Etiological and Serological Studies in Epidemic Influenza

SEROLOGICAL EVIDENCE FOR THE OCCURRENCE OF INFECTION WITH HUMAN INFLUENZA VIRUS IN SWINE

Antibodies capable of neutralizing human influenza virus were present in the sera of old swine on two New Jersey institution farms, but absent from the sera of young swine on the same farms. The old animals had lived through the winter of 1936-37 in which outbreaks of upper respiratory tract disease were prevalent among the human inmates of the two institutions, while the young swine studied were born long after these outbreaks. It is believed that the swine whose sera neutralized human influenza virus had undergone an unrecognized human influenza virus infection acquired from man. The possible bearing of these observations upon the theory that swine influenza was originally of human origin is discussed.

STUDIES ON THE NASAL HISTOLOGY OF EPIDEMIC INFLUENZA VIRUS INFECTION IN THE FERRET : I. THE DEVELOPMENT AND REPAIR OF THE NASAL LESION

A study has been made of the nasal histology in normal ferrets and in ferrets during and after infection with epidemic influenza virus. During the acute stage of infection the respiratory epithelium of the nasal mucous membrane undergoes necrosis with desquamation of the superficial cells and exudation into the air passages, and an inflammatory reaction occurs in the submucosa. Repair begins on the 4th day after infection, and from the 6th to the 14th day the respiratory area is covered successively by a transitional, a stratified squamous, and finally a stratified columnar epithelium. By the 21st day after infection the epithelium has been largely restored to normal but repair in the submucosa and cartilage is still in progress. The respiratory mucosa is substantially normal in structure 1 month after infection although minor abnormalities of cellular arrangement and type can still be distinguished.

STUDIES ON THE NASAL HISTOLOGY OF EPIDEMIC INFLUENZA VIRUS INFECTION IN THE FERRET : III. HISTOLOGICAL AND SEROLOGICAL OBSERVATIONS ON FERRETS RECEIVING REPEATED INOCULATIONS OF EPIDEMIC INFUENZA VIRUS

A study of the respiratory mucous membrane was made in the turbinates of ferrets which had received repeated inoculations of influenza virus. There was no evidence that persistent immunity is related to the presence of a structural modification of the respiratory epithelium. In fact, the respiratory epithelium in fully immune animals differs histologically only in minor respects from that of the normal, untreated ferret. On the other hand, a functional difference exists between the normal and the previously infected animals as evidenced by a marked acceleration of the repair process in the latter. Serological studies at the time of reinfection, 4 months or more after the previous attack, indicate that a relation exists between the height of antibody titer and resistance. The degree of immunity is probably a product of serological immunity and the rate of tissue repair. The implications of these studies to the problem of influenza in man are discussed.

NEUTRALIZATION OF EPIDEMIC INFLUENZA VIRUS : THE LINEAR RELATIONSHIP BETWEEN THE QUANTITY OF SERUM AND THE QUANTITY OF VIRUS NEUTRALIZED

A linear relationship exists between the logarithm of the quantity of epidemic influenza virus neutralized and the logarithm of the quantity of antiserum which is capable of achieving this result. This relationship is the same for the serum of a ferret convalescent from experimental influenza as for the serum of a rabbit immunized with the virus. By means of the linear relationship between virus and antiserum it is possible to determine a fixed, rather than a relative, value for the neutralizing capacity of a serum.

AN UNIDENTIFIED VIRUS PRODUCING ACUTE MENINGITIS AND PNEUMONITIS IN EXPERIMENTAL ANIMALS

An infectious agent is described which belongs apparently to the class of filtrable viruses, but which, on the basis of the evidence at hand, is not to be identified with any virus previously described. The virus has multiple tropisms and is pathogenic for mice, ferrets, and monkeys of both M. rhesus and M. cynomolgos species. Intranasal infection of mice and ferrets causes extensive pneumonic lesions of fatal severity. Intracerebral inoculation of the virus produces in monkeys a lymphocytic choriomeningitis from which the animal recovers, while in mice a rapidly fatal choriomeningitis is produced. Fatal paralysis occurs in a moderate proportion of mice which receive the virus by intraperitoneal or subcutaneous routes, while the remainder become immune to the intracerebral test but not to the intranasal test. Subcutaneous inoculation of mice, monkeys, ferrets, rabbits, and guinea pigs causes local granulomatous induration of the skin with enlargement of the regional lymph nodes. The virus was repeatedly recovered in 1936 from ferrets inoculated with throat washings of patients suffering from an epidemic disease clinically indistinguishable from epidemic influenza. It is impossible, however, to conclude whether the virus is of ferret or human origin. Although possessing many features in common with the virus of lymphocytic choriomeningitis and the virus of lymphogranuloma inguinale, cross immunity tests have failed to yield any evidence that the new agent is immunologically related to either of the aforementioned viruses. For purposes of identification the name virus of acute meningopneumonitis is suggested.

