Effect of delayed exposure of cattle to Mycobacterium avium subsp paratuberculosis on the development of subclinical and clinical Johne's disease

Association between the severity of histopathological lesions and Mycobacterium avium subspecies paratuberculosis (MAP) molecular diversity in cattle in southern Chile

The objective was to evaluate the association between the severity of histopathological lesions caused by Mycobacterium avium subspecies paratuberculosis (MAP) infection and the molecular diversity of this pathogen. Blood, ileum, and mesenteric lymph node samples were collected at slaughter, from 1,352 adult cattle [source population 1 (SP1)]. In addition, 42 dairy herds (n = 4,963 cows) were followed for 2 years, and samples from compatible paratuberculosis clinical cases [source population 2 (SP2)] were collected. MAP infection was confirmed using an ELISA test, liquid media culture, and PCR. Isolates were genotyped using five MIRU-VNTR markers. Tissues from confirmed samples were subjected to a histopathological examination. A histopathological severity score (HSS) system was developed and used to grade (0 to 5) the magnitude of lesions caused by MAP. In general, the HSS system assesses the number of foci and degree of macrophage infiltration, together with the presence of multinucleated giant cells (MGCs) and acid-fast bacilli (AFB), in addition to the fusion of the intestinal villi and hyperplasia of the crypts. Despite the large sampling effort, only 79 MAP isolates were successfully genotyped, where 19 different haplotypes were described. A mixed-effect Poisson regression model was used to assess the relationship between haplotypes and HSS values. The model was controlled by animal age, and the farm was used as a random effect. Haplotypes were grouped based on their relative frequency: the most frequent haplotype (group i, 49.4%), the second most frequent haplotype (group ii, 12.7%), and all other haplotypes (group iii, 37.9%). Model outputs indicated that group i had significantly higher HSS values than group iii. In addition, group i was also associated with higher optical density (OD) values of the ELISA test. These results support the existence of differences in pathogenicity between MAP haplotypes. However, results were based on a relatively small sample size; thus, these should be taken with caution. Despite this, study findings suggest that haplotypes would be associated with differences in disease progression, where the dominant haplotype tends to generate more severe lesions, which could be linked to a greater shed of MAP cells than non-dominant haplotypes, increasing their chances of transmission.

Modeling Paratuberculosis Transmission in a Small Dairy Herd Typical of Slovenia Suggests That Different Models Should Be Used to Study Disease Spread in Herds of Different Sizes

This study aimed to investigate the possible dynamics of paratuberculosis or Johne’s disease in a typical Slovenian dairy herd of about 17 cows. Paratuberculosis is a worldwide endemic disease of cattle caused by Mycobacterium avium subsp. paratuberculosis (MAP) and is associated with significant economic losses. We developed a stochastic compartmental model with two pathways of disease progression, infections of adult cows and infections of young animals through horizontal and vertical transmission, and transmission through animal movements. The average proportions of subclinically and clinically infected cows were 4% and 0.47%, respectively. The prevalence within the herd, which included latently infected animals, averaged 7.13% and ranged from 0% to 70.59%. Under the given circumstances, the results showed a relatively high rate of spontaneous elimination (0.22 per herd per year) of the disease and a high rate of reinfection (0.18 per herd per year) facilitated by active animal trade. To our knowledge, this stochastic compartmental model is the first to be developed specifically to represent a small dairy herd and could apply to other countries with a similar structure of dairy farms. The results suggest that different models should be used to study MAP spread in herds of various sizes.

