Pathogenesis of influenza

A review of mathematical models of influenza A infections within a host or cell culture: lessons learned and challenges ahead

Most mathematical models used to study the dynamics of influenza A have thus far focused on the between-host population level, with the aim to inform public health decisions regarding issues such as drug and social distancing intervention strategies, antiviral stockpiling or vaccine distribution. Here, we investigate mathematical modeling of influenza infection spread at a different scale; namely that occurring within an individual host or a cell culture. We review the models that have been developed in the last decades and discuss their contributions to our understanding of the dynamics of influenza infections. We review kinetic parameters (e.g., viral clearance rate, lifespan of infected cells) and values obtained through fitting mathematical models, and contrast them with values obtained directly from experiments. We explore the symbiotic role of mathematical models and experimental assays in improving our quantitative understanding of influenza infection dynamics. We also discuss the challenges in developing better, more comprehensive models for the course of influenza infections within a host or cell culture. Finally, we explain the contributions of such modeling efforts to important public health issues, and suggest future modeling studies that can help to address additional questions relevant to public health.

Mechanisms of infection in the respiratory tract

Related to its potential vulnerability the respiratory tract has a very complex and effective defence apparatus. The interaction between these defence mechanisms and certain characteristics of aetiological agents results in a pattern in which initial infections by these agents tend to occur at specific sites in the tract. Infections in which the primary portal of entry is in the upper respiratory tract include Bordetella bronchiseptica and Haemophilus spp in pigs; Pasteurella spp in cattle, sheep, pigs; Mycoplasma spp in cattle, sheep, pigs and poultry; equine herpesvirus 1 in horses; infectious bovine rhinotracheitis in cattle; parainfluenza 3 in cattle and sheep; infectious laryngo-tracheitis and infectious bronchitis in poultry; feline viral rhinotracheitis and calicivirus in cats; Aujeszky's disease virus and swine influenza in pigs; and equine influenza in horses. Infections in which the primary portal of entry is in the lower respiratory tract include Aspergillus fumigatus in poultry and mammals, respiratory syncytial virus in cattle, distemper virus in dogs and adenovirus in cattle and dogs. A fuller understanding of the interactions between an agent and the host at the point of entry would make it much easier to develop effective vaccines and therapeutic agents.

Acute alterations in the regulation of lipid metabolism after intravascular reexposure to a single bolus of homologous virus during influenza B infection in ferrets: possible model of epiphenomena associated with influenza

Metabolic responses occurring 24 h following the secondary haematogenous dissemination of influenza B virus during convalescence from infection were examined in the ferret as a possible model for epiphenomena which can occur following infection with influenza. Among the major changes found were a further rise in the mean fasting serum free fatty acid (FFA) level to three times the control mean value and a 50% drop in the mean serum triglyceride (TG) concentration after the intravascular administration of a single bolus of virus compared to levels found in uninfected or convalescent animals. In adipose tissue, hormone-sensitive and lipoprotein lipase activities were increased six and three-fold, respectively, over mean control values, probably accounting for the changes that were observed in serum lipid concentrations. In the liver, total carnitine palmitoyltransferase (CPT) activity was affected only slightly and the total lipid content of the liver remained unchanged. These findings indicate that 24 h after the intravascular dissemination of homologous virus in a single bolus during convalescence from influenza B infection, major distortions in the regulation of lipid metabolism occur in the ferret. Loss of the synchronous regulation of the two adipose tissue lipases is a significant consequence leading to the mobilization of a large amount of FFA during fasting from both adipose tissue and the circulating plasma TG stores.

The effects of maternal influenzal viraemia in late gestation on the conceptus of the pregnant ferret

Pregnant ferrets were inoculated intra-cardially on day 30 of gestation with influenza virus. The animals were sacrificed on days 5 to 11 after inoculation and the products of conception including the uterus were examined virologically and histopathologically. The results indicate that the initial site of infection of the conceptus is the haemophagous organ and that spread occurs from this site to the endometrium, placental labyrinth and fetus. Lesions in the fetus are confined to the liver and respiratory tract. In the liver they may represent either a viral hepatitis or a secondary response to placental damage resulting in the stimulation of erythropoiesis. In the respiratory tract they first occur in the nasal sinuses and upper airways suggesting that infection is via the amniotic fluid rather than via the blood stream. The relevance of these findings to human pregnancy is discussed.

Thermal stability and interaction with ferret inflammatory exudates of two clones of influenza virus of differing virulence for both ferrets and man

After intranasal inoculation of ferrets with influenza virus the upper respiratory tract infection diminishes during the second day and the onset of this reduction occurs earlier for an attenuated clone (64d) than for a virulent clone (7a) of the recombinant virus A/PR/8/34-A/England/939/69 (H₃N₂). The relevance of pyrexia and the nasal inflammatory response to this reduction in infection has been investigated.

Egg-grown Clone 64d was more thermobile than Clone 7a at normal and pyrexial temperatures when suspended in egg allantoic fluid or phosphate-buffered saline. However, in infected nasal washes, both clones were rapidly inactivated when the washes were incubated at these temperatures.

