In addition, control of prevalent diseases by vaccination reduces the need to use antibiotics as therapy for secondary infections including E.coli.
The process of administering vaccines does not necessarily assure the desired level of protective immunity. A successful vaccination program requires following accepted standard techniques that result in uniform vaccine coverage. Instituting a monitoring system provides necessary feedback and accountability for the entire process of maintaining health.
Protecting the immune system
For any vaccination program to be successful, one must first ensure that each flock is capable of responding appropriately to the vaccines given, which requires an intact and functional immune system.
Vaccines stimulate humoral immunity (circulating antibodies) and local cellular immunity (activation of macrophages and Tlymphocytes). While resistance to some infections such as Newcastle disease and avian influenza rely mostly on antibody production, others including infectious bronchitis and laryngotracheitis rely on cellular immune function. The two most important diseases negatively affecting the immune system are Marek's disease (compromising cellular immunity) and infectious bursal disease (depressing antibody production).
The Disease: Marek's disease, caused by a herpesvirus, is known for its ability to cause variable to high mortality in pullets over eight weeks of age due to tumors affecting nerves and internal organs. Marek's disease virus also causes significant immune suppression. The "silent" effect on the immune system is frequently more significant than the impact of mortality.
Vaccination: Chicks should receive a full dose of a combination serotype 2 and 3 Marek's vaccine, usually comprising SB-1 and HVT strains. In areas of high challenge, a serotype 1 Rispens vaccine, with or without HVT, is highly recommended. The choice of Rispens vaccine is critical in high challenge areas as low-passaged (less attenuated) products have been proven to offer significantly more protection than higher-passaged (milder) vaccines.
Monitoring: Marek's disease vaccines are applied to each bird at hatch and do not spread efficiently within the flock so that accurate vaccination is critical for protection. Monitoring of vaccine administration at the hatchery should reveal over 99.5% of chicks with vaccine dye under their skin and none on the feathers.
Currently polymerase chain reaction (PCR) technology is available to quantify the take level of the vaccine virus in feather pulp which is correlated with a proper immune response. Figure 1 shows the contrast in bird-to-bird replication of Rispens vaccines of different passage levels.
Infectious Bursal Disease (IBD)
The Disease: Depending on the strain of IBD virus to which flocks are exposed, unprotected birds can become mildly to severely depressed accompanied by acute mortality. Partially immunized flocks sometimes show no obvious clinical signs although feed consumption, body weight gain and uniformity may be reduced.
If adequate protection is not accomplished through vaccination, field challenge will cause some level of immune suppression. Infection with IBD virus before three weeks of age can result in permanent and serious compromise of the immune system. Infections after three weeks can often lead to temporary yet significant immune suppression. Damage to the immune system reduces the effectiveness of subsequent vaccinations against respiratory infections and bacterial pathogens.
Vaccination: A typical vaccination program includes administration of at least three live intermediate IBD vaccines between two and five weeks-of-age. The vaccines should be spaced seven days apart to maximize the immunization of susceptible birds. Varying levels of maternal antibody result in different time points when birds will respond to vaccination. A history of successive flocks experiencing field challenge by or before four weeks-of-age indicates the need to administer the first vaccination by two weeks-of-age. It is important to remember that there is a wide range of attenuation levels among intermediate IBD vaccines.
Milder vaccines will not immunize as many birds during each application, resulting in suboptimal protection. Without a solid vaccine take in the bursa of Fabricius, the source of B-lymphocytes, protection against a strong field challenge is difficult to achieve.
Monitoring: The extent of microscopically detectable changes in the bursa of Fabricius indicates vaccine response or field exposure to IBD virus. Various procedures have been developed to measure either the gross size or histological integrity of the bursa.
In addition, PCR analysis can reveal whether the bursa has been infected with either vaccine or field virus. Vaccination induces less atrophy of the bursa compared to field viruses and can be assessed by microscopic examination. Regeneration of the bursa is an excellent indication of protection.
B-lymphocyte depletion at the time of pullet transfer indicates the bursa was severely challenged weeks earlier and that the pullet is not fully immunocompetent. Table 1 shows data from two flocks experiencing a normal vaccine take followed by solid bursal regeneration.
The bottom line
Developing a sound vaccination program for pullets is essential for a productive layer flock and a safe egg supply. Proper administration of vaccine in a structured, appropriate program will help ensure that flocks are well immunized. This requires periodic monitoring of vaccination techniques and confirmation of the expected immune response.
Marek's disease and infectious bursal disease are the two greatest threats to the immune system. Proper immunization against these infections will make it possible for vaccinations against other diseases to stimulate immunity as intended.