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Does Lactation Stage Affect Bacterial Shedding?

05 November 2013

Iowa State University Extension

Evidence suggests cattle with Johne's disease shed bacteria into colostrum and milk, say a team of specialists at Iowa State University.

Cows in advanced stages of Johne's Disease (JD) and in early lactation were highlighted as the most relevant group to shed Mycobacterium avium subsp. paratuberculosis, the Johne's causing bacteria. 

After sampling milk from 42 Holsteins heavily affected with JD over a seven year period, the team has concluded that cow to calf transmission through milk is a viable route of transmission.  

The group consisted of Suelee Robbe-Austerman, of USDA APHIS, Professor Donald Beitz, Department of Animal Science, Judith Stabel, Livestock Disease Expert USDA-ARS and Graduate Assistant, Laura Bradner. 

Introduction

Mycobacterium avium subsp. paratuberculosis (MAP), the causative agent of Johne’s Disease (JD), is an enteric pathogen that is primarily shed in the feces, the team said.

JD is a slowly progressing wasting disease that does not manifest physical symptoms in ruminants until late in the disease, making early diagnosis difficult and costly.

It is estimated that 91.1 per cent of dairy herds are infected with this disease. The primary route of exposure for neonates is fecal-oral; however, MAP is also shed into the milk and calves can be exposed to this pathogen by suckling the dam or being fed colostrum or waste milk from infected cows.

Despite this possibility, there is little information in the literature to document the shedding of MAP into the colostrum and milk of infected dams, particularly, the bacterial load and how this relates to the infection status of the dam and the stage of lactation.

Yet, if producers could understand the association of disease with bacterial load in the milk, they might be more willing to make critical management decisions to further prevent dissemination of infection within the herd. 

The objective of this study was to determine if an association exits between disease status and days in milk with the amount of MAP shed into milk and colostrum.

Results and Discussion

Of the 20 clinical cows, 11 died from JD within one year of calving. Samples collected from these cows accounted for 12 of the 13 milk samples that were culture positive in BACTEC 12 B medium and all of the milk samples that were culture positive on HEY medium.

In addition, 33 of the 45 milk samples that were positive for MAP via direct PCR were collected from this group of clinical cows. These data suggest that as JD progresses, the amount and frequency of MAP shed into milk and Of the 20 clinical cows, 11 died from JD within one year of calving.

Samples collected from these cows accounted for 12 of the 13 milk samples that were culture positive in BACTEC 12 B medium and all of the milk samples that were culture positive on HEY medium.

In addition, 33 of the 45 milk samples that were positive for MAP via direct PCR were collected from this group of clinical cows. These data suggest that as JD progresses, the amount and frequency of MAP shed into milk and colostrum continuously increases until the cow succumbs to JD.

This study definitively demonstrates that MAP is shed into milk and colostrum, preferentially by cows in advanced stages of JD and in early lactation. This study proves dam to calf transmission of MAP by milk consumption is a viable route of transmission.

To compound this issue, research has found that MAP that has been exposed to high-osmolarity environments like those found in milk and the mammary gland possess an invasive phenotype. In addition, calves less than 6 months of age are most susceptible to MAP infection.

Therefore, if a newborn calf is allowed to suckle even once, it is experiencing the highest likelihood of infection by MAP. Additionally, if producers feed pooled colostrum, many calves could be at risk of infection by MAP. 

Veterinarians now have critical information to educate dairy producers on the risk of MAP transmission through milk and colostrum and the need to change calf rearing practices to accommodate these findings.

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