Coccidioidomycosis, a fungal infection endemic to the desert Southwestern United States, regions of South America, Central America, and Mexico is caused by the dimorphic fungi Coccidioides immitis and C. posadasii. Dogs are typically exposed by inhalation of the spores from soil, and they often develop only subclinical or mild respiratory disease. In severe cases, dissemination of the fungus occurs leading to lesions in skin, bone, CNS, lymph nodes, eyes, GI tract, and other sites. Diagnosis presents challenges as Coccidioides spherules are often not identified from cytology or histopathology samples, and the index of suspicion may be low if veterinary practitioners do not commonly see coccidioidomycosis cases. The clinical presentation may mimic other infections or neoplasia.
Serology by agar gel immunodiffusion (AGID) is used most frequently to screen for exposure to Coccidoides; however, there are limitations to this diagnostic tool. Positive titers up to 1:8 have been observed in 5 to 20% of healthy dogs from endemic areas. Titers between 1: 2 and 1: 8 occur in a small proportion of dogs with coccidioidomycosis. The initial AGID results (positive or negative) are typically available after 3 days, but the titer value will usually require another 3 days. In addition, AGID titer does not reflect the severity of disease [1, 2].
The research team at MiraVista developed the MVista® Canine IgG Antibody EIA to improve antibody testing for Coccidoides, as the technology is more rapid and highly reproducible (Figure 1). Our recent study examined 74 dogs with pathology-proven or probable coccidioidomycosis and 143 healthy control dogs from endemic and non-endemic areas . The sensitivity of the IgG EIA and AGID were 89% and 92%, respectively, and the specificity in endemic controls was 94% for both methods. All probable cases were diagnosed based upon positive AGID results precluding comparison of the sensitivity of the IgG EIA.
Diagnosis in the proven group was based upon pathology or culture allowing comparison of the sensitivity of AGID and IgG EIA. AGID alone was positive in 2 case and IgG EIA in 4 cases. AGID or IgG EIA were positive in 20 of the 22 (91%) proven cases.
Detection of Coccidioides antigen by EIA in serum or urine is another diagnostic tool; however, antigen detection alone has low sensitivity, which may be related to the fungal burden of the patient. We also examined antigen detection in the study recently published in Medical Mycology, and we confirmed the low overall sensitivity (34%). We demonstrated, however, that some dogs with proven coccidioidomycosis may have detectable antigen yet lack an antibody response. One proven case with no detectable antibody response had detectable antigen levels in serum and urine, and another proven case had low titer (<1:2) by IDCF, negative IgG in EIA, but was positive for serum and urine antigen. We also found that serum antigen alone was positive in 3 of 19 cases (16%) and urine antigen only in 3 of 27 cases (11%). This highlights the value in testing serum and urine for antigen by EIA and anti-Coccidioides antibody by IgG EIA and AGID to gain the highest combined sensitivity, positive in 73 of the 74 cases (99%).
In summary, detection of anti-Coccidioides antibodies using MVista® Coccidioides Canine IgG Antibody EIA and Coccidioides antigen in the urine and serum in the MVista® Coccidioides Antigen EIA has potential to aid in diagnosis of coccidioidomycosis in dogs by offering high sensitivity and a faster turn-around time than pathology, culture or AGID antibody detection. Combined serum and urine antigen testing, IgG antibody testing by EIA and AGID may improve sensitivity for diagnosis of coccidioidomycosis in dogs.
Note: Study included dogs with pathology proven coccidioidomycosis, as well as those with “probable” disease based on AGID results >1:8. The better estimation of sensitivity can be made using the proven cases. It is unclear if the dogs with positive AGID, but negative results in other assays, represent true coccidioidomycosis cases.
Interassay agreement of EIA unit values from repeat testing of sample from dogs with coccidioidomycosis (n = 57) demonstrate a strong day to day correlation. Coefficient of determination is equal to 0.996.
Credits for Table and Figure: ED Holbrook, RT Greene, Novel canine anti-Coccidioides immunoglobulin G enzyme immunoassay aids in diagnosis of coccidioidomycosis in dogs, Medical Mycology, 2019 Vol. 00, No. 00, by permission of Oxford University Press.
(1) Shubitz, L.E., et al., Incidence of coccidioides infection among dogs residing in a region in which the organism is endemic. J Am Vet. Med Assoc, 2005. 226(11): p. 1846-1850.
(2) LF, B.C.a.S. A RETROSPECTIVE REVIEW OF CANINE COCCIDIOIDOMYCOSIS CASES AT A TERTIARY CARE CENTER IN TUCSON. in 62nd Annual Coccidioidomycosis Study Group Meeting. 2018. Flagstaff, AZ.
(3) Holbrook, E.D., et al., Novel canine anti-Coccidioides immunoglobulin G enzyme immunoassay aids in diagnosis of coccidioidomycosis in dogs. Med Mycol, 2019.
(4) Kirsch, E.J., et al., Evaluation of coccidioides antigen detection in dogs with coccidioidomycosis. Clin. Vaccine Immunol, 2012.19(3): p. 343-345.