Background

The H1N1 form of swine flu is one of the descendants of the Spanish flu that caused a devastating pandemic in humans in 1918–1919. As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza. However, direct transmission from pigs to humans is rare, with 12 cases in the U.S. since 2005.

The flu virus is perhaps the trickiest known to medical science; it constantly changes form to elude the protective antibodies that the body has developed in response to previous exposures to influenza or to influenza vaccines. Every two or three years the virus undergoes minor changes. Then, at intervals of roughly a decade, after the bulk of the world’s population has developed some level of resistance to these minor changes, it undergoes a major shift that enables it to tear off on yet another pandemic sweep around the world, infecting hundreds of millions of people who suddenly find their antibody defenses outflanked. Even during the Spanish flu pandemic, the initial wave of the disease was relatively mild, while the second wave was highly lethal.

In 1957, an Asian flu pandemic infected some 45 million Americans and killed 70,000. Eleven years later, lasting from 1968 to 1969, the Hong Kong flu pandemic afflicted 50 million Americans and caused 33,000 deaths, costing approximately $3.9 billion. In 1976, about 500 soldiers became infected with swine flu over a period of a few weeks. However, by the end of the month investigators found that the virus had “mysteriously disappeared”, and there were no more signs of swine flu anywhere on the post. There were isolated cases around the U.S., but those cases were supposedly to individuals who caught the virus from pigs.

Medical researchers worldwide, recognizing that the swine flu virus might again mutate into something as deadly as the Spanish flu, were carefully watching the latest 2009 outbreak of swine flu and making contingency plans for a possible global pandemic.

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Diagnosis

Your health care provider will probably be able to diagnose your case of flu from a physical examination and a description of your symptoms. He or she may take a chest X ray if there is concern about complications such as pneumonia.

Preventive Care

Annual flu shots are recommended if you:

• Are 50 years of age or older
• Have chronic heart, lung, or kidney disease
• Live in an institution (such as a nursing home)
• Have a weakened immune system
• Have sickle cell anemia

You should not receive the vaccine if you are allergic to eggs.

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Clinical signs

The incubation period of the disease is very short, as little as 12-48 hours. When the virus first enters the herd two or three animals may be observed sick for the first two days, followed by a rapid explosive outbreak of inappetence and clinically very ill pigs. The effects on the reproductive system follow the sudden onset of a rapid spreading respiratory disease with coughing, pneumonia, fevers and inappetence. Acute respiratory distress persists over a period of 7-10 days (depending on the amount of contact between groups of sows). There are three important periods when infection causes infertility. First, if sows are ill in the first 21 days post-service pregnancy their developing embryos may not get established and an increase in 21 day returns results. If pregnancy has been established 14-16 days after mating, and it then fails returns will be delayed. Second if infection occurs in the first five weeks of pregnancy, there could be total embryo mortality and absorption with sows becoming pseudo-pregnant and not in-pig. Litter size may also be affected at this stage due to absorption of embryos. Towards the end of the pregnancy period abortions or late mummified pigs at farrowing may also be experienced. The third major effect is on the boar, where high body temperatures affect semen and depress fertility for a 4 to 5 week period.

SI in large herds may become endemic with intermittent bouts of disease and infertility and different strains may also sequentially infect the herd. Immunity to influenza viruses is often short lived (6 months) and the immunity profile in the breeding herd varies considerably with time.

At a herd level the following may also be seen:

• A sudden and rapid onset of acute illness in sows.
• Coughing and pneumonia spreading rapidly.
• A return to clinical normality over 7-10 days.
• Delayed returns to heat after post-weaning.
• Increased repeats at 21 days.
• Increased repeats outside the normal cycle.
• Increased numbers of sows coming through not in-pig.
• Increased numbers of abortions, particularly late term.
• Increased numbers of stillbirth rates and slow farrowings.
• Occasionally an increase in mummified pigs.

During the phases of high temperatures other diseases present in the herd may be triggered off. A typical example would be an increase in abortions associated with leptospira infection.

