Pigs and infectious disease
Pigs can be carriers of several infectious diseases that can, in rare instances, be transmitted to humans. The most significant diseases of concern include swine flu, brucellosis, erysipelas, leptospirosis, and infections caused by the bacteria Pasturella multocida. Swine flu, particularly, gained attention during the 2009 pandemic, although it was found that the strain did not originate from pigs. Transmission typically occurs through direct contact with infected animals, making certain occupations, such as farming and veterinary work, at higher risk. In addition, food-borne illnesses like salmonellosis can arise from consuming contaminated pork products. While many of these diseases are of low incidence in the United States, they can have implications for public health, particularly in instances of food safety and occupational exposure. Preventative measures include proper hygiene practices and vaccination for swine flu. Understanding the dynamics of these diseases and their transmission is crucial for those working with pigs and for public health efforts.
Pigs and infectious disease
Definition
Transmission of infectious diseases from pigs to humans is rare and is usually a result of direct contact of humans with pigs. The infectious diseases transmissible from pigs to humans include erysipelas, leptospirosis, brucellosis, swine flu, and Pasturella multocida. Of these infectious diseases, swine flu is of primary concern. The food-borne disease salmonellosis is one exception to direct-contact transmission; it is caused by eating contaminated foods, including meat from pigs.
Direct Contact
The swine flu received its name because of similarities in the genetic makeup of the flu strain with a strain that lived in pigs. The World Health Organization declared a swine flu pandemic in 2009; however, there is no indication that humans contracted this strain from pigs. In fact, this strain has not been observed in pigs. To avoid using this rather misleading name, some have preferred to call swine flu the H1N1 virus, a name derived from the presence of two surface antigens on the virus. Although cross-species (pig-to-human and human-to-pig) infections have been known to occur, these infections were caused by direct contact of humans with pigs and have been limited to local areas.
The genetic code of influenza viruses consists of eight ribonucleic acid (RNA) segments. The H1N1 virus consists of one segment from a human flu segment, two from avian strains, and five from swine strains. This mixing of RNA segments occurs through a process called antigenic shift. If a pig is infected simultaneously by a pig and human flu virus, for example, segments from both species can be incorporated into a new virus with different surface antigens. The host species is then vulnerable to infections because its immune system does not recognize the different antigens. Pigs are particularly susceptible to cross-species infection.
The symptoms of swine flu are similar to other flu infections and include fever, lethargy, lack of appetite, and coughing. Other symptoms can include runny nose, sore throat, nausea, vomiting, and diarrhea.
Practical prevention methods include washing hands frequently; avoiding touching one’s mouth, nose, and eyes; avoiding crowds, especially those in which people are coughing or sneezing; and avoiding drinking or eating foods touched by others. Swine flu vaccines are readily available and can be administered either by injection or by nasal spray.
Four antiviral drugs are approved for the treatment of swine flu; however, the virus has developed resistance to two, amantadine and rimantadine. The effective drugs, oseltamivir andzanamivir, should be taken within forty-eight hours of developing symptoms. One should get an official diagnosis of swine flu because taking the drugs will lead to side effects. Though the most recent H1N1 pandemic ended in August 2010, according to the Cleveland Clinic in 2022, it remains a seasonal flu virus across much of the world.
In 2024, the first case of a pig infected with H5N1 influenza was reported in Oregon. Though the disease primarily affects birds, and has been circulating in wild bird populations for years, it has recently infected cows, mink, and marine animals. Experts are concerned that this might make it more likely for the virus to make the jump to humans, potentially resulting in another pandemic.
Brucellosis is a serious disease in livestock and humans in many regions of the world, but it is now rare in the United States because of veterinary control measures. Humans contract the disease through direct contact with domestic species, especially cattle, sheep, goats, and swine, or through dairy products or meat derived from them. Brucellosis can be chronic or acute and is characterized by intermittent fever, malaise, anorexia, and prostration.
Pasturella multocida causes respiratory disease in swine, and infection in humans is rare. Although respiratory infection from P. multocida has been reported in pig farmers and others who work with pigs, most human cases come from dog or cat bites.
Erysipelas is caused by group A beta-hemolytic streptococci. It is spread to humans only by direct contact with affected pigs by farmers, butchers, or veterinarians. The bacterium typically enters through a person’s wound while that person is handling an infected animal. The disease causes classic fiery-red plaques on the face, although the legs are most often affected. Death is very rare. The disease is typically restricted to isolated cases and has declined in importance since the mid-twentieth century.
Leptospirosis is caused by several Leptospira species. Although swine and other domestic animals can transmit the disease, wild animals are the most important reservoirs. Leptospirosis is spread by contact with the urine of infected animals or with the food, water, or other substances contaminated by the urine of infected animals. The organisms have flagella that allow them to burrow through the skin or mucous membranes. Most cases of leptospirosis are asymptomatic or mild. Although a variety of symptoms can occur, the most serious is a type of meningitis with a severe headache and a stiff neck. A more severe form with multiple organ failure occurs in rare cases. The Centers for Disease Control estimated in June 2024 that roughly one million cases occur worldwide each year, resulting in nearly sixty thousand annual deaths.
Food-borne Diseases
Salmonella infection (salmonellosis) is caused by eating foods contaminated with Salmonella organisms. Poultry and eggs are most commonly implicated, although other meats, including that from pigs, and some fruits and vegetables, can also cause salmonellosis. Many species of Salmonella can cause the infection, whose symptoms include severe diarrhea, nausea, and abdominal pain. Cleanliness is essential to prevention and includes handwashing after handling raw foods.
Impact
Most of the diseases transmitted from pigs to humans are also transmitted from other animals, both domestic and wild. Likewise, most of the diseases described are of low incidence and have little impact on the population of the United States. Some diseases affect only workers who have direct contact with swine or swine meat. Foodborne illness and influenza outbreaks are the exceptions.
Bibliography
"About Leptospirosis." Centers for Disease Control, 24 June 2024, www.cdc.gov/leptospirosis/about/index.html. Accessed 4 Feb. 2025.
Hendrickson, Susan, et al. “Animal-to-Human Transmission of Salmonella typhimurium DT104A Variant.” Emerging Infectious Diseases 10, no. 12 (December 2004): 2225-2227.
“Leptospirosis.” Centers for Disease Control and Prevention, 30 Jan. 2018, www.cdc.gov/leptospirosis/pdf/fs-leptospirosis-clinicians-eng-508.pdf. Accessed 4 Feb. 2025.
Malcom, Kelly. "Why the Bird Flu's Jump to Pigs Is Concerning." Michigan Medicine, 14 Nov. 2024, www.michiganmedicine.org/health-lab/why-bird-flus-jump-pigs-concerning. Accessed 4 Feb. 2025.
“Swine Flu (H1N1): What Is It, Causes, Treatments & Prevention.” Cleveland Clinic, 5 Aug. 2022, my.clevelandclinic.org/health/diseases/23928-swine-flu-h1n1. Accessed 4 Feb. 2025.