In recent months, the topic of colostrum management in herds with Johne's disease has surfaced frequently. We have known for some time that the causative organism, Mycobacterium avium subspecies paratuberculosis (MAP), may be found in colostrum and milk taken from the udders of infected cows. Work done in Ohio and published in 1995 reported that MAP was isolated from the colostrum of 22% of infected cows and from the milk of about 8% of infected cows. Cows that were heavy fecal shedders of MAP were more likely to shed the organism in their colostrum than were light fecal shedders. Research done in Pennsylvania showed a slightly higher shedding level in milk (11.6%) and shedding in milk was also more likely in cows that were heavy fecal shedders. Colostrum was not examined in that study. It is important to note that the numbers of MAP recovered from milk in these studies would be considered quite low -- from 2 to 8 colonies from a 50 ml (cc) milk sample.
In recognition of both the potential risk that infected colostrum might pose in establishing an infection in a newborn calf and the importance of colostrum in protecting against other diseases in young calves, some producers and veterinarians have attempted to identify cows that might be of higher risk of having MAP in their colostrum in order that their colostrum might not be used for heifer replacements. Although it was designed to be a screening test to select HERDS (and animals) for additional testing using a more definitive test, such as fecal culture, ELISA (enzyme-linked immunosorbent assay) is commonly being used for this purpose because it is cheap and fast relative to culture. It is usually performed at dry-off.
To our knowledge, only one research study has attempted to address the ability of ELISA to detect cows that are shedding MAP through their mammary gland. In that work, Sweeney and colleagues reported the results of tests on samples of feces, supramammary lymph nodes, milk, and serum taken at slaughter from 86 cows not showing clinical signs of Johne's disease. The samples were tested by culture (feces, lymph nodes, and milk) and complement fixation, ELISA, and agar-gel immunodiffusion (AGID; done on blood serum).
Relevant findings included:
- Heavy fecal shedders of MAP were more likely to have culture positive milk and lymph nodes than intermediate or light shedders, and "The trend for an increased proportion of culture-positive milk or supramammary lymph node samples with an increased fecal shedding rate is significant (P < 0.05)." In this study, 7 of 37 (19%) cows that were heavy fecal shedders had MAP in their milk.
- "There is a significant difference (P < 0.001) in the ELISA scores between cows with culture-positive and -negative lymph nodes, but no difference was demonstrated for milk samples." This implies that ELISA might be useful to detect cows with MAP in the mammary lymph nodes but not the milk.
The authors concluded "Thus, the serologic status of the cow [as assessed by ELISA, AGID, or complement fixation] appears to be of little use in predicting the risk of milk or supramammary lymph node infection." It is important to note that the ELISA technique used in this study was not the same as current ELISA in use in the USA today, although it is similar to them.
The current practice of discarding colostrum from ELISA-positive animals seems to stem from our observations that these animals are most likely to be moderate to heavy fecal shedders of MAP, and the inference from the studies cited is that heavy shedding animals are more likely to have MAP in their milk. Although on the surface this approach seems appropriate, there are several significant cautions of which producers should be aware. The first of these is that currently available ELISA probably only detect about 70% of heavy shedders (including from unpublished Ohio observations, 2006). This means that up to 30% of heavy shedders will not be detected by ELISA. Furthermore, ELISA detection of low to moderate fecal shedders is variously reported to be only 2 to 30% (including from unpublished Ohio observations, 2006). In the Sweeny study, 1/9 (11%) intermediate fecal shedders and 1/31 (3%) light fecal shedders DID have positive milk samples. In addition, 2 of 9 intermediate shedders (22%) and 2 of 33 light shedders (6%) had culture positive supramammary lymph nodes.
In light of these observations, it would seem well for producers to be aware that ELISA-negative animals may have a risk of having MAP in their milk similar to that of ELISA-positive animals.
Secondly, in the milk and colostrum culture studies cited above, samples were collected from the udder after thorough teat end cleaning and disinfection of the skin. Therefore, the results reflect the status of the mammary gland itself. Because fecal material from infected cows in amounts as small as a gram (about the size of a large pea) may contain billions of MAP bacteria, a few infected animals can contaminate the dairy environment to a very significant degree. Work done on farms in Ohio in 2005 and 2006 has shown that contamination of the environment with MAP in an infected herd can result in heavy contamination of the skin of the teat and udder - in some cases in a very high percentage of the herd. In fact, skin swabs taken from the skin of the teat and udder of fecal culture-negative cows, using just a 4-inch square gauze sponge, have yielded MAP upon culture in amounts equivalent to fecal samples taken directly from the rectum of infected cows. A very small amount of this contamination can result in a bucket of colostrums, potentially containing an infectious dose of MAP for every calf fed from it. Consequently, If producers are not paying close attention to teat and udder cleaning at the time of colostrum (or milk) collection, small amounts of fecal contamination from environmental sources could completely wipe out any value of assessing an individual cow's infection status with either blood or culture tests. This very real potential for bacterial contamination during harvest has recently been shown.
Some producers may wish to consider the purchase of a pasteurizer for waste milk and colostrum. Some studies have shown significant economic and calf health benefits from pasteurization of waste milk. Although there are mixed results from studies examining the efficiency of pasteurization of milk or colostrum to kill MAP, most studies have shown at least a 99.9% reduction in MAP numbers. In addition, recent work has shown that heating colostrum at a lower temperature for a longer time than typical batch pasteurization may fully inactivate MAP without turning it into a semi-solid and without major losses in antibody content (1400 F for 60 to120 minutes). Because some of this work was done in a laboratory setting, it will need to be determined whether these study findings can be successfully replicated using commercial on-farm batch pasteurization equipment. However, these results are very encouraging and suggest that, for some producers, on-farm pasteurization may be a very useful tool to reduce MAP in colostrum and waste milk.
The "take homes" are:
1. ELISA-positive cows are more likely to be heavy shedders of MAP than ELISA- negative cows, and removing ELISA-positive cows as colostrum/calf milk donors may decrease the burden of MAP in the colostrum/calf milk pool, since some (8 to 22%) of these positive cows may shed MAP in colostrum.
2. However, ELISA will not identify the majority of MAP-positive cows shedding intermediate or low levels of MAP in their manure, yet from 3 to 11% of these cows appear to shed MAP into milk. Because there are usually many more intermediate and low shedders in a herd than heavy shedders, they may be responsible, collectively, for more direct mammary gland shedding of MAP than the heavy shedders identified by ELISA. In addition, ELISA will not identify all heavy shedders.
3. The level of MAP in the manure (organisms/ml) of infected cows is potentially millions of times HIGHER than in milk. Therefore, fecal contamination of the colostrum/milk at milking is probably a greater factor driving the level of MAP in milk/colostrum than MAP infection originating in the udder and subsequently shed directly into milk.
4. Finally, MAP infection in a herd can result in TREMENDOUS contamination of cow contact surfaces. Cows not infected with MAP have been shown to have large numbers of MAP on their teat and udder skin (picked up from fecal contamination in the environment) so even milk/colostrum from truly uninfected cows may be contaminated with MAP if milk is contaminated at milking (i.e. pre-milking preparation is inadequate).
Using ELISA testing to identify cows to remove from the colostrum/calf milk pool will decrease the MAP burden but will likely do so in a relatively small way. In herds infected with MAP, data suggest that fecal contamination remains the main way colostrum/calf milk is contaminated. A focus on environmental management (clean cows) and meticulous pre-milking cow preparation should remain centerpieces of a MAP control program. Pasteurization offers a promising adjunct to provide a method to greatly reduce contamination of colostrum/calf milk.
*A complete list of research references is available on request.