Of course, we can't diagnose the doggie over the Internet. But I've
gathered some information that I hope you'll find useful in
understanding the situation.
The medical term for an enlarged spleen is "splenomegaly." An enlarged
spleen is sometimes a minor problem, sometimes a serious one. This is
one of those symptoms that can be related to so many medical
conditions that it's not possible to say much about the dog's
condition without further testing.
Here's an excerpt from a veterinary question-and-answer page, "Ask Dr. Mike":
"Lot of things make spleens enlarge, unfortunately. Splenic
enlargement occurs when pets are anesthetized with some anesthetics,
in stressful or shock inducing situations, when blood parasites are
present, if there is bruising (hematomas) affecting the spleen, if
there is splenic cancer (particularly hemangiosarcoma), when the
stomach torses (twists) and for other reasons, as well. By itself,
splenic enlargment is a pretty non-specific sign and doesn't match up
well with any particular disorder. A lumpy enlarged spleen, or one
that obviously contains a lump in it, is a different story. In this
instance, it cancer or hematomas are much more likely. Often, the only
way to tell these apart is to biopsy the spleen or to remove it and
have a pathologist determine what the disease process is. Hematomas of
the spleen and hemangiosarcoma tumors are very hard to tell apart when
doing surgery, even after good visualization of the spleen."
Veterinary Information for Dog Owners: Spleen Problems in Dogs
This veterinary health center gives a very good explanation of canine splenomegaly:
"Splenomegaly is enlargement of the spleen. It is not a disease;
rather it is a clinical sign associated with an underlying disease
process or disorder. There are many underlying problems that may cause
splenomegaly... Systemic bacterial, fungal, and parasitic infections
affecting other organs and tissues may affect the spleen as well and
cause splenic enlargement. Diseases that cause destruction of red or
white blood cells or platelets and the resulting anemia, leukopenia,
and thrombocytopenia, respectively, may, by a variety of
pathophysiologic mechanisms, cause splenomegaly. Processes that cause
portal hypertension (high blood pressure in the venous system of the
liver) and congestive heart failure may lead to splenic congestion and
enlargement. Bleeding and hematoma formation (bruising) in the spleen
can enlarge it. Tumors such as hemangioma and hemangiosarcoma, a
common malignancy, can lead to splenomegaly. Some primary problems of
the spleen, such as splenic torsion, may cause it as well."
Pet Health Center: What is splenomegaly?
Here's a vet site which gives an excellent description of various
diseases involving the spleen (there is a graphic surgical photo of a
splenic tumor about halfway down the page; if you are squeamish about
such things, you might want to stop reading when you reach the heading
Vet Surgery Central: Spleen
A discussion of splenic masses called "WHEN THE SPLEEN GROWS A MASS:
A DOG?S PERSPECTIVE":
Mar Vista Animal Medical Center: Splenic Masses
Here you'll find some good info on splenic torsion and tumors of the
spleen, two of the many conditions that can cause an enlarged spleen:
Shadetree Kennels: Splenic Torsion
VetCentric: Tumors of the Spleen in Cats and Dogs
Regarding your concerns as a dog-sitter, the best you can do for the
little fellow is to keep him on his prescribed diet (no fatty foods,
no table scraps and such). If his abdominal distention becomes
noticeably worse, if he becomes extremely listless or difficult to
rouse, if he begins vomiting, if there is blood in his urine or feces,
or if he cries out while moving his bowels, seek veterinary attention.
Try to keep him from being extremely active; if he has a splenic tumor
of some sort, activity might cause tearing of the splenic capsule (the
tissue that covers the spleen).
Google search strategy:
Google Web Search: "enlarged spleen" + "dog"
Google Web Search: "splenic mass" + "canine"
Google Web Search: "splenomegaly" + "canine"
Bless you for caring so much about your friend's dog! If anything I've
said is unclear, or if a link doesn't work for you, please request
clarification; I'll be glad to offer further assistance before you
rate my answer.