What you should know about foodborne zoonotic diseases in pigs

Saturday, August 8, 2015

In the first of two articles, veterinary consultant Ernest Sanford reviews the effects of salmonella as a cause of infection in pigs and humans. While clinical disease in pigs is rare, salmonella food poisoning in humans is not

by ERNEST SANFORD

A "zoonotic disease" or "zoonosis" is defined as an infectious disease transmitted from animals (in this cases, pigs) to humans. Direct zoonoses are ones in which the zoonotic agent or pathogen is passed directly from the animal (pig) to humans through media such as the air (for example, the influenza virus).

A large number of zoonotic infections, however, are transmitted via the animal's (pig's) food products (pork) rather than by direct contact with the pig. This is called foodborne transmission.  There are many examples of zoonotic foodborne transmissions emanating from animals, including pigs. I've chosen three bacterial pathogens as examples to highlight – Salmonella, Campylobacter and Yersinia.

I expect everyone will be familiar with Salmonella, but fewer will know about Campylobacter and fewer still – maybe almost none – will be familiar with Yersinia.

I'll be covering Salmonella in this issue of Better Pork and following up with coverage of Campylobacter and Yersinia in a subsequent issue.

Historically, Salmonella infection in pigs has taken second place to Salmonella infection in poultry meat and eggs. The poultry industry has worked assiduously, with success, to overcome this negative image, leaving the pig as the current primary source of Salmonella infections and foodborne disease in humans.

Salmonella spp. infections are common in pigs, but clinical disease is much less frequent. Pigs can be infected by a large number of different serotypes of Salmonella spp. Salmonella Choleraesuis and S. Typhimurium, the most common in pigs, cause very different types of clinical diseases. Salmonella Choleraesuis is a host-adapted Salmonella. It causes septicemic disease in pigs, meaning that the S. Choleraesuis bacterium infects many organs in the pig, resulting in fever, loss of appetite, pneumonia, meningitis, arthritis, diarrhea and death, depending on which organs and systems the bacterium has invaded. Extremities (ears, feet and legs) become blue (cyanotic). Pigs can become long-term, subclinical carriers of S. Choleraesuis. The organism does this by establishing itself in the mesenteric lymph nodes that drain the intestines. The bacteria are then shed in the feces when the pig is stressed. Shedding may be intermittent over an indeterminate period of time, but is ultimately self-limiting.

S. Choleraesuis had been a persistent, intermittent problem in Ontario and Quebec for decades, until about 25 years ago when the organism spontaneously disappeared from pigs in both provinces and, for the most part, has not returned, except for rare, isolated cases. Prior to 25 years ago, it caused outbreaks in individual herds. S. Choleraesuis had never been a problem in pigs in Western Canada and still isn't.

Diarrhea is the primary result of infection by S. Typhimurium. In addition, however, S. Typhimurium is a major cause of food poisoning in humans. In the intestines of pigs, it can contaminate carcasses at slaughter, hence the potential public health concern and source of Salmonella food poisoning in humans from pork.

Food-borne salmonellosis (Salmonella food poisoning) occurs when Salmonella bacteria are ingested in uncooked or undercooked food such as poultry, meats, milk and eggs. Although meat and meat products are the usual source of Salmonella food poisoning, fruits and vegetables (melons, spinach, lettuce), can also do so.

Contamination of pork by Salmonella occurs when feces from pigs shedding Salmonella come into contact with pork. This most often happens when intestinal contents are accidentally spilled onto the carcass at a slaughter plant, but can also occur in the home via cross-contamination after handling raw pork or other meats that are contaminated with Salmonella.

It is estimated that more than four million human Salmonella infections occur in the United States annually, resulting in 20,000 hospitalizations and some 500-600 deaths. Because of under-reporting or non-reporting, as the symptoms abate spontaneously after a period of illness, the actual number of Salmonella infections in humans in the United States might be up to 40 times the number reported above. In Canada, estimates run to more than 87,500 infections annually, resulting in about 4,500 hospitalizations.

Salmonella infection in humans can be completely symptomless, but most commonly consists of diarrhea  (including bloody diarrhea), abdominal cramps, vomiting, body aches and fever of 37.8 C (100 F) to 38.9 C (102 F). Symptoms start about six to 72 hours after eating contaminated food and can last for three to seven days if not treated.

In short, Salmonella infections are common in pigs but clinical disease is uncommon. Clinical disease in pigs is caused primarily by two of the several thousand serotypes of Salmonella; Salmonella Choleraesuis and S. Typhimurium. S. Typhimurium is a major cause of salmonella food poisoning in humans and comes from people eating foods that have been contaminated with S. Typhimurium bacteria. Pork is a contributor to food contamination with S. Typhimurium, as are uncooked or undercooked poultry, other meat products, fruits and vegetables. BP

S. Ernest Sanford, DVM, Dip Path, Diplomate ACVP, is a swine veterinary consultant.