Swine Influenza Virus: A focus on the respiratory syndrome
By Gomez, G., Ferro, P., and B.T. Velayudhan
Three, approximately two-month-old (two females and one male), pigs of unspecified breed were submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for necropsy and ancillary testing. The owner reported multiple deaths in animals of various ages over the past 3 to 4 weeks due to respiratory disease. According to the owner, the clinical signs started soon after Hurricane Harvey. Animals responded poorly to treatment with Nuflor, Draxxin, and Naxcel. Clinical signs included labored breathing and deep cough. The history did not specify the size of the herd and the percentage of animals affected and whether any recovered. Gross examination of all three animals revealed significant lesions only in the lungs. The cranioventral aspect (approximately 50% of the lung tissue) of the lungs of one animal (animal #1) was red to dark-red and firm. The lungs of the remaining two animals (animals #2 and #3) were diffusely red to dark-red and slightly firm. On microscopic examination, the lung of animal #1 contained areas where the lumen of the airways was distended with a large number of neutrophils admixed with necrotic debris. This inflammatory infiltrate extended to the surrounding alveolar spaces. The alveolar septa in other areas of the lung were expanded with macrophages and lymphocytes. In the heart of animal #1, the epicardial surface was lined with fibrin admixed with neutrophils. The lung of animal #2 had alveolar septa that were thickened with lymphocytes and plasma cells and the airways were surrounded with moderate numbers of lymphocytes and plasma cells. The airways in the lung of animal #3 were distended with a large number of neutrophils admixed with necrotic debris that extended to the surrounding alveolar spaces. The alveolar septa in the surrounding tissue were thickened with moderate numbers of lymphocytes and plasma cells. Multifocally, the alveolar spaces were lined with hyperplastic type II pneumocytes. Testing (PCR) on fresh tissue of all three animals was negative for porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus 2 (PCV-2) but positive for swine influenza virus (SIV). Lung tissue from animal #1 also tested positive for Mycoplasmasp. on PCR. Bacterial culture on the lung of all three animals resulted in the isolation of Candida albicansand Escherichia coliwith Pasteurella multocidaas the primary bacterial agent recovered from the lung of animal #1.
Respiratory Disease in Swine
Respiratory disease is one of the most common disease syndromes affecting the swine population. Proper treatment and management rely upon a prompt and accurate diagnosis. However, arriving at a diagnosis starts with gross and microscopic lesions that suggest an infectious disease agent that is confirmed with additional testing, most commonly molecular and/or serological testing. The most common organisms associated with respiratory disease in swine are PRRSV, SIV, PCV2, P. multocidaand Mycoplasma hyopneumoniae. Streptococcus suis, Actinobacillus pleuropneumoniaeand Haemophilus parasuishave also been implicated in the Porcine Respiratory Disease Complex (PRDC).
Respiratory disease due to influenza was first detected during the 1918 Spanish flu pandemic; however, virus isolation from nasal discharge in pigs was not done until 1931. Swine influenza is caused most frequently by a Type A influenza virus whose genome is segmented (8 segments) negative sense single stranded RNA belonging to the family Orthomyxoviridae. Important from an epidemiological standpoint, influenza viruses undergo frequent point mutations and, less frequently, genetic shift resulting in new strains. Influenza viruses are further subtyped based on two surface proteins: hemagglutinin (HA) and neuraminidase (NA). There are sixteen different HA (H1-16) and nine different NA (N1-9) which allows for 144 possible combinations (e.g. H1N1, H2N3, H3N1, H3N2, H7N9). The HA protein plays an important role in the attachment to the host’s cell and subsequent cellular internalization and fusion of the viral capsid to the endosome. Type A avian influenza viruses are further characterized based on pathogenicity into low-pathogenic (LPAI) and highly-pathogenic (HPAI). Human and swine strains are not, at this time, characterized in this way. The NA protein acts to prevent aggregation of virus after its release from infected cells and is also considered a factor contributing to virulence. Identification of the HA and NA proteins helps to understand the epidemiology of outbreaks, changes in disease pattern and are also helpful in ensuring that available vaccines are protective against circulating strains. While influenza viruses are typically host specific, there is a low level of cross-species transmission that occurs. The pig is an important and unique host for influenza viruses in that they have receptors for both human and avian influenza strains, therefore are susceptible to infection with avian, human, and swine strains. Historically, swine were considered the “mixing vessel” from which new strains would arise, particularly ones that could affect humans (e.g. pH1N1).
