January 28, 2002
Animal Health - Swine Influenza, H3N2


Porcine circovirus and swine influenza virus are newly emerged and recently re-emerged viral pathogens of swine that cause substantial economic loss for swine producers.
Swine influenza subtypes H1N1 and H3N2 have re-emerged as important viral pathogens in swine. Efficient and reliable vaccines and diagnostic reagents are lacking for the H3N2 subtype. Mixed infections with two or more subtypes of influenza virus may yield reassortant viral progeny that escape the animal's existing defenses.
There is a pressing need to develop multivalent vaccines and polyvalent differential diagnostic tests that identify viral subtypes associated with disease outbreaks.


Evolution of swine H3N2 influenza viruses in the United States - Webby RJ, Swenson SL, Krauss SL, Gerrish PJ, Goyal SM, Webster RG, Department of Virology and Molecular Biology, St. Jude Children's Research Hospital. J Virol 2000 Sep;74(18):8243-51.
During 1998, severe outbreaks of influenza were observed in four swine herds in the United States. This event was unique because the causative agents, H3N2 influenza viruses, are infrequently isolated from swine in North America. Two antigenically distinct reassortant viruses (H3N2) were isolated from infected animals: a double-reassortant virus containing genes similar to those of human and swine viruses, and a triple-reassortant virus containing genes similar to those of human, swine, and avian influenza viruses. Because the U.S. pig population was essentially naive in regard to H3N2 viruses, it was important to determine the extent of viral spread. Hemagglutination inhibition (HI) assays of 4, 382 serum samples from swine in 23 states indicated that 28.3% of these animals had been exposed to classical swine-like H1N1 viruses and 20.5% had been exposed to the triple-reassortant-like H3N2 viruses. The HI data suggested that viruses antigenically related to the double-reassortant H3N2 virus have not become widespread in the U.S. swine population. The seroreactivity levels in swine serum samples and the nucleotide sequences of six additional 1999 isolates, all of which were of the triple-reassortant genotype, suggested that H3N2 viruses containing avian PA and PB2 genes had spread throughout much of the country. These avian-like genes cluster with genes from North American avian viruses. The worldwide predominance of swine viruses containing an avian-like internal gene component suggests that these genes may confer a selective advantage in pigs. Analysis of the 1999 swine H3N2 isolates showed that the internal gene complex of the triple-reassortant viruses was associated with three recent phylogenetically distinct human-like hemagglutinin (HA) molecules.
Acquisition of HA genes from the human virus reservoir will significantly affect the efficacy of the current swine H3N2 vaccines. This finding supports continued surveillance of U.S. swine populations for influenza virus activity.


Herd factors associated with the seroprevalences of four major respiratory pathogens in slaughter pigs from farrow-to-finish pig herds - Maes D, Deluyker H, Verdonck M, Castryck F, Miry C, Vrijens B, de Kruif A. Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, University of Ghent, Belgium. Vet Res 2000 May-Jun; 31(3):313-27.
The objective of this study was to investigate sero-epidemiological aspects of Mycoplasma hyopneumoniae (Mh), influenza H1N1 and H3N2 viruses and Aujeszky disease virus (ADV) in fattening pigs from 150 randomly selected farrow-to-finish pig herds. Different herd factors were examined as potential risk indicators for the percentage of pigs with antibodies against the 4 pathogens. The median within-herd seroprevalences of the pathogens were: Mh 76%, H1N1 100%, H3N2 40% and ADV 53%. There was a positive association between the seroprevalences of both influenza viruses, and a negative association between the seroprevalences of ADV and H1N1. The percentage of pigs seropositive for Mh increased with the purchase of gilts and with the season (slaughter date in March-April). The within-herd seroprevalences of both influenza viruses were higher in the case of a higher density of pig herds in the municipality. A higher number of fattening pigs per pen additionally increased the risk of being seropositive for H3N2. The percentage of pigs with anti-gE-antibodies against the wild type ADV increased with higher airspace stocking density in the finishing unit, increasing herd size, increasing number of pig herds in the municipality and slaughter date in March-April.
Increased seroprevalences for these 4 respiratory pathogens were mostly associated with pig density in the herd and its vicinity, the winter period, and with the purchase of gilts. Purchase of gilts, number of fattening pigs per pen and airspace stocking density are risk factors that can be managed directly by farmers striving to attain a high respiratory health status of pigs.

