Valley Fever in dogs is most commonly diagnosed through a blood test that detects antibodies against the Coccidioides fungus. This antibody test, called a titer, is the standard first step, but vets often combine it with imaging, bloodwork, and sometimes tissue samples to confirm the diagnosis and assess how far the infection has spread.
The Antibody Titer: The Primary Test
The cornerstone of Valley Fever diagnosis is a blood test that measures your dog’s immune response to the Coccidioides fungus. Rather than looking for the fungus itself, the test detects antibodies your dog’s body has produced to fight it. There are two main types of antibody tests used in veterinary medicine, and they differ in speed and sensitivity.
Agar gel immunodiffusion (AGID) is considered the gold standard. It can detect both early-response antibodies (IgM, which appear first after infection) and later-response antibodies (IgG, which indicate ongoing or established infection). The trade-off is time: AGID requires 24 to 48 hours of incubation to produce results. Enzyme immunoassay (EIA), on the other hand, can return results in under two hours and is more sensitive, meaning it picks up lower levels of antibodies. However, most EIA tests only detect IgG antibodies, so they may miss very early infections where the body has only produced the initial IgM response.
Your vet will typically order one or both of these tests when Valley Fever is suspected based on your dog’s symptoms and geographic history.
What Titer Numbers Mean
Titer results are reported as ratios: 1:2, 1:4, 1:8, 1:16, 1:32, and so on. The lab dilutes your dog’s blood sample repeatedly until the antibody reaction disappears. A result of 1:16 means the test stayed positive through more dilutions than 1:4, indicating a stronger immune response and, broadly speaking, more severe disease.
Asymptomatic dogs, those infected but not visibly sick, often have low titers like 1:4 or 1:8, sometimes up to 1:16. Dogs with titers at 1:32 or higher are 7.5 times more likely to have disseminated disease, meaning the infection has spread beyond the lungs to bones, joints, skin, or other organs. At the 1:32 cutoff, the titer correctly identifies about 74% of disseminated cases. That said, it’s not a perfect predictor on its own. Some very sick dogs have low titers or even negative results, and some dogs remain positive at a low titer for the rest of their lives without ever getting seriously ill.
Because a single titer is just a snapshot, vets often retest every few months to track whether the number is rising (worsening infection), falling (responding to treatment), or staying stable.
Timing Matters: When Tests Turn Positive
Antibodies take time to develop after a dog inhales fungal spores. It can take several weeks after exposure before a titer test comes back positive. IgM antibodies appear first, followed by IgG. This means a dog in the earliest stage of infection can test negative even though it’s already sick. If your vet suspects Valley Fever but the initial test is negative, a repeat test two to four weeks later is standard practice. A negative result early on does not rule out the disease.
Imaging: X-Rays and Beyond
Because Valley Fever most commonly affects the lungs first, chest X-rays are a routine part of the workup. In a study of 19 dogs with confirmed coccidioidomycosis, 13 showed pulmonary infiltrates (cloudy areas in the lungs indicating inflammation or infection), and 10 had visibly enlarged lymph nodes near the base of the lungs. An interstitial pattern, where the tissue between the air sacs looks hazy or thickened, was the most common finding.
If your dog is limping or has swollen joints, your vet will likely X-ray those areas too. Valley Fever can cause bone lesions that show up as areas of bone destruction on radiographs. These images help determine whether the infection has disseminated and guide treatment decisions.
Tissue Sampling for Definitive Diagnosis
The only way to definitively confirm Valley Fever is by finding the actual fungal organisms in a tissue or fluid sample. A vet can collect samples through fine needle aspiration (inserting a thin needle into a swollen lymph node, skin lesion, or other affected area) or through a surgical biopsy. Under the microscope, Coccidioides appears as distinctive thick-walled spherules, roughly 50 to 60 microns across, packed with smaller endospores inside. Reactive immune cells typically surround these spherules.
This method is highly specific: if spherules are found, the diagnosis is certain. But it’s not always practical. Sampling requires that there’s an accessible lesion to aspirate, and the fungal organisms aren’t always present in every sample. In one case report, aspirates from enlarged lymph nodes showed no spherules despite the dog having confirmed disseminated disease elsewhere. For these reasons, tissue sampling tends to be used alongside serology rather than as a standalone screening test.
Why PCR Testing Falls Short in Dogs
PCR tests, which detect the fungus’s genetic material directly, might seem like an ideal diagnostic tool. In humans with pulmonary coccidioidomycosis, PCR on respiratory samples catches between 56% and 94% of cases depending on the study. In dogs, though, the picture is starkly different. A study that ran 72 PCR tests on nasal, throat, and eye swabs from 24 dogs with confirmed pulmonary Valley Fever found zero positive results. Not a single swab detected fungal DNA. By comparison, AGID serology caught 46% of those same dogs on IgM and 71% on IgG, while EIA detected IgG in 79%.
The takeaway: PCR swab testing is not a reliable way to screen dogs for Valley Fever. Serology remains far more useful.
Routine Bloodwork as a Supporting Clue
A complete blood count and chemistry panel won’t diagnose Valley Fever on their own, but they often raise suspicion. Dogs with active coccidioidomycosis frequently show elevated globulin levels in their blood, a sign the immune system is ramping up antibody production. Inflammation markers also tend to be elevated. Dogs with IgG titers at 1:16 or above showed markedly higher levels of C-reactive protein (a general inflammation marker) compared to dogs with lower titers.
These routine blood tests are valuable because they can prompt a vet to order a Valley Fever titer in the first place, especially in dogs living in or traveling through endemic areas like Arizona, central California, or southern Nevada. They also help monitor your dog’s overall organ function during the months of antifungal treatment that Valley Fever typically requires.
False Negatives and Repeat Testing
False negatives are a real concern with Valley Fever testing. Both AGID and EIA can miss infections, particularly early in the disease course before antibodies have built up. But timing isn’t the only factor. Some dogs with histologically confirmed disease (the fungus was found in their tissue) still had negative serology results, and this wasn’t linked to any specific body location of the infection. Immunosuppression, whether from medications or from the disease itself overwhelming the immune system, can also blunt the antibody response.
If your dog has symptoms consistent with Valley Fever, lives in or has visited an endemic area, and tests negative, your vet may recommend retesting in a few weeks, pursuing imaging, or attempting tissue sampling. A single negative titer in a symptomatic dog is not enough to cross Valley Fever off the list.