In vitro tests showed that both clones adsorbed to the phagocytes of peritoneal exudates from uninfected ferrets and nasal inflammatory exudates of ferrets infected with both clones. About 90% of the virus was adsorbed after 30 min at 0° or 370° and only 2-14% of this was recovered after treatment with receptor-destroying enzyme followed by freeze-thawing the cells. In contrast, high recoveries (36-112% of that adsorbed) were obtained from red blood cells that were treated similarly. Significant differences were not detected between the clones in either adsorption by or recovery from phagocytes of the different types of exudates.

Thus pyrexia and the nasal inflammatory cells probably play a major role in the reduction of nasal tract infection but, while pyrexia may have had some influence, no evidence was obtained to indicate that the cells contributed to the earlier reduction of Clone 64d.

Isolation of influenza virus from liver and muscle biopsy specimens from a surviving case of Reye's syndrome

Influenza virus (A/Ohio/7/76) was demonstrated by haemagglutination and by direct electron microscopy in chick embryo fluids inoculated with liver, muscle, cerebrospinal fluid, and nasotracheal secretions obtained from a boy with Reye's syndrome, 8 days after the onset of the upper-respiratory-tract symptoms and at the height of neurological and hepatic involvement. This finding accords with the hypothesis that Reye's syndrome may be an epiphenomenon of influenza-virus infection. It is postulated that the pathogenesis of Reye's syndrome may be related to the other influenzal eipiphenomena including influenzal pneumonia in previously healthy persons and influenza-associated myopathy, both of which, like Reye's syndrome, become clinically evident about 7 days after the first clinical signs of influenza.

The behaviour in ferrets of two closely related clones of influenza virus of differing virulence for man

Clones 7a and 64d of the recombinant influenza virus A/PR/8/34-A/England/939/69(H3N2) which are of different virulence for man as judged by clinical score (7a more virulent than 64d) showed similar differences in ferrets. With intranasal inoculation the approximate 50% minimal infectious doses of both clones were similar (between 10(0) and 10(2) EID(50)) as were their titres in nasal washes 24 h after inoculation and the histologically evident damage they caused in the nasal turbinates. However, clone 7a persisted in the nasal washes more than 64d and produced a more prolonged pyrexia. Furthermore, 7a consistently produced a lung infection which was produced only occasionally by 64d and then to a lesser extent than 7a. In contrast to nasal mucosa, histological damage in the lung was slight with both strains. Differences in replication of 7a and 64d in organ cultures of nasal turbinates appeared only after 24 h incubation. They were not sufficiently large to explain the markedly superior ability of 7a to persist in the nasal tract in vivo. This persistence, which coincides with the production of pyrexia, may be due to a greater ability of 7a to resist induced systemic host defences. Spasmodic isolations of infective virus of both clones were made from extra-respiratory tissues such as liver, spleen and kidney.

Replication of influenza virus in organ cultures of human and simian urogenital tissues and human foetal tissues

A survey of human adult tissues in organ cultures showed that influenza viruses (A/Moscow/1019/65 (h2n2) or a recombinant virus virulent for man (PR/8-A/England/939/69 Clone 7a(H3N2)) could infect uterus, bladder and conjunctiva but not oesophagus under the conditions employed; simian bladder and uterus were also susceptible. These results were similar to those already described for corresponding ferret tissues. Organ cultures of human foetal nasal mucosa, trachea, oesophagus, small and large intestine, and bladder consistently supported replication over 4 days or more with high virus yields. Lung, conjunctiva and umbilical cord were less consistently susceptible and gave lower yields. Placenta and kidney cultures allowed replication of virus in one of 8 and one of 4 experiments respectively, the yields being low and of short duration. Organ cultures of neural tissue (meninges and brain), lymphopoietic tissue (spleen, liver and thymus) and amnion did not support significant viral replication. The results are discussed in relation to possible infection of the foetus in utero with influenza virus.

The localization of influenza virus: minimal infectious dose determinations and single cycle kinetic studies on organ cultures of respiratory and other ferret tissues

A comprehensive survey of ferret tissues in organ culture showed that influenza virus (A/Moscow/1019/65 (H₂N₂)) can infect bladder, uterus, oviduct and conjunctiva in addition to respiratory tissues. Alimentary tract tissues were insusceptible, except oesophagus and pharynx which were infected at high inocula. Muscle, reticuloendothelial tissues, blood vessels and kidney were also insusceptible.

Minimal infectious dose determinations on the susceptible tissues showed significant differences in the ease of initiation of infection in the order nasal turbinates > bladder > uterus > trachea > lung > conjunctiva > oviduct > oesophagus. On the other hand, the susceptible tissues were similar with respect to the kinetics of virus replication over one infection cycle, new virus appearing from all tissues at about the same time (5-6 hours) after inoculation.

Large inocula of virus disappeared very quickly from the blood stream of ferrets in vivo, yet in vitro ferret blood could not inactivate large quantities of virus although it inhibited smaller quantities.

The results are discussed in relation to the degree of confinement of influenza infection to the respiratory tract.