The spleen is a blood-filled (vascular) organ found in mammals and birds. It is located in the abdomen and lies near the stomach. It has no function in digestion but is a primary storage site for blood – as much as twenty percent of the total blood volume can be stored in the spleen. The spleen also makes both red and white blood cells although the bone marrow is the primary source of these cells. The spleen also acts to filter out old red blood cells and to reclaim the iron in these cells. The spleen is a blood filter and is of primary importance in the immune system; antibodies to viruses, bacteria, parasites are stored in the spleen and can be activated to neutralize these pathogens when they are recognized in the body. In medieval times the spleen was considered to be the site of the “humors” – emotional states such as anger or melancholy. “Venting one’s spleen,” means to “tell it like it is”, to speak one’s mind. We no longer believe the spleen has any connection to our emotions, but it is certainly a major organ in immune function and blood storage.
The spleen is located in the abdomen and lies near the stomach.
The major problem with ferret spleens is they get big! Just about everybody’s spleen gets big due to certain types of diseases – tumors being the major one and autoimmune problems being another. Ferret spleens certainly can get big for these reasons but they also get big seemingly just because they get big! “Idiopathic Splenomegaly” (or spleens getting big because we don’t know why they get big) is the major reason for a big spleen in ferrets. This is very different from other mammals where big spleens are associated most definitely with disease. Dr. Bruce Williams, a well-known veterinary pathologist, has probably looked at more ferret spleens than anybody else and he estimates that only about 5% of all the ferret spleens he examines are cancerous.
He also states that a large proportion of older ferrets have enlarged spleens without obvious clinical disease.
Benign extramedullary hematopoiesis is the term for enlarged spleens, which are large due to massive accumulations of red and white blood cells. Why it occurs and what, if anything, it means for the general health of ferrets is a controversial subject. EMH has been linked to adrenal disease, Helicobacter infections, and other gastrointestinal infections, to Marshall Farms ferrets, and probably this year, in California, to the influence of El Niño!
I cannot comment on El Niño and think is useless to demonize Marshall Farms, but I will try to discuss these other diseases as well as lymphosarcoma, the most common tumor of the ferret spleen.
Lymphosarcoma is the most common malignant tumor of the ferret. Insulinomas and adrenal tumors are more common and certainly cause problems, but they are far less likely to metastasize or spread than lymphoma. Lymphosarcoma is defined as a tumor of the lymphatic system; the lymphatic system is comprised of the spleen, liver, peripheral blood, peripheral lymph nodes and the bone marrow. Two types of lymphoma are seen in ferrets; the most common type affects older ferrets and can cause enlarged lymph nodes or just general lethargy, loss of appetite and depression (lethargy in a ferret – how do we define that?). The other type affects young ferrets under two years of age, often younger than one year. The youngest ferret I personally ever saw with lymphoma was ten months old. Young ferrets with lymphoma are very severely affected very rapidly and usually die from the disease despite treatment. The young ferret I mentioned previously was playing happily at home in the morning, lethargic at noon and was dead of massive splenic hemorrhage by 4 o’clock that afternoon despite emergency surgery and blood transfusions. These young ferrets almost always have enlarged spleens and livers and there is nothing benign about them. These ferrets are obviously ill, usually shocky, cold, and pale; sometimes they are having trouble breathing due to enlarged lymph nodes or an enlarged thymus gland in the chest.
Diagnosis of lymphoma or any other tumor affecting the spleen is usually made either via a surgical biopsy of the spleen, or a fine-needle aspirate of the spleen. A fine-needle aspirate involves using a small needle and syringe to take a direct sample of the spleen through the skin. Since one is usually doing this on animals with big spleens, finding the spleen is not an issue and most ferrets need only mild sedation for this procedure. Enlarged lymph nodes can also be aspirated, but ferret lymph nodes are surrounded by fat and make aspirating the node somewhat difficult. Lymph node biopsies are generally better than aspirates.
Lymphoma can be treated in a variety of ways, everything from aggressive chemotherapy to benign neglect. Splenectomy is usually not beneficial except in a case such as the one I mentioned where a ferret is bleeding internally due to splenic hemorrhage. Most older ferrets with lymphoma are slowly progressive in terms of their symptoms and many can survive for months to years without treatment as long as something else doesn’t get them first. There are several different chemotherapeutic protocols for ferrets, which vary widely in terms of cost and efficacy. Most ferrets tolerate chemotherapy quite well and side effects are usually not severe. Other types of cancer such as hemagiosarcoma or adenocarcinoma are much less common in ferrets; in these cases the spleen should be removed.
