE M2 channel is very important for virus replication
These compounds clog the channel and prevent it from pumping protons into the virion.
In the presence of amantadine, viral RNAs remain bound to M1 and cannot enter the nucleus. Virus replication is inhibited.
Resistance to amantadine occurs by changes in amino acids that line the M2 channel. These changes prevent the drug from plugging the channel.
In certain strains of influenza A virus, the pH changes that result from M2 inhibition alter t
In certain strains of influenza A virus, the pH changes that result from M2 inhibition alter the conformation of hemagglutinin during its intracellular transport later in replication and thus block the viral assembly also.
Neuraminidase Inhibitors
Inhibitor of neuraminidase [NA] enzyme synthesized by Influenza A and B viruses.
Oseltamivir (Tamiflu)
Laninamivir
Zanamivir
Peramivir
Oseltamivir phosphate (Tamiflu) is a
Oseltamivir phosphate (Tamiflu) is a prodrug that, after its metabolism in the liver, releases an activate metabolite that inhibits neuraminidase.
What are the major glycoproteins found on the surface of the Influenza virus?
Neuramindase (NA) and hemagglutinin (HA) are major membrane glycoproteins found on the surface of influenza virus.
Neuramindiase activity
HA of influenza virus binds to receptors containing sialic acid on host cell membrane.- where the hemmaglutin spikes attach!
After budding, HA of progeny influenza virions are still bound to sialic acid containing receptors on infected host cell surface. NA present on virus will cleave the sialic acid containing cell surface receptors and release HA. The virus is freed from the infected cell.
NA is thus critical in cell to cell spread of influenza viruses
Neuramindiase Inhibitors
Blocking the function of neuraminidase with NA inhibitors is an effective way to treat influenza.
This prevents release of virus and spread of infection, as the HA of virus is still bound/attached to the sialic acid containing receptors on surface of already infected host cell.- they do not prevent replication
Inhibition of neuraminidase, therefore, slows virus spread, giving the immune system the opportunity to “catch up” and mediate virus clearance.
Nucleoside Analog Reverse Transcriptase Inhibitors (NRTIs)
Zidovudine (ZDV) or AZT [Azidothymidine]
ddI [Didanosine]
ZDV/AZT
ZDV/AZT belongs to the family of nucleoside analog reverse transcriptase inhibitors (NRTIs)
Nucleoside analog of thymine, i.e. resembles the deoxyribonucleotide containing the base thymine.
AZT/ZDV is phosphorylated by
AZT/ZDV is phosphorylated by kinases present in host cell to AZT-triphosphate (AZT-TP).
Since ZDV/ AZT resembles thymine deoxyribonucleotide-triphosphate
Since it resembles thymine deoxyribonucleotide-triphosphate, the reverse transcriptase cleaves two phosphates and inserts AZT monophosphate into the cDNA that is being synethized from viral RNA
STOPS CHAIN ELONGATION
Synthesis of cDNA from Viral RNA by Reverse Transcriptase
In order for a cDNA strand to elongate, the phosphate group of a free deoxyribonucleotide bonds to the hydroxyl (OH) on the 3 prime carbon of the deoxyribose of the last deoxyribonucleotide in the growing cDNA strand.
What happens then?
Competitive inhibition of Reverse Transcriptase activity: AZT-triphosphate competes with thymine deoxyribonucleotide triphosphate for reverse transcriptase.
Insertion of AZT-monophosphate into cDNA blocks the growth of the cDNA being transcribed from the viral RNA by reverse transcriptase.
Zidovudine (ZDV, AZT) has an
Zidovudine (ZDV, AZT) has an azide (N3) group instead of a hydroxyl (OH) group on its pentose sugar. Once the phosphate group of the zidovudine bonds to OH of the last deoxyribonucleotide in the strand, no further free deoxyribonucleotides can attach. (The phosphate groups of free deoxyribonucleotides can only bond to OH groups, they are unable to bond to N3groups.). This results in an incomplete cDNA/provirus.
Since reverse transcription of virus genome takes place
Since reverse transcription of virus genome takes place in the cytoplasm where the drug appears first and is in highest concentration, the RNA-dependent DNA polymerase (Reverse Transcriptase) of the virus is 100 times more sensitive to AZT/ZDV than the cellular DNA polymerase (in host cell nucleus), resulting in selective activity and low mammalian toxicity.
