By Dr Marijke Henton, Iddexx, on behalf of Pig Veterinary Society
This article is the second of two articles (see part 1) below and covers the bacteria involved in the zoonotic diseases of pigs.All living animals are afflicted by disease. Some diseases are specific to one type of animal, others affect groups of related animals, such as rinderpest which only affects cattle and related wildlife, and yet others affect all animals, even fish and birds.
Diseases that are transferred from animals to man are called zoonotic diseases. The zoonotic disease can affect man directly, or be foodborne. Pork is not often implicated as a cause of foodborne disease, as the injunction “cook pork well” has been taken to heart by most cooks.
However, if all the causes of foodborne disease are analysed, pigs and pork could theoretically be implicated in over 40% of cases. The CDC (Center for Disease Control) in 2003, found that in America, Salmonella caused 26% of all food poisoning cases [fig 1]. Salmonella is commonly found in the intestine of pigs. The other causes of foodborne disease that pigs might carry, as illustrated in fig 1, are Clostridium perfringens (4%), E. coli (6%) and Campylobacter (4%). These last three are also common in the intestine of pigs.
People vary in their susceptibility to disease. Healthy young adults, on a good diet, are far more resistant to most diseases than the very old and the very young. Pregnant women may be more susceptible to certain diseases, and certain genetic groups are far more susceptible to some diseases. People who are immuno-incompetent are naturally also far more susceptible to most diseases. Immuno-incompetence includes AIDS, of course, but people who are on cortisone, those on cancer treatment, people with transplants, those who have had their spleens removed, and alcoholics are also immuno-incompetent.
Zoonotic diseases caused by pigs can therefore affect the farm workers, pig handlers, abattoir workers, those who consume pork and anyone else who comes into contact with any part of a pig. Improved bio-security and hygiene are crucial in controlling zoonotic diseases, ensuring that farm workers are protected and that pork is a safe product.
The most important zoonotic diseases for South Africa are discussed in more detail below. Immuno-incompetent people would be susceptible to far more infections than these, and there are a few, such as ringworm and anthrax, which are so rare in South African pigs, that they have not been discussed further.
Great strides have been made in recent years by farmers determined to improve hygiene and bio-security on their farms. The decrease in the number of pigs positive for Salmonella during the last 15 – 20 years is remarkable. Salmonella is a good benchmark bacterium to use for disease control schemes. As it is found in the intestine, together with E. coli, Clostridium perfringens and Campylobacter, decreasing Salmonella also leads to decreases in the other bacteria and viruses carried in the gut. It is therefore no surprise that Salmonella is the target organism of many hygiene programmes.
There are about 2, 500 serotypes of Salmonella, and they are very variable in pathogenicity, which means they vary in their ability to cause disease. Some strains of Salmonella are of very low virulence, and have never been implicated in any disease in any animal.
The tests demanded by international bodies are for the whole group of Salmonella, and there is no leniency if an environmental strain instead of a pathogenic one is isolated. To pass the tests, no Salmonella may be present. This makes the test so much more severe, but the benefits are also greater. By controlling Salmonella on your farm, you are also diminishing all the other diarrhoea causing bacteria, viruses, worms, and so forth.
Salmonella causes both enteritis and systemic disease. Some serotypes are specific for one species of animal, such as Salmonella Choleraesuis, which only affects pigs, and Salmonella Dublin, which only affects cattle. Such strains are not considered to be zoonotic, but all the others are.
The most common pathogenic Salmonella in South Africa, in all farm animals, including pigs, is Salmonella Typhimurium, and Salmonella Typhimurium is the second most common Salmonella in man. This is because it is carried by rats and mice, and seldom causes disease in the rodents. The carriers spread the infection via the feed. Rodents are attracted to feed, at all stages of processing. Salmonella contamination could affect the primary feed ingredients, the feed mill, food stored on the farm and also spilled feed in the pens.
Pelleted food is free of Salmonella, but pellets could become contaminated after processing. As rodents consume the feed, they leave their faecal pellets containing the infecting Salmonella scattered about the food. Rodents may carry any of the 2, 500 serotypes of Salmonella, and not necessarily the pathogenic Salmonella Typhimurium. Other sources may be insects such as flies and cockroaches, birds and man. Insects and birds can visit a large number of feed troughs, moving from pen to pen, in a short time.
Man can become infected directly, by coming into contact with diseased or carrier pigs, being in contact with their dung, or via poorly cooked, unhygienic pork.