The epidemiology and evolution of influenza viruses in pigs - IH Brown. Veterinary Laboratories Agency-Weybridge, Addlestone, UK. Vet Microbiol 2000 May 22;74(1-2):29-46.
Pigs serve as major reservoirs of H1N1 and H3N2 influenza viruses which are endemic in pig populations world-wide and are responsible for one of the most prevalent respiratory diseases in pigs. The maintenance of these viruses in pigs and the frequent exchange of viruses between pigs and other species is facilitated directly by swine husbandry practices, which provide for a continual supply of susceptible pigs and regular contact with other species, particularly humans.
The pig has been a contender for the role of intermediate host for reassortment of influenza A viruses of avian and human origin since it is the only domesticated mammalian species which is reared in abundance and is susceptible to, and allows productive replication, of avian and human influenza viruses. This can lead to the generation of new strains of influenza, some of which may be transmitted to other species including humans. This concept is supported by the detection of human-avian reassortant viruses in European pigs with some evidence for subsequent transmission to the human population. Following interspecies transmission to pigs, some influenza viruses may be extremely unstable genetically, giving rise to variants which could be conducive to the species barrier being breached a second time. Eventually, a stable lineage derived from the dominant variant may become established in pigs. Genetic drift occurs particularly in the genes encoding the external glycoproteins, but does not usually result in the same antigenic variability that occurs in the prevailing strains in the human population. Adaptation of a 'newly' transmitted influenza virus to pigs can take many years. Both human H3N2 and avian H1N1 were detected in pigs many years before they acquired the ability to spread rapidly and become associated with disease epidemics in pigs.

Antigenic drift in swine influenza H3 haemagglutinins with implications for vaccination policy - JC de Jong, AP van Nieuwstadt, TG Kimman, WL Loeffen, TM Bestebroer, K Bijlsma, C Verweij, AD Osterhaus, andEC Class. Research Laboratory of Infectious Diseases, National Institute of Public Health and the Environment (RIVM), Netherlands. Vaccine 1999 Mar 17; 17(11-12):1321-8.
In order to explore the occurrence of antigenic drift in swine influenza A(H3N2) virus, we examined virus strains from outbreaks of respiratory disease among finishing pigs in the Netherlands in 1996 and 1997 and from earlier outbreaks. In contrast to swine H3N2 strains from the 1980s, the recent isolates did not show significant cross-reactivity with human influenza A(H3N2) viruses from 1972-1975 in haemagglutination inhibition tests. These new strains form a separate branch in the phylogenetic trec of the HA1 parts of HA. We conclude that recently there has been considerable antigenic drift within the swine H3N2 viruses in the Netherlands and Belgium and recommend replacement of the A/Port Chalmers/1/73 (H3N2) strain in the current vaccine by a more recent swine H3N2 isolate.

Evaluation of a protective immunity induced by an inactivated influenza H3N2 vaccine after an intratracheal challenge of pigs - MH Bikour, E Cornaglia, and Y Elazhary. Virology Section, Faculte de medecine veterinaire, Universite de Montreal, Quebec. Can J Vet Res 1996 Oct; 60(4):312-4.
A challenge study was conducted to evaluate the safety and efficacy of an inactivated influenza H3N2 virus vaccine combined with Quil A/Alhydrogel mixture under controlled conditions in piglets. Twenty-four piglets from 12 sows were allocated to 2 groups; injected intramuscularly with 2 doses of the tested vaccine or with PBS at 2 wk intervals and challenged intratracheally with 105TCID50 of the H3N2 swine influenza virus 6 d after the 2nd immunization. Clinical and virological parameters were recorded for 4 d after the challenge. The use of the tested vaccine produced high serum hemagglutination-inhibition titers against the swine H3N2 strain virus. This strong immune response suppressed all clinical signs and viral shedding and reduced pulmonary lesions due to the challenge in the vaccinated group, without causing any secondary effects. Our results suggest that the serum HI titers correlated with the degree of protection induced by an inactivated swine influenza H3N2 vaccine.


