Manitoba Agriculture, Food and Rural Initiatives
Health Status as a Risk Factor: A Systems and Unit Perspective
Cate Dewey and Bob Friendship
Departmenet of Population Medicine, University of Guelph,
Guelph, Ontario, N1G 2W1
Introduction
Disease is most important when a naive herd becomes positive. Then the cost of disease is easy to calculate and the need for disease prevention is obvious. But it is too late, the piglets die, the sows abort, the growers cough, and the finishers stop growing. If your herd has every pig disease it is time to repopulate, if not, it is time to formulate a disease prevention strategy. This paper will outline disease prevention strategies.
Why do pigs get sick? The number of sick animals in a farm at one point in time is determined by the number of naive or non-infected animals, the chance of that animal being exposed to enough bacteria particles to become ill, and the length of time it takes for the bacteria to cause clinical problems. A naive herd is one that has not been exposed to the bacteria or virus before. Often we call this an SPF or Specific Pathogen Free farm. These herds are not free of all diseases but are free of specific pathogens. The pathogens in pigs are typically bacteria or viruses. In a naive herd we can assume we have susceptable animals. In other herds susceptable animals are those without immunity to disease. An example of these animals are nursery pigs when the passive immunity has worn off.
What determines the chance of disease? An animal has to be exposed to a large enough number of viruses (or bacteria) for the viruses to multiply in the animal’s body and then cause clinical signs. For example, all animals are exposed to Ecoli bacteria but if a piglet is exposed to enough, they will develop diarrhea. The number of organisms required to cause disease depends on the bacteria or virus, the age of the pig, and the amount of stress in the pig’s environment. A pig in a cold room will need fewer Ecoli bacteria than a pig in a warm room. Pigs infected with TGE will shed the virus in large numbers for 2 weeks and then the volume of viruses shed decreases over time. The incubation period is the time between exposure to the organism when the first clinical signs are evident.
Disease Entry
Incoming pigs are the one most likely source of new diseases into a naive herd. But we should consider other routes as well, to develop a comprehensive security program. General considerations include:
- Location of the herd away from potential sources of infection, including other farms, abattoirs, sales barns, and roadways.
- Totally enclosed facilities that are rodent and bird proof.
- Perimeter fences and locked doors to prevent entry by visitors.
- Disallowing vehicles used to transport pigs from approaching the farm unless previously emptied, cleaned, and disinfected.
- Secure loadouts that prevent pigs from returning to the farm once they have been exposed to the trucks.
- Excluding dogs and cats from the farm complex.
- Farm personnel have no contact with swine outside the herd.
- Keeping people away from other pigs for 24 hours prior to visiting the farm
- Providing a change of clothing and boots for the farm visit.
The chance that a herd will break with a disease is dependent on the probability of bringing enough bacteria or viruses (disease organisms) into the barn to provide an adequate contact for the susceptible pigs.
Pig-to-Pig Spread of Disease
By far, the greatest risk of disease entry occurs from the introduction of pigs unto the farm. There are two important principles involved in preventing disease entry when newly purchased breeding-stock are brought into the herd. Firstly, it is important to know the health status and biosecurity program of the herd selling the gilts and boars.
Table 1. Diseases found in various herd types
| Cesarian pigs | Diseases acquired over time | Long term freedom |
| Ecoli Erysipelas Illeitis Parvovirus |
Enzootic pneumonia Glassers |
APP Mange Rhinitis Swine dysentery |
Breeding-stock supplier herds should be inspected by veterinarians to assess the herd for clinical evidence of disease and rate the biosecurity of the farm by awarding points for such measures as location away from other pig farms, locked doors, no introduction of live animals, and precautions taken to wash and disinfect trucks. The higher the biosecurity score, the more certain the purchaser is that the disease status of the supplier herd will remain the same. Breeding-stock herds can be tested for the presence of diseases using serology, bacterial cultures, and slaughter check evaluations.
