Anaplasmosis, antibodies vs antigens

(This post is part of the April A to Z blogging challenge.)

In August of 2013, Keeper had a SNAP test come up positive for Anaplasmosis, a tick-borne illness similar to Lyme, that can cause joint damage and disable the dog. As Keeper's my service dog in training, this was Bad News... if it was true. It was a weak positive. So I went and hit the documentation, and checked with Dad, who's more of an immunology nerd than I'll ever be. I kind of skipped over it at the time, as if talking about it would bring about the worst possible outcome.

There are at least two known strains of the Anaplasmosis bacteria that will cause the SNAP test to come up positive. The SNAP test uses antigens for heartworm and antibodies for the bacterial diseases it tests. Antigens are the thing that the immune system reacts to- viruses, bacteria, worms, etc. Antibodies are how the immune system reacts. Both antigen-antibody binding and antibody-antibody binding can be measured from a blood sample to check for disease or infection.

I suppose an analogy would be that antigens are like thieves and antibodies are like cops. If you've got thieves roaming around stealing things, and people report the crimes, you'll have cops show up. Thieves lead to cops, antigens lead to antibodies.



The antigen test is usually very, very accurate. If the dog tests positive on an antigen, the causative agent of disease is very likely there (if you're catching theives, you've probably had robberies.) Antibody tests can be very, very accurate, too... but there is a much higher chance you're cross reacting (if you see a cop, there may be no robberies. It could be worse- a homicide cop, for example- or it could be completely unrelated- cop in uniform giving a talk at career day, or not a cop at all, say a blue Navy uniform.)

Documentation says weak positives on antibodies become strong positives in the presence of disease in one to three weeks for Anaplasmosis, and most show on a SNAP test 8 days after infection. Waiting to retest made much more sense than hitting a young dog with a harsh antibiotic. The retest also came up weak positive, not strong positive.

And that's when I chose to spend way more money sending off blood for titer tests than on the antibiotics. If it had become a strong positive, I might have waited for signs of disease, or I might have treated preventatively. But a second weak positive, weeks later, with no sign of disease in the dog, seemed less like we had the disease (thieves, in the analogy) and more like we had a cross reaction to antibodies (cops from a different department) or a lingering antibody response after the disease had already been eliminated by her immune system (cops at the crime scene after the thief had fled or been arrested.)

It was still possible she had the infection, but the evidence was pointing toward a prior exposure without active, current infection, or that the antibodies to some other disease had grabbed on to the Anaplasmosis test spot on the 4-way SNAP kit.

In addition, a pathologist stained and looked at Keeper's blood under a microscope. One strain of Anaplasmosis that SNAP covers is fairly nasty, infecting white blood cells, causing joint pain, fever and loss of appetite. The other one infects platelet cells and is usually pretty mild, if it ever shows signs (though increased bleeding is a risk). Both strains can be seen inside the blood cells under a scope in the case of an active infection.

As all the more expensive testing came back negative, we're in a low-risk area [the vet had seen 2 cases in 15 years of practice], and Keeper continued to look healthy, I opted to ignore the SNAP test results. It's a good product, especially for heartworm, but there's always exceptions. I'll keep using the SNAP test to check for hearworm, Lyme and Ehrlichia. If the Anaplasmosis ever comes up strongly positive, we'll likely have a pathologist double checking her blood through a microscope before treatment, as 30 days of Doxycycline is rough on a body. Testing is important, but interpreting the results can be a bit more than yes/no.