Immune deficiencies explained
Q: Last week I had to put down my 7-year-old American Saddlebred due to an illness called common variable immunodeficiency (CVID). I had never heard of this disease. In retrospect, I realize that all the little issues we had over the last two years were a sign of an immune system that was failing. The care we received from the University of Missouri Equine Clinic was wonderful, and I allowed my horse to be necropsied in the hopes that more can be learned about this disease and that maybe one day my horse’s death could help save another horse’s life. Please let more EQUUS readers know about CVID. The conclusion of this disease is nearly always death, and by the time you realize you have a major issue on your hands, it is too late. Rebecca Peck Liberty, Missouri
A: Immunodeficiencies are rare conditions in humans and animals. They can be difficult to diagnose, and veterinarians may not consider them when initially evaluating a horse. We often suspect an underlying failure of the immune system when infections with fevers occur a few times during the year, and antibiotics are needed more than once to fight them. When immunodeficiencies are hereditary they may manifest in early age; those that we refer to as late-onset manifest later in life. In other cases, transient immunodeficiency is caused by immunosuppressive drugs, malnutrition or certain viral infections.
Different kinds of immunodeficiencies reflect the immune system’s various and overlapping strategies for fighting diverse types of organisms, including bacteria, viruses, fungi and parasites. These strategies involve different cell types and proteins that can prevent pathogens from invading the
body and replicating, promote their removal and destruction, and trigger inflammation or a state of alertness. If one of these strategies is not functional or absent, certain pathogens find a way to infect the body, which means that the different types of immunodeficiency result in distinct susceptibility to particular pathogens. Nevertheless, recurrent infections and fevers are common features of immunodeficiencies. Antimicrobials may help control them but, when the course of treatment is over, the infection occurs again.
Common variable immunodeficiency (CVID) in the horse is a late-onset immunologic disorder of inadequate antibody production. Most horses with this condition are reported to have had a healthy life until recurrent bacterial infections and fevers start to occur, usually after 10 years of age (horses diagnosed with CVID have been as young as 2 years and as old as 23). The condition can appear in any breed and both genders can be affected. The clinical signs include repeated episodes of pneumonia, sinusitis or diarrhea; infection of the abdominal cavity and liver; inflammation of the eye; skin abscesses; meningitis or neurologic disorders; and susceptibility to gastrointestinal parasites. Weight loss and/or muscle loss are common. Infections are often caused by common types of bacteria.
Blood testing in a horse with CVID shows low antibody levels, specifically the serum concentrations of the antibodies IgG and IgM. Typically, the cells that produce antibodies, named B cells are also low because their production in the bone marrow is impaired. In some cases, the B cells develop, but they do not function properly. The lymph nodes in horses with CVID also tend to be small and lack B cells.
A diagnosis of CVID can be made based on a horse’s clinical history of recurrent bacterial infections combined with immunologic testing of blood samples. Some cases may be confirmed by examining the lymph nodes during a necropsy and ruling out other diseases, such as lymphoma. There is no genetic test available since no genetic mutations relating to CVID have been identified.
To date, CVID is a fatal disease in horses. Some cases may be managed for a few years with antibiotic therapy, but eventually the recurrent bacterial infections will become overwhelming. Antibody replacement therapy has been successful in treating human patients with CVID but is not cost-effective in horses. Bone marrow transplantation was attempted in one horse but proved to be difficult. Studies are currently underway to explore the use of stem cells to restore B cell production in the bone marrow of affected horses.
Please accept our condolences for the loss of your horse. We share your hope that what we’ve learned will help us to one day be able to cure this dreadful disease.
M. Julia B. Felippe, MedVet, MS,
PhD, DACVIM Cornell University College of
Veterinary Medicine Ithaca, New York.