Implications of Cattle Trade for the Spread and Control of Infectious Diseases in Slovenia

The objectives of this study were to gain insight into the structure of the cattle trade network in Slovenia and to evaluate the potential for infectious disease spread through movements. The study considered cattle movements between different types of premises that occurred between August 1, 2011 and July 31, 2016 with the exclusion of the movements to the end nodes (e.g., slaughterhouses). In the first part, we performed a static network analysis on monthly and yearly snapshots of the network. These time scales reflect our interest in slowly spreading pathogens; namely Mycobacterium avium subsp. paratuberculosis (MAP), which causes paratuberculosis, a worldwide economically important disease. The results showed consistency in the network measures over time; nevertheless, it was evident that year to year contacts between premises were changing. The importance of individual premises for the network connectedness was highly heterogeneous and the most influential premises in the network were collection centers, mountain pastures, and pastures. Compared to random node removal, targeted removal informed by ranking based on local network measures from previous years was substantially more effective in network disassociation. Inclusion of the latest movement data improved the results. In the second part, we simulated disease spread using a Susceptible-Infectious (SI) model on the temporal network. The SI model was based on the empirically estimated true prevalence of paratuberculosis in Slovenia and four scenarios for probabilities of transmission. Different probabilities were realized by the generation of new networks with the corresponding proportion of contacts which were randomly selected from the original network. These diluted networks served as substrates for simulation of MAP spread. The probability of transmission had a significant influence on the velocity of disease spread through the network. The peaks in daily incidence rates of infected herds were observed at the end of the grazing period. Our results suggest that network analysis may provide support in the optimization of paratuberculosis surveillance and intervention in Slovenia. The approach of simulating disease spread on a diluted network may also be used to model other transmission pathways between herds.

Characteristics of subclinical Mycobacterium avium ssp. paratuberculosis infection in a captive white-tailed deer herd

Paratuberculosis (Johne's disease) is caused by Mycobacterium avium ssp. paratuberculosis (MAP), and affects both domestic and wild ruminants, including cattle, goats, sheep, and deer. In cattle, most infections occur during calfhood followed by a prolonged incubation period of 1-2 y or more before cows shed culturable numbers of MAP bacilli in their feces. As disease progresses, infected animals develop protein-losing enteropathy, intractable diarrhea, and weight loss. In a cohort of 32 clinically normal deer from a herd with a history of periodic clinical paratuberculosis, we found that subclinical infection was characterized by high rates of infection, common involvement of mesenteric lymph nodes, minimal lesion formation, few intralesional acid-fast bacilli, and low-level fecal shedding of MAP. The characteristics of subclinical paratuberculosis in white-tailed deer resemble those of cattle and red deer, although microscopic lesions were less common in subclinical deer than reported for subclinical cattle, and we did not see necrotizing granulomas as described in subclinical red deer and elk.

A nested compartmental model to assess the efficacy of paratuberculosis control measures on U.S. dairy farms

Paratuberculosis, also known as Johne's disease (JD), is a chronic contagious disease, caused by Mycobacterium avium subsp. paratuberculosis (MAP). The disease is incurable, fatal and causes economic losses estimated to exceed 200 million dollars to the U.S. dairy industry annually. Several preventive and control measures have been recommended; however, only a few of these measures have been validated empirically. Using a nested compartmental (NC) modeling approach, the main objective of this research was to identify the best combination of control and preventive measures that minimizes the prevalence and incidence of JD and the risk of MAP occurrence in a dairy herd. The NC model employs both MAP transmission estimates and data on pen movement of cattle on a dairy to quantify the effectiveness of control and preventive measures. To obtain reasonable ranges of parameter values for between-pen movements, the NC model was fitted to the movement data of four typical California dairy farms. Using the estimated ranges of the movement parameters and those of JD from previous research, the basic reproduction number was calculated to measure the risk of MAP occurrence in each pen environment as well as the entire dairy. Although the interventions evaluated by the NC model were shown to reduce the infection, no single measure alone was capable of eradicating the infection. The numerical simulations suggest that a combination of test and cull with more frequent manure removal is the most effective method in reducing incidence, prevalence and the risk of MAP occurrence. Other control measures such as limiting calf-adult cow contacts, raising calves in a disease-free herd or colostrum management were less effective.

