Disease Surveillance and Food safety: Post mortem inspection of Lymph nodes in Food animals
P SENTHAMIL SELVAN*, K RAJALAKSHMI* and POOBITHA S**
*Assistant Professor, Department of Veterinary Anatomy and Histology, RIVER,
** Assistant Professor, Department of Veterinary Pathology, RIVER
Puducherry-605 009. Email- drsenvet@gmail.com,
INTRODUCTION:
Meat inspection is to ascertain quality and food safety to the consumers. It helps to determine the heath of the food animal both before death and after death. Veterinary inspection of the carcass after slaughter to determine its suitability for human consumption is referred to as the Post mortem inspection. Examination of lymph nodes that are associated with the head, viscera and carcass is an important recommended procedure to identify any local or generalized pathology. These pathologies affect the meat quality and these contaminated lymph nodes harbors pathogens that are zoonotic and gets transmitted to the humans posing a public health hazard world wide. Veterinarians play a crucial role in examining food animals for pathologies if any and associated lymph nodes for disease diagnosis and judgment during meat inspection. In examining the carcass for any disease condition and to certify them it is imperative that the veterinarian should be well aware of identifying, locating, palpating and incising (if required) these important and indicated lymph nodes in food animals.
LYMPH NODES TO BE INSPECTED
| Species | Head | Viscera | Carcass | Technique |
| Cattle and calf upto 6 weeks of age, Equine, Sheep and Goat, Porcine | Submaxillary/Mandibular l.n., Parotid l.n., Medial and lateral retropharyngeal l.n., | Bronchial l.n., mediastinal l.n., Mesenteric l. n., Supramammary l.n.,
Hepatic/portal l.n., |
Prescapular l.n., Femoral l.n., popliteal l.n., superficial inguinal l.n., External and Internal iliac l.n., pre pectoral l.n.,
and Renal l.n.,
sub rhomboideal l.n., (melanosis) in horse |
All lymph nodes are visual inspected, Palpated and incised if required
|
Abnormal lymph nodes detected on palpation can be incised except for abscess in them to determine consistency. Two to three incisions parallel to each other right through the node is appropriate. Suspected nodes are also excised and referred for more detailed histopathological or microbiological examination if required (disease surveillance and eradication) before passing it for human consumption..
POSITION AND IDENTIFICATION OF THESE LYMPH NODES
1.Lymph nodes in Head inspection
a.Parotid lymph node: It is situated ventral to the temporo mandibular joint on the caudal part of the masseter muscle and is partly covered by the parotid salivary gland. This lymph node is Incised in bovines to detect tuberculosis
b.Mandibular lymph node: This lymph node is located between the sternocephalicus muscle and the ventral part of the mandibular salivary gland. It is incised in porcine to detect tuberculosis and Actinobacillosis in cattle,
c.Retropharyngeal lymph node: They are medial and lateral. Medial nodes are present near the great cornu of the hyoid bone, medial to the stylo hyoid muscle between the pharynx and ventral straight muscles of the head. The lateral nodes are small and are lateral to the pharyngeal wall under the edge of the wing of the atlas.
- Atlantal Lymph node:It is situated ventral to the wing of the atlas, on the tendon of cleido-mastoideus muscle, and is partly under the cover of the mandibular salivary gland.
2.Lymph nodes in Visceral inspection
In postmortem examination of viscera, the lymph nodes associated with the visceral organs are also examined for any pathologies associated with infectious and zoonotic diseases in food animals. These lymph nodes and their location are as given under.
a.Bronchial Lymph nodes: These lymph nodes are examined when examining the lungs. They are three or four in number and include a left bronchial gland, right bronchial gland, dorsal bronchial gland above the tracheal bifurcation and an apical gland at the level of the apical bronchus to apical lobe of lungs. These nodes are often incised to check for tuberculosis.
b.Mediastinal lymph nodes: The mediastinal lymph nodes are examined when examining the thoracic viscera. They are dorsal mediastinal, ventral mediastial, anterior mediastinal and posterior mediastinal lymph nodes. The dorsal mediastinal lymph nodes are present on either side of the thoracic aorta. The ventral mediastinal lymph node are present along the transverse thoracis mucle on the floor of the thorax. The anterior mediastinal lymph nodes are situated at the thoracic inlet, and along the esophagus, trachea, anterior venacava and brachiocepahlic trunk. The posterior mediastinal lymph node are along the esophagus posterior to the aortic arch.
c.Mesenteric lymph nodes : Mesenteric lymph nodes include a large number of elongated nodes that lies in the mesentery between the peritoneal folds. They receive lymph from the intestine. According to their distribution and supply along the length of the small and large intestine they are referred to as the duodenal, Jejunoileal, caecal, colic, rectal and pancreatico-intestinal lymph nodes. These lymph nodes are examined during inspection of the intestines.
d.Hepatic/Portal Lymph node: These lymph nodes are associated with the liver and are examined during its inspection. They lie ventral to the portal fissure and are in groups around the portal vein, hepatic artery, bile duct and are covered by pancreas.