Major toxicities of AZT/ZDV include
Major toxicities of AZT/ZDV include anemia and granulocytopenia, which can occur in up to 45% of treated patients.
LESS TOXIC TO HOST AND MORE TOXIC TO THE VIRUS
AZT/ZDV is considered as a
AZT/ZDV is considered as a maintenance drug, because, at best, it slows down the progression of HIV infection, but does not eradicate or eliminate the virus.
AZT has been shown to reduce
AZT has been shown to reduce clinical signs in FIV-positive cats when administered at a dose of 10mg/kg twice a day, subcutaneously, for a period of 3 weeks.
Proteases are required to cleave the
Proteases are required to cleave the HIV polyproteins into functional proteins
HIV must use a
HIV must use a HIV encoded enzyme called protease in order to cleave a large Gag- Pol polyprotein (p120), a Gag polyprotein (p55), and an Env polyprotein (gp160) into functional proteins essential to the structure of HIV and to its RNA packaging. The active site of the HIV protease binds to the polyproteins and cleaves them into functional proteins
Proteases inhibitors inhibit the proteases. HIV polyproteins cannot be cleaved
Proteases inhibitors inhibit the proteases. HIV polyproteins cannot be cleaved into functional proteins.- REPLICATION CAN GO FORWARD
Protease inhibitors bind to the active site of the HIV protease and prevent the enzyme from cleaving HIV polyproteins into functional proteins. As a result, HIV can not mature and noninfectious viruses are produced.
The four ‘W’s of immunization:
A. Where?
Primarily populations in endemic areas.
B. When?
If the disease has a distinct “season,” such as seen with Vector-borne agents, immunization just before the season will provide the maximum efficiency.
Outbreak of a nonendemic disease occurs
C. Who?
Population at Risk.
D. Why?
For a program of vaccination to be justifiable, the loss caused by the disease must be greater than the cost of immunization.
Features of a Good Vaccine:
Safe to use
Effective against diverse strains of the same Pathogen
Few side effects
Give long lasting, appropriate protection
Low in cost
Stable with long shelf life (no special storage requirements)
Easy to administer
Inexpensive
Benefit outweighs the risk
Prevention and Control of Viruses
A.1. Live-Attenuated Virus Vaccines:
A.1. Live-Attenuated Virus Vaccines:
A. Vaccination:
Vaccines Produced from Naturally Occurring Attenuated Viruses
The original vaccine, introduced by Jenner in 1798 for the control of human smallpox, utilized cowpox virus. Cowpox virus produced only a mild infection in humans, but offered immunity against the deadly small poxvirus. This is because cowpox is antigenically related to smallpox virus.
Other examples:
Protection of chickens against Marek’s disease using a vaccine derived from a related herpesvirus of turkeys.
Protection of piglets against porcine rotavirus infection using a vaccine derived from a bovine rotavirus.
A. Vaccination:
Life attenuated Vaccines Produced by ______________
A. Vaccination:
Vaccines Produced by Attenuation of Viruses by Serial Passage in Cultured Cells
Most of the live-attenuated(virus looses it’s virulence but keeps it antegenesity— THEREFORE IT HELPS WITH IMMUNITY) virus vaccines in common use today were derived empirically by serial passage of virulent viruses (wild-type or field strains) in cultured cells. The cells may be of homologous or, more commonly, heterologous host origin.
During repeated passage in cultured cells, viruses typically accumulate nucleotide substitutions in their genome, which in turn lead to attenuation.
A. Vaccination:
A.1. Live-Attenuated Virus Vaccines:
Vaccines Produced by Attenuation of Viruses by Serial Passage in Heterologous Hosts
Rinderpest (deadly disease of cattle) and classical swine fever (deadly disease of pigs) viruses were each adapted to grow in rabbits and, after serial passage, became sufficiently attenuated to be used as vaccines.
- in hosts that it doesn’t induce disease- it just looses it virulence
A. Vaccination:
A.1. Live-Attenuated Virus Vaccines:
Vaccines Produced by Attenuation of Viruses by Selection of Cold-Adapted Mutants and Reassortants
Cold-adapted mutant viruses would be safer vaccines for intranasal administration, in that they would replicate well at the lower temperature of the nasal cavity (about 33°C in most mammalian species), but not at the higher temperature (37°C) of the more vulnerable lower respiratory tract and pulmonary airspaces.
Cold-adapted influenza vaccines that contain mutations in most viral genes do not revert to virulence, and vaccines against equine influenza have been developed utilizing the same principle.
