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.
Evaluation of phagocytic cell function
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.