H3N2 Subtype Continues to Spread in U.S. Swine Herds; Vaccination Effective, But Producers Urged To Confirm Flu Strain First - MySwine.com
The new H3N2 strain of swine influenza virus is spreading quickly through U.S. hog operations and now accounts for about half the flu cases reported, according to veterinarians speaking at a recent tele-news conference.
"In terms of the relative number of isolates, we're seeing a 50/50 distribution of H3N2 and H1N1,"said Dr. Gene Erickson of the Rollins Animal Disease Laboratory, North Carolina Department of Agriculture, Raleigh.
At the National Veterinary Service Laboratories (NVSL) in Ames, Iowa, "We've had 54% as H1N1 and 46% as H3N2" since the fall of 1998, when the first H3N2 sample was identified, through January 2000, said Dr. Sabrina Swenson, of NVSL.

Schering-Plough Animal Health Introduces New H3N2 Swine Flu Vaccine with Broader Protection: USDA Issues Conditional License - MySwine.com.
After months of closely monitoring the spread and evolution of swine influenza virus in the United States, Schering-Plough Animal Health Corporation has introduced a new vaccine offering broader protection against the new and costly H3N2 subtype.
The inactivated vaccine, which now contains two isolates of the new H3N2 subtype, was issued a conditional license by USDA after the company demonstrated a "reasonable expectation of efficacy." Schering-Plough Animal Health is pursuing full license for the product.

H3N2 strain of swine influenza spreads rapidly in Missouri herds - Missouri Commercial Agriculture News, Spring 2001.
Missouri hog producers have a new disease to contend with, according to Tom Fangman, Commercial Agriculture swine veterinarian at the University of Missouri-Columbia.
"It's a new strain of swine influenza--SIV H3N2--first noted in the United States in North Carolina in 1998 and confirmed to be present in Missouri in the fall of 1999," Fangman says. "The new influenza strain is now being found in hog operations throughout the state.
"I suspect that a third of Missouri swine operations have been infected and that H3N2 will continue to spread," the MU veterinarian predicts.

"What should I do when I know I have the H3N2 strain in my herd is a question producers might ask.

H4N6 influenza virus isolated from pigs in Ontario - Susy Carman DVM PhD, Animal Health Laboratory-Guelph; Chris Olsen DVM PhD, Dept Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI; Margaret Stalker DVM PhD Dipl ACVP, Gaylan Josephson DVM Dip Path, Animal Health Laboratory-Guelph.
Classical H1N1 influenza viruses are the most common influenza A viruses found in swine in Ontario. H3N2 influenza viruses, similar to currently circulating human strains, have been found in Ontario swine, but with much lower frequency. H3N2 influenza viruses recovered from Quebec swine in 1990 have previously been shown to be similar to earlier 1975 human strains, suggesting that pigs act as a reservoir for human influenza virus. Newer strains of H3N2 influenza viruses, which are triple reassortments of human, swine and avian strains, have emerged in the USA over the last few years. In addition, H1N2 reassortments of H1N1 and H3N2 viruses have also been identified in American swine.
In the fall of 1999, the AHL isolated 2 influenza viruses from an Ontario swine herd that could not be typed using either H1 or H3 antisera. The viruses have recently been typed by reference laboratories to be a totally avian H4N6 influenza virus, and to be similar to the influenza viruses found to replicate in the epithelial cells of the gastrointestinal tract of ducks. H4N6 influenza viruses are the most common influenza viruses found in the Canadian wild duck population. The protoype virus has been identified as A/Swine/Ontario/01911/99.

More articles to examine:

Efficacy and safety of Fort Dodge Animal Health's bivalent Swine Influenza Vaccine -Javaraiah Srinivasappa,; Catherine M. Jennen; Deborah A. Champ, Proceedings of the 32nd annual meeting of the American Association of Swine Veterinarians, Nashville, 2001 171-

Characterization and performance of a new bivalent, H1N1 & H3N2, swine influenza vaccine: End-FLUence - Rich Schlueter,; Wei Lu, ; Marc Eichmeyer, Proceedings of the 32nd annual meeting of the American Association of Swine Veterinarians, Nashville, 2001. 213-

Characterization of H3N2 swine influenza viruses in Iowa swine - John David Schneider; Kyoung-jin Yoon, Proceedings of the 32nd annual meeting of the American Association of
Swine Veterinarians, Nashville, 2001. 23-