Field notes on Marek’s disease in poultry farms
Professor Dr. Salah Mahdi Hassan / Poultry Health and Production Consultant and Expert
A few days ago I watched a video clip of a seminar on the poultry industry and the speaker mentioned in a lecture about Marek’s disease that there is a subclinical case of Marek’s disease! Of course, this is inaccurate and cannot be applied to Marek’s disease for a very simple reason: Merck’s disease is one of the diseases characterized by the fact that the causative agent, which is the cell-associated herpes virus, causes a latent phase, which is considered one of the most important stages of its pathogenesis, which takes long periods that may be weeks or months until clinical signs appear on the infected bird.
Therefore, I would like to explain to my fellow veterinarians working in the poultry industry the most important matters related to this disease in terms of field work, and I will outline them in points to facilitate understanding and follow-up.
- Marek’s disease is a severe viral disease associated with lymphoma, and therefore it is considered a cancerous disease with a known viral cause. The disease affects chicks from the first hours after hatching, but its clinical signs and pathological lesions do not appear until after the eighth week of the birds’ age, and therefore this means that its pathological importance is not meaningful in broiler fields! Which witness rapid growth and the age of the marketed broiler (5-6 weeks), and therefore it is of interest in the fields of hatching egg mothers and table egg chicken fields.
- The disease was announced in 1907 and the causative virus was successfully isolated in 1967, which is the cell-associated herpes virus. This is an important point in identifying the virus because there is another disease in poultry, which is the infectious tracheal disease (ILT), also caused by the herpes virus, which is a free virus not associated with the cell.
- There are 3 serotypes of Marek’s disease virus that have been identified: A– Serotype I, which is the oncogenic Marek’s disease virus, such as the vaccine strain CVI988 called (Rispens). B– Serotype II, such as the vaccine strain SB-1. C– Serotype III, such as the vaccine strain isolated from turkey HVT. It should be noted that the virulence and carcinogenicity of the disease in chickens is concentrated only in viruses of the first serotype, while the pathogenicity of the other types in chickens has not been determined.
- Although these serotypes can be identified by serological tests, the three serotypes are characterized by having many common antigenic determinants, which suggests that these determinants may interfere in the immune response events in chicks vaccinated with Marek vaccines. This characteristic is followed in many countries by using the trivalent vaccine.
- The virus that causes Marek’s disease is a strongly cell-bound virus, and the free virus of the disease is produced only by the epithelium of the feather follicles of the infected bird, which is the main source of infection and its continuation, noting that one of the field signs in infected flocks is the presence of large quantities of feathers on the bedding. (Therefore, the process of rinsing the poultry breeding halls with water after the end of the flock meal and removing the bedding was the most important process to eliminate the Marek’s virus because it has what is called water phobia (hydrophobia) which leads to its death).
- The free carcinogenic Marek virus naturally infects chicks through the respiratory tract, which quickly moves to be located in the lymphatic organs by phagocytic cells. In the lymphatic organs, the causative virus quickly works at first to cause destruction in the B lymphocytes, which are the cause of the state of immunosuppression in the humoral immune response of the infected chicks. (Of course, this condition must be taken into account when investigating the positive causes of cases of immunosuppression in broilers).
- After the first page of cellular destruction occurs, the Marek disease virus begins to create a latent infection of the lymphocytes, which in this case are mostly T cells, as these cells become active as a result of their response to the infected and destroyed B cells. (Some scientific sources have indicated that Schwann cells and satellite cells in the spinal cord of infected chicks are also a target for the virus causing the latent infection.)
- During the latent infection period of the disease, the virus begins to cause a state of suppression of the cellular immune response (Cellular Immunity), which is very important for the development of lymphoma. The acceleration of the state of programmed death (Apoptosis) of T lymphocytes caused by the Merck’s disease virus is the main reason for the state of immunosuppression caused by the virus. Also, the state of mutation (modulation) of the level of expression in the active molecules of T lymphocytes can have an important role in the state of cellular immunosuppression of chicks infected with the Marek’s disease virus. (One of the most important indications of the possibility of infection of broiler or layer flocks with the Merck’s disease virus in the first weeks of age (3-4 weeks) is the infection of those flocks with coccidiosis, because coccidiosis depends mainly on the cellular immune response, and therefore caution must be exercised when coccidiosis occurs in those flocks. (This is what we have observed in the field in The outbreaks of the Marek virus in 1999-2000 during the so-called Oil for Food and Medicine Program, from which the National Poultry Program emerged.