Pigs provide the best opportunity for the introduction of disease. Bacteria and viruses that cause pig diseases multiply very well in the pig. Because of this, pigs with clinical disease shed large volumes of disease organisms. Viruses are found in urine, manure, and in the fluid when pigs cough or sneeze. Pigs that carry disease organisms, (perhaps on their tonsils) tend to shed large numbers of disease organisms after they have been stressed. A good example of stress is the transportation of breeding stock animals.
Herds that maintain a "closed" herd policy so that new genetic material is introduced only by artificial insemination, Cesarian-derived piglets, or embryo transfer greatly reduce the risk of introducing disease, but certain diseases can still be spread using these techniques. In all likelihood, embryo transfer is the safest but the most impractical of the three techniques. Diseases that can be transmitted in semen include pseudorabies, PRRS, parvovirus, and leptospirosis.
Methods used to reduce the pig-to-pig transmission of diseases include maintaining a closed herd, purchasing pigs from only one source and isolation and testing of breeding stock prior to introduction to your herd (Tables 2, 3). The cost of a closed herd is reduced genetic diversity which may mean lower productivity as measured by reproductive parameters, weight gain, feed to gain and/or back fat. The advantages of a closed herd include maintaining freedom from diseases that are spread by pig-to-pig contact. Purchasing pigs from multiple sources increases the chance of disease introduction exponentially. If the chance of disease introduction from one source is 5%, the chance from two sources is 25%.
Table 2. Prevention of pig-to-pig transmission of disease.
| Factor | Disease and/or Agent |
Possible | |
| All-in / All-out | Clostridium Enzootic pneuomonia Salmonella |
Rotavirus PRRS |
|
| Artificial Insemination | PRRS (-ve) | ||
| Batch farrow | Chronic TGE | PRRS | |
| Breeding stock testing | Leptospirosis (type) Pasteurella type D Salmonella |
APP PRRS Glassers |
|
| Depopulate & C-section | Most diseases | Illeitis | |
| Isolation unit & testing | TGE | PRRS | |
| Medicated Early Weaning | Atrophic rhinitis M hyopneumonia |
Illeitis ? Strep suis ? |
|
| Segregated Early Weaning | Swine dysentery Atrophic rhinitis |
APP | PRRS ? |
| Single source | Strep. suis | ||
| Vaccines | Atrophic rhinitis Erysipelas Leptospirosis Enzootic pneumonia |
Ecoli Glassers Parvo |
PRRS?
|
Even purchasing from one source can be a risk because the herd may have recently broken with a new disease. To decrease the chance of diseae introduction, reduce the frequency of gilt purchases. To do this you will have to buy gilts of various ages and have sufficient room to house the gilts in your own facility.
To decrease the chance of introducing disease, set up an isolation unit. How long you keep the gilts in isolation will depend on which disease you are trying to prevent and whether or not you wish to include an acclimatization period for the gilts. Each disease has a specific incubation period ( the time from exposure to the disease to the onset of clinical signs) and a time from exposure to a measurable immune response (Table 3). Isolation time provides a good opportunity to prepare the gilts for the breeding herd with vaccination, boar exposure for the onset of puberty, and perhaps breeding with a DIDI boar.
Table 3 Expected time of occurrence of clinical signs and antibodies to appear in weaned pigs exposed to gilts carrying disease agents
ORGANISM/ DISEASE |
Clinical Signs |
Antibodies |
| A. pleuropneumonia | 3 weeks | 30 days |
| Atrophic rhinitis | 6 weeks | 30 days |
| Enzootic pneumonia | 2 weeks | 28 days |
| Leptospirosis | unknown | 30 days |
| PRRS | unknown | 30 days |
| Strep suis | 4 weeks | 40 days |
| Swine Dysentery | 4 weeks | 30 days |
| TGE | 1 week | 14 days |
Procedures for the use of an isolation unit.
