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Causes of Spleen Enlargement in Dogs

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Spleen Enlargement in Dogs

The causes of spleen enlargement in dogs can be various and warrant investigation by a veterinarian so to identify the underlying cause. A variety of disease processes cause secondary changes to the spleen including enlargement and therefore it's important to have several diagnostic tests run so to identify or rule them out. Whether or not the dog displays symptoms as a consequence of the enlargement depends on several factors such as the underlying disease, size of the spleen and how advanced the disease is. Following are several potential causes of spleen enlargement in dogs.

 Spleen enlargement in dogs is common in middle-aged and older dogs.

Spleen enlargement in dogs is common in middle-aged and older dogs.

Spleen Enlargement in Dogs 

Splenomegaly, is the medical term for the presentation of an enlarged spleen in dogs. It is used to depict the enlargement only, while the term splenic masses is used to depict any asymmetric or solitary areas of enlargement.

A dog's spleen is located in the left side of the dog's abdomen, just following the curve of the stomach. This organ can be described as a sponge with the capability of soaking up large amounts blood. Indeed, at a closer insight, a dog's spleen has been known for storing anywhere between 10 and 20 percent of the total volume.

The spleen carries out several important functions. It produces red blood cells through a process known as erythropoiesis, it produces a variety of immune-related cells, it removes foreign invaders through a process known as phagocytosis, it stores platelets, helps metabolize iron and removes old, damaged or dying red blood cells. The spleen is divided into four main components: white pulp, red pulp, marginal zone and fibromuscular capsule.

In many cases, the first signs of a dog's enlarged spleen are seen on x-ray. Palpation of the dog's abdomen by an experienced vet may reveal an enlargement, but it's not always detectable. A much deeper insight on the state of the dog's spleen and other organs can be obtained through ultrasound or through advanced imaging.

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Symptoms are generally mostly related to the underlying condition rather than the enlargement per se. Affected dogs may exhibit loss of appetite, weight loss, weakness,pale mucous membranes, vomiting, diarrhea, increased drinking and increased urination, abdominal distension and even shock and collapse.

In general, the causes of spleen enlargement in dogs are divided into four distinct categories: inflammatory enlargement, hyperplastic enlargement, congestive enlargement and infiltrative enlargement.

Inflammatory Enlargement of the Dog's Spleen

xrays

In this case, the dog's spleen enlarges significantly because of inflammatory changes. The medical term for an inflamed spleen is splenitis. There can be several underlying systemic disease processes that can cause a spleen to become inflamed. The list can be quite long considering the spleen's tendency to enlarge due to presence of bacteria, viruses, protozoa fungi, and foreign items.

Causes of suppurative splenitis, that is, inflammation of the spleen associated with the production of pus, include penetrating abdominal wounds as seen with dog bites to smaller dogs, migrating foreign bodies such as plant awns, (foxtails), bacterial endocarditis; that is, a bacterial infection affecting the inner lining of the dog's heart, bacterial infections secondary to splenic torsion (when the spleen twists on itself), septicemia; a serious bloodstream infection, and certain viral diseases such as acute infectious canine hepatitis.

Some forms of inflammation of the spleen may cause necrosis, that is, death of cells due to disease, injury, or failure of the blood supply. These causes include splenic torsion due to torsion causing the blood flow to be cut off, acute canine infectious hepatitis, salmonellosis and some types of spleen cancer (splenic lymphoma, splenic hemangiosarcoma).

In some cases, one cause for the enlargement and inflammation of the spleen is a condition known as eosinophilic gastroenteritis. In this condition, a form of inflammatory bowel disease (IBD), there is an infiltration of certain types of white blood cells (known as eosinophils) in the dog's digestive tract. Affected dogs often have a history of chronic vomiting and diarrhea.

Several subacute and chronic infectious disorders known to cause enlargement include pyometra, commonly found in intact, non-spayed female dogs, brucellosis (a contagious disease that affects the reproductive organs of male and female dogs), chronic ehrlichiosis (a bacterial illness transmitted by ticks), haemobartonellosis (another tick transmitted disease) and the chronic form of infectious canine hepatitis.

