Many people have partial immunity to seasonal H1N1 viruses
Many people have partial immunity to seasonal H1N1 viruses
Swine flu, also known as 2009 H1N1 type A influenza, is a human disease. People get the disease from other people, not from pigs. The disease originally was nicknamed ‘Swine Flu’ because the virus that causes the disease originally jumped to humans from the live pigs in which it evolved.
The virus is a reassortant - a mix of genes from swine, bird, and human flu viruses. Scientists are still arguing about what the virus should be called, but most people know it as the H1N1 swine flu virus.
The swine flu viruses that usually spread among pigs aren’t the same as human flu viruses. Swine flu doesn’t often infect people, and the rare human cases that have occurred in the past have mainly affected people who had direct contact with pigs. But the current “swine flu” outbreak is different.
It’s caused by a new swine flu virus that has changed in ways that allow it to spread from person to person - among people who haven't had any contact with pigs.
That makes it a human flu virus. To distinguish it from flu viruses that infect mainly pigs and from the seasonal influenza A H1N1 viruses that have been in circulation for many years, the CDC calls the virus "2009 H1N1 virus." Other names include "novel H1N1" or nH1N1, "quadruple assortant H1N1," and "2009 pandemic H1N1."
Many people have at least partial immunity to seasonal H1N1 viruses because they've been infected with or vaccinated against this flu bug. These viruses "drift" genetically, which is why the flu vaccine has to be tweaked from time to time.
But the H1N1 swine flu is not the usual "drift variant" of H1N1. It came to humans from a different line of evolution. That means most people have no natural immunity to H1N1 swine flu. The normal seasonal flu shot does not protect against this new virus.
Some people who may have had seasonal H1N1 flu before 1957 might have a little bit of protective immunity against the new virus. That's because seasonal H1N1 flu strains that circulated before 1957 (and which were replaced by the 1957 pandemic flu bug) were genetically closer to the 2009 H1N1 swine flu.
This protection is not complete. While relatively few elderly people have had H1N1 swine flu, many of those who did get the disease became severely ill
H1N1 swine flu are like regular flu symptoms and include fever, cough, sore throat, runny nose, body aches, headache, chills, and fatigue. Many people with swine flu have had diarrhea and vomiting. But these symptoms can also be caused by many other conditions. That means that you and your doctor can't know, just based on your symptoms, if you've got swine flu.
Health care professionals may offer a rapid flu test, although a negative result doesn't mean you don't have the flu. The accuracy of the test depends on the quality of the manufacturer’s test, the sample collection method, and how much virus a person is shedding at the time of testing.
Like seasonal flu, pandemic swine flu can cause neurologic symptoms in children. These events are rare, but, as cases associated with seasonal flu have shown, they can be very severe and often fatal.
Symptoms include seizures or changes in mental status (confusion or sudden cognitive or behavioral changes). It's not clear why these symptoms occur, although they may be caused by Reye's syndrome.
Reye's syndrome usually occurs in children with a viral illness who have taken aspirin -- something that should always be avoided.
Only lab tests can definitively show whether you've got swine flu. State health departments can do these tests. During the peak of the pandemic, these tests were reserved for patients with severe flu symptoms.
U.S. cases of H1N1 swine flu have been in children and young adults. It's not clear whether this will change as the 2009 H1N1 pandemic wanes and the virus becomes a seasonal flu bug.
But certain groups are at particularly high risk of severe disease or bad outcomes if they get the flu:
- Pregnant women are six times more likely to have severe flu disease than women who are not pregnant.
- Young children, especially those under 2 years of age.
- People with asthma.
- People with COPD or other chronic lung conditions.
- People with cardiovascular conditions (except high blood pressure).
- People with liver problems.
- People with kidney problems.
- People with blood disorders, including sickle cell disease.
- People with neurologic disorders.
- People with neuromuscular disorders.
- People with metabolic disorders, including diabetes.
- People with immune suppression, including HIV infection and medications that suppress the immune system, such as cancer chemotherapy or anti-rejection drugs for transplants.
- Residents of a nursing home or other chronic-care facility.
- Elderly people are at high risk of severe flu disease -- if they get it. Relatively few swine flu cases have been seen in people over age 65.
People in these groups should seek medical care as soon as they get flu symptoms.
A striking number of adults who developed severe swine flu complications have been morbidly obese. While the vast majority of extremely obese people suffer respiratory problems and/or diabetes, which make flu worse, obesity itself is now considered a risk for serious 2009 H1N1 flu.
You come into close contact with a person who has the flu -- especially if that person did not cover a cough or sneeze when you were within 6 feet -- you've been exposed. Exposure does not guarantee infection or illness, so there's still a good chance you won't get the flu.
What you should do next depends on your risk for getting severe disease and on the risk of severe disease in others with whom you cannot avoid contact.
