Toxicology Brief: Too much of a good thing: Zinc toxicosis in dogs - Veterinary Medicine
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Toxicology Brief: Too much of a good thing: Zinc toxicosis in dogs
Excessive amounts of this essential mineral can lead to serious consequences, and the condition can easily be mistaken for immune-mediated hemolytic anemia. Be sure to follow these guidelines to detect and treat zinc toxicosis in your patients.


VETERINARY MEDICINE


LABORATORY FINDINGS

For a patient that is presented for evaluation of pigmenturia and anemia, a complete blood count, a serum chemistry profile, a urinalysis, a direct Coombs test, a saline agglutination test (SAT), and abdominal and thoracic radiography may be indicated.

Complete blood count

Clinical pathologic findings on a complete blood count can include regenerative anemia, thrombocytopenia, and leukocytosis with a mature neutrophilia. Heinz bodies were reported in 33% of canine patients in a retrospective study of zinc toxicosis.15 The presence of spherocytes is also an inconsistent finding in zinc-related hemolysis.15 In one study, 20% of dogs with zinc-induced hemolytic anemia had mild spherocytosis.15

Serum chemistry profile

Abnormalities that may be seen on a serum chemistry profile include hyperbilirubinemia, hyperproteinemia (not often seen), azotemia, and elevated amylase, lipase, and hepatic enzyme activities.15 Also, a disproportionate elevation in blood urea nitrogen (BUN) concentration compared with creatinine concentration may be seen due to dehydration or gastrointestinal (GI) blood loss.

Urinalysis

Urinalysis may reveal proteinuria and pigmenturia from increased bilirubinuria or hemoglobinuria.15 Proteinuria and casts are often seen.18 To differentiate hematuria from hemoglobinuria, simply centrifuge the urine. Unlike blood cells in the case of hematuria, hemoglobin will not settle out but will remain in the supernatant.

Direct Coombs test

Direct Coombs testing, which is a direct antiglobulin test, is used to diagnose primary or secondary immune disorders causing hemolytic anemia. This test documents complement fixation or antibodies on the surface of RBCs. A positive result can be seen with zinc toxicosis. This can occur because serum proteins are damaged by RBC injury or zinc may interact with proteins on the surface of RBCs and induce an immune response.2

Saline agglutination test

An SAT is an easy in-house assessment of intravascular auto-agglutination of RBCs. Most cases of zinc-induced hemolytic anemia will have negative SAT results. However, because of the possibility that an immune response can be mounted in some cases of zinc toxicosis, the test result may occasionally be positive. Thus, the test is helpful in differentiating IMHA from zinc-induced hemolytic anemia but is not foolproof.


2. This figure represents rouleaux vs. microagglutination. Rouleaux appears as stacks of RBCs like coins. Agglutination appears as clusters of RBCs.
To perform this test, apply a drop of saline solution to a small drop of blood on a glass slide, and assess for agglutination via microscopy (Figure 2). Rouleaux are stacks of RBCs that can be a normal finding and should not be mistaken for agglutination.

Blood coagulation profiles

Activated clotting times, prothrombin times, and partial thromboplastin times may be prolonged due to direct or indirect effects of zinc on specific clotting factors and because the liver fails to synthesize these factors.2

Serum zinc concentration

In many cases, determining the serum zinc concentration is unnecessary to diagnose zinc toxicosis if a zinc-containing object or known source of zinc toxicity is found. Definitive diagnosis of zinc toxicosis is obtained by assessing serum zinc concentrations. Normal serum zinc concentrations in dogs and cats are 0.7 to 2 μg/ml.25 The exact toxic serum concentration of zinc is unknown. Avoid zinc contamination of serum samples by using royal blue top, trace element-free collection tubes.2,3

Radiography


3 & 4. Abdominal radiographs revealed a metallic object, a penny, within the stomach of a dog.
Radiographic identification of a metallic foreign object within the GI tract is highly suggestive of zinc toxicosis in a patient with hemolytic anemia. They are usually found within the stomach (Figures 3 & 4) but can be found anywhere along the GI tract. The absence of a metallic object does not rule out the possibility of zinc toxicosis. Obtaining serum zinc concentrations may also be indicated.

Postmortem findings

Necropsy findings can vary based on the wide variety of clinical derangements caused by zinc toxicosis. A zinc-containing object may be found within the GI tract along with catarrhal gastritis or enteritis and edema or ulceration of the mucosa. The liver may contain centrilobular to diffuse hepatocellular vacuolar degeneration and necrosis. Pancreatic necrosis and fibrosis may be noted. The kidneys may have diffuse tubular degeneration and epithleial necrosis. The brain's grey matter may display shrunken neuronal cells that become triangular with pyknotic nuclei and dark-red cytoplasm. The white matter can develop edema, astrocytosis, and astrogliosis.2,16,19


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Source: VETERINARY MEDICINE,
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