The patient that presents with recurrent infections may have an underlying immune deficiency or dysfunction. Depending upon the age of the patient and the age of onset, the problem may be genetic (primary immunodeficiency) or acquired. In either case it is possible to carefully dissect the function of the immune system to determine which if any of the immune functions are abnormal. A typical history might be that the patient developed a bacterial infection, which responded well to antibiotics, and then when the antibiotics were discontinued, the patient relapsed. If the recurrent infections have been occurring since early in life, the possibility of a primary immunodeficiency is high. Many primary immunodeficiencies have been described in dogs, including those affecting innate immunity, humoral, cellular, and complement. Older dogs may have predisposing conditions that have compromised one or more of their immune constituents.

A focused approach to immune system evaluation

There are a variety of assays available to test immune function in the dog. The choice of assay should be based on: 1) the type of pathogen causing the infection and the body system of the patient that is most frequently infected, 2) breed, age, and history of the patient, 3) evaluation of the complete blood count (CBC). For example, a dog that has a high neutrophil count and evidence of a bacterial infection that recurs after antibiotic withdrawal may have a neutrophil defect (such as inability to phagocytose and kill). In contrast, a dog with chronic fungal infection would be more likely to have a T cell defect. A young German Shepherd with recurrent lung or gastrointestinal infection may have a selective IgA deficiency. For each of these there are diagnostic assays that can assist with pin pointing the defect.

The first parameter to assess is cell number and morphology. Dogs afflicted with Cyclic Neutropenia will show normal neutrophil numbers when they are non-symptomatic, but will have drastically lowered numbers when they are sick. This syndrome occurs in grey coat color collies, which is a big hint that neutrophils need to be examined. For other dogs neutrophil numbers may be normal, increased, or decreased in the presence of bacterial infection. Once it is established that the level of cells is not abnormally low, the various processes required for them to find and kill infecting pathogens must be examined.

Determining the ability of the neutrophils to carry out their function involves independent assessment of: adhesion molecule expression, response to chemotactic stimuli, ability to engulf, and finally intracellular killing (both oxidative and non-oxidative systems are involved). The genetic defect resulting in defective expression of the integrin CD11b/CD18 causes an inability of the neutrophil to respond to chemotactic stimuli and emigrate from the blood to the site of infection in tissues. This uncommon condition is called Canine Leukocyte Adhesion Deficiency (CLAD) has been described in the Irish Setter breed. Affected dogs die early due to recurrent bacterial infections. An extreme leukocytosis coupled with an absence of pus in the peripheral tissues in the presence of bacterial infection is typical of a CLAD afflicted dog. Defects in chemotaxis can be evaluated using a Boyden chamber, in which the patient's neutrophils are separated from a chemotactic stimulus by a semi permeable membrane. When patient is compared with control cells depression of chemotactic activity can be documented. Another assay is used to evaluate the ability of the patient's cells to engulf bacteria. Incubation with bacteria or with opsonized particles followed by fixation of the cells and observation of the stained slide under the microscope is used to calculate a phagocytic index, to evaluate intake by phagocytosis. There are several tests available to evaluate killing. Oxidative killing is examined using the nitroblue tetrazolium test. Finally, overall ability to engulf and kill can be determined using a bactericidal assay. This assay is laborious and thus expensive, but it is perhaps the best way to evaluate the effectiveness of neutrophil killing. These assays are performed in clinical veterinary immunology laboratories, but they do require prior notice and cells that are fresh and viable. Most labs will also ask that you submit a sample from a normal control for comparison when the bactericidal assay is performed.