Aerosolized virus or infected secretions are the primary sources of influenza viruses. Once in the susceptible host, viral infection is typically restricted to the respiratory tract in the majority of cases. There are few reports that describe infection of the heart, muscle, lymph nodes and brain in horses, swine, and humans. This disseminated presentation is poorly understood but presumed to be due to viremia (most associated with highly pathogenic strains). The site of viral infection is the epithelial cells lining the respiratory tract that extend from the nasal mucosa to the alveolar spaces. Virus has also be detected in the glandular epithelium of the bronchi.
Disease due to SIV can have two clinical manifestations in swine. The first clinical manifestation is that of a severe, non-fatal disease that spreads rapidly in naïve populations. The second clinical manifestation is a long standing, subtle and insidious disease that forms part of the PRDC in endemically infected herds. Disease due to SIV infection can be exacerbated with stress, abrupt changes in the climate, infection with PRRSV, or other concurrent diseases. Disease outbreaks are more pronounced during the winter in cold regions and the summer in hot climates. The clinical signs of the acute and severe form can include coughing, stiffness, and fever, and it can spread rapidly but have low mortality with affected animals recovering in 5-14 days. The clinical manifestation of the endemic disease shares many features with other primary causes of PRDC, where the primary manifestation is a secondary bacterial bronchopneumonia. Laboratory diagnostics are necessary to distinguish the underlying cause of disease in PRDC.
The most common gross finding of SIV infection in pigs is consolidation and congestion of the cranio-ventral lobules. The affected areas are often clearly demarcated from the normal lung tissue. Emphysema of the adjacent lobules is sometimes seen; however, pulmonary edema may be generalized. The mediastinal and bronchial lymph nodes may be enlarged and edematous. The submucosa of the bronchi and trachea may be moderately expanded with edema and the mucosal surface may be congested. When there is a secondary bacterial infection in the lung, the lobules may be raised, red and firm and may obscure the lesions of SIV.
Microscopically, the major feature of SIV infection is necrosis of the airway epithelium. As early as 24 hrs post infection, the lumen of the bronchioles is distended with neutrophils that are admixed with necrotic debris. Lymphocytes and plasma cells may be seen expanding the interstitium and alveolar septa but also surrounding blood vessels and bronchioles. Mononuclear cell infiltration starts at 48 hrs post-infection. In the majority of the cases, the lesions are only found in the respiratory tract and the draining lymph nodes. Hyperplasia of the airway epithelium is seen in the regenerative stage of the disease.
For a definitive diagnosis, PCR on fresh tissue (trachea, lung, tracheal swab), serology (serum), immunohistochemistry on fixed tissue and virus isolation on fresh lung are tests available to help confirm an SIV infection. TVMDL currently offers necropsies and microscopic examination, virus isolation, serology, and PCR testing on fresh tissue.
To learn more about this case or about Swine Influenza Virus, contact Dr. Gabriel Gomez or Dr. Pam Ferro at the College Station laboratory or Dr. Binu Velayudhan at the Amarillo laboratory. For more information on TVML’s test catalog, visit tvmdl.tamu.edu or call 1.888.646.5623.
- Maxim MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. 6th St. Louis, MO: Elsevier; 2016.
- H. Janke. 2014. Influenza A Virus Infections in Swine: Pathogenesis and Diagnosis.Veterinary PathologyVol 51 (2): 410-426.