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Bovine paratuberculosis: modelling to control within-herd spread  

To better study bovine paratuberculosis, or Johne's disease, and to control its spread, scientists in the INRA-Oniris* Joint Research Unit for Biology, Epidemiology and Risk Analysis in Animal Health (BioEpAR, UMR 1300) built a computer model that has enabled study of the spread of the causal bacterium within a dairy cattle herd. This model has demonstrated how herd management and structure can lead to the persistence or disappearance of the disease on a farm. This computer simulation will now enable the development of decision-making tools.

Colonie de Mycobacterium avium ssp. paratuberculosis sur gélose (milieu de Herrolds au jaune d'œuf (X 15)). © GRAYON Maggy
Updated on 02/12/2013
Published on 07/17/2012

A persistent disease

Mycobacterium avium subsp. Paratuberculosis (Map) is the pathogen that causes paratuberculosis or paratuberculous enteritis, an incurable and fatal disease in cattle. This pathogen infects calves up to the age of a year, either in utero through the infection of a gestating cow (vertical transmission) or via the consumption of milk or colostrum from an infected cow, or as a result of indirect contamination from the environment (feed, etc.) via dejections from infected young or adult animals (horizontal transmission). It may be years (up to 15 years) before newly-infected animals will present clinical signs of the disease, but because they have since become contagious, they shed bacteria that contaminate animal housing and neighbouring pastures. Particularly resistant, Map can live in the environment for several years. Once infection has spread within a herd, often following the introduction of an infected animal, it can cause a drop in milk yield and a deterioration in beef carcass weight and quality. The slaughter of sick animals is usually recommended to control outbreaks within a herd.

Modelling: an exploratory tool

In order to control the spread of Map, herd protection measures are necessary. However, although between-animal transmission pathways have been identified, it has never been possible until now to quantify their relative contributions to within-herd infection dynamics, or to determine the rearing conditions that influence the spread of Map. Indeed, studies on the epidemiology of this disease are very difficult to perform in the field and extremely costly because of the problems encountered in diagnosing the infection, detecting Map in samples and, above all, due to its lengthy incubation period. Using an integrative modelling approach that can separately or simultaneously vary herd management parameters constitutes an excellent means of studying Map spread within a herd.

An original model

In the context of the SANCRE project** (Animal Health, Food Safety, and Competitiveness of Regional Animal Supply Chains; Santé animale, sécurité des aliments et compétitivité des filières animales régionales), the scientists modelled the dynamics of Map spread within a dairy cattle herd. This original model, called ParatubSpread_Intra (filed jointly by INRA and Oniris with the French Software Protection Agency as no. IDDN.FR.001.240022.000.R.P.2011.000.31230) takes account of the survival of Map in the environment and its shedding by infected calves and cows. Depending on their age or parity, animals have different probabilities of switching between stages of infection: susceptible vs. resistant to Map infection, temporarily infectious, latently infected, sub-clinically infectious (high levels of shedding but no apparent signs), clinically affected (rapidly culled). The researchers modulated herd management parameters based on data describing the technical choices within a regulatory framework (duration of maintaining calves in individual pens), observed data (grazing periods, duration of the susceptibility of animals to infection, exposure of animals to Map, etc.), and expert opinions (variable efficiency of cleaning procedures) for the management of a herd over 25 years. They studied the case of the contamination of a herd by Map due to the entry of a single infected animal (a "closed" herd differing from an "open" herd where numerous animals may be introduced).

Valid priorities whatever the herd size

Using this model, the scientists were able to demonstrate that following the occurrence of at least two clinical cases of the disease within five years of Map introduction into a dairy cattle herd, the infection persisted over the long term in 96% of cases if no control measures were implemented other than the culling of clinical cases within 6 months of their detection. Furthermore, the infection was mainly transmitted in utero and via the environment; the other known pathways only played minor roles at the herd scale. Preventing contacts between calves had no impact on Map spread. On the other hand, preventing calf exposure to an environment contaminated by adult cows, and rapidly culling clinical cases, were essential to enabling an effective reduction in disease prevalence. Whatever the herd size, the conclusions of the simulations – performed in each case for a timeframe of 25 years – were the same.

Based on the results obtained, the research team recommended the introduction into farms of a calf rearing zone that is very securely separated from the areas occupied by adults, with biosafety measures similar to those applied for pigs or poultry. The very early elimination of animals displaying any clinical signs was also highlighted as a means of reducing the presence of Map within herds.

Since this initial study, the scientists have launched a new project (MIHMES, ANR Investments for the Future) which, based on this modelling, will enable the development of a software that also takes account of animal movements and the risks of contamination between farms, so that appropriate sanitary measures can be drawn up at a regional scale. Once developed, this tool will enable the evaluation of a broad range of control actions, implemented alone or in combination.

*Oniris: Nantes-Atlantic National College of Veterinary Medicine, Food Science and Engineering (Ecole Nationale Vétérinaire, Agroalimentaire et de l'Alimentation Nantes-Atlantique): http://www.oniris-nantes.fr/
** SANCRE is a project under the "For and On Regional Development in Western France" programme (PSDR GO) funded by INRA, CEMAGREF, and the Basse-Normandie, Brittany, Pays de la Loire and Poitou-Charentes Regional Councils. http://www4.inra.fr/psdrgo/Qu-est-ce-que-PSDR-GO

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Find out more

  • Marcé, C. ; Ezanno, P. ; Seegers, H. ; Pfeiffer, D.U. ; Fourichon, C. Predicting fadeout versus persistence of paratuberculosis in a dairy cattle herd for management and control purposes: a modelling study. Veterinary Research. 2011, 42 (1):36
  • Marcé, C. ; Ezanno, P. ; Seegers H. ; Pfeiffer D.U. ; Fourichon, C. Within-herd contact structure and transmission of Mycobacterium avium subspecies paratuberculosis in a persistently infected dairy cattle herd. Preventive Veterinary Medicine. 2011, 100:116-125
  • Marcé, C. ; Ezanno, P. ; Weber, M.F. ; Seegers, H. ; Pfeiffer, D.U. ; Fourichon C. Invited review: Modeling within-herd transmission of Mycobacterium avium subspecies paratuberculosis in cattle: a review. Journal of Dairy Science., 2010, 93:4455-4470
  • Marcé, C. ; Guatteo, R. ; Bareille, N. ; Fourichon, C. Dairy calf housing systems across Europe and risk for calf infectious diseases. Animal. 2010, 4:1588-1596
  • ParatubSpred_Intra, IDDN.FR.001.240022.000.R.P.2011.000.31230, co-dépôt INRA/Oniris du 16 juin 2011