A. Vaccination:
A.2. Non-Replicating Virus Vaccines
Vaccines Produced from Inactivated Whole Virions
Inactivated (syn. killed) virus vaccines are usually made from virulent virus
Chemical or physical agents are used to destroy infectivity while maintaining immunogenicity.
Need to contain relatively large amounts of antigen to elicit an antibody response commensurate with that induced by a much smaller dose of live- attenuated virus vaccine.
Killed vaccines usually must be formulated with chemical adjuvants to enhance the immune response
Vaccines Produced from Purified Native Viral Proteins
The virion is solubilized and its components released, including the glycoprotein
spikes of the viral envelope.
Differential centrifugation is used to semi-purify these glycoproteins, which are then formulated for use as so-called split vaccines
A. Vaccination:
A.3. Vaccines Produced by Recombinant DNA and Related Technologies
Vaccines Produced by Attenuation of Viruses by Gene Deletion or Site-Directed Mutagenesis
Subunit Vaccines Produced by Expression of Viral Proteins in Eukaryotic (Yeast, Mammalian, Insect), Bacterial, or Plant Cells
Vaccines Utilizing harmless Viruses as Vectors for Expression of Other (Heterologous) Viral Antigens
Vaccines Utilizing Viral DNA (“DNA Vaccines”)
DIVA (Differentiating Infected from Vaccinated Animals)
Vaccination with the only live attenuated vaccines (LAV) leads to production of an antibody response that does not differ from the antibody response developed after natural infection.
Thus, it is impossible to differentiate animals vaccinated with LAV vaccines from animals infected with the field strain using serology.
How does DIVA work?
DIVA (Differentiating Infected from Vaccinated Animals):
Subunit ‘marker vaccines’ DIVA vaccines have only a portion (subunit) of the pathogen in the vaccine, i.e. has less antigens than natural strains.- ONLY TEST POSTIVE FOR THE MARKER
If antibodies to other parts/antigens of the pathogen not included in the vaccine are detected the animal has been infected with the pathogen.
If only antibodies to the vaccine subunit/antigens are detected, the animal has not been infected. An accompanying diagnostic test allows us to actually make that differentiation.
Vector Control
- source reduction- espesically controlling where they bred
- Biological control- Use of natural enemies to manage mosquito populations., such as predatory fish that feed on mosquito larvae (Gambusia affinis, carps and minnows).
- . Chemical control: Insecticides-
Larvicides: Insecticides that specifically targets the larval life stage of mosquitoes.
Adulticide: Insecticides against adult mosquitoes.
Prevention and Control of Viruses C. Reducing Contact Potential:
Isolation: Applies to animals/persons who are known to be ill with a contagious disease.
Reduces the probability of contact with a susceptible host
Facilitates treatment
Not effective, if detectable pathogen shedding does not occur, as in carriers or
during incubation period.
Relies on Sensitivity of Diagnostic tool.
Quarantine: Applies to those who have been exposed to a contagious disease.
Enforced for the longest incubation period of the disease
Not effective with diseases involving chronically infected healthy shedders.
Population control programs:
Population reduction may be used to control spread of a zoonosis within a
reservoir population.
Biological control of wildlife reservoirs.
Prevention and Control of Viruses
Quarantine & Culling:
To separate and restrict the movement of animals.
Culling (killing) of Animals
Proper Disposition of Culled Animals
D. Protection of Portals of Entry:
Personal protection:
Protective clothing
Repellents
Nets on doors and windows
Mosquito nets
Decontamination
Decontamination is a term used to describe a process or treatment that renders a medical device, instrument, or environmental surface safe to handle.
A decontamination procedure can range from sterilization to simple cleaning with soap and water.
Sterilization, disinfection and antisepsis are all forms of decontamination.
Sterilization
Sterilization describes a process that destroys or eliminates all forms of microbial life/
Pathogens, including highly resistant pathogens, such as Bacteria with Spores.
No degrees of sterilization: an all-or-nothing process
Disinfection
Disinfection describes a process that eliminates many or all pathogenic microorganisms, except bacterial spores, on inanimate objects.
Less effective than sterilization, does not necessarily kill all microorganisms.
Antisepsis
Antisepsis is the application of a liquid antimicrobial chemical to skin or living tissue to inhibit or destroy microorganisms.
Sterilization Methods:
Moist Heat:
- Use of steam. Autoclave (Use of steam heated
to 121°C (250 °F) for at least 15 min in 15 psi pressure).