- It must be noted that the latent infection page of the disease is where secondary destructive infection of the feather follicle epithelium occurs, as a result of which the feathers fall out and the free virus is shed from the infected bird continuously throughout the bird’s life. (Here I repeat the discussion about one of the field signs of flocks infected with Marek’s is the dense presence of feathers on the litter).
- Subsequently, lymphoma will grow in many body organs, such as: nerves, muscles and skin of infected chickens, and in a short life span that may be 3 weeks after infection.
- Because I do not want to bother colleagues with the precise details of Marek’s disease, I would like to point out that the morphological analysis of the cell lines of the lymphoblast infected with the Merck virus, where the lymphatic lesions showed that the virus primarily infects the following lymphocytes: CD3+, CD4+, and CD8- inside the bird’s body.
- Because the Merck’s disease virus is cell-associated, attempts to prepare a vaccine were not as easy as in the case of other disease viruses such as Newcastle and Gumboro. However, in the early seventies of the last century, researchers discovered that early vaccination immediately after hatching with a live vaccine and a non-pathogenic strain worked to prevent lymphoma, and thus the use of the vaccine became one of the most important successful prevention methods in reducing Marek’s disease. Therefore, the three viral types are used today as live vaccines for the disease.
- All three serotypes of vaccines induce a humoral and cellular immune response in the vaccinated bird, and thus neutralizing antibodies can be detected within 1-2 weeks after vaccination and may continue throughout the bird’s life. The role of active immunity shown by maternal immunity has also been detected in hatched chicks. (It should be noted here that there is no cross-reaction between acquired immunity and active immunity resulting from the use of live vaccine in the hatchery.)
- The function of antibodies is to reduce, not stop, the level of infection of the disease by hindering its spread. One of the most striking features of Marek’s disease vaccines is their ability to prevent the formation of cancerous lesions without protecting chicks from infection with Merck’s disease. Therefore, it seems clear that the effect of Marek’s disease vaccines is through their anticancer effects rather than their antiviral effects. Thus, this situation indicates a greater role for cellular immunity provided by Marek’s disease vaccines compared to the role of humoral immunity in reducing the severity of Marek’s disease.
- I have previously indicated that the three serotypes are currently used as live vaccines for the disease, but it must be remembered that not all of these types can provide 100% protection for vaccinated chicks and therefore there were some cases of vaccination failure. As a single vaccine, it can be said that the live attenuated vaccine for serotype I, known worldwide as “CVI988” and also called “Rispens”, is the most effective vaccine for Marek compared to the vaccines for serotypes II (SB-1) and III (HVT). Accordingly, vaccination using two vaccines (Bivalent) or using 3 vaccines (Trivalent) seems to increase the immunogenicity against Marek’s disease compared to using vaccination with a single vaccine. Therefore, these observations clearly confirm that the immunogenicity of the three serotypes and the antitumor (antitumor) seems to differ from one serotype to another. Based on my field experience, which I claim is somewhat long, I recommend By using the triple in broiler chicks and laying hens because it provides sure protection if the vaccination process is controlled.
One of the most important recommendations that must be mentioned is that the chicks vaccinated with the Marek’s vaccine must remain in the hatchery for an acceptable period of time of not less than 3-4 hours in order to stabilize the vaccine virus in the chick’s body and to avoid the field virus interfering with the work of the vaccine virus when the chicks are taken out of the hatchery because it is really a game of chairs and the vaccine virus must succeed in entering its correct place to prevent the field virus from its destructive effect. Therefore, it must be noted that there is no meaning in the saying of re-vaccination with Marek in the field to make the birds’ immunity high because it is not based on a correct scientific fact about the nature of the Marek field virus.
Finally, I would like to point out that all cases of Marek’s disease outbreaks in mother and laying hens are the result of failed vaccinations. This is a major topic that I will discuss at a future time, God willing, because it is very, very important.
In the prevailing custom in the Iraqi poultry industry, broiler flocks are not vaccinated against Marek’s for the reasons I mentioned earlier, but the reality of the current commercial hybrids of broiler chickens, including the commercial hybrid Rose 308, their rapid growth and high production performance, and the great progress in the production of advanced veterinary vaccines, especially for poultry, including vaccines for Newcastle and Gumboro diseases, which are very important in broiler chickens, including vaccines called recombinant vaccines, which is a vaccine composed of the HVT virus of Marek’s , which is the carrier virus, and the (F) protein of the Newcastle virus is integrated into it, and there is also a similar vaccine for Gumboro, and the VP2 protein is integrated into it. Therefore, the use of such vaccines in broiler chickens will be of great importance in warding off the danger of the two diseases mentioned, in addition to a greater importance, which is contributing to controlling Marek’s disease in Iraq by taming the field virus and killing it outside the body.