When a vaccination fails, the quality of the vaccine is typically blamed. Although this is certainly an important consideration, there are other factors that must be evaluated to determine the cause of the failure. A thorough understanding of these factors will help prevent losses associated with vaccination failure.

Maternal immunity

A high level of maternal antibodies in the young chicken may interfere with the multiplication of live vaccines thereby reducing the level of immunity provided. For example, if a chick comes from a breeder hen with high levels of antibody against Gumboro (infectious bursal disease), the chick will typically have high levels of (maternal) antibodies for several weeks. If vaccination is attempted while these antibodies are still present, some of the vaccine viruses will be neutralised and response to the vaccine results will be diminished. On the other hand, delaying vaccination until the maternal antibodies have catabolised may leave the birds susceptible to field challenges.

Feed factors

The presence of mycotoxins in poultry rations can impair the immunity acquired through vaccination and eventually lead to diseases occurring, such as infectious bursal disease virus (IBDV) or Adenovirus. Mycotoxins induce immunosuppression by depressing T or B-lymphocyte activity, impairing macrophage/neutrophil-effector functions, reducing the level of antibodies and the activity of phagocytic cells following vaccination. This can then increase the production of steroids from the adrenal glands which decreases lymphocytes and increases neutrophils. Poultry producers therefore need to keep feed troughs free of mould and control mould growth by using mould inhibitors containing acetic acid, sorbic acid, and propionic acid.

There are many reasons for vaccination failure which can be prevented by carefully considering every aspect. Photo: Jan Willem Schouten

Environmental factors

Under heat stress conditions, corticosterone is released by the hypothalamus–pituitary–adrenal axis. This hormone has been shown to deregulate both the lymphocyte phenotype characteristics and total antibody production in broiler chickens. Chickens exposed to heat stress (38 ± 2°C) over long periods of time will thus have impaired vaccine responses and consequently will be susceptible to pathogenic environmental challenges. Therefore they should be kept in a thermo-neutral environment, especially in the early stages of their life.

If the ammonia level in the house is high (above 100 ppm), this too will have a negative effect on the bird’s ability to produce local immunity, as the cells on the surface of moist membranes are adversely affected. In the absence of local immunity, the bird becomes susceptible to viral diseases, particularly those which proliferate in the respiratory tract.

Ammonia concentration can be controlled by the following management and feeding strategies:

• Management:
  Ammonia problems can be alleviated with management practices, such as the use of nipple drinkers, balancing bird density with ventilation facilities and using litter materials with a high water-retention capacity.
• Protein nutrition:
  Feeding reduced protein diets (2% less) can reduce nitrogen excretion, with ammonia emissions reduced by up to 24%. Further decreases in diet protein levels, however, will require that all essential amino acids are included, preferably in a crystalline form, while ensuring a careful balance of amino acids.
• Fibre nutrition:
  Fibre provides energy to bacteria in the lower gastrointestinal tract where the bacteria use nitrogen for bacterial protein synthesis that would otherwise have been excreted as uric acid.
• Feed additives:
  There are some additives that can be added to the diet to sequester or trap ammonia. Among these is zeolite, a type of mineral with a porous or lattice-like structure. When included in the diet, it binds ammonia in the faeces and prevents it from being emitted to the air.
• Minimising feed and water waste:
  Poultry will waste a significant amount of feed if feeders are overfilled, mounted too low, or poorly designed. In which event, faecal nitrogen may increase by 1.5% for every 1% increase in feed waste, and ammonia emission rates will therefore also increase. Feeders should be designed such that it is difficult for the birds to push the feed out of the feeders. The feeder height should be adjusted so that the top of the feed pan is level with the birds’ neck, with the feed fill level set to only 25% of the feeder pan. Too much water in the house also results in excess ammonia emission and poor air quality. This problem can be alleviated by adopting the nipple drinker system which gives better control over water intake and wastage.

