A mouse model for the study of contact-dependent transmission of influenza A virus and the factors that govern transmissibility.

Influenza A virus transmission by direct contact is not well characterized. Here, we describe a mouse model for investigation of factors regulating contact-dependent transmission. Strains within the H3N2 but not H1N1 subtype of influenza virus were transmissible, and reverse-engineered viruses representing hybrids of these subtypes showed that the viral hemagglutinin is a determinant of the transmissible phenotype. Transmission to contact mice occurred within the first 6 to 54 h after cohousing with directly infected index mice, and the proportion of contacts infected within this period was reduced if the index mice had been preinfected with a heterologous subtype virus. A threshold level of virus present in the saliva of the index mice was identified, above which the likelihood of transmission was greatly increased. There was no correlation with transmission and viral loads in the nose or lung. This model could be useful for preclinical evaluation of antiviral and vaccine efficacy in combating contact-dependent transmission of influenza.

Experimental Infection of Amblyomma americanum (Acari: Ixodidae) With Bourbon Virus (Orthomyxoviridae: Thogotovirus).

Following the recent discovery of Bourbon virus (BRBV) as a human pathogen, and the isolation of the virus from Amblyomma americanum (L.) collected near the location of a fatal human case, we undertook a series of experiments to assess the laboratory vector competence of this tick species for BRBV. Larval ticks were infected using an immersion technique, and transstadial transmission of virus to the nymphal and then to the adult stages was demonstrated. Transstadially infected nymphs transmitted virus to adult ticks at very high rates during cofeeding, indicating the presence of infectious virus in the saliva of engorging ticks. Vertical transmission by transstadially infected females to their progeny occurred, but at a low rate. Rabbits fed on by infected ticks of all active life stages developed high titers of antibody to the virus, demonstrating host exposure to BRBV antigens/live virus during tick blood feeding. These results demonstrate that A. americanum is a competent vector of BRBV and indicate that cofeeding could be critical for enzootic maintenance.

Epidemic threats to the European Union: expert views on six virus groups.

In recent years, several animal disease epidemics have occurred within the European Union (EU). At the 4th Annual Meeting of the EPIZONE network (7-10 June 2010, St. Malo, France), an interactive session was run to elicit the opinions of delegates on a pre-defined list of epidemic threats to the EU. Responses from over 190 delegates, to questions relating to impact and likelihood, were used to rank six virus groups with respect to their perceived threat now (2010) and in 2020. The combined opinions of all delegates suggested that, from the pre-selected list of virus groups, foot-and-mouth disease and influenza are currently of most concern. Delegates thought that influenza would be less of a threat and zoonotic arboviruses would be more of a threat in 2020. Although the virus group rankings should not be taken as definitive, the results could be used in conjunction with experimental and field data, by scientists, policy-makers and stakeholders when assessing and managing risks associated with these virus groups.

Transmission of aerosolized seasonal H1N1 influenza A to ferrets.

Influenza virus is a major cause of morbidity and mortality worldwide, yet little quantitative understanding of transmission is available to guide evidence-based public health practice. Recent studies of influenza non-contact transmission between ferrets and guinea pigs have provided insights into the relative transmission efficiencies of pandemic and seasonal strains, but the infecting dose and subsequent contagion has not been quantified for most strains. In order to measure the aerosol infectious dose for 50% (aID(50)) of seronegative ferrets, seasonal influenza virus was nebulized into an exposure chamber with controlled airflow limiting inhalation to airborne particles less than 5 µm diameter. Airborne virus was collected by liquid impinger and Teflon filters during nebulization of varying doses of aerosolized virus. Since culturable virus was accurately captured on filters only up to 20 minutes, airborne viral RNA collected during 1-hour exposures was quantified by two assays, a high-throughput RT-PCR/mass spectrometry assay detecting 6 genome segments (Ibis T5000™ Biosensor system) and a standard real time RT-qPCR assay. Using the more sensitive T5000 assay, the aID(50) for A/New Caledonia/20/99 (H1N1) was approximately 4 infectious virus particles under the exposure conditions used. Although seroconversion and sustained levels of viral RNA in upper airway secretions suggested established mucosal infection, viral cultures were almost always negative. Thus after inhalation, this seasonal H1N1 virus may replicate less efficiently than H3N2 virus after mucosal deposition and exhibit less contagion after aerosol exposure.

