M. H. Collie

Differential distribution of virus and histological damage in the lower respiratory tract of ferrets infected with influenza viruses of differing virulence.

The distribution of four strains of influenza virus [A/PR/8/34 (H0N1) and clone 64d (attenuated for ferrets) and clones 64c and 7a (virulent for ferrets) of the recombinant virus A/PR/8/34--A/England/939/69 (H3N2)] in the lower respiratory tract (trachea, bronchi and the hilar, intermediate and outer alveolar zones of the lung) of ferrets was monitored daily for 4 days after intranasal inoculation. On day 1, some animals had high virus titres in all the tissues but in other animals virus was undetectable, irrespective of the virus strain. Two days after inoculation increase of virus contents of all tissues tended to be restricted. On days 3 and 4, the virulent clones (64c and 7a), in contrast to the attenuated strains (A/PR/8/34 and clone 64d), consistently infected the lower respiratory tissues. However, for all infected animals the virus contents of the hilar zones of the lungs were higher than those in the intermediate zones, while the alveolar zones were relatively free from virus. Quantitative estimations of the mild histological damage occurring in the lower respiratory tract 3 to 6 days after inoculation also indicated that bronchial and bronchiolar tissue were more susceptible to influenza virus than alveolar tissue and that clones 64c and 7a produced more damage than the other two strains. In agreement with the relative viral contents of clones 64c and 7a in the bronchi and in the hilar and intermediate zones of the lung, clone 64c produced more damage than clone 7a in the bronchi and less in the bronchioles of the lung parenchyma.

Distribution of Viral Antigen within the Lower Respiratory Tract of Ferrets Infected with a Virulent Influenza Virus: Production and Release of Virus from Corresponding Organ Cultures

Using fluorescent antibody techniques, a semi-quantitative survey has been made of the distribution of influenza virus antigen in the trachea, main bronchi, and three zones (hilar, intermediate and alveolar) of all four lung lobes of ferrets following intranasal inoculation of a virulent clone (7a) of the recombinant influenza virus A/PR/8/34-A/England/939/69 (H3N2). The results confirm the indications from our previous quantitative surveys of infectious virus and histological damage in these areas, namely that infection is confined largely to airway epithelium and is rare in the alveoli. Furthermore, in the lung zones, viral antigen resided mainly in the bronchial rather than bronchiolar epithelium. In attempts to identify the reasons for lack of alveolar involvement organ cultures of alveolar tissue, from which all major airways had been removed, produced levels of virus similar to cultures of bronchus and trachea and the hilar and intermediate lung zones which contain airway and alveolar tissue. Hence, the lack of alveolar infection in vivo must be due to factors which prevent virus attack of susceptible alveolar cells. However, these organ culture experiments showed that a contributing factor could be very poor release of virus from any alveolar cells that do become infected. In contrast, although cultures of bronchi produced less virus than those of nasal turbinates (the most susceptible tissue in vivo) they released a high proportion of their yield and this ease of release may contribute to spread of infection in vivo.

The localization of influenza virus in the respiratory tract of ferrets: susceptible nasal mucosa cells produce and release more virus than susceptible lung cells.

Infectious virus production by ferret nasal mucosa and lung organ cultures has been monitored in both tissue pieces and medium over 24 h following inoculation with an Asian (H2N2) strain of influenza virus. Freshly prepared cultures of nasal mucosa produced approx. 10-fold more virus per cell than fresh lung cultures. Also the nasal mucosa cells liberated into the medium a greater proportion (mean 31%) of the total virus produced than did fresh lung (mean 6%). Maintenance of lung explants for 24 h prior to inoculation resulted in a 20- to 100-fold increase in the amount of virus released. However, total virus production by fresh and maintained lung was similar. Trypsin did not increase the infectivity of virus released from any of the cultures, indicating that the haemagglutinin in the virus particles was cleaved. Similar results were obtained with a Hong Kong (H3N2) virus strain. Hence, one factor operating in the lower susceptibility of the lung compared with the nasal mucosa in vivo may be a lower capacity of lung cells both to produce and release influenza virus.

Infection of neonatal and adult mice with non-passaged influenza viruses

SummaryIn contrast to adult mouse influenza, infection of neonates with non-passaged influenza viruses (7a and 64c) resulted in approximately 50 per cent mortality. Virus predominated in the upper respiratory tract though the neonatal lung infection was greater than in adults.