The cost of introduction of diseases onto farms varies widely but it is unequivocally a major source of potential and real loss for most swine producers. The effects of individual diseases vary widely, and the pathogenicity of the disease, the length of time of effects, and the size of the herd at risk are of chief concern. Added to this, the management level of the herd can have an effect upon the pathogenicity of the disease.

As most entries of livestock into swine herds are as breeding stock into the top part of a pyramidal structure of pig distribution, the effects can be large. The pigs at risk are any pigs that are in contact either directly or indirectly with the breeding herd. Thus, if multiple source nurseries or finishing barns are part of the system, this can be considered a part of the herd at risk.

The direct costs of a disease can include a decreased reproductive ability, decreased growth performance both in growth rates and feed conversion, increased mortality and increased medication costs. Finally, we should not forget disease outbreaks may be demoralizing to staff and create a less pleasurable environment for employees.

The costs of disease outbreaks has long been the subject of study and conjecture. A review of the literature is listed in Table 1. It should immediately be recognized that the costs are dependent upon the characteristics of the farm, including historical levels of productivity. It is probable that some of the reported values are underestimates of the economic damage on individual farms.

Though there are many methods of introducing a disease into a herd, including carriage by machinery, transferal through the air, and human transferal through human infections, by far the most common method of disease introduction is through pig-to-pig contact on the farm. Though this method of transferal is most common, it is surprising that in many cases few steps are taken to minimize the opportunity of disease transferal. Conversely, in some cases, the closure of swine herds with no introduction of live animals into the herd has compromised the ability to improve genetic characteristics of the herd.

However, the transferal of the disease is not easily predicted. Transferal of disease through entry of the disease through livestock is a function of many different factors. It is a function of the probability that the disease is actually in the source herd. It is also a function of the probability that the disease is not in the target herd, as it should be recognized that in many cases, the disease may not have been diagnosed in the target herd but still be present. It is also a function of the chance that the pigs being transferred are infected and the chance that the infected pigs will actually transfer the disease.

For instance, if a herd producing 10,000 pigs annually expects the cost of transmissible gastroenteritis (TGE) introduction to be $1.50 per pig per year and the duration of chronic effects is five years, the cost of the disease is calculated to be $5.68 per pig of annual production. The value of $5.68 is the net present value (NPV) of five years' losses. This is calculated by adding together each year's losses but discounting future losses at a rate of 10% per year.

The benefit of isolation is simply the reduction in the cost of disease when using this system. If we use the values for TGE listed previously, and the probability of detecting the disease in the source herd is 80%, the probability of detection in a pig is 80%, and the probability of communication of that condition is 75%, the benefit of isolation is calculated to be 98.4% of the cost of disease introduction, equaling $1,269 in the case where two pigs are introduced.

Another case study may be useful. A 1,000 head sow herd that supplies common off-site nurseries and finishing floors producing 200,000 pigs per year has gilts delivered on a monthly basis. Each month's delivery consists of 425 gilts. The introduction of a new serovar of Actinobacillus pleuropneumoniae is of concern as it is estimated that, conservatively, the added costs of it's control and chronic effects is $2.00 per pig. The NPV, based on a discount rate of 9% and a five year horizon is $7.88. In this example the cost of disease transferal is $13,049 per month or almost $160,000 per year. With this size of group the disease can be controlled very effectively by isolation. It is only when the probability that the disease is detected is below 1% that the efficacy is affected.

As this paper suggests, under certain conditions there are real benefits to isolation. This illustration just shows the benefits for one disease. In most cases, however, the concern is over a multiplicity of diseases and thus the benefits can become extraordinary. The main problem in identifying the benefits is in identifying rational estimates of each of the components of the model. The literature on the most part is sparse and, in most cases, the risk factors involved in varying levels of prevalence and odds of infection among different source herds is especially poorly documented. Table 1 lists some estimates of the variables for diseases of concern. This may be inaccurate for individual farms but does give guidelines for estimation of the model. The effects of changes in productivity create different economic effects across farms and is dependent upon the constraints and the abilities to capitalize upon the benefits of lack of a specific disease.

In many cases, a good source of information is the experience of a local veterinarian both in classifying occurrences within a herd as well as defining prior probabilities of disease transferal. Cost of disease can be calculated using partial budgets and prior estimations of disease effects. The probability of disease occurring in the source herd can be made on the basis of surveys, but usually the best estimates are based on the knowledge of details of the source herd. The probability of disease in the target herd is also a function of the knowledge of the disease as well as use of diagnostic tests.

The prevalence of disease in the animals at risk is less likely to be known and this may be a function of the dynamics of immunity within the herd. This may be a variable that should be analyzed in some detail or assumed to be at a low prevalence. The ability to detect disease in infected animals is a function of certain diseases, and will vary with the housing facilities, the attention given to these pigs, and the use of diagnostic procedures such as serology during isolation. The probability of detection of disease in the source herd is similarly affected. Proper communication should always be available and reliable sources of breeding stock should be used.

Take-Home Message

New diseases are often devastating to a herd and the effects can be widespread and long standing. Isolation is a neglected but financially important part of swine production.