Survival of influenza viruses on environmental surfaces.

To investigate the transmission of influenza viruses via hands and environmental surfaces, the survival of laboratory-grown influenza A and influenza B viruses on various surfaces was studied. Both influenza A and B viruses survived for 24-48 hr on hard, nonporous surfaces such as stainless steel and plastic but survived for less than 8-12 hr on cloth, paper, and tissues. Measurable quantities of influenza A virus were transferred from stainless steel surfaces to hands for 24 hr and from tissues to hands for up to 15 min. Virus survived on hands for up to 5 min after transfer from the environmental surfaces. These observations suggest that the transmission of virus from donors who are shedding large amounts could occur for 2-8 hr via stainless steel surfaces and for a few minutes via paper tissues. Thus, under conditions of heavy environmental contamination, the transmission of influenza virus via fomites may be possible.

Saliva-activated transmission (SAT) of Thogoto virus: dynamics of SAT factor activity in the salivary glands of Rhipicephalus appendiculatus, Amblyomma variegatum, and Boophilus microplus ticks.

Thogoto (THO) virus is transmitted from infected to uninfected ticks when co-feeding on uninfected guinea-pigs, even though the guinea-pigs do not develop a detectable viraemia. This form of non-viraemic transmission is potentiated by a factor(s) secreted by the saliva of ticks and hence has been termed saliva-activated transmission (SAT). The synthesis of the SAT factor by the salivary glands of three ixodid tick species was determined by placing uninfected nymphal ticks on guinea-pigs that were subsequently inoculated with a mixture of THO virus and salivary gland extract (SGE) derived from one of the tick species. SAT factor activity was measured by determining the number of nymphs that acquired THO virus. For the three-host ixodid species, Rhipicephalus appendiculatus and Amblyomma variegatum, maximum enhancement of THO virus transmission was observed when salivary glands were derived from uninfected female ticks that had fed for a period of 6 or 8 days, respectively. In contrast, when salivary glands were derived form uninfected female Boophilus microplus, a one-host ixodid tick species, enhancement of THO virus transmission was observed throughout the tick feeding period. Thus, the natural feeding behaviour of ticks appears to be an important factor in determining the relative importance of these vectors in mediating SAT.

Saliva activated transmission (SAT) of Thogoto virus: relationship with vector potential of different haematophagous arthropods.

Tick saliva (or salivary gland extract) potentiates the transmission of Thogoto (THO) virus to uninfected ticks feeding on a non-viraemic guinea-pig. This phenomenon has been named saliva activated transmission (SAT). To investigate the potential of different haematophagous arthropods to mediate SAT, guinea-pigs were infested with uninfected R.appendiculatus Neumann nymphs and inoculated with THO virus and salivary gland extract (SGE) derived from a range of ixodid (metastriate and prostriate) or argasid ticks, or mosquitoes; control guinea-pigs were inoculated with virus alone. Enhancement of THO virus transmission was observed only when SGE was derived from metastriate ticks. Comparison with the vector potential of these various arthropod species revealed that enhancement of THO virus transmission was specific for ticks which were competent vectors of the virus. The data indicate a correlation between vector competence and the ability of haematophagous arthropods to mediate SAT of THO virus.

Mx1-based resistance to thogoto virus in A2G mice is bypassed in tick-mediated virus delivery.

The interferon-induced mouse Mx1 protein has intrinsic antiviral activity against orthomyxoviruses, including Thogoto virus. Thus, Mx1(+) A2G mice are apparently resistant to infection following needle- or tick-borne virus challenge. However, tick-borne challenge and, to a lesser degree, injection of virus mixed with tick salivary gland extract resulted in virus transmission to uninfected ticks feeding on the A2G mice. The data indicate that immunomodulatory components in tick saliva can overcome a natural antiviral mechanism.