SWINE INFLUENZA

Carol G. Woodlief
College of Veterinary Medicine
North Carolina State University
Raleigh, NC 27606

Swine Influenza is an acute, febrile respiratory disease of swine with high morbidity and low mortality. It is commonly known in the industry as Swine Flu, Hog Flu, and Pig Flu. The etiology of Hog Flu is a Type A Influenza virus which belongs to the Orthomyxoviridae family. It is an enveloped, RNA virus with a lipid envelope and a helical nucleocapsid.

There are two structural proteins present on the virus. A Hemagglutinin and Neuraminidase protein. There are 13 different types of Hemagglutinin proteins. This hemagglutin protein is responsible for the attachment of the virus to the host-cell and for fusion of the viral envelope to the host-cell membrane. This results in agglutination of red blood cells. There are 9 different types of the neuraminidase protein. It is responsible for elution of virus from erythrocytes. It may play a role in the release of virus from infected cells.

These proteins play important roles in antibody formation and function. Antibodies to the hemagglutinin are responsible for preventing infection with an influenza virus containing the same hemagglutinin protein. Where as antibodies against the neuraminidase restrict the spread of virus from infected cells.

The antigenic characteristics of the 2 surface glycoproteins are the bases for dividing the virus into subtypes. The four main subtypes which have been isolated in swine are H1N1, H1N2, H3N2, and H1N7. Studies have shown that 30% of the entire swine population in the U.S. have been exposed to H1N1. More specifically, 51% of swine in the north central U.S. have been exposed.

Transmission

Influenza is transmitted primarily pig-to-pig by the nasopharyngeal route. Nasal secretions are laden with virus during the acute febrile stage. The virus is easily carried and spread by avian species, particularly waterfowl, and humans. Care should be taken to prevent spread from and between birds and humans to swine. The virus can be shed for 30 days post-inoculation and has been recovered from clinically normal animals.

Clinical Signs

The clinical signs of Swine Influenza are typical flu-like symptoms. The pigs will develop a fever, (40.5oC - 41.7oC), nasal and ocular discharge, and severe spasms of coughing. They will appear lethargic and become anorectic. These signs will appear very suddenly, spread rapidly throughout the entire barn, and persist for about 7 days. Then the pigs will almost recover about as quickly as they develop clinical signs. These outbreaks typically occur in the fall and early winter, and are more frequently seen in nursery through finishing age pigs.

Pathogenesis and Pathology

It is speculated that pigs maintain the virus by passing the virus to young susceptible animals. The virus is probably in constant circulation but there is not any evidence to confirm the existence of a long term true carrier state. The incubation period for swine influenza virus ranges from 1 to 3 days. The virus is inhaled and deposited on the surface of the lower respiratory tract. Research has shown that a widespread interstitial pneumonia will exist up to 21 days after infection and hemorrhagic lymph nodes will be seen. A major concern with Swine Influenza is the economic loss seen with uneven growth and prolonged finishing time. It has been documented that these pigs will lose from 5 to 12 pounds of body weight over a 3 to 4 week period.

Typically, uncomplicated infections have changes seen in the cranial ventral lung lobes. There is generally a sharp line of demarcation between normal and affected lung tissue. The involved areas are often purple and firm. Some interlobular edema may also be evident. The airways are likely to be filled with blood-tinged fibrinous exudate. Often the bronchial and mediastinal lymph nodes are enlarged. Severe cases may result in a fibrinous pleuritis. Microscopic lesions usually consist of airways filled with exudate, widespread alveolar atelectasis, interstitial pneumonia and emphysema. Peribronchial and perivascular cellular infiltration is also seen.

Diagnosis

Diagnosis of Swine Influenza can be based on clinical signs, virus isolation, histopathological confirmation of lesions, paired serology, and antigen detection.

Treatment

The treatment for Swine Influenza is pretty basic. The primary treatment is supportive therapy. These infected pigs require a dry, clean, dust free environment. We need to decrease any drafts which may be in the barns so that the pigs will be as comfortable as possible. Be sure to avoid any stress to the animals. Do not move any animals or mix animals from different pens. Antibiotics are also essential to treat and control any secondary bacterial infections that usually develop. Water medication does not seem to work as efficiently as parenteral injections to clinically infected animals. Expectorants are commonly used as a herd treatment and are administered in the drinking water.

Prevention and Control

The best way to deal with Swine Influenza is to prevent the occurrence and spread of the disease. There are certain parameters that one can follow that will prevent and control the disease in a herd.

Limited movement ia a herd is essential. Try to avoid purchasing new stock and avoid taking animals to exhibits where they will be in contact with other animals. One should also avoid moving any animals within the barn itself to eliminate any excess stress. If any new herd additions are to be entered into a herd, be sure to isolate these animals, preferably in a separate building. Screen these animals by serology to determine if these animals have been exposed to Swine Influenza, and if so virus isolation should be considered.

Standard sanitary measures can also help prevent and control the spread of virus. Influenza A viruses are inactivated by soap, heat, and formalin. Be sure to clean and disinfect trucks, trailers, and any equipment which may be contaminated. If a group of finishing pigs had Swine Influenza, be sure to clean and disinfect the house completely before the next group enters.

Vaccination is another means of controlling Swine Influenza. There have been commercial vaccines available in Europe for a while which have had considerable variation in antibody response and protection from infection following vaccination. Only within the last year has there been a vaccine licensed for use in swine in the U.S. It was developed by Syntro Vet Incorporated and is an oil-in-water vaccine containing Influenza A, H1N1 subtype. Results from various studies show that vaccinated animal exposed to Swine Influenza Virus have markedly reduced nasal shedding, virus infection in lung tissue, and lung pathology than nonvaccinated animals. Studies have also shown than maternally derived antibody in vaccinated sows protected 5 week old pigs from clinical disease, virus infection in the lung, and lung pathology. Maternal antibody did not prevent nasal viral shedding. There is more research underway to determine when pigs from vaccinated sows can be actively immunized and to determine the duration of immunity.

Suggested Reading

1. Bey, Russ. Influenza Virus. Proc. Allen D. Leman Swine Conference; (1994) 159 -161.

2. Brown, GB, Janis McMillen. Maxivac-Flu: Evaluation of the Safety and Efficacy of a Swine Influenza Vaccine. Proc. AASP; (1994) 37 -39.

3. Brown, IH, DJ Alexander. Swine Influenza Virus in Europe. Proc. AASP; (1994) 246 -249.

4. Chase, Chris, RD Lockwood. Clinical Perspectives on Swine Influenza. Proc. Allen D. Leman Swine Conference; (1994) 162 - 163.

5. Done, Stan, IH Brown. Pathogenesis of Swine Influenza. Proc, Allen D. Leman Swine Conference; (1994) 154 - 158.

6. Easterday, BC, VS Hinshaw. Swine Influenza. In: Leman AD, et al, eds. Diseases of Swine. 7th ed. Ames, Ia: Iowa state University Press, 1992, 359 - 357.

7. Hinshaw, VS. Swine Influenza in the U.S.- The Past and the Present. Proc. AASP; (1994) 244 - 245.

8. Sibbel, Rick, Janet McMillen, GB Brown. Field experiences with Maxivac-Flu. Proc. Allen D. Leman Swine Conference; (1994) 166 - 167.

9. Umbaugh, Jerry, Russ Bey, Randy Simonson, Scott Lang, Mary Larson. Swine Influenza Vaccine Efficacy. Proc, Allen D. Leman Swine Conference; (1994) 164 - 165.

http://mark.asci.ncsu.edu/HealthyHogs/book1994/woodlief.htm