Canine Methemoglobinemia: Understanding the Condition

Characterized by a persistent cyanosis of the visible mucous membranes, the clinical features of this condition are quite striking. Pathological tests reveal a significant increase in the level of methemoglobin in the blood, accompanied by a marked decrease in the activity of the NADHmethemoglobin reductase enzyme within the red blood cells.

Causes of the Disease

The root cause of this disease lies in mutations or alterations in the structural or regulatory genes responsible for the production of NADHmethemoglobin reductase, leading to a congenital deficiency of this enzyme. The primary pathogenic mechanism involves the inhibition of the reduction of methemoglobin to ferrous hemoglobin, resulting in a reduced methemoglobin reduction rate.

In affected dogs, the activity of the NADHmethemoglobin reductase in red blood cells is significantly lower, reaching only about a quarter of normal levels. The concentration of methemoglobin in the blood is notably elevated, accounting for approximately 33% of the total hemoglobin. This leads to a series of functional disturbances and clinical manifestations, including cyanosis and hypoxia.

Methemoglobin levels exceeding 1% of the total hemoglobin content are classified as methemoglobinemia. When methemoglobin levels reach 20% to 50%, symptoms of hypoxia may appear. If methemoglobin levels exceed 60% to 70%, circulatory collapse, coma, and death can occur.

Key Symptoms and Lesions

The disease is familial in certain dog breeds, with the genetic type yet to be determined. It is believed that this condition may share similarities with human counterparts, possibly being an autosomal recessive inheritance. Affected dogs of both genders can become symptomatic shortly after birth or during lactation.

The primary symptom is varying degrees of cyanosis in the visible mucous membranes. The blood in the entire circulatory system, including arterial, venous, and capillary blood, appears extremely dark and brownish. At rest, the dog's body temperature and pulse usually show no significant changes.

Moderate exercise or contact with certain oxidizing agents can trigger symptoms such as rapid breathing, a rapid pulse, muscle weakness, unsteady gait, and mental depression, potentially leading to fainting.

The course of the disease is typically slow, lasting for years or even a lifetime, unless accompanied by other complications. Dogs with this condition generally do not die unless other health issues arise.

Diagnostic Tests

Three diagnostic tests are used to confirm the disease: the color of the blood, the level of methemoglobin in the blood, and the measurement of the NADHmethemoglobin reductase activity in red blood cells.

The characteristic of methemoglobinemia is a chocolatebrown color that does not turn bright red when shaken in the air. Standard hemoglobinometers show a significantly lower hemoglobin content, leading to calculated red blood cell indices such as MCH being lower than normal. However, when tested using the cyanomethemoglobin method, both the hemoglobin content and the corresponding red blood cell indices are within the normal range.

Testing reveals that the methemoglobin content in the blood of affected dogs ranges from 40 to 70 grams per liter, which is about a hundred times higher than the normal value (0.6 ± 0.4 grams per liter). The proportion of methemoglobin in the total hemoglobin can reach 28.6% to 38.8%, which is about 60 times higher than the normal value (0.5% ± 0.3%). The NADHmethemoglobin reductase activity in the red blood cells of affected dogs is 0.5 international units per gram of hemoglobin, which is only 25% of the normal activity (2.12 ± 0.7 international units per gram of hemoglobin).

Diagnostic Criteria

The key points for diagnosing this disease include a family history of the disease, onset at birth or during lactation, persistent cyanosis, brownish blood, and a chronic, stable course of the disease. Clinical manifestations such as the brownish color of the blood that does not turn red when shaken in the air, significantly elevated methemoglobin levels and proportions, and a marked decrease in the activity of NADHmethemoglobin reductase, are also indicative of the disease.

Preventive and Treatment Measures

This disease typically presents a chronic, stable course, with symptoms primarily limited to mucosal cyanosis and brownish blood color. In most cases, treatment is not necessary. In cases where the condition worsens rapidly due to exposure to oxidizing agents like nitrates or nitrites, emergency treatment with agents such as methylene blue and ascorbic acid can be administered. In everyday life, high doses of vitamin C can be taken to keep methemoglobin levels in the blood low, thereby alleviating the condition.