WHAT ARE THE PRACTICAL USES THAT CAN BE OBTAINED?

Serological studies in swine farms are mainly used for:

• Disease prevention in newly arrived animals.

• Choosing the most appropriate time for vaccination.  

• Surveillance of vaccination programs and assessment of the sanitary status of the farm. 

Preventing disease appearance.

Preventing the introduction of new diseases to the farm is one of the problems that can be solved by serology. One of the main ways by which diseases enter a farm is the arrival of new animals without having performed any serological analysis or without quarantine measurements. The first step in order to prevent infections is to avoid the incorporation of sick animals and/or animals with subclinical or latent infections. If the serum of an animal is analyzed previously to its joining the farm, we will avoid the spreading of the disease to the rest of the animals, as well as important economic losses. It is recommendable to know the serological status of the farm of origin, and to perform a quarantine period of between 3 to 8 weeks. This depends on the disease we want to control. It is important to remember that each infectious disease has an incubation period and an elimination period which are considerably variable.

Just one analysis on the day of arrival is not enough to know the sanitary status of the newly arrived animals. It is possible that they are in the incubation period or in the first stages of the infection; thus, the results obtained will be negative This is why it is so important to keep the right quarantine period, giving enough time to allow the clinical or serological manifestation of each disease.  Analyzing the presence or absence of antibodies against different diseases during the quarantine will allow us to know the actual sanitary status of the animal before incorporating it to our farm. 

The suitable quarantine period depends on the infectious agent, its incubation period and its capability of expression. Usually, 4 to 8 weeks is the recommended time for porcine diseases.

Choosing  the right vaccination time.

Choosing the right time for vaccination is one of the most important applications of the serological assessment of a farm.

Determining the most adequate moment to vaccinate piglets, is also something that a correct serological survey can provide. One of the objectives of this survey is to find the immunological window in which the animal is not protected anymore by maternal antibodies, and so can start producing its own antibodies. If we vaccinate while the piglet has lots of maternal antibodies, it is possible that this vaccine will be neutralized by them, and so will not produce the correct immunity. If, on the other hand, we wait too long, the animal will be without any protection and any disease can appear. When is the right moment? It is of the utmost importance to know when maternal antibodies reach a low level (non- protective level) in order to give the vaccination before this happens.

The right moment for vaccination (yellow line). Maternal antibodies do not interfere with vaccination, but are still protective (blue line). This situation allows the production of vaccination antibodies (green line), and at the same time prevents infection and a posterior production of infection antibodies.

Surveillance of the vaccination program.

In order to  perform this surveillance, sera from different piglets will be taken at different days after birth, from day 0 to day 40. It is possible to know, in this way, the immunological status of piglets and when they reach the lowest level of antibodies.Therefore, an accurate assessment of the best moment for vaccination will be done. Another way of studying the piglets serological status is to choose a number of piglets from different sows and study the presence of antibodies against the disease for several days (from day 0 to day 40). In this case, the same animals have to be evaluated.  

It is important to remember that sera profiles only allow one to know the sanitary status under the given conditions that are present during the study. If any of the parameters changes (different vaccination, age of vaccination, different animals, etc) the obtained results could also change. Thus, it would be necessary to perform different serological studies each time any change occur in the farm.

The knowledge of the sanitary status. 

Serology is also useful when we want to know if there is any infectious disease and how it evolves on a given pig farm. Knowing the sanitary status of our farm must be one of the main objectives in order to increase quality and reduce production costs. To gain this knowledge, serological studies along with slaughterhouse inspections and even the use of sentinel animals, are good tools in this kind of studies. But when we want to perform a study of epidemiological surveillance, it is important to remember that antibodies take a certain time to appear and be detected with conventional techniques. This time ranges from 10 to 30 days. This is why serology is not an appropriate technique when we need to establish an early diagnose; it is possible that all the collected samples could give a negative result  because the animals are within the incubation period. 

 Serology is more useful in the case of subclinical or latent infections than in the case of acute stages of the disease.  In order to study the presence of a given disease in our farm we must perform two different tests in the same animal, with a 20 days span between each determination. This is true in every case with the exception of Aujeszky´s disease, in which if one has the gE negative vaccines it is possible to discern between vaccinated animals and sick animals; in this case, then one determination is enough. 

In the case of Aujeszky´s disease when a gE negative vaccine and indirect competition ELISA method is used to study anti gE and gB antibodies, it is possible to know, with just one determination, if a serum belongs to an animal which has been (1) vaccinated (it only has anti gB antibodies), (2) infected (it has anti gB antibodies) (3) whether it has the disease or not, and if it has been vaccinated or not (it doesn't have either anti gE or anti gB antibodies)     

The results of both determinations will show two different situations:

1. The  first titre is higher than the second one. In this case, the animal is recovering from an infection (only if the animal has not been previously vaccinated): seronegativitation. 

2. The titre obtained in the first determination is lower than that obtained in the second one. The animal is then going through the acute stage of an infection and its immune system is reacting to it adequately. Seroconversion.

3. Both determinations are similar. In this case the response is a basal response, and depending on the disease, it could be possible that the animal is an asymptomatic carrier (basal status). 

So, with this type of studies it is possible to know if an animal is starting or finishing the acute stage of an infection; if it is a carrier or if it is recovering from the disease.

SERONEGATIVIZATION

SEROCONVERSION

BASAL STATUS

Serological surveillance of an area.

Serology is also indicated when we want to know the sanitary status of an area that has been affected by a given disease, and when it is necessary to recommend the cacellation of the existing restrictions on animal movement. For this kind of studies, serological analyses of the different farms are performed. They usually start 30 days after the day of the last declared case. The restrictions can be canceled if the serology is negative. 

Indicators of protection. 

As we will see in chapter 7, the level of actual "in vivo" protection against a given infectious agent depends on the capability of the immune  response (both humoral and cellular). This is why the determination of just the humoral response does not give complete certainty, especially in  negative cases. So, when the humoral response is low, it does not necessarily mean that the animal is not well protected. Assessing the level of neutralizing antibodies does however,give a representative indication of the animal's capability to fight this infectious agent. Seroneutralization is the technique of choice for assessing the  level of protection of an animal.The assessment of specific antibodies using other serological methods can give an indication of the level of protection, but the correlation is inferior to that obtained by seroneutralization.  

The seroneutralization technique is used to assess the capability of the serum (at different dilutions) of neutralizing the infectivity of a virus upon a sensitive cell line. If the unknown antibody can neutralize the virus,  viral replication will not take place (and neither will there be a cytopathic effect). If the antibody is not neutralized, the virus will replicate and we will see a cytopathic effect. Some viruses do not produce a cytopathic effect; in these cases, viral replication can be determined using immunofluorescence or peroxidase techniques.

SOME OTHER APPLICATIONS OF SEROLOGY.

Serology is also used for blood typing (with hemolysis techniques). 15 blood groups have been determined in pigs, and named with a letters from A to O. The most frequent group is the A group. A and O substances are soluble antigens that can be found in the pig sera which are bound to the surface of red blood cells of each animal after birth. There are anti-A antibodies and an A-negative animal may have them; in this case, if the animal receives a transfusion from an A-positive pig, a transitory collapse with hemoglobinuria can take place.   

Blood groups are regulated by two locus. One of them is known as locus A and contains two alleles: A and O. The other locus receive the name of S (secretor) and also has two alleles: S and the recessive one s. Locus S controls the expression of the A system, so only A or O will be expressed if the animal has a recessive s gene. 

The genotypes that have been found are:

Different combinations of these genotypes can be found in the animals: