By Dr Tom Spencer, University of Pretoria on behalf of the Pig Veterinary Society
This is the first of two articles on Zoonoses, diseases that affect pigs and humans.
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, Norovirus caused about 36% of all food poisoning cases.
This Norovirus sounds serious but in fact it is not involved in food poisoning directly and definitely not from pork. This virus is actually transferred from man to man and is found in gastroenteritis cases or loosely diagnosed stomach flu. Remember the scare at Johannesburg Hospital in about March this year? Norovirus was incriminated there. It was first found in children in Norwalk, Ohio in 1968, hence its name. It has been cultured in outbreaks from Colleges in the USA. So we must accept it is around, is not a zoonotic pathogen and will appear in areas of high human concentration coupled with decreased immunological competency.
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, sound and wholesome product.
The most important non-bacterial zoonotic diseases for South African humans and pigs are discussed in more detail below. Immuno-incompetent people would be susceptible to far more infections than these.
Beginning with protozoal conditions, these are little organisms that exist in the intestines of pigs and can infect humans.
– Balantidium coli, lives quite happily in the colon of pigs without causing much harm or announcing its presence. This organism may multiple in the colon and form cysts of a group of protozoa or trophozoites that are expelled with the faeces and by poor hygiene the people can be infected. As in the pig the humans do not show much normally but in severe cases there can be diarrhoea.
– Cryptosporidium spps., can infect the intestines of piglets with a shorter lifecycle than that of Coccidia spps. Fortunately, the infection is more common in calves in South Africa. Treatment for animals and humans is not necessary or that effective so just maintain hydration. It is termed “traveler’s diarrhea” but only serious in immune-compromised persons and animals.
– Lyme disease or post-trip illness is caused by Giardia spps. The name indicates one gets sick after the trip because of the longer incubation period (time from infection to clinical signs). Although the pig is incriminated, the infection for humans commonly comes from a waterborne source.
– Toxoplasma gondii, first found in 1952, this parasite shares its zoonotic life with cats and pigs and other mammals are sub-clinically infected. Humans are infected via unhygienic behaviour and accidental infection from cat feaces. Symptoms are mild in all mammals but the ability of the tachyzoites to pass to the uterus does result in an abortion.
The biggest risk of a zoonosis from pigs is the tapeworm or measles. Here measly pork (pork infected with rice grain-like cysts in the muscles) is infective to humans if this pork is eaten. The little cysts develop in the intestine of the human to form a tapeworm. This long worm attaches its head into the lining of the intestine and segments develop along its length. As these segments develop until they mature and become full of infective proglottids. The pig being an omnivore, will eat the human feces as a food source and can become severely infected.
The pig shows no clinical signs but the progress of the infection is to form cysts within the muscles and so the infection can process. It is claimed by speculators that by looking under the tongue of the pig one can diagnose measles in the live animal. This is not so unless the infection is so high that the cysts protrude out of the muscle bundles. The diagnosis is done at the abattoir during routine meat inspection and the number of cases condemned at abattoirs in South Africa has decreased over the years so now it becomes a rarity.
The problem arises where there is no meat inspection. The pig production in the Eastern Cape rural areas has been doing well over many decades and unfortunately tapeworms were introduced so the pigs became infected and without meat inspection and cooking the infected pork well the cycle has been perpetuated. A major medical concern has been the fact that due to poor personal hygiene, especially among children, they have infected themselves after passing a stool and not washing their hands and any contamination off.
The proglottids perceive the gastric acids of the human stomach as the right environment to develop and develop as if in the pig. The onchospheres are released from the intestine and form cysts in the muscles or take an aberrant route and land up in the brain producing a cyst. This is neurocysticercosis and humans have persistent headaches or if the cyst is formed in an important part of the brain there are learning disabilities or muscle control (motor neuron) impairment. Hence education on personal hygiene and meat inspection is paramount to control the infection persisting.
Trichinella spiralis is a similar type of condition but not so severe. In the polar regions there is a polar bear-walrus cycle, in temperate climates a human-pig cycle and fortunately it is rarely found within the tropics. In South Africa there is a source of infection in the warthog found near the Kruger National Park (another reason to stop warthog meat being moved around the country.)
The list of viral agents involved in zoonoses is short and rarely there are cases.
– Rabies is theoretically possible to pass from a pig to human. Most aggressive pigs (read boars) are normally slaughtered as they pose a risk to the handlers.
– Foot and Mouth Disease. Strangely there is a form of FMD in humans caused by a virus (not the usually accepted human connotation). The viruses involved in animals and humans are different so it is thus NOT a zoonosis.
– Swine flu. The world knows what the novel H1N1 flu virus from Mexico did to the pig industry. It must be remembered that H1N1 is primarily a swine flu virus and pigs and humans were both infected by it, the symptoms are mild unless there is a complicating factor. The pig is considered a good incubator for flu viruses as there is close human-pig association in many lands and the infected pig hardly shows any symptoms, except maybe to go off their food for a day or so. Many pigs exported from South Africa have tested positive for human strains of flu, so it is a case of the worker infecting the pig.
– Nipah virus was found in 1998/9 at Nipah in Malaysia. 105/265 people developed an encephalitis which led to a coma and finally death. 90% of the persons affected were closely associated with pigs, being pig farmers. To control the epidemic Malaysia slaughtered 1.1 million pigs and found out later that the fruit bat was the vector transmitting the virus to pigs.