INTRODUCTION
Johne’s disease (JD) is a chronic, enteric disease of ruminants caused by infection with Mycobacterium avium subsp. paratuberculosis (MAP), otherwise known as Mycobacterium paratuberculosis. This disease is endemic within the United States, with at least 22% of dairy herds classified as Johne’s positive (NAHMS, 1997). Production, reproduction, and culling losses caused by JD can be significant (Goodell et al., 2000). Potential economic losses attributable to this disease are substantial.
No effective, practical treatment for JD exists; however, it can be controlled through implementation of strategies that are at least partially dependent on the identification and management of MAP-infected individuals. On-farm biosecurity, such as feeding pasteurized milk or milk replacer to neonatal calves, increasing farm sanitation to minimize fecal-oral transfer of MAP, and the purchasing of animals from known low-risk sources, are also thought to be important in controlling spread of the disease. To best implement appropriate measures, however, producers must first be aware of the extent of the presence of JD in their herds. In an effort to improve awareness and minimize the spread of JD, the US Animal Health Association, (USAHA, 1998) in conjunction with federal and state agencies, initiated a Voluntary Bovine JD Status Program for cattle (USDA-APHIS, 2002). One of the program’s objectives is to identify low-risk herds to minimize the inter-herd spread of disease through outside purchase of replacement animals. Programs such as the Voluntary Bovine JD Status Program utilize standardized diagnostic tests to assess infection status; thereby providing increased confidence for freedom from disease at the herd level, provided that all animals test negative during strategically designed testing schemes.
Detecting subclinical animals (i.e., infected but not exhibiting clinical signs) is considered by many to be extremely important to the control of the spread of disease. Unfortunately, the pathogenesis of JD complicates the application and interpretation of these tests (e.g., fecal culture and/or serological testing). Test specificity is usually high, but a low sensitivity makes it very difficult to detect all animals in a herd that are infected (Dargatz et al., 2001; Nielsen et al., 2002). Because the role of these tests is crucial to JD control programs, potential problems affecting sensitivity and specificity also need to be identified and well characterized.
Background
In early 2001, during the conduct of a herd-health program designed to minimize calfhood exposure to MAP on a dairy farm in Texas, a seasonal pattern of serological response to MAP was observed. Roughly once per month, dairy cows that were confirmed pregnant were tested for serum antibodies to MAP using a commercial ELISA test. Upon preliminary examination of those data, a decrease in the proportion of animals testing positive for JD during summer months was noted (May through August). This decrease is illustrated in a categorical scale (i.e., proportion testing positive) in Figure 1
. Previous studies have shown that cattle experiencing hyperthermia (i.e., heat stress) have reduced productivity and feed intake, as well as altered endocrine function and energy balance (Wolfenson et al., 2000; De Rensis and Scaramuzzi, 2003), so it is biologically plausible that elevated temperatures also will have an effect on cattle immune response to MAP infection. Indeed, although varied and conflicting results exist in the literature concerning the relationship between heat stress (climate-related seasonality) and immunological traits such as serum and colostral immunoglobulin concentrations (Lacetera et al., 2002), the degree of heat stress experienced by a dairy cow seems to play an important role in the response to infection.
Based on our preliminary cross-sectional data, the primary objective of this project was to examine seasonal temperature effect on the interpreted results of serological testing using a commercial ELISA for MAP in a large Texas dairy cattle herd. An epidemiological field study design was employed to control for the effects of age (i.e., birth cohort) and other potential confounders on the interpretation of a seasonal decrease in the proportion or risk of adult cows being categorized as positive for JD under a contemporary cohort sampling scheme. In addition, the potential for a change in test result (negative to positive or vice versa) for the same cow was examined with a repeated seasonal sampling schedule.
A secondary objective of this project was exploration of the possibility of a reciprocal increase in the risk of fecal shedding for those periods during which seasonal changes related to heat might reduce humoral response as measured by the sample-to-positive (S/P) ratio of the commercial ELISA.
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