Pigs are an ideal model for vaccine research. They are relatively
cheap and because of their size can be housed in large groups in
standard animal isolation rooms. They do not require special
handling facilities, and can be routinely bled and immunized using
well-established standard procedures. Vaccines can be administered
either intramuscularly, subcutaneously, intradermally, orally
or intranasally. In addition, more advanced methods of administration
such as needle-free injectors, gene-guns and microneedles
have been successfully used in pigs. As described earlier, pigs offer
easy access to the various immune compartments including
systemic and mucosal lymphoid tissues. Compared to mice, large
numbers of immune cells can be isolated from each of these
compartments and used for various immune assays. Depending on
the age and breed, this can vary significantly. In addition, secretions
such as saliva, nasal secretions, urine and feces can be easily
collected and assessed for the presence of secretory IgA, IgM and
IgG. Furthermore, pigs offer the advantage of the ability to test
vaccines in an outbred population, and thus allow a more accurate
assessment of the efficacy of potential human vaccines. Pigs have
an epitheliochorial placentation, which means that there is no
transplacental transfer of antibodies or larger molecules during
gestation. Thus, the neonate depends on passively transferred
immunity via colostrum and milk, which includes antibodies,
proteins and immune cells. Maternal immunization is a common
practice in the swine industry, and both colostrum and milk can be
collected from the lactating sow. The litter size in domestic pigs is
about 10–12 piglets per litter. Interestingly, during the first 7 days,
piglets can be exchanged between litters, and vaccines can therefore
be tested in the presence or absence of maternal antibodies.