If you have any of the conditions that put you at increased risk of severe H1N1 swine flu -- pregnancy, asthma, lung disease, diabetes, heart disease, neurologic disease, immune suppression, or other chronic conditions -- it may be riskier for you to get the flu than for other people.
That also goes for children under age 2 years, young people under age 19 taking daily aspirin therapy, and people over age 65. And if you are a caretaker for an infant under age 6 months, that child is at risk of severe disease if he or she catches the flu from you.
The CDC advises against the use of the anti-flu drugs Tamiflu and Relenza to keep exposed people from getting the flu. That's because most of the few cases of drug-resistant H1N1 swine flu have popped up in people taking Tamiflu to prevent flu.
Transmission between pigs
Influenza is quite common in pigs, with about half of breeding pigs having been exposed to the virus in the US. Antibodies to the virus are also common in pigs in other countries.
The main route of transmission is through direct contact between infected and uninfected animals. These close contacts are particularly common during animal transport. Intensive farming may also increase the risk of transmission, as the pigs are raised in very close proximity to each other.
The direct transfer of the virus probably occurs either by pigs touching noses, or through dried mucus. Airborne transmission through the aerosols produced by pigs coughing or sneezing are also an important means of infection.
The virus usually spreads quickly through a herd, infecting all the pigs within just a few days. Transmission may also occur through wild animals, such as wild boar, which can spread the disease between farms.
Transmission to humans
People who work with poultry and swine, especially those with intense exposures, are at increased risk ofzoonotic infection with influenza virus endemic in these animals, and constitute a population of human hosts in which zoonosis and reassortment can co-occur.
Vaccination of these workers against influenza and surveillance for new influenza strains among this population may therefore be an important public health measure. Transmission of influenza from swine to humans who work with swine was documented in a small surveillance study performed in 2004 at the University of Iowa.
This study, among others, forms the basis of a recommendation that people whose jobs involve handling poultry and swine be the focus of increased public health surveillance.
Other professions at particular risk of infection are veterinarians and meat processing workers, although the risk of infection for both of these groups is lower than that of farm workers.
Interaction with avian H5N1 in pigs
Pigs are unusual as they can be infected with influenza strains that usually infect three different species: pigs, birds and humans. This makes pigs a host where influenza viruses might exchange genes, producing new and dangerous strains.
Avian influenza virusH3N2 is endemic in pigs in China, and has been detected in pigs in Vietnam, increasing fears of the emergence of new variant strains. H3N2 evolved fromH2N2 by antigenic shift. In August 2004, researchers in China found H5N1 in pigs.
The CDC recommends real-time RT-PCRas the method of choice for diagnosing H1N1. This method allows a specific diagnosis of novel influenza (H1N1) as opposed to seasonal influenza. Near-patient point-of-care tests are in development.
Prevention of swine influenza has three components: prevention in swine, prevention of transmission to humans, and prevention of its spread among humans.
Methods of preventing the spread of influenza among swine include facility management, herd management, and vaccination (ATCvet code: QI09AA03). Because much of the illness and death associated with swine flu involves secondary infection by other pathogens, control strategies that rely on vaccination may be insufficient.
Control of swine influenza by vaccination has become more difficult in recent decades, as the evolution of the virus has resulted in inconsistent responses to traditional vaccines.
Standard commercial swine flu vaccines are effective in controlling the infection when the virus strains match enough to have significant cross-protection, and custom (autogenous) vaccines made from the specific viruses isolated are created and used in the more difficult cases. Present vaccination strategies for SIV control and prevention in swine farms typically include the use of one of several bivalent SIV vaccines commercially available in the United States.
Of the 97 recent H3N2 isolates examined, only 41 isolates had strong serologic cross-reactions with antiserum to three commercial SIV vaccines. Since the protective ability of influenza vaccines depends primarily on the closeness of the match between the vaccine virus and the epidemic virus, the presence of nonreactive H3N2 SIV variants suggests current commercial vaccines might not effectively protect pigs from infection with a majority of H3N2 viruses.
The United States Department of Agriculture researchers say while pig vaccination keeps pigs from getting sick, it does not block infection or shedding of the virus.
Facility management includes using disinfectants and ambient temperature to control viruses in the environment. They are unlikely to survive outside living cells for more than two weeks, except in cold (but above freezing) conditions, and are readily inactivated by disinfectants. Herd management includes not adding pigs carrying influenza to herds that have not been exposed to the virus.
The virus survives in healthy carrier pigs for up to three months, and can be recovered from them between outbreaks. Carrier pigs are usually responsible for the introduction of SIV into previously uninfected herds and countries, so new animals should be quarantined. After an outbreak, as immunity in exposed pigs wanes, new outbreaks of the same strain can occur
Author is a microbiologist.