Dry Heat:
- Hot air oven, at least two hours at 160 °C (320 °F).
Chemical Methods:
- Gases like Ethylene oxide, Ozone. - Chemicals like Hydrogen peroxide
at high concentrations.
Radiation:
- Non-ionizing: Ultraviolet Radiation. - Ionizing: Gamma rays, X-Rays
Sterile Filtration:
- Microfiltration using membrane filters
(pore size <0.2 μm remove most microbes)
Distribution of Transmissible GastroEnteritis (TGE) in Pigs
TGE has been reported in USA, the Americas, Europe and many parts of Asia, primarily in the northern hemisphere. In densely pig farming areas, such as Midwestern USA, TGE is one of major causes of mortality and morbidity in piglets.
Sentinel Surveillance in forecasting Equine encephalitis, a vector-borne disease of horses and humans transmitted by mosquitoes.
- Trapping and testing mosquitoes for viruses
- Use of sentinel animals/birds to monitor presence of viruses, such as chicken.
- Sentinel chicken serology
- Dead bird reporting and testing
5.
Farm Biosecurity:
Farm Biosecurity: Biosecurity on a farm comprises all measures taken to minimize the risk of the introduction and the spread of infectious agents.
2 components:
External biosecurity- Measures taken to prevent an infectious disease from entering or leaving the farm
Internal Biosecurity-Measures taken to combat spread of an infectious disease within the farm
Biosecurity Measures:
A. Housing and Management:
Minimize contact between young and older animals or consecutive production batches.
Maintain optimal Stocking density. High stocking density facilitates disease spread, and
also increases stress, lowering immunity and predisposing animals to infectious disease.
Adoption of the All-in and All-out housing system- when a new batch of animals comes and you isolate them for the period of the incubation period of the diseases that they can have
Biosecurity Measures:
B. Vermin and bird control:
Prevent contact with free roaming animals (e.g. wildlife, cats, dogs, etc.).
Minimize bird contact
Maintain a rodent and insect control program.
Secure all feed storage areas and clean up spilled feed to minimize access by pests. Pasture management, for Microbes and Parasitic Diseases.
Biosecurity Measures:
C. Purchasing policy:
Adopt a Closed Herd System, avoid buying animals from outside. Difficult to follow.
Reduce the number of new animals brought to the farm. More animals-More Risk.
Limit the number of farms or sources from where you are buying the animals.
Determine the vaccination & health status of newly purchased animals and of the herd of origin.
Farms from which you buy animals or semen should have a higher sanitary status
Quarantine or keep newly arrived animals in isolation, away from the main herd.
The quarantine period should be long enough and depend upon the incubation period of important infectious diseases.
Use the quarantine period to test the animals for possible and important infectious diseases.
Vaccinate, if necessary.
Biosecurity Measures:
E. People (Visitors and Farm Workers):
Keep Visitors to the minimum.
Current Health record/history of Visitor and Workers.
Maintain a log book of all entering and leaving the Farm.
Make Visitors aware of farm protection methods. Train and Educate Farm workers.
Discourage visitors from entering the housing and feeding areas, and touching animals.
Ensure supply of clean rubber boots or plastic disposable boots and clean coveralls.
Provide a footbath containing disinfectant before entering Stables.
Insist workers wash their hands before and after handling animals.
Insist workers wear protective plastic or rubber gloves when required, such as for calving cows
Establish a working line. Attend animals in order of increasing age groups, and at the last, visit sick animals.
incubation period
is the time elapsed between infection and when clinical symptoms are first apparent.
What are the 3 ways that you can treat viral diseases?
- Antivirals
- Immune system stimulation:
- Synthesize antibodies or administration of natural antiserum (antibodies)
Antivirals can target which parts of the virus replication cycle?
receptor binding, uncoating, nucleic acid and protein synthesis, assembly, release, or modulating the immune system
Acyclovir is a synthetic nucleoside of….
synthetic nucleoside analog of deoxyguanosine.
It stops elongation of DNA chains
Also, competitive inhibition of Viral DNA polymerase, as acyclovir- triphosphate compete with dGTP for viral DNA polymerase
Targets for Anti-retroviral Therapy
- inhibit protease
- inhibit reserve transcriptase
- inhibit integrase
- inhibit fusion
survillence
tracking a virus through different method of infection
i.e. reservoirs, route/ portal of entry, mode of transmission etc
