**8. Immunoprophylaxis of Marek's disease**

The one effective way of preventing Marek's disease is prophylactic vaccinations, mainly performed in 1-day-old chickens. *In ovo* vaccination is also used, however, due to its high costs, it is used to a very limited extent. At this point, it should be recalled once again that vaccination against Marek's disease does not protect birds against infection, but against clinical symptoms and pathological changes in the internal organs of the birds. Marek's disease continues to cause heavy economic losses on account of these latter aspects. In 2020 and 2021, a slight upward trend was observed in the number of cases of the clinical form of the disease in field conditions. Practically from the beginning, when the immunoprophylaxis against Marek's disease was introduced, vaccines based on Marek's disease virus serotype 1 and serotype 3 have been used. Marek's disease virus vaccines based on serotype 3, contain the

turkey herpesvirus strain HVT (FC126), which occurs naturally in turkeys and is a non-pathogenic strain for these birds. This strain comes in two forms: as a targetassociated virus and as a cell-free virus. Cell-bound virus must be stored at liquid nitrogen temperature (−1960°C) and cell-free virus in lyophilisate form at 2–8°C. Vaccines based on the serotype 1 (strain Rispens CVI988) are in the form of a liquid suspension and absolutely must be stored at liquid nitrogen temperature (−196°C). There are also vaccines on the market consisting of two strains: HVT (Fc126) and the Rispens CVI988 strain, and here, also, liquid nitrogen temperature is required.

In the United States, a vaccine based on serotype 2 (strain SB1) is additionally used. However, this vaccine is not used in Poland, although the results obtained in several laboratories indicate that this strain may already be present in poultry flocks in Poland [70].

Administration of the vaccine against Marek's disease in the correct manner causes the reduction of the multiplication process in the body of the virulent field strain and the spreading of the infection in the flock horizontally (lower level of replication in feather follicles). As a result, the formation of pathological changes in internal organs, mainly in lymphatic tissues, is limited. The viral load of infected birds is also reduced [24, 71]. A very important aspect of vaccination against Marek's disease is the fact that vaccine immunity significantly reduces the risk of immunosuppression. It is important due to the possibility of contact of birds on the farm with immunosuppressive factors, mainly from the infectious background [72].

There are a few important information's to keep in mind when prophylactic vaccinations against Marek's disease have been conducted:


Most often, the birds are vaccinated on the first day of their life in the hatchery or immediately after being placed on the farm.

An important aspect at the time of vaccination is the fact that Marek's disease vaccine should not be combined with other vaccines. The exceptions are, of course, the recommendations of the vaccine recommendations.

The information that in the field conditions very different volumes of the vaccine are used (not to be confused with the dose of virus in 1 dose of the vaccine) arouse much controversy of scientific and field nature (in terms of vaccine effectiveness). The recommended volume for a 1 day old chickens is 0.2 ml, which contains approximately 2000–3000 PFU (focus-forming units in cell cultures).

Research studies indicated that the use of a 0.1 ml volume of vaccine will not provide adequate protection for the birds from clinical Marek's disease. On the other hand, a vaccine volume of 0.5 ml is used in areas where Marek's disease virus may be endemic. Neither one solution nor the other can find any justification [unpublished data]. According to our own observations, there is a very frequent breakdown of post-vaccination (cellular) immunity, and then we are dealing with the classic course of Marek's disease in a given flock. A very important step in the correct vaccination

#### *Marek's Disease Is a Threat for Large Scale Poultry Production DOI: http://dx.doi.org/10.5772/intechopen.98939*

against Marek's disease is the storage and transport of the vaccine. Recently, vaccines based solely on vaccination with HVT (FC 126) strain in the form of lyophilisate have been used. Therefore, vaccines based on the Rispens CVI 988 strain or in combination with HVT (FC 126) must be strictly stored under the conditions recommended by the vaccine protocol [73]. In some cases, after removing the vaccine, the entire contents of the vaccine remain in the upper part of the ampoule. This clearly proves that the contents of such an ampoule were previously thawed and then frozen again. This vaccine is no longer usable and should be disposed of. The fall in the titter of the vaccine virus is then 100% [73]. Storing the vaccine at a temperature of 40°C or over 25°C for more than 1 hour causes a significant decrease in the viral load. After 24 hours, the decrease in titter is close to 80–90% [73]. The vaccine after removing from the container with liquid nitrogen should be dissolved up to 2 minutes, because a longer time causes a significant decrease in the titter of the vaccine virus in 1 dose of the vaccine. We should not use water over 37°C to defrost the vaccine.