Some forms of spleen inflammation trigger the immune system to attempt to wall off substances perceived as foreign. This is often the case of systemic mycoses, infections caused by a fungus. Examples of such condition include blastomycosis (a fungal infection caused by fungi living in moist soil), histoplasmosis (infection by a fungi found in the droppings of birds and bats in humid areas) and sporotrichosis (a rare, albeit potentially serious fungal infection).

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Hyperplastic Enlargement of the Dog's Spleen

The term hyperplastic simply means an increase in tissue resulting from cell proliferation. This increase in tissue is meant for defensive purposes in reaction to insults such as red cell destruction or presence of foreign substances. This increase in tissue is therefore responsible for the enlargement of the dog's spleen.

Conditions known to cause this type of enlargement include systemic lupus erythematosus (an autoimmune disease where the body mistakenly attacks healthy tissue), immune hemolytic anemia (another autoimmune disease where the body attacks its own red blood cells).

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Congestive Enlargement of the Dog's Spleen

At the Vet's Office

As mentioned, the spleen can store a great amount of blood. However, excessive pooling of blood may lead to an outflow issue causing an increase in the amount of red pulp. This accumulation of blood is ultimately what causes it to enlarge. This form of enlargement can be seen with torsion of the spleen as blood flow may be compromised and would end up pooling in the spleen.

Excessive pooling causing enlargement may also be seen with the use of some types of sedatives (acepromazine, thiopental, or propofol) and certain anticonvulsants such as barbiturates. This form of spleen enlargement in dogs can therefore often be seen as a consequence of sedation or anesthesia. Halothane in particular has been associated with splenic enlargement.

Other causes include portal hypertension secondary to severe hepatic disease, thrombosis and right-sided congestive heart failure.

"It should be remembered that tranquilization or anesthesia usually results in congestive splenomegaly, making interpretation of splenic size on plain radiographs extremely difficult."~C Guillermo Couto and Alan S. Hammer, Textbook of Veterinary Internal Medicine

Infiltrative Enlargement of the Dog's Spleen 

In this case, the spleen is infiltrated with cells, and the infiltration of neoplastic cells (cancerous) are unfortunately the most common cause of spleen enlargement in dogs.

The types of cancers known for causing infiltrative splenomegaly in dogs include acute and chronic leukemias, malignant histiocytosis, multicentric lymphomas, multiple myelomas and sometimes, although not very common, the cancer can occur as a result of cancer spread from other organs.

Sometimes benign accumulations of cells normally found in the spleen can cause the formation of benign masses that may cause an irregular border on the spleen. Hyperplastic nodules are benign masses, but they cannot be differentiated malignant cancerous lesions by ultrasound.

Non-cancerous causes of infiltrative splenectomy are not very common. They include amyloidosis (the build-up of amyloid an abnormal protein produced by the bone marrow) and extramedullary hematopoiesis.

Did you know? In a study involving 325 dogs, 58 percent of dogs with an enlarged spleen were diagnosed with splenic malignancy, and 32 percent being hemangiosarcomas.

The tissue sample taken from the liver is checked under a microscope.

The tissue sample taken from the liver is checked under a microscope.

Should the vet find an enlarged spleen on palpation, further diagnostic tests may include x-rays and an ultrasound. While the ultrasound procedure takes place, based on the vet's findings, it is possible to take a fine needle aspirate of the spleen. Bone marrow sampling may be also needed in some cases.

Other diagnostic tests may include blood work. A complete blood count may show several signs suggestive of splenic enlargement such as neutropenia; an abnormally low level of neutrophils, thrombocytopenia; a low blood platelet count and regenerative anemia; the bone marrow responding to anemia by increasing production of red blood cells and releasing immature red blood cells.

Based on the vet's findings, removal of the spleen (splenectomy) may be necessary. Removal may be suggested for splenic torsion, splenic rupture, splenic enlargement causing symptoms and presence of splenic masses. The risks for the dog developing sepsis, a life-threatening complication of an infection following surgery are estimated to be around 3 percent.

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