Akotaibi and colleagues from the University College Dublin reported on the correlation between a commercially available PCR for MAP and a commercial liquid culture system for detection of MAP in fecal samples. Their study population was 115 sika deer with known culture results, 79 from a MAP-infected herd and 36 from a MAP-free herd. Their publication is in the June 30 online issue of Veterinary Diagnostic Investigations and is Open access (free to everyone).
ABSTRACT
Mycobacterium avium subsp. paratuberculosis (MAP) infection in deer causes paratuberculosis (PTb; Johne disease), a slowly progressive chronic granulomatous enteritis. Accurate and rapid detection of MAP shedding by subclinical animals is essential for effective control of infection in deer herds. The VetMAX MAP 2.0 qPCR assay targeting IS900 was developed for cattle, sheep, and goats. However, the performance of the assay in sika deer (Cervus nippon) has not been evaluated. Here, we describe the use of the VetMAX MAP 2.0 qPCR assay on fecal samples from sika deer for the detection of MAP and its correlation with bacterial culture. DNA was extracted from fecal samples from 115 sika deer with known culture results (79 from a PTb-endemic herd and 36 from a PTb-free herd), using the QIAamp PowerFecal Pro DNA extraction kit. Diagnostic sensitivity and specificity were 100% (76 of 76) and 92.3% (36 of 39), respectively, with an overall accuracy of 97.4% compared with bacterial culture. A strong positive correlation was observed between culture time to detection and the qPCR Cq-value (r = 0.70, area under the curve = 0.96). Our findings offer correlation between bacterial culture and VetMAX MAP 2.0 qPCR assay results in feces of farmed sika deer and highlight the quantitative value of the qPCR assay as a rapid indicator of bacterial load and shedding intensity.
COMMENTS
While the report used fecal samples from Sika Deer, the findings are broadly applicable to many other animal species infected by MAP. This publication verifies the findings of others that PCR is as, if not more, accurate at detection of MAP in fecal samples than culture methods. PCR has replaced bacterial culture for MAP detection in most labs for several reasons:
- PCR is faster than culture. In this study MAP shedding was detected in one day by PCR (the assay takes one day but labs typically batch samples for efficiency and report findings weekly). By contrast in this study, it required 10 to 40 days (mean of 15 days) for samples collected from clinically affected animals and 15-81 days (mean 36 days) from subclinically affected animals due to the lower numbers of MAP/gram of feces.
- PCR is quantitative. PCR Cq or Ct values (same thing) are directly correlated with the number of MAP/gram feces, i.e. how infectious the source animal is, which helps rank animals for culling. Low Cq/Ct values indicate more MAP. Animals shedding fewer MAP/gram have higher Cq/Ct values and also take much longer to detect by culture methods.
- PCR is less expensive. As one example, the fee for liquid culture for MAP detection at the WVDL is $53.70 while that for PCR is $36.85. Presumptively positive cultures must always be confirmed by PCR which contributes to the cost of culture methods.
- PCR has far fewer assays that fail or yield unreadable results due to substances in the fecal sample that interfere with the assay or, in the case of culture methods, cause culture contamination.
- PCR is more easily standardized among laboratories and requires less MAP-specific laboratory experience than culture methods.
- PCR for MAP detection is more widely available.
So, why is culture still offered?
When animals are exported to other countries, it is the receiving country that sets requirements for MAP testing. Some countries are slow to adapt to changing MAP detection technology and thus still require the old fashion methods, even if inferior. In time, countries will catchup.
A little about sika deer (from Wikipedia):
The sika deer (Cervus nippon), also known as the northern spotted deer or the Japanese deer, is a species of deer native to much of East Asia and introduced to other parts of the world. Previously found from northern Vietnam in the south to the Russian Far East in the north, it was overhunted to the brink of extinction in the 19th century. Protection laws were enacted in the mid-20th century, leading to a rapid recovery of their population from the 1950s to the 1980s.
Velvet antler (dried precalcified antlers) is a popular ingredient in traditional Chinese medicine, and sika deer were domesticated by deer farms in China long ago for the antler trade, along with several other species. In many countries sika deer are farmed for velvet antlers.
PS – Deer (Cervids) are extremely susceptible to MAP infection and Johne’s disease is a major challenge for the farmed deer industry. In this study 39 fecal samples came from deer with clinical disease in a single herd.