After dissolving the contents of the vaccine in a suitable solvent, use the entire contents within 1 hour or a maximum of 2 hours. Research carried out at PIWet-PIB in Puławy showed a decrease in the viral load with the duration of vaccination.

Another important aspect of correct vaccination is the number of chickens vaccinated subcutaneously or intramuscularly. On this point, veterinarians are much divided. It is recommended to vaccinate about 2000–2500 birds within 1 hour. The more vaccinated birds per hour, the lower the number of correctly vaccinated birds should be presumed. You should also take into account the method of vaccination, whether we vaccinate with an automatic syringe or a special vaccination machine. Many companies offer suitable dyes that are added to the solvent. In this way, the vaccination veterinarian can very easily assess the quality of the vaccination performed. It should also be remembered that birds should not be given any antimicrobials or substances with immunosuppressive activity (e.g. immunostimulators) with the vaccine. In the case of vaccination in breeding flocks and commercial hens it is recommended to use vaccines based on the Rispens CVI 988 strain or vaccines containing the Rispens CVI 988 and the HVT (FC 126) strains. In broiler chicken flocks, vaccines based on the HVT (FC 126) strain are used, however, due to the presence of Marek's disease virus strains with high pathogenicity, it is recommended to use vaccines based on the Rispens CVI 988 strain. For turkey herds, the use of vaccines based on Rispens (CVI 988) is recommended. If vaccination is already performed on the farm, it should be remembered that all proper sanitary and hygienic conditions are maintained. We should apply the very simple but extremely effective "full farm - full empty farm" principle. This means that only one-age birds should be existed on the farm. This is a barrier protecting young birds against the possibility of transmission of pathogens from older birds. Among these pathogens there may be immunosuppressive pathogens, which adversely affect the effectiveness of the vaccination against Marek's disease.

After vaccination, it is very important to deal with chickens for the first 2 weeks of life, i.e. until immunity is developed. In this period, there should be close cooperation between the hatchery, the poultry producer and the veterinarian providing services to the poultry producer [74]. The in ovo method is used to inoculate the embryos on the 17.5-19th day of incubation, most often when the embryos are transferred from the incubation chamber to the brood chamber. The site of *in ovo* vaccination is the amniotic, allantoic or yolk sac (extra embryonic - EE), but also the body of the embryo (intra embryonic - IE) [75]. The place where the vaccine is administered depends on the age of the embryo, the egg placement, the size of the egg and the breed of the

parent flock. Within one hour, up to 50,000–60,000 embryos can be vaccinated. Properly conducted *in ovo* vaccination reduces the incidence of vaccine breakdown and clinical form/expression of Marek's disease in the field. In addition, it protects or significantly reduces hatched chickens from the possibility of early infection of chickens with virulent, field strain of Marek's disease virus. Like any vaccination method, it has advantages and disadvantages [76]. The advantage of the *in ovo* vaccination is that there is no stress that may occur in vaccinated chickens. There are significantly fewer infections associated with the *in ovo* technique alone compared to the subcutaneous or intramuscular vaccination of 1-day-old chickens. It is important to compare the effectiveness of *in ovo* vaccination compared to that of 1-day-old chickens vaccination. Our own observations at NVRI show that the effectiveness of both methods is probably at a similar level. When *in ovo* vaccination was used, a lower percentage of so-called seizures at the slaughterhouse associated with coetaneous Marek's disease were observed in broiler chickens.

Another controversial issue is the vaccination of chickens already vaccinated on 1 day of life. Immunization is used even in the first day of life after a few hours' break from the first vaccination. However, it seems that there is no scientific justification for this, and moreover, it was not observed in the field that vaccination, e.g. on the 3rd, 7th or even 10/21 days, had an impact on the possible course of Marek's disease in infected birds.

Recently, there has also been a limited amount of data collected on the field (unpublished data) on additional vaccination of birds, especially in breeding flocks 35–37 weeks of age. Also, such a scheme is a bad scheme and should not be used, especially since the vaccination process itself is a great stress for the birds.
