8. Asfarviridae & Iridoviridae Flashcards
MEMBERS OF THE FAMILY ASFARVIRIDAE (1 species)
- GENUS: ASFIVIRUS
- AFRICAN SWINE FEVER VIRUS (ASFV)
What disease is hog cholera?
Classical Swine Fever
Arthropod hosts for African Swine Fever Virus
Argasid ticks. Soft ticks of the genus Ornithodoros.
E.g. O. moubata in Africa
O. erraticus in Europe
Species that can act as reservoir hosts but do not show signs of African Swine Fever
African wild swine, such as warthogs (Potamochoerus aethiopicus) and bushpigs (Potamochoerus porcus).
[African swine fever virus infects domestic swine and
other members of the family Suidae, including warthogs, bushpigs, and wild boar (Sus scrofa ferus)]
Geographic distribution of African Swine Fever Virus (ASFV)
ASF is found in countries around the world, particularly in sub-Saharan Africa. More recently, it has spread through China, Mongolia and Vietnam, as well as within parts of the European Union. It has never been found in the United States – and we want to keep it that way.
Virus-encoded genes used for genotyping African Swine Fever Virus
Various virus-encoded genes, including p72 (also referred to as p73), can be used for genotyping the virus
Persistance of African Swine Fever virus (resist to temp, pH, chemicals, survival in tissues)
Temperature: Highly resistant to low temperatures. Heat inactivated by 56C/70 minutes; 60C/20 minutes.
pH: Inactivated by pH < 4 or > 12 in serum-free medium. Serum increases the resistance of the virus, e.g., at pH 13.4—resistance lasts up to 21 hours without serum, and 7 days with serum.
Chemicals/disinfectants: Susceptible to ether and chloroform. Inactivated by 8/1000 sodium hydroxide (30 minutes), hypochlorites—2.3% chlorine (30 minutes), 3/1000 formalin (30 minutes), 3% ortho-phenylphenol (30 minutes) and iodine compounds.
Survival: Remains viable for long periods in blood, feces, and tissues; especially infected uncooked or undercooked pork products. Can multiply in vectors (Ornithodoros sp.).
African swine fever virus is thermolabile and
sensitive to lipid solvents.
However, the virus is very resistant to a wide range of pH (several hours at pH 4 or pH 13), and survives for months and even years in refrigerated meat.
Modes of transmission for ASFV
- Direct transmission occurs when infected animals come into contact with healthy animals through contact with infected saliva, respiratory secretions, urine and feces.
- Indirect transmission (fomites), an example of which is the practice of “garbage-feeding” in which swine become infected when fed food waste contaminated with uncooked pork products
- vector-borne: Soft ticks (Ornithodoros spp.) serve as a vector for transmission, passing the virus to swine hosts when taking their blood meal (any stage).
- In sub-Saharan Africa, ASF is maintained through the sylvatic cycle—reoccurring transfer between bushpigs, warthogs, and Ornithodoros species ticks. Infected ticks are also able to transmit ASFV to other ticks (sexual), to their offspring (transovarial), and from one life cycle to another (transstadial)
Clinical symptoms of African Swine Fever
The acute and peracute forms of African swine fever in susceptible swine are characterized by a severe, hemorrhagic disease with high mortality.
After an incubation period of 15 days (3-21d), (<5d by tick bite) swine develop fever (105–107.6F/40.5-42C), which persists for about 4 days. Starting 1-2 days after the onset of fever, there is inappetence, incoordination, and prostration. Swine may die at this stage without other clinical signs. In some swine there is dyspnea, vomiting, nasal and conjunctival discharge, reddening or cyanosis of the ears, snout and body, and bleeding from the nose and anus. Pregnant sows often abort. Mortality is often 100%, with domestic swine dying within 1-3 days after the onset of fever. Infected adult warthogs do not develop clinical disease.
Pathogenesis ASFV
- African swine fever virus infection of domestic swine results in leukopenia, lymphopenia, thrombocytopenia, and apoptosis of both lymphocytes and mononuclear phagocytic cells.
- The ability to induce cytopathology in macrophages is a critical factor in viral virulence.
- If infection is via respiratory, it replicates first in the pharyngeal tonsils and lymph nodes draining the nasal mucosa, disseminates rapidly throughout the body via a primary viremia; virions are associated with both erythrocytes and leukocytes.
- A generalized infection follows, with very high virus titers (up to 109 infectious doses/mL of blood), and all secretions and excretions contain infectious virus.
- Swine that survive the acute infection may appear healthy or chronically diseased, but both groups may remain persistently infected.
Morbidity and mortality of African Swine Fever
- For all forms of the disease, morbidity rates are very high.
- Mortality rates vary by form:
- For the peracute form, mortality can reach 100% and occur in the absence of any clinical signs within 7–10 days after exposure to the virus.
- The acute form rates approach 100%, often with death occurring within 6–13 days post inoculation.
- The subacute form is dependent on the age of the affected populations; younger pigs have higher rates (70–80%), while older pigs experience significantly lower rates (<20%).
- For those affected by the chronic form of ASF, mortality is typically low.
What protein may responsible for a fatal hemorrhagic disease in domestic pigs but mild, persistent infection in its natural host, the African warthog.
The viral gene A238L encodes a protein that is similar to an inhibitor of the cellular transcription factor, nuclear factor κB (NFκB).
The viral protein inhibits activation of NFκB and thus down-regulate the expression of all of the antiviral cytokines that are controlled by NFκB.
Mechanistically, the A238L protein acts as an analogue of the immunosuppressive drug cyclosporin A, which represents a novel viral immune evasion strategy.
Control and Eradication Strategy for ASFV
The primary control and eradication strategy is stamping-out. There is no effective vaccine.
Stamping-out is the depopulation of clinically affected swine and, as appropriate, swine that are considered to be exposed to the virus. This strategy is supported by critical activities such as quarantine and movement control, biosecurity, surveillance, and cleaning and disinfection
Confirmatory diagnostic testing for ASF in the US
Confirmatory diagnostic testing for ASF will be performed at National Veterinary Services Laboratories (NVSL) Foreign Animal Disease Diagnostic Laboratory (FADDL) approved at Plum Island.
Tests include: virus isolation, antigen detection through direct fluorescent antibody (DFA) testing, enzyme-linked immunosorbant assays (ELISA), and immunofluorescence assays (IFA), and PCR tests
Sample Collection for Diagnostic Testing for ASFV as FAD in the US
Specimen /// Medium
- Serum /// Red top tube (10ml)
- Whole blood Heparin – VI /// Green top tube (10ml)
- Whole blood EDTA – PCR /// Purple top tube (10ml)
- Fresh tissue: spleen, kidneys, and the distal part of the ileum. Also, brain, the maxillary, mesenteric, gastrohepatic and renal lymph nodes should all be submitted /// Separate Whirlpak bag per tissue type
- Set of tissues /// Formalin (10:1)
Shipping preservative (for all of the above): Ice pack
