4. Discussion
A possible limitation of the study is the use of antibody
ELISA as the indicator of MAP infection. ELISA sensitivity
(Se) is a direct function of the distribution of the infection
stages in the test population (Collins and Sockett, 1993).
False negative results are more likely to occur in the initial
stages of the infection; as the infection progresses and the
animal begins to show signs, which could be indirectly
related to MAP (such as metabolic or immune disorders,
infertility or unproductivity), the Se of the ELISA increased
(Sweeney et al., 2006), having this limitation a smaller
impact on the results.
Nielsen and Toft (2008) indicated that the specificity of
several commercial ELISAs ranged between 91% and 98%.
The expected proportion of false positive results is
expected to be low considering that the samples were
pre-absorbed with Mycobacterium phlei. Either way in all
the 8 studied farms, the presence of the bacteria had been
confirmed by means of fecal culture at least in one cow.
As a part of the control program, it is recommended to
dispose of the animals that are culture positive. Consequently,
those animals were not included in the study.
However, given the epidemiological situation of MAP in
Galicia, where the animal seroprevalence in infected herds
ranges from 5.3% to 7.7% (Diéguez et al., 2007), and the
existence of programs to fight other pathogens that may
also require the culling of animals, seropositive but culture
negative cows are not recommended to be culled although
farmers are encouraged to not breed them. A previous
study, performed at herd instead of animal-level indicated
that farms with both ELISA-positive and fecal culturepositive
animals had higher culling rates than those with
ELISA positive but no fecal-culture positive animals. In
addition, substantial differences in the causes of loss were
found between the two groups (Arrazuría et al., 2014)