Comparison of the Q-fever complement fixation test and two commercial enzyme-linked immunosorbent assays for the detection of serum antibodies against Coxiella burnetti (Q-fever) in ruminants : Recommendations for use of serological tests on imported animals in New Zealand

AIM: To make valid recommendations  on the use of serological test methods for the detection of serum antibodies  in ruminants against Coxiella burnetii (Q-fever), by comparing the  performance of the complement fixation test (CFT) and two ELISA, and by  identifying reasons for discrepancies between the test methods.  
METHODS: A total of 73 serum  samples from infected cattle, 69 from infected goats, and 100 samples from  non-infected cattle and 57 samples from non-infected sheep, as well as 95 samples  from infected cattle herds (mix of seropositive and seronegative samples),  were tested using the CFT, the IDEXX ELISA (I-ELISA) and the Pourquier ELISA  (P-ELISA). A mixed panel of 12 serum samples from sheep from inter-laboratory  proficiency testing (proficiency panel) was also tested using the CFT and  both ELISA, and further investigated using IgG- and IgM-specific ELISA.  
RESULTS: Generally, the two  commercial ELISA were more sensitive than the CFT for the detection of  infected ruminants. Good agreement between ELISA for positive and negative  results was found for samples from the infected herd, while results for the  positive panels varied between the two ELISA. For the total of the positive  serum panels, the I-ELISA detected 95% of samples as positive or suspicious,  while the P-ELISA detected only 81%. In the P-ELISA, more samples were  considered suspicious (18%) than in the I-ELISA (14%). All sera from non-infected  sheep and cattle tested negative in the serological test methods employed,  except for one positive sample from a sheep in the P-ELISA. Further  investigation revealed that a CFT-positive but ELISA-negative result was due  to high IgM and low IgG reactivity.  
CONCLUSIONS: The two commercial  ELISA were more sensitive than the CFT in all panels from infected ruminants.  However, they could only detect IgG. The I-ELISA should be the serological  test method of choice for cattle, sheep and goats for import testing of  animals into New Zealand because it was more sensitive than the P-ELISA and  was equally specific to the PELISA and the CFT. For other animal species,  such as deer and camelids, the CFT should still be used since none of the  ELISA has been evaluated for these species. This study has shown that the two  commercial ELISA will detect the majority of infected ruminants but may miss  animals that have not developed an IgG response.  
KEY WORDS: Q-fever, Coxiella  burnetii, enzyme-linked immunosorbent assay, ELISA, complement fixation test