Now that we have dealt with cancer as a cause of big spleens in ferrets, we are left with extramedullary hematopoiesis! Once again, this is by far the most common reason for big spleens in ferrets and no one really knows why. Dr. Bruce Williams and several other veterinarians who are very experienced in ferret medicine believe that EMH is due to chronic infection with Helicobacter mustelae. Helicobacter is a bacterium, which generally infects the stomach and surrounding lymph nodes. Its kissing cousin Helicobacter pylori is a common cause of gastric ulcers in people.
Helicobacter mustelae is believed to be a cause of ulcers and chronic gastrointestinal disease in ferrets as well. Dr. Williams makes the case for EMH due to Helicobacter infection based on its common, almost ubiquitous, occurrence in older ferrets that also have enlarged spleens. By definition, extramedullary hematopoiesis is caused by demand for more blood cells, both white and red. Anemia could certainly create more demand for red blood cells but, aside from ferrets with lymphoma, ferrets with big spleens are rarely anemic. However chronic infection will decrease both red and white blood cells and the body will try to make more of these in order to fight off the infection. Helicobacter is an infection, which smolders but rarely becomes a forest fire consuming its host. Very recently a test for Helicobacter has become conveniently available; we may be able to test more of these ferrets with big spleens to see if they have concurrent Helicobacter infections.
The standard treatment for Helicobacter infection is a combination of Amoxicillin, Metronidazole, and Pepto-Bismol. This can work extremely well for some ferrets; for others it seems to make little difference or it helps the ferret when she is under treatment, but signs recur when she has finished the medication. In my experience it helps most those ferrets that have definite signs of gastro intestinal illness – loose stool, nausea, retching, and hypersalivation. For older ferrets that are just lethargic (excepting those with isulinoma) I have not seen a great deal of improvement. Probably Helicobacter is one of those infections that we may be able to control but not wholly cure.
Adrenal disease is another postulated cause of EMH in ferrets; once again, adrenal disease is so common in older ferrets that it would have to exist at least coincidentally in a large number of ferrets with big spleens.
Once again, the link between adrenal disease and EMH is not definitively proven; once again it is postulated that chronic stimulation of the adrenal glands causes a chronic stimulation of the spleen.
I have done a fair number of adrenalectomies on ferrets; in some, I have seen the spleen decrease in size post-operatively, in some it has made no difference. Not all ferrets on which I have performed adrenalectomies had big spleens to begin with. I am personally more dubious about the link between adrenal disease and EMH than I am about Helicobacter and EMH, but personal belief and anecdotal evidence only get you so far; sometimes it’s so far in the wrong direction.
So what do we do about ferrets with big spleens that do not have lymphosarcoma or Helicobacter or adrenal disease? Well, we set up a breeding colony of all three of these ferrets and they go wild and kill cattle and poultry because their natural killer instances are unfettered by clinical disease! Oh, sorry, that’s what Fish and Game would say. All kidding aside, I do see ferrets with big spleens who feel good, who eat well and have normal stools and who have all their hair. What do I do to these ferrets? Nothing, as long as the ferrets show no signs of illness. I do like to do splenic aspirates when possible. Sometimes ferret owners hesitate because I do have to warn them about the small risk of bleeding from a splenic aspirate and because I prefer to sedate ferrets for this brief procedure. Although a splenic aspirate can certainly be done on a conscious ferret if they are cooperative, I feel that the procedure is a little uncomfortable and I really do not want a ferret jumping around on the end of a needle.
If the spleen is very enlarged and the ferret does not feel well, then splenectomy is often the best course. It must be understood however that splenectomy removes a major organ and ferrets can be more predisposed to infection post-splenectomy. But if the poor ferret is dragging his abdomen on the ground there is not much choice. The bottom line is that ferrets with big spleens need to be evaluated for underlying disease in the same way that ferrets with enlargements of any other organ need to be evaluated. Only if there is no evidence of any other disease process and the ferret is comfortable and shows no clinical symptoms should we shrug our shoulders and move on to the next case.