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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.

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What You Can Do
First, wash your hands frequently after exposure to animals and avoid contact with ill appearing animals. If you or your family becomes ill with flu-like symptoms, let your doctor know if you have been around pigs that could have the flu. A nose or throat swab is needed to determine if you might be infected with a swine flu virus.

Most cases of influenza in humans are caused by human flu viruses. However, in the unusual event that you are infected with a swine flu virus, the health department will want to talk with you about your illness and make sure that other people you live and work with are not sick with swine flu. Influenza medications are available to treat swine flu illness in people. These medicines should be started in the first 2 days of being ill to be most effective.

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Can humans catch swine flu?
Swine flu viruses do not normally infect humans. However, sporadic human infections with swine flu have occurred. Most commonly, these cases occur in persons with direct exposure to pigs (e.g. children near pigs at a fair or workers in the swine industry). In addition, there have been documented cases of one person spreading swine flu to others. For example, an outbreak of apparent swine flu infection in pigs in Wisconsin in 1988 resulted in multiple human infections, and, although no community outbreak resulted, there was antibody evidence of virus transmission from the patient to health care workers who had close contact with the patient.

How common is swine flu infection in humans?
In the past, CDC received reports of approximately one human swine influenza virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with swine influenza have been reported.

What are the symptoms of swine flu in humans?
The symptoms of swine flu in people are expected to be similar to the symptoms of regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Some people with swine flu also have reported runny nose, sore throat, nausea, vomiting and diarrhea.

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The following steps are potentially useful to reduce transmission of influenza viruses between pigs and people:

• Influenza virus vaccination of pigs – While the swine influenza virus vaccines used today may not induce sterilizing immunity nor completely eliminate clinical signs of infection, vaccination of pigs can reduce the levels of virus shed by infected animals, and thus reduce the potential for human exposure and zoonotic infections.
• Influenza virus vaccination of swine farm workers – The vaccines produced on a yearly basis for the human population contain only human, not swine, strains of influenza viruses. Nonetheless, these vaccines are likely to provide some level of protection against infection with swine viruses of the same hemagglutinin subtype.
  Conversely, vaccination of farm workers will reduce the amounts of viruses they shed if infected during human influenza outbreaks, and thereby limit the potential for human influenza virus infection of their pigs.
• Sick-leave policies – To further reduce the chances for infection of pigs with human influenza viruses, the farm owner should provide sick-leave policies for employees that encourage them to remain away from work when they are suffering from acute respiratory infections. People typically shed influenza viruses for approximately 3-7 days, with the period of peak shedding correlated with the time of most severe clinical illness.
• Ventilation – Ventilation systems in containment production facilities should be designed to minimizere-circulation of air within animal housing rooms. This is important to reduce the exposure of pigs to viruses from other pigs, to reduce their exposure to human influenza viruses, and conversely, to reduce exposure of workers to swine influenza viruses.
• Basic hygiene practices -Workers should change clothes prior to leaving swine barns for office facilities, food breaks or their homes. In addition, hand-to-face contact should be minimized and hand-washing stations should be available throughout the animal housing areas. Influenza viruses spread not just by inhalation of aerosolized virus, but also by eye and nose contact with droplets of respiratory secretions.
  Interspecies transmission among pigs and birds: The global reservoir of influenza viruses in waterfowl, the examples of infection of pigs with waterfowl-origin influenza viruses, the risks for reassortment of avian viruses with swine and/or human influenza viruses in pigs, and the risk for transmission of influenza viruses from pigs to domestic turkeys all indicate that contact between pigs and both wild and domestic fowl should be minimized. The following factors are potentially useful to reduce transmission of influenza viruses between birds and pigs:
• Bird-proofing – All doorways, windows and air-flow vents in swine housing units should be adequately sealed or screened to prevent entrance of birds. Although small birds such as sparrows, swallows, finches, wrens etc. are not thought to be important in the overall ecology of influenza viruses, they may carry influenza viruses from waterfowl feces into barns on their bodies.

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