The Importance of Marek’s Disease and Lymphoid Leukosis in Family and Backyard Poultry Farming – Part II

In the first part of this article, the importance of family poultry farming in rural areas and the health challenges it faces, especially due to viral diseases such as Marek’s Disease and Lymphoid Leukosis, was addressed.

• Both present similar clinical manifestations and anatomopathological lesions, making differential diagnosis difficult and affecting the health of the flocks.

This second part delves into the histopathological, diagnostic, and prevention and control strategies.

The objective is to provide rural veterinarians with technical tools to protect native birds and ensure the sustainability of family poultry farming.

HISTOPATHOLOGY OF MAREK’S DISEASE

The histological lesions in Marek’s disease are characterized by:

Figure 6. Histological section of nerve. H.E. staining. A Wallerian degeneration with fragmentation and curling of the myelin sheaths. B mild lymphocytic infiltration. Marek’s Disease.

Figure 7. Histological section of nerve. H.E. staining. A focus of lymphocytic infiltration with cellular pleomorphism. The arrows indicate foci of edema and demyelination. Marek’s Disease.

Figure 8. Histological section of nerve. H.E. staining. Severe lymphocytic infiltration with marked cellular pleomorphism. Marek’s Disease.

Figure 9. Histological section of myocardium. H.E. staining. Separation of cardiac fibers by abundant interstitial infiltration of pleomorphic lymphocytes, including the so-called Marek’s cells. Marek’s Disease.

HISTOPATHOLOGY OF LYMPHOID LEUKOSIS

The microscopic lesions in lymphoid leukosis consist of homogeneous infiltrates of lymphoblastic cells.

• The neoplasms originate in the bursa of Fabricius, in the follicles, where there is a tumoral transformation of B lymphocytes.
• This occurs around the sixth week of age. These are large, very prominent tumoral follicles that compress and displace the normal follicles. (Figures 10, 11, 12 and 13).

Figure 10. Histological section of the Bursa of Fabricius. H.E. staining. Two central follicles with transformation of neoplastic lymphocytes. The other follicles are atrophied. Lymphoid leukosis.

Figure 11. Histological section of the Bursa of Fabricius. H.E. staining. Neoplastic follicle with a uniform population of lymphoblasts. Note the presence of mitotic figures. Lymphoid leukosis.

Figure 12. Histological section of myocardium. H.E. staining. Focus of infiltration with neoplastic cells, predominantly lymphoblasts. Lymphoid leukosis.

Figure 13. Histological section of liver. H.E. staining. Focus of perivascular neoplastic infiltration consisting of a uniform population of lymphoblasts. Lymphoid leukosis.

COMPARATIVE TABLE MAREK VS LEUKOSIS

Table 1. The comparative table allows for a clear differentiation of the clinical, macroscopic, and microscopic characteristics between Marek’s Disease and Lymphoid Leukosis.

DIAGNOSIS IN MAREK’S DISEASE

Clinical history:

• It is crucial to know whether the birds have been vaccinated against Marek’s Disease, which vaccine was used, and the vaccination schedule.

Complete necropsy:

• It must include the inspection of the nerve plexuses and peripheral nerves.

Histopathology:

• Histopathology is essential for an accurate diagnosis, especially depending on the type of lymphoid infiltration and its location.
• The lymphocytic infiltrates in the nerves are pathognomonic of the disease.

Diagnostic tests:

• Immunohistochemistry and PCR are useful for identifying the disease.
• Molecular techniques, such as quantitative PCR, are used to detect the viral genome in tumors.

Serological tests:

• The serological tests include:
  • Immunofluorescence
  • Immunohistochemistry
  • ELISA
  • Agar precipitin test
  • Virus neutralization.

Previously, the key methods for diagnosing Marek’s Disease, from clinical history to specialized tests, are highlighted.

DIAGNOSIS OF LYMPHOID LEUKOSIS

Detection age:

• Lymphoid leukosis is not observable in broiler chicken flocks, since the detection of tumors is generally performed from 16 weeks of age onwards.

Differentiation from Marek’s Disease:

• It is important to emphasize the absence of lesions in the nerve plexuses and peripheral nerves, as these are pathognomonic of Marek’s Disease.

Histological characteristics:

• In lymphoid leukosis, the tumor cell infiltrates are uniform and are composed of lymphoblasts.
• In contrast, in Marek’s Disease, the lymphoid infiltrates are pleomorphic (they present different forms).

Tests for diagnosis:

• Several biological tests have been described for the detection of lymphoid leukosis, including:
  • Virological and serological criteria to detect the virus, viral antigens, and specific antibodies.
  • These tests are useful to ensure the absence of infection in pathogen-free birds and breeders. They also help to ensure that the vaccines are free of viruses.

Limitations of the tests:

• The diagnostic value of these tests is limited, since oncogenic viruses in birds frequently produce infection without inducing neoplasms.

PREVENTION AND CONTROL OF MAREK’S DISEASE

The biosecurity measures currently used in poultry farming influence the prevention of all infectious diseases, including Marek’s Disease; however, vaccination is essential and must be carried out on the first day of life or at 18 days in ovo.

Vaccines with the following serotypes are commercially available:

• Serotype 3 HVT: Strain FC-126.
• Serotype 2: Strains SB-1 and 301B/1
• Serotype 1: Attenuated strain CVI-988 and HPRS-16.

There are also bivalent vaccines such as:

• SB-1 (serotype 2) + FC-126 (serotype 3).
• CVI-988 (serotype 1) + HVT.

And trivalent vaccines such as:

• Serotype 1 + serotype 2 + serotype 3.

Among the recombinant vaccines are:

• HVT + IBD viral gene + ILT viral gene.
• HVT + IBD viral gene + VND viral gene.

PREVENTION AND CONTROL OF LYMPHOID LEUKOSIS

Likewise, implement all biosecurity measures and, if possible, permanently monitor breeder flocks with biological tests for viral detection, viral antigens, and specific antibodies, in order to eliminate birds carrying the virus or those presenting antibodies. There is no vaccination against avian leukosis.

CONCLUSION

• Marek’s Disease and Lymphoid Leukosis are serious threats to family poultry farming. Their control requires accurate diagnosis, strict biosecurity, and vaccination (only for Marek’s Disease).
• It is essential to train rural veterinarians and protect native birds to maintain avian health, preserve genetic diversity, and ensure the sustainability of production in non-technified systems.