An agent-based model evaluation of economic control strategies for paratuberculosis in a dairy herd

This paper uses an agent-based simulation model to estimate the costs associated with Mycobacterium avium ssp. paratuberculosis (MAP), or Johne's disease, in a milking herd, and to determine the net benefits of implementing various control strategies. The net present value (NPV) of a 1,000-cow milking herd is calculated over 20 yr, parametrized to a representative US commercial herd. The revenues of the herd are generated from sales of milk and culled animals. The costs include all variable and fixed costs necessary to operate a representative 1,000-cow milking herd. We estimate the NPV of the herd with no MAP infection, under an expected endemic infection distribution with no controls, and under an expected endemic infection distribution with various controls. The initial number of cows in a herd with an endemic MAP infection is distributed as 75% susceptible, 13% latent, 9% low MAP shedding, and 3% high MAP shedding. Control strategies include testing using ELISA and fecal culture tests and culling of cows that test positive, and culling based on observable milk production decrease. Results show that culling cows based on test results does not increase the herd's NPV and in most cases decreases NPV due to test costs as well as false positives and negatives with their associated costs (e.g., culling healthy cows and keeping infected cows). Culling consistently low producing cows when MAP is believed to be present in the herd produces higher NPV over the strategy of testing and culling MAP infected animals, and over the case of no MAP control.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): paratuberculosis

Paratuberculosis has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of paratuberculosis to be listed, Article 9 for the categorisation of paratuberculosis according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to paratuberculosis. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, paratuberculosis can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria in Sections 3, 4 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (c), (d) and (e) of Article 9(1). The animal species to be listed for paratuberculosis according to Article 8(3) criteria are several species of mammals and birds as susceptible species and some species of the families Bovidae, Cervidae and Leporidae as reservoirs.

A new compartmental model of Mycobacterium avium subsp. paratuberculosis infection dynamics in cattle

Models of Mycobacterium avium subsp. paratuberculosis (MAP), a chronic infectious agent of cattle, are used to identify effective control programs. However, new biological findings show that adult infections occur and that infected animals can be separated into 2 paths: animals that will become high-shedding and, eventually, experience clinical disease (high-path); and animals that will shed only small quantities of MAP and will remain subclinical (low-path). Longitudinal data analysis found that high-path animals progress more quickly than previously believed. A standard model of MAP transmission in dairy herds was modified to include adult low-path infections and 2 infection pathways for infected calves. Analysis of this model showed that adult infection may play an important role in MAP dynamics on a dairy farm, and that the increased rate of progression for high-path animals influences both the prevalence and the persistence of MAP on a dairy farm. This new model will be able to determine the effectiveness of MAP control programs more accurately than previous models.

Evaluation of the Johne's disease risk assessment and management plan on dairy farms in Ontario, Canada

Johne's disease (JD) is a production-limiting gastrointestinal disease in cattle. To minimize the effects of JD, the Ontario dairy industry launched the Ontario Johne's Education and Management Assistance Program in 2010. As part of the program, trained veterinarians conducted a risk assessment and management plan (RAMP), an on-farm questionnaire where high RAMP scores are associated with high risk of JD transmission. Subsequently, veterinarians recommended farm-specific management practices for JD prevention. Milk or serum ELISA results from the milking herd were used to determine the herd ELISA status (HES) and within-herd prevalence. After 3.5 yr of implementation of the program, the aim of this study was to evaluate the associations among RAMP scores, HES, and recommendations. Data from 2,103 herds were available for the analyses. A zero-inflated negative binomial model for the prediction of the number of ELISA-positive animals per farm was built. The model included individual RAMP questions about purchasing animals in the logistic portion, indicating risks for between-herd transmission, and purchasing bulls, birth of calves outside the designated calving area, colostrum and milk feeding management, and adult cow environmental hygiene in the negative binomial portion, indicating risk factors for within-herd transmission. However, farms which fed low-risk milk compared with milk replacer had fewer seropositive animals. The model additionally included the JD herd history in the negative binomial and the logistic portion, indicating that herds with a JD herd history were more likely to have at least 1 positive animal and to have a higher number of positive animals. Generally, a positive association was noted between RAMP scores and the odds of receiving a recommendation for the respective risk area; however, the relationship was not always linear. For general JD risk and calving area risk, seropositive herds had higher odds of receiving recommendations compared with seronegative herds if the section scores were low. This study suggests that the RAMP is a valuable tool to assess the risk for JD transmission within and between herds and to determine farm-specific recommendations for JD prevention.

