Photo courtesy of MarVistaVet.com
Red Blood Cell Destruction
Most people have at least a passing familiarity with hemoglobin. Hemoglobin is the iron-containing protein that is responsible for the red color of blood. It binds the oxygen we breathe in and carries it to our tissues, exchanging oxygen for carbon dioxide and carrying that to our lungs to be exhaled away. Our red blood cells have no nucleus and no DNA, and they do not participate in most normal cellular functions; they are simply bags of hemoglobin being conducted around our blood vessels.
A normal canine red blood cell lives approximately 120 days (60 days for feline red cells), and it dies when it either becomes too stiff to fold itself through tiny capillaries and bursts trying to do so, or it simply cannot generate enough energy to keep itself alive.
The spleen is responsible for removing old red blood cells. The spleen possesses many tortuous, winding blood vessels where red blood cells may break if they are not supple enough to pass through. The spleen then collects the hemoglobin for recycling, but since hemoglobin by itself is not safe for release into the bloodstream, a conversion is necessary. The hemoglobin is broken into a protein part (which is broken apart and recycled), iron (also broken off even further and recycled) and heme which is converted into a substance called bilirubin for transport. Bilirubin is sent to the liver to be chemically processed for removal (the process is called conjugation). This process enables the bilirubin to dissolve in water so that it can be excreted in bile and disposed of through the intestinal tract. Bilirubin is responsible for the brown color of feces.
If too many red blood cells are being destroyed at once, the systems described above get overwhelmed.
What is Hemolysis?
Hemolysis is the destruction of red blood cells. Red blood cells may be destroyed by:
- immune-mediated pathways (in which the immune system treats red blood cells as foreign cells to be destroyed)
- the spleen’s pathways described above
- parasites that live in or on the red blood cell (such as Mycoplasma haemofelis).
- toxins such as zinc or copper
- infections or tumors.
When too many red blood cells are destroyed, the body’s mechanisms for processing hemoglobin are overwhelmed, and bilirubin builds up. The patient may be obviously yellow (jaundiced or icteric) when the mucous membranes are examined. The serum may be yellow when checked in a blood sample. If the blood cells are being destroyed in the bloodstream rather than in the spleen, the urine will turn red or rust-colored. Hemoglobin is toxic to the kidneys, and kidney failure is a strong possibility. Excessive hemolysis must be stopped.
This dog has swallowed a coin (readily visible as a white disk in the stomach). Photo courtesy of Jennie Marvelle, DVM, Boothbay Animal Hospital Boothbay, ME
In 1983, the U.S. government began minting pennies made of zinc wafers coated in copper rather than out of pure copper. As it is not uncommon for young animals to swallow pennies, zinc toxicity became recognized.
Other zinc sources include nuts, bolts, zippers (some metal zippers contain zinc), and zinc oxide-based skin creams, such as diaper rash cream and sunscreen. You can see a penny or other radiodense object of about the right size on a radiograph, or there may be a known history of penny eating. It is important to rule out immune-mediated hemolysis as this is a common condition requiring specific (and very different) treatment.
The clinical signs of zinc toxicosis include:
- red urine
- icterus (yellow mucous membranes)
- liver failure
- kidney failure
Vomiting and diarrhea generally precede the destruction of red blood cells. How zinc produces hemolysis is not known. With zinc-containing ointments, usually, only stomach upset is seen after the ointment is licked, but if the ointment is applied and licked regularly, full zinc toxicity can develop.
If an object possibly made of zinc is seen on an X-ray, it should be removed promptly. This might involve surgery, endoscopy, or possibly inducing vomiting if the object does not have sharp edges. Support then becomes crucial. Fluid therapy is important to keep adequate circulation to the kidneys and help prevent failure. Transfusion may be necessary to combat anemia, and anti-nausea therapy is justified. Zinc is corrosive to the stomach so medication to protect the stomach will be needed. Antacids are helpful in reducing the absorption of zinc, which is enhanced by stomach acid. Research is looking at methods for binding excess zinc in the circulation similar to the way lead poisoning is treated.
Once the zinc source is removed, most pets are able to recover within 48-72 hours with supportive care, but of course, this depends on the severity of the poisoning.