Update: Diagnostic Options for Canine Brucellosis
By Dr. Sasha Thomason
Recently, the KSVDL added additional test options to aid our clients in diagnosing canine brucellosis. Below is an updated article, originally printed in the September 2019 issue, describing the utility of each test for diagnosing this disease.
Brucellosis is a contagious, zoonotic disease caused by a small, gram-negative, aerobic, coccobacillus bacteria in the genus Brucella. Clinical signs can vary from species to species and from animal to animal. Reproductive failure is the most common clinical sign in all animals. Based on the levels of lipopolysaccharide (LPS), the organism can either have a rough morphology (Brucella canis), or it can have a smooth morphology (B. abortus, B. suis and B. melitensis). The differences in rough vs. smooth are important when choosing a brucellosis test as most tests can only detect one type.
Brucella organisms tend to have a host preference, but most can infect other host species. Brucella canis is the most common causative agent of canine brucellosis, but dogs can also become infected with Brucella abortus (cattle) and Brucella suis (pigs) after ingesting contaminated placentas and aborted fetuses from livestock. It has also been reported that attenuated vaccine strains of B. abortus and B. melitensis (sheep, goats) can infect dogs.
At the KSVDL, we offer six different test options for canine brucellosis:
- Brucella BAPA (buffered acidified plate antigen). This serology test detects smooth Brucella spp. such as abortus, B. suis and B. melitensis. It will not detect B. canis. This is a good screening test for dogs that intermingle with farm animals. A negative result is reliable. A positive result requires confirmatory testing.
- Brucella canis Rapid Slide Agglutination Test (RSAT). This serology test will detect antibodies to canisonly. It has a higher sensitivity, making it a good screening test for Brucella canis. (Be aware that a ‘higher sensitivity’ for brucellosis is still only around 71%. This is why serial testing is recommended.) False positives are possible, so all positive results will require a second confirmatory test.
- Brucella canis 2-Mercaptoethanol Tube Agglutination Test (2-ME TAT). This serology test will detect antibodies to B. canis only. Adding the 2-mercaptoethanol to this test helped increase the specificity of the test, but decreased the sensitivity in the process. This makes it a less reliable test to use for screening purposes.
- Brucella canis 2-Mercaptoethanol Rapid Slide Agglutination Test (2-ME RSAT). This serology test is only used as a confirmation test and cannot be requested as an initial test for brucellosis. It has similar attributes to the Brucella 2-ME TAT test described above. The sensitivity of this test is 32%; the specificity is 100%.
- Brucella PCR. Our PCR test is a two-target, real-time PCR. One will detect all Brucella spp. The second will detect canis only, so it can differentiate between B. canis and other Brucella spp. It can detect 2CFU/ml, making it a highly sensitive and specific test. This detection level is approximately 5 times more sensitive than blood culture. This test can be used as both a screening test and a confirmatory test. The result will not report each target separately, but as a single positive or negative result.
- Brucella canis blood culture. Although this is the traditional gold standard diagnostic test for the disease, it is best used only as a confirmatory test. False negative results are possible because of the typically low levels of bacteria in each specimen, intermittent shedding and slow-growing nature of the organism, just to name a few.
Brucellosis can be difficult to diagnose, and there is not a standard protocol to follow to help guide the diagnostic process in dogs. A good rule of thumb is to use more than one type of test to help increase the chances of detecting Brucella. It can take anywhere from 2 weeks to several months for titers to become detectable, so performing multiple tests throughout that time period will also help. Antibody titers and bacteremia are usually highest during proestrus, estrus, pregnancy and immediately following abortion in infected female dogs. Testing during those time periods will increase the chances of identifying a dog with brucellosis.
Brucellosis is a reportable disease in most states, including Kansas. State animal health officials will help guide you regarding confirmatory testing recommendations in the event of a positive result with any of the testing methods.
For more information about brucellosis control in breeding kennels, read Best Practices for Brucella canis Prevention and Control in Dog Breeding Facilities: https://www.aphis.usda.gov/animal_welfare/downloads/brucella_canis_prevention.pdf
Sasha Thomason, DVM is the Section Head for Accessioning at the Kansas State Veterinary Diagnostic Laboratory.