Health Status as a Risk Factor:
A Systems and Unit Perspective
Cate Dewey and Bob Friendship
Department of Population Medicine, 50 Stone Road E., University of Guelph, Guelph, ON, Canada, N1G 2W1
Introduction
Disease is most important when a naive herd becomes positive. Then the cost of disease is
dramatic 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 transmission and prevention strategies.
Adequate Contact
Why do pigs get sick? The number of sick animals on a farm at a specific time is determined by
the number of susceptible animals, the probability of an effective contact, and the incubation
period of the disease (Reed-Frost Model). In a naive herd we can assume we have susceptible
animals. In other herds, susceptible animals are those without immunity to disease. Adequate
contact is the number of E coli organisms required to cause diarrhea. For example, if a baby pig
was exposed to one E coli bacterium, it probably would not get diarrhea. The number of
organisms required depends on the disease, the age of the pig, and the amount of stress in the
pig's environment. A pig in a cold room will need fewer E coli bacteria than a pig in a warm
room. The incubation period is the time between exposure to the organism to the time when the
first clinical signs are evident. Pigs infected with TGE will shed the virus in large numbers for two
weeks and then the volume of viruses shed decreases over time. TGE has an incubation period of
18 hours, but other diseases such as APP (A. pleuropneumonia) and enzootic pneumonia will not
spread to all naive animals as quickly.
Cost of Disease
A sow unit that breaks with TGE can expect to lose four to six weeks of newborn pigs and have a
reduced farrowing rate for six weeks. A similar herd that breaks with PRRS loses half of the
weaned pigs for two months and one-quarter of the weaned pigs for a further two months and
experiences ongoing morbidity and mortality in the nursery.
Disease Entry
A knowledge of all potential entry routes for disease agents is an important prelude to developing a comprehensive security program. General considerations include:
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 onto 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.
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. In addition,
breeding-stock herds can be tested for the presence of diseases using serology, bacterial cultures,
and slaughter check evaluations.
Table 1. Diseases found in various herd types.
| Herds derive by cesarian have |
Diseases clean herds acquire over time |
Diseases kept out of clean herds |
| E coli | Enzootic pneumonia | APP |
| Erysipelas | Glassers | Mange |
| Illeitis | Rhinitis | |
| Parvovirus | Swine dysentery |
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. 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. However, closed herds can maintain 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%.
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 disease 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.
Table 2. Diseases prevented or clinical signs reduced by various management factors.
| 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 |
E coli
Glassers Parvo |
PRRS?
|
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 ).
PRRS has its own set of diagnostic problems. Even if gilts come from a PRRS positive herd, they may be negative on serological tests at six months of age. By transporting gilts, we expect some will shed the virus and eventually test positive serologically or at least transmit the virus to susceptible nursery age pigs. Gilts must be kept in isolation units for 60 days if we are to ensure that they will not bring PRRS into a unit. This is because PRRS antibodies do not last a long time in positive animals and PRRS may not be transmitted easily from the gilts to the naive nursery pigs.
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
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 |
| Pseudorabies | sows | 10% / 28 |
| 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
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 an infertile boar.
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 Infection
The 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 -20oC. When stored at +21oC 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. Keep rendering trucks far from
the swine facility as they can harbour large numbers of disease organisms - even those that are not
currently in your facility. 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
(manure, mucus, and dust) |
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) |
||
| 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 |
|
Birds, Rodents, and Other Animals
Birds can transmit avian tuberculosis, TGE, 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
(ileitis).
Trucks, Boots, and Equipment
Vehicles 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 inanimate objects.
| Organism | Survival |
| TGE virus | Sensitive to sunlight and warm temperatures. Will survive for only 6 hours in sunlight, for about 1 week at 20oC, and indefinitely if frozen (> 1 year) |
| Pseudorabies virus | Can survive for 14 days in urine, 7 days in well water, and about 2 days in manure |
| 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 |
Airborne Spread
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), 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.
The risk analysis of the location of isolation units includes a balance of the chance of disease entry, given the modes of disease transmission listed above, and the costs of building and managing the isolation facility. The isolation unit requires power, water, manure disposal system, and labour. The location of the unit needs to take into consideration wind direction, natural disease barriers such as hills and woods, and road access to the unit. Those constructing the unit need to focus on manure disposal, rodent control, loading facilities for pigs and feed, and shower facilities for employees.
Table 7. Distances between herds that broke with disease and the period over which the closest neighbour was free of the disease, as observed by Muirhead (1989).
| 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.
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 TGE from one herd to another. A shower policy and the requirement of pig freedom of 24 to 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.
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.
Table 8. Precautions implemented on 122 farms in Britain and the number of herds that broke with disease.
| Precaution | ||||
|
Time away from pigs (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 |
a It is assumed that enzootic pneumonia was spread in the air.
Adapted from Muirhead, Pigletter 1989;9(10):38.
References
Muirhead, M.R. (1989) Payoff can justify high health status. Pigletter 9:36-38.