The results of the present study suggest that the apparent prevalence of anaplasmosis on arrival in Iowa feedlots (15.17%) is considerably higher than the prevalence of 1.93% previously reported for yearling cattle imported into Alberta feedlots from Montana in 2001 (14
). A follow-up study in the same region reported prevalence rates of 1.82% and 1.35% in 2002 and 2003 respectively (20
). Anaplasmosis has been reported in all 48 contiguous states in the United States although clinical anaplasmosis is uncommon in the northern states such as Montana, Idaho, Washington, and North Dakota (14
). Apparent seroprevalence reports range from very low in northern states (1.93% in Montana) to moderately high in the Midwest (7.6% in Illinois) (21
), Gulf (3.8% to 11.2% in Louisiana) (4
), and southern states (4.7 to 17% in Oklahoma) (22
). There is no published literature on serological surveys of the prevalence of anaplasmosis in feedlots in the southern US. To our knowledge, this is the first study to report the prevalence of anaplasmosis in calves at processing in US feedlots and to examine associations with morbidity, mortality, production parameters, and carcass traits.
The number of calves potentially exposed to anaplasmosis during the feeding period could not be quantified in the present study as sampling only occurred on arrival. This is an important consideration in interpreting the results of the present study given that serological status on arrival was used as the explanatory variable for mortality and morbidity events that happened subsequently. Vector-borne transmission of anaplasmosis is considered uncommon in feedlots and is unlikely to have occurred in winter in Iowa since this is the non-vector season. However, iatrogenic transmission via contaminated equipment at processing is a consideration. Calves may therefore have been misclassified as seronegative impacting subsequent associations between serological status and production parameters.
The κ statistic measures the agreement between 2 tests on a scale from 0 to 1. When applied to diagnostic test results, common interpretations of κ are that < 0.2 is slight agreement, 0.2 to 0.4 is fair agreement, 0.4 to 0.6 is moderate agreement, 0.6 to 0.8 is substantial agreement, and > 0.8 is almost perfect agreement (19
). In this study, a κ statistic of 0.74 ± 0.036 (30% cutoff) was calculated indicating substantial, but not perfect, agreement between the 2 duplicate ELISA tests. One explanation for the discrepancy between tests is that chance-corrected measures of association require test results to be converted to discrete dichotomous outcomes (positive or negative) based on a pre-determined cutoff value. This implies that the agreement between tests may change depending on which cutoff value is selected to determine serological status.
In the United States, the cELISA test manufacturer (VMRD) suggests a 30% inhibition cutoff to determine Anaplasma
positive status which was therefore used to determine serostatus in the present study (13
). However, the Canadian Food Inspection Agency has previously selected a 42% inhibition cutoff in serological surveys of Canadian cattle using the VMRD test (14
). When the 42% cutoff was applied to our results, the apparent and adjusted prevalence was reduced from 15% to 5%. This is consistent with a reduction in apparent prevalence from 1.93% to 0.73% noted when cELISA results from Montana-sourced calves were interpreted using a 42% inhibition cutoff (14
When the cELISA test cutoff was increased from 30% to 42% inhibition, the κ statistic still indicated substantial, but not perfect, agreement between the 2 duplicate ELISA tests. However, when test results interpreted using a 30% inhibition cutoff were compared with results interpreted with a 42% inhibition cutoff, the κ statistic indicated only moderate agreement between tests. This finding suggests that increasing the cutoff has a substantial effect on the interpretation of anaplasmosis cELISA test results. This has implications for disease certification and international trade in livestock, especially if territories apply different cutoff levels for results obtained from the same diagnostic test. Furthermore, this indicates that cELISA results in the 30% to 40% inhibition range may be inconclusive and the tests should likely be repeated. We suggest that diagnostic laboratories report percent inhibitions with the cutoff specific test interpretation to allow clinicians to determine if animals have been accurately classified as anaplasmosis positive or negative.
Procedures such as vaccination, ear tagging, hormone implanting, dehorning, and castration performed soon after cattle arrive at feedlots are collectively known as processing (22
). Blood-contaminated needles and surgical instruments are recognized methods of transmission of anaplasmosis. In a review of several reports of anaplasmosis outbreaks attributed to iatrogenic infections, Dikmans referenced a case involving 1500 animals associated with exposure to contaminated dehorning and horn tipping equipment (23
). Stiles similarly published a report of 105 cases of anaplasmosis that occurred following vaccination (24
). This observation has subsequently been confirmed experimentally (8
). The apparent prevalence of anaplasmosis in calves arriving at feedlots reported herein emphasizes the importance of using clean equipment to minimize disease transmission at processing.
The term “undifferentiated fever” (UF) is used to describe feedlot cattle which have signs of depression and pyrexia in the absence of abnormal clinical signs referable to a specific organ system other than the respiratory system (26
). The reason for the weak association between anaplasmosis serostatus and UF reported in the present study is not known. Since the number of calves enrolled in this study was not sufficient to detect a statistically significant association between these parameters, a larger investigation is required to elucidate this. With the proportions of disease positive and negative animals in this study a minimum sample size of 985 animals would be required for a statistical power of 0.80. Fever is considered the first recorded sign of anaplasmosis and fever in excess of 40°C usually persists throughout the period of increasing parasitemia (27
). In acute anaplasmosis, phagocytosis of parasitized erythrocytes causes severe anemia, resulting in increased heart and respiratory rates (27
). It is therefore possible that some cases of UF may be attributable to clinical anaplasmosis rather than BRD.
Calves with UF attributable to anaplasmosis may therefore be inappropriately treated with antimicrobials that are only effective against BRD pathogens. Macrolides, florfenicol, and tetracyclines are the primary antimicrobials used as part of the initial treatment of calves presenting with UF (30
). Of these, only the tetracyclines are effective against acute anaplasmosis (29
). It is also noteworthy that anaplasmosis infections are not sterilized at the usual recommended tetracycline doses (31
). These cattle will remain persistently infected and may thus serve as lifelong reservoirs of infection in the feedlot (23
). However, since A. marginale
infection in calves less than 1-year-old is generally self-limiting and rarely fatal (28
), the resolution of UF in these cases may in some cases be incorrectly attributed to a response to antimicrobial therapy.
Although anaplasmosis outbreaks in calves are uncommon, there are reports that cattle on a higher plane of nutrition develop more severe anaplasmosis than animals maintained on a lower energy plane (32
). In 1 study, Thompson et al (34
) reported 122 cases of clinical anaplasmosis in 2036 feedlot calves aged between 5 and 10 mo. Regression analysis of the data found that calves diagnosed and treated for anaplasmosis during the late finishing period (d 35 to slaughter) had a reduction in average daily gain (ADG) of 96 g (P
< 0.001). In addition to a reduced ADG, prehepatic icterus associated with tick-borne infections is considered a major cause of carcass condemnation in countries where hemoparasitic diseases are prevalent (35
). Consequently, although no significant association between anaplasmosis serostatus and performance was found in the present study, clinical anaplasmosis has significant economic implications in feedlot operations.
In conclusion, the results presented here indicate that the apparent prevalence of anaplasmosis in Iowa feedlots is higher than previously reported in Montana-sourced calves arriving in Canadian feedlots. These data emphasize the importance of implementing biosecurity and biocontainment strategies to prevent iatrogenic transmission of anaplasmosis during processing. Calves that were seropositive for A. marginale did not have higher morbidity, mortality, or individual treatment costs than seronegative cohorts. Although a weak association between anaplasmosis serostatus and undifferentiated fever was found, additional studies are needed to characterize this association further.