Lymph nodes (a.k.a. lymph glands) are composed of lymphoid tissue and form part of the lymphatic system which represents an integral and extensive component of the circulatory and immune system. The lymphatic system drains tissue fluid, called lymph, and filters it via the lymph nodes, spleen and thymus before returning it to the bloodstream. Aside from its role concerning immunity, the lymphatic system also helps regulate body fluid levels.
Lymph nodes are found in clusters at certain locations on the dog’s body e.g. armpits, groin. They help filter microscopic foreign particles, bacteria and viruses as well as cancer cells, which are then attacked and destroyed by the residing white blood cells (lymphocytes – B and T cells – and other immunity cells).
The causes of enlarged lymph nodes can include cancer, or a response to either infectious agents e.g. bacterial or fungal, or non-infectious agents, such as those associated with allergens or rarer conditions e.g. Canine hypereosinophilic syndrome (eosinophil overproduction) Prakash et al.2012. The correct medical term for enlarged lymph nodes is Lymphadenomegaly, however it is common to see the term Lymphadenopathy (lymph node pathology) used in a synonymous manner.
Variability in lymph node response means that enlargement may range from barely detectable through to significant, and though it usually occurs in those lymph nodes local to an allergic response, this may not always be the case. Areas commonly affected include the armpit, groin, below the jaw, back of leg and shoulder. Distinct from other allergic response symptoms, and depending where the enlarged lymph node occurs, the actual physicality of enlargement may make it difficult for the dog to breathe, eat, defecate or urinate.
Diagnosis involves an examination – both health history and physical – of the dog, as well as various blood and urine tests. In addition, further diagnostic tests may need to be employed to rule out other conditions e.g. scans, ultrasounds, microscopic examination of samples (aspirates and biopsy).
Lymphadenopathy treatment will be determined by its cause, and when due to an allergic reaction, the allergic condition per se will be treated in ways as discussed in-depth in the section dog allergy treatment. This may include symptomatic treatment e.g. antihistamines, or the application of immunotherapy e.g. immunosuppressive drugs such as cyclosporine. However, in some cases an infection or secondary infection can develop that may require antimicrobial treatment e.g. antibacterial or antifungal medication.
A tick bite is a good example of the last point. Although well-documented in humans, tick-related infections and diseases are being increasingly acknowledged as an emerging problem in dogs and this includes temperate/urban environments (Shaw et al. 2001). The bite from various ticks can cause an allergic reaction as well as transmit a range of disease-causing bacteria, protozoa or viruses such as:
- Canine anaplasmosis/ Ehrlichiosis (Anaplasma phagocytophilum or Anaplasma platys)
- Babesiosis (Babesia gibsoni or Babesia canis vogeli )
- Bartonellosis (Bartonella vinsonii or Bartonella henselae)
- Lymes disease (Borrelia burgdorferi) or
- Rocky Mountain Spotted Fever (Rickettsia rickettsii)