QUANTITATIVE RELATIONSHIPS BETWEEN THE IMMUNIZING DOSE OF EPIDEMIC INFLUENZA VIRUS AND THE RESULTANT IMMUNITY

A direct proportion exists between the concentration of epidemic influenza virus used for intraperitoneal immunization of mice and the degree of immunity to intranasal infection which develops. Mice vaccinated with virus of a given strength resist infection with virus of the same concentration but not more. An irreducible minimum exists since mice vaccinated with less than ten intranasal lethal doses do not develop sufficient immunity to overcome intranasal infection with virus of the same strength. The fact that there exists a limiting threshold for the degree of immunity which a certain strength of virus will induce indicates that the virus does not multiply after intraperitoneal inoculations. In ferrets a state of partial immunity is induced as a result of subcutaneous vaccination with active influenza virus. Vaccination with doses containing 100 or more intranasal infectious units is required for the production of circulating antibodies, protection of the animals from pulmonary involvement, and modification of the severity of the disease. On the other hand, intranasal inoculation with one infectious unit results in a firm, immediate immunity, although the duration of immunity may bear a relation to the severity of the original infection and consequently to the size of the infecting dose. Ferrets in a state of partial immunity resulting from subcutaneous vaccination, or from the waning of a firm immunity following infection, respond to intranasal inoculation of influenza virus with an accelerated production of neutralizing antibodies. The antibody titer under these conditions reaches a much higher level than occurs following a primary infection. Fully immune animals, however, show no further antibody response to a second inoculation.

Serological studies during the 1953 epidemic of influenza A in New York state

A number of workers have shown that antibodies to the influenza A viruses which are demonstrable in the sera of normal adults undergo periodic fluctuations in titre related to epidemic occurrence of the disease. The rise in antibodies accompanying infection and demonstrable by neutralization, agglutination-inhibition and complement-fixation tests is used as a method of diagnosis which is admitted to be of greater sensitivity than actual recovery of virus from the throat. Studies of the population during an epidemic have also revealed the existence of subclinical infection with rise in antibodies comparable to that occurring in those suffering clinical illnesses. But surveys of sera from large samples of the population show no general upward shift in antibody levels unless an actual epidemic occurs (Martin, 1940). The studies made by Francis, Magill, Rickard & Beck (1937), Hoyle & Fairbrother (1937), Rickard, Lennette & Horsfall (1940) and Martin (1940) also indicate the relative impermanence of the enhanced antibody levels consequent upon an epidemic. A relatively rapid decrease in antibody is shown both by the neutralization and complement-fixation tests during the 3–6 months after an epidemic and then a slower fall occurs until the next epidemic again causes a rise in titres. There have, however, been relatively few studies on large samples of populations situated in different geographical areas before and during an outbreak.

Influenza antibodies in the population of the USA; an epidemiological investigation

Sera taken from persons of various ages in 1951 and collected from adults yearly from 1943 to 1951, inclusive, were tested by the haemagglutination-inhibition method with influenza viruses selected to represent the subgroups of each antigenic type. These were influenza A-WS (1933) and PR8 (1934); influenza A-prime-FM1 (1947) and FW-1-50 (1950); influenza B-Lee (1940) and IB1 (1950); influenza C-1233 (1947). The sera tested with influenza A and B viruses were treated with cholera filtrate to remove non-specific inhibitor. Since influenza C virus was not affected by the non-specific substance, the sera tested against this agent were not so treated.Children's sera showed high antibody level, attained at an early age, for FM1 and FW-1-50 viruses, and essentially no antibody for WS or PR8. By contrast, adult sera revealed high antibody content for PR8 and moderate titres for WS and the A-prime viruses. In adult sera, antibody against the PR8 virus increased significantly in 1944, after the 1943-4 epidemic, and remained at a relatively constant level for the eight succeeding years. The antibody pattern for WS was similar to that for PR8, but the values for its titres were only half as great. Antibody against the A-prime strains rose steadily from inconsequential levels in 1943 to high mean titres in 1951. These findings were consistent with virus isolation studies which suggested that the A-prime viruses, such as FM1, were introduced about 1946 and have been continuously prevalent since that time, while the WS and PR8 agents have been recovered only occasionally in recent years. Sera tested with Lee and IB1 viruses showed essentially identical titres. Antibody to these strains was low in the sera of children, indicating that there had been little previous exposure to the B agents. Adult sera showed marked elevation in titre between 1944 and 1946, and the titres have remained at a high level since that time. The increase in 1946 followed the influenza B outbreak of 1945-6; the increase in the previous year occurred in spite of the fact that there was no epidemic. Children developed significant antibody to influenza C in early life, and high antibody levels were observed in adult sera collected over the nine-year period. These observations indicate that the virus was widespread in the population and was prevalent before 1943.

STUDIES ON THE COMPLEMENT FIXATION ANTIGENS OF INFLUENZA VIRUSES TYPES A AND B

Two types of specific particles can be obtained from allantoic fluid preparations of influenza A and B virus. The larger particle which possesses all the attributes of the virus and which shows a sedimentation constant of about 600 S was compared with the smaller component (30 S) by complement fixation technic. The 30 S component can be differentiated from the 600 S particle by the patterns of the optimal antigen-antibody relationships and by cross-absorption of the sera with the two particles. Some of the 30 S-type antigen can be demonstrated in higher concentrations of the 600 S particles by the use of sera containing only anti-30 S; i.e., sera carefully absorbed with 600 S component. Also, upon sonic vibration of the 600 S particle, serologically active material was released which behaved in every respect like the 30 S antigen. The response of human beings to these antigens was found to vary. Antibodies to the 30 S component developed in the majority of subjects exposed to active virus either under epidemic or experimental conditions, but only rarely following vaccination with three different vaccines. In selecting sera without 30 S antibodies a fairly close correlation between the antibody titers obtained by inhibition of hemagglutination and complement fixation with the 600 S component was obtained. The presence of 30 S antibodies prevented such a correlation. The reaction with the 30 S antigen may be of value in the diagnosis and study of the epidemiology of influenza.