3.Lymph nodes in carcass examination
a.Prescapular lymph node: This lymph node is identified along the cranial border of the supraspinatus muscle and is covered by brachiocephalicus and omotransversarius muscle.
b.Pre Femoral lymph node: It is present in front of the cranial border of the Tensor fascia latae muscle in the middle of the line connecting tuber coxae and patella.
- Popliteal lymph node:This is situated in the fat on the gastrocnemius muscle between the gluteobiceps and semitendinosus muscle.
d.Superficial inguinal lymph node:
Mammary lymph node: in females: This is located above the caudal border of the base of the udder. These nodes are to be examined even in dry cows when slaughtered.
Scrotal Lymph node: in males: It is present below the prepubic tendon at the neck of the scrotum
e.External iliac Lymph node: It is present at the bifurcation of the circumflex iliac artery.
f.Internal iliac Lymph node: This lymph node is present at the termination of the abdominal aorta.
- Posterior cervical/ Pre pectoral lymph node: They lie at the level of the manubrium sterni near the thoracic inlet.
h.Renal lymph node: It is situated along the course of renal vessels.
i.Subrhomboidial lymph node in horse: It is beneath one of the two scapular cartilages of all grey or white horses. This lymph node is to be examined for melanosis after loosening the attachment of one shoulder.
PATHOGENS IN LYMPH NODES OF PUBLIC HEALTH IMPORTANCE
Lymph nodes represent microbiological filters and may therefore be a source of contamination of the carcass and meat during inspection and in further processing. They are the potential sources of pathogenic bacteria in ground meat. Contamination levels differ between animals. The microbial load is higher in the lymph nodes of culled animals than animals reared exclusively for meat.
Cattle: Lymph nodes in cattle are reported to harbor pathogenic organisms such as multiple drug resistant Salmonella enterica, E. coli O157:H7/ Shiga-toxigenic E. coli (STEC), Campylobacter, Yersinia spp, Listeria monocytogenes, Mycobacterium bovis, Mycobacterium avium paratuberculosis, Clostridium spp, Cryptosporidium parvum, Toxoplasma gondii. etc., Unless these suspected animals and lymph nodes are removed from the processing chain they are to pose a significant food safety threat to the consumers. It has also been reported that the prevalence of a specific pathogen is also influenced by the seasonal changes and body regions. Example, Salmonella enterica is reported to be higher especially in the subiliac and inguinal lymph nodes during the summer season and in early fall in cattles slaughter for meat.
Sheep and goat: Some zoonotic bacterial species, often localize in lymph nodes and induces characteristic lesions such as granulomatous inflammation in them. Bacterial isolates from the mediastinal, trachea-bronchial and mesenteric lymph nodes in sheep and goat are reported to contain C. pseu dotuberculosis that cause caseous lymphadenitis in sheep and goat. Other organisms include, S. aureus, S. dysgalactiae subsp. equisimilis, or T. pyogenes. Further slaughter house workers, veterinarians or other industry employees who are in direct contact with these infected animals and tissues are exposed to these zoonotic infections with these microorganisms.
Porcine: In pigs, the prevalence of Salmonella enterica is more in the mandibular lymph node. of sows. Other lymph nodes such as ventral superficial cervical, Inguinal, popliteal and subiliac lymph nodes are also reported to harbor them. Other zoonotic pathogens that are transmitted through lymph nodes include Streptococcus spp, Mycobacterium spp, Staphylococcus aureus, Rhodococcus equi, Toxoplasma spp, Brucella abortus, Brucella suis. Japanese encephalitic virus, Psedorabies virus etc.,
Rabbit: In rabbit, Francisella tularensis is a zoonotic gram negative non spore forming bacteria that lives intracellular (inside macrophages) can contaminate meat through lymph nodes and can cause Rabbit fever in humans. Two out of four subspecies are responsible for majority of the human disease, these are F. tularensis biovar tularensis (or Jellison type A) and F. tularensis biovar palaearctica (or Jellison type B). The other two less prevalent subspecies of F. tularensis include mediasiatica and novicida. Some of the other zoonotic bacteria that are transmitted through contaminated lymph nodes include E coli, Pasturella multocida, Salmonella spp, Staphylococcus aureus and Hepatitis E virus etc.,
Prevention of foodborne disease due to contamination of meat through infected lymph node in future should be given importance during meat inspection procedures and elimination of these LN from meat and meat products is mandatory. Inclusion of these lymph nodes can be permitted only if the meat in general or its products are destined for cooking before human consumption.
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