Lately several inquiries have been submitted to SESC regarding pig carcasses with erythema (skin redness) and alterations of lymph nodes.

  • Lymphadenomegaly: enlarged lymph nodes.
  • Lymphadenitis: inflammation of lymph nodes, which when generalized is often an indicator of a systemic infectious disease in that carcass. However, this is not always the case.
  • Lymphadenopathy: term that refers to a non-specific change (at a macroscopic level) of the lymph nodes, that present with increased size and which may be regional or generalized.

In this post we have tried to review the possible causes that can cause lymph node changes:

  • Benign lymphoid hyperplasia: nonspecific immune reaction in the lymph node, where no inflammatory exudate can be observed . It may be accompanied by oedema or congestion. It can be a physiological process or as a result of a viral, bacterial or protozoan infection. In pigs it may have cystic appearance due to the lobular architecture of their lymph nodes.
  • Stress Response: pigs respond in a peculiar manner to stress during handling. Poor management or overexcitement can cause hyperacute bleeding / blood reabsorption in lymph nodes and erythematous lesions of the skin and subcutis. These need to be distinguished from lesions associated with sepsis or viremia.
  • Lymphadenitis: presence of an inflammatory reaction in the lymph nodes, can be acute or chronic and can have different causes.
    • Suppurative lymphadenitis: characterized by the presence of purulent exudate. Associated with bacterial infections
    • Necrotizing Lymphadenitis: presence of necrosis. Associated with Salmonella typhisuis infection , mycobacteria, fungi and Actinomycetal-type bacteria (Nocardia spp).
    • Granulomatous lymphadenitis: hardened, well defined or encapsulated nodules can be observed, sometimes mineralized (nodular form), or an increase in size without apparent macroscopic lesion (diffuse). Microscopically macrophage infiltration can be observed. The nodular form is compatible with TB, fungal or parasitic migration, the diffuse form is typical of PMWS.
  • Neoplasia: can be primary (a malignant lymphoma is relatively common in pigs) or, less frequently, it might be a metastasis (carcinoma, mesothelioma, etc.).
  • Other changes:
    • Oedema: increased size and humid appearance when cut, remember that lymph nodes drain fluid extravasated from the tissues.
    • Reabsorption of blood / hemorrhage: dark reddish appearance, lymph nodes drain extravasated blood in the tissues. It can also be primary vascular damage due to viremia or sepsis. If it is blood drainage red areas correspond to peritravecular and subcapsular sinuses, while if it is bleeding itself the reddish coloration is diffuse throughout the lymph node parenchyma.
    • Emphysema: presence of air bubbles adhered to the lymph node itself, in pigs, associated with intestinal emphysema. Increase in size and they float in the water.
    • Hemosiderosis: hemosiderin deposits, mainly observed in subcapsular and peritravecular sinuses and especially visible in young animals that received injection of iron, giving a dark brown colouration to lymph nodes.
    • Anthracosis: deposits of carbon pigment giving a black discoloration.
    • Melanosis: congenital deposits of melanin pigment.
    • Ceroid pigment deposition: greenish, metabolism byproducts of lipid substances.

List of most common diseases that may present with lymphadenopathy:

Disease / Agent

Localization

Macroscopic lesion

BACTERIA
Arcanobacterium pyogenes Widespread or focal Suppurative lymphadenitis.
Bacillus anthracis Mandibular and suprafaringeal LN.

Increase in size.

Haemorrhage and necrosis.

Clostridium perfringens Peritoneal LN Haemorrhage. Emphisema.

Exhudative epidermitis

(Staphylococcus hyicus)

Superficial LN

Increase in size. Oedema.

Suppurative lymphadenitis.

Erysipelas

(Erysipelothrix rhusiopathiae)

Widespread Increase in size, congestion, subcapsular haemorrhage.
Mycoplasma hyopneumoniae Bronchial and mediastinic LN  Increase in size.
Rhodococcus equi Cervical and submaxilar LN Necrotizing lymphadenitis
Salmonelosis Peritoneal LN Oedema, haemorrhage and necrosis.
Septicaemia by Escherichia coli

Starts in mesenteric LN

Widespread.

Increase in size. Congestion.

 

Septicaemia by Salmonella Widespread, but particularly in gastrohepatic LN. Oedema and increase in size.
Septicaemia by Streptococcus (S.equisimilis, S.suis) Widespread or focal

Increase in size.

Suppurative lymphadenitis.

Tuberculosis

(M. bovis, M. avium)

Widespread or focal Granulomatous/necrotizing lymphadenitis.
VIRUS
Aujeszky’s disease Mainly in oral cavity and upper respiratory tract LN. Increase in size, Haemorrhage and necrosis.
Porcine Circovirosi (PMWS) Widespread

Increase in size.

Granulomatous lymphadenitis.

Cytomegalovirus Thoracic LN Increase in size.
Japanese B encephalitis Widespread Congestion.
Porcine Influenza Bronchial and mediastinic LN. Increase in size.
Classical swine fever and other pestiviruses (BD, BVD) Widespread Increase in size, oedema and haemorrhages.
African swine fever Widespread, mostly gastrohepatic LN. Increase in size, oedema and severe haemorrhages.
PRRS Thoracic LN, Widespread Oedema, brown colouration and increase in size.
OTHERS
Toxoplasmosis Widespread Increase in size.
Lymphoid neoplasias (Multicentric lymphoma, lymphosarcoma). Widespread Increase in size.

When lymphadenopathy is observed other lesions must be looked for in the skin or viscera suggesting viremia or septicemia such as bleeding (petechiae, ecchymosis, purpura), pustules, papules, crusts …

To diagnose these processes formalin fixed samples shoud be submitted of the affected skin, organs and lymph nodes. Keep in mind that the process of blanching the carcass causes significant artifacts to the skin and often precludes the obtention of an accurate diagnosis. To facilitate the diagnosis is also advisable to attach samples of the  unaffected skin of the same animal to compare it with the lesioned parts.

If septicaemia is suspected liver or spleen samples can be submitted (organs with high blood content) refrigerated for microbiological culture.

Moderate lymphadenomegalia and blood resorption (regional drainage). In this case the most likely cause was a dermatitis due to exposure to solar radiation with a secondary reactive lymph nodes hyperplasia.

Moderate lymphadenomegalia and blood resorption (regional drainage). In this case the most likely cause was a dermatitis due to exposure to solar radiation with a secondary reactive lymph nodes hyperplasia.

A second example of the case described in Figure 1.

A second example of the case described in Figure 1.