Pesticide pollution

A study was conducted in Egypt to determine the effect of pesticide pollution on the vaccination efficacy of free-range chickens vaccinated against Newcastle disease using Hitchner-B1, Lasota, and inactivated vaccines. Newcastle antibody titres were found to be inversely proportional to the quality and number of pesticide pollutants in the sera, with the disease widely prevalent even on vaccinated farms leading to higher morbidity and mortality (Table 1), probably due to the immuno-toxicity and immunosuppressive effects of pesticides.

Diseases and parasites

The health status of the flock should be checked 1 to 2 days before vaccinating, devoting particular attention to all chronic or sub-clinical pathologies which could either reduce the effect of the vaccination or cause secondary infections spurred by a post-vaccination reaction (mycoplasma Pasteurella, E. coli, coccidiosis mycotoxins, etc.). These potential risks therefore need to be addressed by giving preventive treatment before and after vaccination.

It is not advisable to vaccinate sick birds because the capacity of their immune systems is already diminished. Introducing a live virus vaccine into these birds often results in a reduced immune response and an adverse reaction because, in many cases, they are unable to develop a response even to the milder vaccine virus.

A basic rule for vaccination is always to delay vaccination until the birds are healthy. It is better to skip a vaccine in a diseased flock than to vaccinate in the presence of a concurrent disease.

Parasite infection also affects chickens’ response to vaccination. A study was conducted to investigate the influence of helminths on antibody response after vaccination against Newcastle disease of free-range chickens naturally infected with the parasite Ascaridia galli. The response was compared with vaccinated groups that had been dewormed with fenbendazole and niclosamide. The results showed that the antibody titres rose in the dewormed groups but not in the parasite-infected groups. Among the latter, the synthesis of immunoglobulins was reduced owing to an absolute loss of protein and this might have resulted in a reduced humoral antibody response. Anthelmintic treatment should therefore be done well before vaccination to allow the birds to recover from the helminth infections and hence improve their immune response.

Points to consider for vaccine handling and administration Live vaccines may be inactivated due to improper handling or administration. Points to consider include: Do not vaccinate birds during stress periods, especially a few days before or after debeaking, transfer, and grading. These create stress conditions that may lower the immunity of birds. Do not vaccinate birds during extremely hot weather. As indicated above, heat stress may lower the birds’ immunity and impair the efficacy of the vaccine. Vaccination should be carried out during the cooler periods of the day (morning and evening). Avoid using metal drinkers and containers for mixing vaccines. Metals may inactivate live vaccines presented in the drinking water. Aluminum and brass containers are also known to have high inactivation properties and are similarly not recommended. Plastic containers should be used instead. Use clean equipment, drinkers and sterile syringes. Do not use chemical disinfectants to clean utensils before vaccination because residues of these chemicals may inactivate the vaccines. The utensils should be rinsed only with fresh water. Always use sterile diluents preferably of the same company for the reconstitution of injectable vaccines. This is because some diluents may contain more than optimum levels of dye which act as a steriliser for the diluent but which at the same time may inactivate the vaccine. Vaccine preparation needs to be done with the right equipment and in accordance with the issued guidelines. Photo: Ruud Ploeg Do not use chlorinated water for vaccine dilution because the chlorine will destroy the vaccine. Suspect municipal tap water should be allowed to stand in an open plastic container for at least two days to allow dissolved chlorine to evaporate. In areas where a high salt concentration in water has been established, the preferred route of vaccination is either by injection or through eye, nose and mouth dropping. The addition of skimmed milk to the water used for oral vaccination aids with the optimum distribution and stabilisation of vaccines. The birds should be made to consume the reconstituted vaccine within one hour, bearing in mind the temperature of the pen and feeding time. Delays in ingesting the vaccine by the birds may lead to a reduction in vaccine efficacy. Do not expose reconstituted vaccines to heat, sunlight, high room temperature or brooding house temperature. Do not open the lid of the vaccine vial in the open because the vial is sealed under negative pressure and will suck in contaminated air. Use a sterile disposable syringe to reconstitute the vaccine or open the vial under water. Avoid reused or disinfected needles and syringes. Always use wide bore needles for vaccine suction or dilution as narrow bore needles may retain some cell culture vaccines, thus possibly making mixing nonuniform.