Salmonella can multiply in the environment, requiring warmth and moisture. The numbers change dramatically if there are climate changes, or if the cleaning cycle is interrupted. The most common route of infection is the faecal/oral route, meaning that Salmonella which may be present in the intestine of any animal, including a littermate, is eaten by another pig.
Salmonella is also a survivor in the environment. It can live for 10 months in anaerobic lagoons, a year in dust, and two years in dry feed. It survives for 56 days in clean water. Salmonella lives twice as long in the winter than in the summer.
E. coli and Clostridium perfringens
These two are similar to Salmonella in that they are also found in the intestine, there are many different types of each, and they are also resistant in the environment. E. coli causes diarrhoea in infants and toddlers, and is also the cause of traveller’s diarrhoea. Clostridium perfringens produces toxins which cause enteritis, and it can also be associated with gangrene.
A common food poisoning strain, E. coli 0157, which is usually carried by cattle, is definitely present in pigs in South Africa. Virulence factors which affect man, such as Stx toxins, verocytotoxins and enterotoxins, are all common in pig E. coli strains isolated in South Africa.
Campylobacter jejuni and Campylobacter coli
Both are commonly found in the intestine of pigs. Pigs are relatively resistant to the effects of Campylobacter, but people easily become ill, showing diarrhoea after only consuming a very few bacteria. Young adults are particularly susceptible. Campylobacter jejuni has been associated with Guillain-Barré syndrome, where the person becomes paralysed, as well as with reactive arthritis.
Yersinia enterocolitica is common in pigs, rodents and dogs. Pigs are asymptomatic carriers, and carry Yersinia in the throat, without showing any symptoms. Even though pigs carry relatively more pathogenic strains than harmless strains, disease in pigs is rare. Yersinia occurs more frequently in cold climates, as it can multiply at 4oC as well as at higher temperatures. Yersinia causes enteritis, liver damage and can be associated with systemic infections.
Brachyspira pilosicoli is less virulent for pigs than the well-known Brachyspira (Treponema) hyodysenteriae, which causes swine dysentery. Brachyspira pilosicoli infects a broad range of animals, including pigs, man, mice, rats, dogs and chickens, causing dysentery. It is definitely present in South Africa. The dysentery in pigs is milder than that caused by B. hyodysenteriae, and the infection may be misdiagnosed in pigs.
Leptospira is associated with water and wet conditions. It commonly causes abortions in pigs, but it can cause a wide range of infections. There are a number of different Leptospira serovars or serotypes.
These are quite species specific, which means that certain serotypes have specific hosts. The hosts are divided into maintenance hosts and accidental hosts. A maintenance host is easily infected by the specific serotype, only a low dose of bacteria is necessary to cause disease, and the animal shows mild or no symptoms. Accidental hosts are resistant to infection, and require high numbers of bacteria before they become ill. Once they are infected, disease is severe. Infected people can show such severe disease, that a haemorrhagic disease such as Congo Fever can be confused with it.
Streptococcus suis is very common in pigs in South Africa. Most pigs carry it in their tonsils. Only young pigs are usually affected, and it causes various purulent infections, such as septicaemia, meningitis, arthritis and pneumonia. It causes meningitis and septicaemia in man, usually only in people, especially the young, who are in close association with pigs, such as farmers and butchers.
This is the cause of Diamond Skin disease in pigs. It is also resistant in the environment in the same way that Salmonella is, and can also multiply at environmental temperatures. It causes erysipeloid in man, which is a specific type of wound infection. The erysipeloid infection, usually limited to the skin, may spread to the local lymph nodes and joints. Healing is slow, and recurrence of the infection is common in man.
Brucella suis does not occur in South Africa, but it is common in South America and South East Asia. It has been reported from Mozambique. It is associated with abortions. It is the second most virulent Brucella for man, after Brucella melitensis, the cause of Malta Fever. The recent outbreaks of PRRS, Foot and Mouth disease and Classical Swine Fever, which probably all originated from ships docking at our ports, makes Brucella suis a real threat to the pork industry in South Africa.
The other recently imported diseases were not zoonoses, but Brucella is a severe threat to the health of man, and the Department of Health as well as the Department of Agriculture, Forestry and Fisheries would be involved in any control programme. Having two different Government Departments involved, would complicate control measures greatly. Brucella canis, which was always absent from South Africa, has recently been isolated from 3 dogs in the Western Cape area. The method by which the infection entered South Africa is unknown. There is a real possibility that a Brucella suis infection could also enter South Africa in the same way as the viral infections or some unknown way.
Causes of Foodborne disease CDC, 2003