Serological tests for Johne s disease

Serological tests for Johne\’s disease have low sensitivity but reasonable specificity. Testing of individual milk specimens yielded a sensitivity of 28% (Collins et al., 2005), slightly higher than serum, and sensitivity increased with age of animal tested (Nielsen et al., 2013). PCR can also be used to test for the presence of MAP DNA in milk (Buergelt and Williams, 2004), as can the peptide-mediated magnetic separation-phage (PMS-phage) assay (Foddai et al., 2011). However, advances in PCR testing for MAP in faeces could negate the need to use antibody based tests.
Antibody based tests (ELISA) are available to measure myeloperoxidase in bulk tank milk to the abomasal parasite O. ostertagi (Forbes et al., 2008). Only an association between ELISA values and milk yield can be made using these test results, rather than confirming true positive nematode infections in the herds, so additional diagnostic testing is required to establish the parasite status of the herd.
In sheep, Q-fever (Coxiella burnetii; Klaasen et al., 2014), Brucella melintensis (Hamidi et al., 2015) and Mycoplasma agalactiae (Poumarat et al., 2012) can be tested for using milk; in goats, milk specimens can be used to test for caprine arthritis and encephalitis (Nagel-Alne et al., 2015). Q-fever outbreaks in humans are associated with C. burnetii infection in small milking ruminants in Africa (Klaasen et al., 2014). Shedding of the organism is intermittent, thus infection was not always detected by PCR and serological tests might also be required. In contrast, PCR testing of milk for B. melintensis detected was more sensitive than serology in one study (Hamidi et al., 2015). Accurate serological classification of the M. agalactiae status of sheep is difficult and PCR testing myeloperoxidase of milk specimens with two PCRs should be used to confirm the presence of the organism. The resultant PCR results also require cross checking with a dot-immunobinding technique (Poumarat et al., 1991).
Milk testing can be utilised for detection of non-infectious conditions. For example, lateral flow devices to test for progesterone concentrations in milk present opportunities to define the oestrus cycle and pregnancy status of cows (Waldmann and Raud, 2016) and technological modifications may allow for testing to occur during milking (Dobson, 2016).

Colostrum
Colostrum is another medium that can be used for animal disease testing instead of milk. Its availability is restricted to a shorter time period, but provides other testing and diagnostic advantages. Testing colostrum for antibodies (as an alternative or add-on to milk testing) is potentially useful because of the higher concentration of immunoglobulins in colostrum compared to milk. The concentration of IgG is estimated to be up to 100 times higher than milk in the first few days after parturition (Korhonen et al., 2000). Recent work in cattle suggests that testing colostrum increases analytical and diagnostic sensitivity compared to milk. This could be most useful for on-going surveillance of animal diseases requiring less frequent checks, as colostrum is only available during the perinatal period (Jenvey et al., 2012, 2015; Cockcroft et al., 2014).
The drawback of colostrum testing is the small window of availability and the difficulty of collection in some species. In dairy cattle, sampling of colostrum might be most usefully applied for diseases where diagnostic tests are hampered by low analytical and/or diagnostic sensitivity (such as is the case with Johne\’s disease; Reichel et al., 1999). Colostrum has also been used successfully for testing for rotavirus and mycoplasma infections (Corthier and Franz, 1981; Zimmermann et al., 1986; Rautiainen, 1998).

Hair and ear notch skin specimens
Hair and ear notch specimens have been used successfully to detect BVDV persistently infected animals (Hill et al., 2007; Lanyon et al., 2014c) and formed the basis of the recent successful Swiss BVDV eradication campaign (Presi and Heim, 2010). In this campaign, detection of BVDV antigen in skin was the specimen of choice to identify persistently infected calves, and was preferred over serum. After the ingestion of colostrum, maternal anti-BVDV antibodies can bind to BVDV antigen and prevent its successful detection in the routinely used antigen-capture ELISA (Fux and Wolf, 2012). Using ear notch skin specimens reduces this complication, as there are fewer antibodies in ear notch tissue. Heating of serum specimens under specific conditions to break up antibody-antigen complexes can overcome the interference of maternal antibody and allow successful serum testing. This adds extra steps to the procedure, but has been used effectively in BVDV testing (Lanyon and Reichel, 2016) and in heartworm serology (Little et al., 2014a, 2014b; Velasquez et al., 2014).