Evaluation of testing strategies to identify infected animals at a single round of testing within dairy herds known to be infected with Mycobacterium avium ssp. paratuberculosis

As part of a broader control strategy within herds known to be infected with Mycobacterium avium ssp. paratuberculosis (MAP), individual animal testing is generally conducted to identify infected animals for action, usually culling. Opportunities are now available to quantitatively compare different testing strategies (combinations of tests) in known infected herds. This study evaluates the effectiveness, cost, and cost-effectiveness of different testing strategies to identify infected animals at a single round of testing within dairy herds known to be MAP infected. A model was developed, taking account of both within-herd infection dynamics and test performance, to simulate the use of different tests at a single round of testing in a known infected herd. Model inputs included the number of animals at different stages of infection, the sensitivity and specificity of each test, and the costs of testing and culling. Testing strategies included either milk or serum ELISA alone or with fecal culture in series. Model outputs included effectiveness (detection fraction, the proportion of truly infected animals in the herd that are successfully detected by the testing strategy), cost, and cost-effectiveness (testing cost per true positive detected, total cost per true positive detected). Several assumptions were made: MAP was introduced with a single animal and no management interventions were implemented to limit within-herd transmission of MAP before this test. In medium herds, between 7 and 26% of infected animals are detected at a single round of testing, the former using the milk ELISA and fecal culture in series 5 yr after MAP introduction and the latter using fecal culture alone 15 yr after MAP introduction. The combined costs of testing and culling at a single round of testing increases with time since introduction of MAP infection, with culling costs being much greater than testing costs. The cost-effectiveness of testing varied by testing strategy. It was also greater at 5 yr, compared with 10 or 15 yr, since MAP introduction, highlighting the importance of early detection. Future work is needed to evaluate these testing strategies in subsequent rounds of testing as well as accounting for different herd dynamics and different levels of herd biocontainment.

Agent-based model for Johne's disease dynamics in a dairy herd

Johne’s disease is an infectious gastrointestinal disease in ruminants caused by Mycobacterium avium subsp. paratuberculosis that causes diarrhea, emaciation, decreased milk production and eventually death. The disease is transmitted in utero and via milk and colostrums to calves, and fecal-orally to all age classes. Financial losses due to the disease are estimated to be over $200 million in the US dairy industry. The goal of this study was to evaluate the cost effectiveness of control measures based on diagnosis with a sensitive ELISA, EVELISA. An agent-based, discrete time model was developed to simulate Johne’s disease dynamics in a US dairy herd. Spatial aspects of disease transmission were taken into account by using six spatial compartments. The effects on disease prevalence were studied with and without transmission routes included in the model. Further, using the model, cost effectiveness of ELISA-based Johne’s disease control was evaluated. Using the parameters we collected and assumed, our model showed the initial prevalence of Johne’s disease (33.1 ± 0.2%) in the farm increased to 87.7 ± 1.7% in a 10 year-simulation. When ELISA-based control measures were included in the simulation, the increase in prevalence was significantly slowed down, especially when EVELISA was used. However, the level of the prevalence was still higher than the initial level after 10 year simulation even with the ELISA-based diagnostic intervention. The prevalence was further reduced when quarterly ELISA testing was included. The cost analysis showed that the quarterly ELISA and EVELISA testing could bring $44.8 and $51.5/animal/year more revenues, respectively, to a dairy farm.