In theory, wild pigs could be infected by any pig pathogen and could spread it between farms. Fortunately they don’t normally come into contact with domestic pigs. When they do enter farms, they may be found in the farm or leave their footprints and possibly faeces around the pig buildings inside the perimeter fence, for example in mud or snow. Furthermore, they live in low numbers so individual wild pigs are unlikely to carry or spread pig pathogens which high-health status domestic herds may be free of. For example, wild pigs are unlikely to be infected with Mycoplasma hyopneumoniae. If an individual wild pig or family group did become infected with M. hyopneumoniae they would soon develop an individual and group immunity and eliminate it. On the other hand, in regions where classical and/or African swine are prevalent, wild pigs could be a threat. However, in the author’s limited experience of wild pigs breaking into domestic pig farms in regions free from CSF and ASF, presumably seeking feed, they have introduced no new diseases.
The main biosecurity barriers to wild pigs are the walls and doors of the pig buildings and the perimeter fence. This would have to be strong with the bottom fixed or buried in the ground. Pigs of course can root under fences and push through small gaps.
Birds commonly carry Mycobacterium avium (and sometimes M intracellulare although this is more commonly environmental), salmonellae, Erysipelothrix insidiosa, campylobacters, and numerous other potential pathogens which generally are shed in their droppings. Most of these agents would probably be in most herds already but some may not. For example it was shown in an outbreak of transmissible gastroenteritis in England that starlings spread the virus between herds sometimes within a radius of many kilometres . They carry the virus passively in their intestines. Birds may also carry influenza viruses and transmit it to pigs in which they may mutate or undergo recombination becoming pathogenic to people although the frequency of this is disputed by some authorities. Sea gulls can infect farms in another way by dropping waste human food and contaminated food wrappers derived from garbage particularly central disposal centres into pig farms. It is safer not to site pig farms near garbage dumps.
It is impossible to prevent contamination from birds in outdoor pig units and difficult in some naturally-ventilated open-sided pig buildings but indoor enclosed units and some open ventilation buildings can be protected with bird-proof netting. This also reduces feed cost preventing the loss through birds. If the feed bins are outside, feed spillage must be removed frequently because of bird contamination.
Flies pose a risk. They can become contaminated on their body surfaces but they can also carry infection in their crops. This was demonstrated with Streptococcus suis type 2. Flies ingesting blood, nasal secretions or carcass fluid from a pig which had been clinically infected with S. suis 2 could carry the organism for several days in their crop. Then, if landing on pig feed, they regurgitate crop contents and contaminate the feed. Flies can travel several kilometres between herds particularly with a light breeze behind them and can be guided to pig farms by the farms’ odour.
Perhaps mosquitoes, which provide the main method of spread of Japanese B encephalitis virus, should be mentioned. This virus does little harm to pigs but causes a severe disease in people visiting infected pig herds.
Rats and mice can become infected by numerous pig pathogens.such as salmonella, coliform bacteria and Brachyspira hyodysenteriae. They do not increase the levels of environmental contamination. For example, if their numbers are greatly decreased by rodent control measures, environmental contamination levels do not decrease. The problem is that they perpetuate the infection in the farm. For example, if a pig farm is depopulated and repopulated to eliminate swine dysentery, then all the mice must be killed because strains of B. hyodysenteriae which are pathogenic to pigs can be maintained indefinitely in mouse populations. Mice tend to remain in a pig farm but rats sometimes migrate from one farm to another. It is difficult to guard against invading rats particularly when surrounding crops such as sugar cane or sugar beet are harvested. Regular rodent control is the only method.
Most dogs and cats are not wild but in some regions guard dogs are kept in the pig compound to deter intruders. Cats may be kept for rodent control. Dogs and cats normally pose little disease threat to the pigs although they can carry organisms such as B. hyodysenteria and salmonella. Dogs can shed TGE virus for 14 days. This is why, if kept in pig farms, they must not be allowed to stray outside the compound. If they do, they should not be allowed to return.
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