Anatomopathological diagnosis in porcine respiratory processes (part I)

Knowing the pathological lesions, sampling and diagnosis of the most common and important swine respiratory conditions, based on the available literature and practical experience is fundamental to address the Porcine Respiratory Disease Complex (PRDC).

Unraveling the mysteries of the respiratory processes of pigs

Respiratory diseases in pigs can be classified into two broad categories depending on the extent and duration of the disease:

1.Respiratory diseases that affect a large number of pigs, can be serious and costly, but of limited duration:

Swine Influenza

Pneumonic form of Aujeszky’s disease

Porcine Circovirus Associated Disease (PCV2)

Porcine Reproductive and Respiratory Syndrome (PRRS)

2. Respiratory diseases that persist in a large number of pigs for indefinite periods (Mycoplasmic Pneumonia and Pleuropneumonia).

In these cases, it is difficult to keep the herds free of respiratory diseases, being endemic in many farms.

Enzootic Pneumonia, for example, when caused only by Mycoplasma is of little relevance. However, when combined with a secondary pathogen such as Pasteurella multocida, the resulting condition can be severe.

 First stop 
Mastering the necropsy technique

Performing a necropsy is a relatively simple process for which there are different procedures available in the literature. In any case, and independently of the method used, the necropsy should always be ordered, systematic and complete, avoiding leaving important organs or tissues unexamined, regardless of the present symptomatology and the presumptive clinical diagnosis.

To obtain the best performance, the necropsy should be performed as early as possible after the death or sacrifice of the animal. In this article, the authors focus on the opening and examination of the thoracic cavity and respiratory system.

1- Placement of the body

Supine position

Generally, the most common is to use the supine position for young or lactating animals.

To stabilize the body, cuts are made in the axillary region and in the coxofemoral joint, letting the extremities fall under their own weight on the table.

Lateral decubitus

In older or adult pigs, the position in lateral decubitus is more manageable, on the right side, cutting in the axillary and coxofemoral region and dropping the extremities or extracting them to expose the entire thoracic and abdominal wall (Image 1).

Image 1. Stabilization of the body in lateral position.

2- Thoracic cavity opening

Costcondral access

In supine position, two cuts are made on the skin and soft tissues from the mandibular symphysis, forming a triangle to the entrance of the thorax.

Later, a cut is made at the costochondral level on each side to expose the thoracic cavity.

Image 2. Opening of the abdominal cavity in lateral decubitus, with cutting direction from last rib – linea alba – lumbosacral vertebrae.

Costovertebral access

In lateral position, the procedure is easier if the abdominal cavity is opened first:

A cut is made from dorsal to ventral, following the path marked by the last rib to the caudal part of the sternum or xiphoid cartilage, and then continuing along the linea alba, from the caudal part of the sternum to the ischiopubic symphysis, and finally dorsally from the pubis to the lumbar vertebrae (Image 2).

  Once the abdominal cavity is open, the diaphragm should be punctured near the sternum and the air intake, or its absence, should be observed/heard as the lungs collapse.

  Then, the insertion of the diaphragm along the costal arch should be cut and released, taking into account the amount and type of fluid.

  To open the chest cavity, the ribs are cut through the costocondral joint.

  Finally, the rib is bent dorsally using the costovertebral union as a support, something that can be difficult in large animals, requiring the use of a costotome (Image 3).

Image 3. Opening of the thoracic cavity in lateral position using the costovertebral junction as support

The previous removal of the skin facilitates this procedure. In this way, we will obtain a complete vision of the thoracic cavity and detect if there are any anomalies such as the presence of liquids or exudates.

3- Extraction of the respiratory system

For a correct and complete examination of the respiratory system, it must be completely removed from the most cranial point.

For this purpose, the tongue is separated from the jaws, the soft palate is sectioned cranially to the palatine tonsils, and the hyoid bones are cut, in order to remove the musculature of the lingual base, larynx, pharynx, esophagus and trachea in block, separating from the cervical musculature up to the entrance of the thorax.

At this point, by traction or using the knife, lungs and heart are extracted together.

The thoracic portion of the aorta and caudal vena cava are sectioned before the diaphragm, as well as the esophagus, which should be caudally linked to the section to preserve the gastric content.

4- Examination of the respiratory system

For a correct examination of the respiratory system, a longitudinal section is made in the cranio-caudal direction from the larynx to the main bronchi of each lung. This is important to identify possible lesions in the mucosa or abnormal contents in the respiratory tract.

Lung

The examination of the lung should always be done by palpation and includes the evaluation of the following characteristics:

Color: color can give valid information, but often color variations are not diagnostic, especially in animals that have been dead for several hours.

Consistency: changes in lung consistency usually have greater diagnostic value, since they provide information about the presence of edema, areas consolidated by pneumonia, neoplasms, parasitic granulomas, etc.

Sections should be made in the thickness of the different lobes in search of small focal or multifocal intraparenchymal lesions, which may go unnoticed externally or on gross palpation.

It is important to examine the nasal cavity, especially in cases of chronic or atrophic rhinitis. For this purpose, a cross section of the nasal region is made at the height of the first premolar. This allows us to easily detect alterations in shape or symmetry of the nasal turbinates (Image 4).

Image 4. Opening of the nasal cavity for assessment of the nasal turbinates

(Bibliographic references and further reading recommended are available upon request)