Posted by Administrator on July 16 2009 13:50
Background
Swine influenza virus (SIV) is an important etiological agent involved both in epizootic and enzootic forms of influenza. Clinical signs of the epizootic form are a deep-dry, “barking” cough, fever above 42.2°C and anorexia. The disease is characterized by nearly 100% morbidity. Sows infected during pregnancy may abort as a result of high fever. Clinical signs of the enzootic form of the disease are coughing, fever, anorexia, and poor performance. While this clinical disease description is relatively the same from year to year with occasional variations in severity of morbidity and mortality, the swine influenza viruses themselves continue to change, making SIV both an interesting and challenging pathogen to study. Equally fascinating to study are the cases of influenza virus interspecies transmission and its evolving role as a zoonotic pathogen.
Subtypes of swine influenza viruses
In swine, three influenza virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the world. Identification of swine influenza virus involves detection by PCR or virus isolation with subsequent characterization and determination of subtype by serotyping, subtyping or genetic sequencing. It is through the characterization methods that we are able to see the changes in the virus.
Between 2002 and 2006, the Minnesota Veterinary Diagnostic Laboratory (MVDL) isolated over 3000 swine influenza viruses. 50% were H1 viruses, 30% were H3 viruses, and 20% were untypable by hemagglutination inhibition (HI) serotyping methods. In response to equivocal HI serotyping results, the MVDL implemented molecular diagnostic subtyping and sequencing methods to more accurately define swine influenza virus subtypes and genotypes. Subtyping by multiplex reverse transcriptase polymerase chain reaction (multiplex RT-PCR) was implemented as a routine diagnostic test on all influenza positive samples in November 2005. Using this limited dataset (Nov 2005 to Jul 2006), the subtypes of swine influenza viruses as determined by RT-PCR at the MVDL are H1N1 (60%), H3N2 (26%), H1N2 (4%), mixed H1N1/H3N2 (4%) and untypable (6%). Because virus subtyping does not reveal any information regarding viral origin or genetic variation, the MVDL performs HA nucleotide sequencing at the submitting veterinarian’s request or when subtyping and serotyping results are unclear
H1 swine influenza virus genotypes
As of July 1, 2006, the swine influenza virus H1 hemagglutinin gene sequence database at the MVDL consisted of 641 field isolates of H1 swine influenza viruses. Sequences were collected between 2001 and 2006. Of the 641 field isolates, the hemagglutinin genotypes are as follows:
- 48% reassortant swine H1N1-like variant genotype
- All swine influenza viruses circulating today are reassortant viruses with viral genes of avian, human, and swine influenza origin
- The reassortment has caused a significant antigenic and genetic change in HA, thus resulting in a new variant of H1N1
- 37% swine H1N2-like genotype
- 6% classical swine H1N1-like genotype
- 9% Human H1-like genotype
- The human H1-like viruses are human/swine reassortant viruses
- The human H1-like viruses are only 70% similar in their HA genetic sequence to swine influenza virus reference strains
- The human H1-like viruses are >97% similar to contemporary human influenza virus strains
In the various geographic regions of the US, there are still differences in the predominant HA genotype. In MN and IA, reassortant H1N1-like H1 viruses predominate. In IL/IN/WI and the Southeastern US, H1N2-like H1 viruses predominate. In the West Central US, reassortant H1N1-like and H1N2-like H1 viruses co-circulate. Rarely, classic H1N1-like H1 viruses are found in the Midwest, Southeastern and West Central regions. Human-like H1s are primarily found in the Southeastern US but have spread throughout the US as pigs are transported to other states.
H3 swine influenza virus genotypes
As of July 1, 2006, the H3 hemagglutinin gene sequence database at the MVDL consisted of 425 field isolates of H3 swine influenza viruses. Sequences were collected between 2001 and 2006. Of the 425 field isolates, the hemagglutinin genotypes are as follows:
- 98% are Sw/IL/99 H3N2-like genotype variants (aka Group III H3N2 genotype)
- 1% are Sw/TX/98 H3N2-like genotype (aka Group I H3N2 genotype)
- 1% are Sw/CO/99 H3N2-like genotype (aka Group II H3N2 genotype)
In all geographic regions of the US, the Group III or A/Sw/IL/99-like H3N2 virus variants predominate.
Other influenza viruses of swine
Less commonly identified influenza viruses in swine in North America include avian H3N3 and H1N1 influenza A viruses isolated from pigs in Canada, avian H4N6 influenza viruses from pigs in Canada, and reassortant H3N1 influenza A viruses isolated from pigs in the United States. Highly pathogenic H5N1 avian influenza viruses (H5N1 HPAI), while capable of infecting pigs, do not appear to be well adapted nor established in any pig populations in the world. H5N1 HPAI surveillance is on-going or being implemented to track infections and determine what role, if any, pigs are playing in the transmission of H5N1 HPAI infections in birds and humans
Control of swine influenza
Vaccinations are delivered in an attempt to control influenza. There are several commercially available SIV vaccines available and, in the United States, autogenous vaccines are also obtainable. Efficacy of vaccination varies depending on strain, antigenic dose, maternal antibody interference, co-infections and adjuvant. Studies have been performed at the University of Minnesota that evaluated the protective immunity induced by vaccination with a commercially available, inactivated H1N1 and H3N2 bivalent swine influenza virus vaccine after challenge with recent field isolates of H1N1 or H3N2 swine influenza viruses. In both studies, pigs were vaccinated twice per label directions then challenged intransally with 1ml of 106TCID50/ml per nostril of heterologous H1N1 virus (A/Swine/MN/00040/2002) in trial 1 or with 1ml of 108TCID50/ml per nostril of heterologous H3N2 virus (A/Swine/Colorado/00294/2004) in trial 2. In both trials, pigs were evaluated daily post challenge for clinical signs of respiratory disease or fever and nasal swab secretions were collected for virus detection. Five to 7 days post-challenge, pigs were euthanized and necropsied for evaluation of pneumonia. Clinical signs and pneumonia lesions were reduced in the vaccinated groups when compared to the unvaccinated groups. However, virus was still detectable in nasal swabs taken from some vaccinated pigs. The partial protection provided by vaccines can, in some cases, allow for continuous virus shedding. The continuous virus shedding is of concern as it may allow for the generation of virus variants.
Summary
The diverse influenza viruses may pose a serious problem for both animal and human populations. Both H1N1 and H3N2 viruses have been well established in pigs and have a propensity for reassortment. While genetic analyses of influenza viruses have told us a great deal about the epidemiology and origin of SIVs in our swine population, they do not provide us with information as to whether SIVs with different genetic sequences will react the same way in the pig or whether disease will be prevented by administering the usual vaccine. Studies in Europe and in the United States have shown that even when antigenic (e.g., serologic) and genetic differences exist among several circulating SIVs, vaccination with a single type of SIV vaccine may protect against the SIV variants if the pigs have sufficiently high concentrations of serum antibodies. However, while pigs are protected against clinical signs, virus infection and shedding still occurs but usually at lower rates. Nevertheless, it is important to continue conduct cross-protection and challenge studies with the variant SIVs that are being found in swine worldwide and to determine whether current vaccines will continue to be effective in our swine populations.