- Pigs enter the isolation facility
- Take blood samples for serology on arrival
- Observe pigs for clinical signs of disease
- Expose gilts to weaners from your own facility
- Take blood from gilts prior to entering your barn
- Expose gilts to cull sows and feces from the farrowing and nursery barns to improve the gilt’s immunity to your herds pathogens
Table 4. Appropriate age and sample sizes required for serological testing
| Agent | Age a | prevb / nc |
| APP (serotypes) | 3-5 mos | 8% / 33 30% / 8 |
| Leptospira (serovar) | sows | 10% / 28 |
| Enzootic pneumonia | growers | 10% / 28 |
| PRRS virus | 8-10 weeks | 80% / 6 40% / 11 20% / 23 |
| TGE | 3-6 weeks | 20% / 13 |
a age group with best chance of positive test b minimum prevalence of positive animals c number of animals required to detect at least one test positive animal CF = compliment fixation ELISA = enzyme linked immunosorbent assay LA = latex agglutination IFA = indirect fluorescent antibody |
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The length of time required for isolation is dependent on the incubation time of the disease and the time it takes for a measurable immune response. The animals are tested on arrival at the isolation facility, exposed to naive herd animals after 30 days and retested prior to moving into the herd at 60 days.
Secondary Sources of InfectionThe second best opportunity to transmit disease is via secretions from pigs such as feces, semen, and mucus. Certain diseases such as PRRS are transmitted very well in semen. To prevent PRRS from entering a herd, the semen used for AI must be certified free of virus. Manure is a very good vehicle for disease transmission. Most viruses and bacteria will live for extended periods of time if kept moist in manure or mucus particularly if it is kept cold. For example, TGE is killed rapidly at room temperature but can survive for 6 months at -20 oC. When stored +21 oC for 4 days there was sufficient virus in the manure to cause TGE in susceptible pigs but by 10 days the virus particles had all died. Dead stock removal is very important for a number of reasons. Keeping rendering trucks far from the swine facility as they can harbour large numbers of disease organisms - even those that are not currently in your facilitiy. If you dispose of dead stock on your own property these pigs can be a source of ongoing contamination. This is particularly true for diseases that can live in rodents, dogs and cats.
Table 5. Diseases transmitted by non-pig methods.
Factor |
Disease and/or Agent |
Possible | |
| Air Distance from other barns |
M hyopneumonia M. hyosynoviae P multocida |
APP Influenza Strep suis |
PRRS ? |
| Birds | PRRS | TGE | |
| Coveralls and boots | Clostridium Rotavirus Swine dysentery |
PRRS TGE APP |
|
| Dogs and cats | Erysipelas Swine dysentery |
TGE | Strep suis ? |
| Feed Bins | Salmonella | TGE | |
| Foot Baths | Salmonella | TGE | |
| Isolation unit | PRRS (60 days) APP (42 days) |
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| People | --- | --- | --- |
| Rodents | Erysipelas Leptospirosis Salmonella Swine dysentery |
EMC Illeitis TGE |
|
| Shower | Swine dysentery | TGE Rotavirus |
APP ? Strep suis ? |
| Time | SIV Glassers |
||
| Truck Clean & disinfected |
PRRS Swine dysentery |
APP TGE |
|
Management techniques used to reduce clinical signs in disease positive herds are aimed at decreasing the volume of disease organisms in the environment and therefore prevent adequate contact. These include all-in all-out, cleaning and disinfecting between batches, and batch farrowing. Segregated early weaning and medicated early weaning are methods to reduce the bacterial and viral load in a group of pigs to prevent adequate contact.
Birds, Rodents, and Other AnimalsBirds can transmit avian tuberculosis, transmissible gastroenteritis, and erysipelas to pigs. PRRS virus which has been shown to be shed experimentally by ducks may be spread from farm-to-farm via birds. Birds have been incriminated in the spread of foot and mouth disease and salmonella but this has seldom been proven. Serpulina hyodysenteriae, the causal organism of swine dysentery, is carried by mice for months. Rats and mice carry and shed Salmonella typhimurium and Leptospira icterohemorrhagiae. Various animals can carry Lawsonia intracellularis, the cause of porcine proliferative enteropathy.
Trucks, Boots, and EquipmentVehicles hauling pigs spread disease through transfer of manure and aerosolized organisms. Many swine disease organisms survive for extended periods of time in manure and urine. The following is a partial list of diseases that survive well in the environment under certain conditions:
Table 6. Survival of organisms on in-animate objects
ORGANISM |
SURVIVAL |
| TGE virus | Sensitive to sunlight and warm temperatures and will survive for only 6 hours in sunlight, for about 1 week at 20oC and indefinitely if frozen (> 1 year) |
| Parvovirus | Even in hot weather, can survive for months |
| Serpulina hyodysenteriae (Swine Dysentery) |
Can survive for several months in manure pits and lagoons, about 1 week in manure at 20oC |
| Salmonella cholerasuis | Months or even years in manure |
| Erysipelathrix rhusiopathiae | Up to 6 months in feces during cool temperatures |
| Leptospira sp | 2 months in dilute urine |
| Streptococcus suis | Several weeks if cool temperature |
| Worm eggs | Many years |
| PRRSV | Heat labile but can survive at 4oC for 1 month |
Organisms exhaled from the pig in large droplets as is common with most respiratory bacterial infections, generally travel a short distance, likely less than 5 m. Examples of diseases that can be spread a short distance by aerosol droplets include Pleuorpneumonia (A. pleuropneumoniae), Streptococcal meningitis (Streptococcus suis), Atrophic rhinitis (Pasteurella multocida), and Glasser’s disease (H. parasuis).
Certain respiratory pathogens, particularly viruses, can be transmitted several kilometres in wind if conditions are ideal. The best examples of long distance (> 5 km) airborne spread are foot and mouth disease and pseudorabies. It has been suggested that PRRS virus, influenza virus, and Mycoplasma hyopneumoniae can travel up to 1 or 2 kilometres in the air.
Table 7. Distances between herds that broke with disease and the closest neighbour that was free of the disease as observed by Dr. Muirhead
| ORGANISM/ DISEASE | DISTANCE | TIME |
| A. pleuropneumonia | 150 m | 6 years |
| Atrophic rhinitis | 300 m | 3 years |
| Enzootic pneumonia | 150 m | 4 months (Depop of infected herd) |
| Strep suis | 300 m | 12 years |
| Swine Dysentery | 300 m | 4 years |
| TGE | 400 m | 4 months (disease died out) |
Adapted from Muirhead, Pigletter 1989;9(10):37 |
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People
The role of people in the transmission of pig diseases between farms is over emphasized. The provision of boots and protective clothing to all visitors greatly reduces any risk of disease spread. Whether visitors and employees should be required to shower prior to entering the swine facility is questionable, unless they have recently been in another swine unit. There are cases where veterinarians and farm workers have transmitted foot and mouth disease and transmissible gastroenteritis from one herd to another. A shower policy and the requirement of pig freedom of 24-48 hours do foster an awareness of disease risk and are important if only for this reason. Likewise, a perimeter fence, locked doors, and signs indicating that entry is restricted are important reminders of the importance of biosecurity.
Table 8 . Precautions implemented on 122 farms in Britain and the number of herds that broke with disease.
Precaution |
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Time (hrs) |
Boots & Coveralls |
None | ||
48 |
24 | |||
| Total number of Farms | 13 |
28 |
50 |
31 |
| Atrophic rhinitis | 0 |
2 |
1 |
1 |
| Enzootic pneumoniaa | 9 |
11 |
1 |
0 |
| Strep suis | 1 |
3 |
2 |
1 |
| Swine Dysentery | 0 |
0 |
1 |
3 |
Adapted from Muirhead, Pigletter 1989;9(10):38 a It is assumed that enzootic pneumonia was spread in the air. |
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