When an infected animal moves from one farm to another, it might introduce a pathogen to that one farm. When a feed supplier, sales person, inspector or veterinarian visits an infected farm, the pathogen could spread to even more farms, depending on the pattern of visits and the “network” of potential indirect contacts between farms.

Researchers in Italy recently reported on a new modeling system for tracking the infection potential for networks of indirect contacts, or fomites, along with direct contact from animals moving from one herd to another.

The research report, titled “The Potential Role of Direct and Indirect Contacts on Infection Spread in Dairy Farm Networks,” is published in the online journal PLOS Computational Biology. The researchers note that the European Union initiated an individual identification and traceability system for cattle in 1997, and the resulting database has helped improve understanding of the patterns of disease transmission through direct contact between animals. Less is known, however, about transmission patterns related to indirect contacts, such as vehicles or people, and the interrelationships between the two sources of transmission. For example, the researches point out that during the 2001 foot and mouth disease (FMD) outbreak in the UK, new farms continued to become infected well after animal movements were halted. “The FMD epidemic stopped only after the implementation of stronger biosecurity measures in UK farms, which mostly targeted the movement of contaminated equipment and personnel,” they note.

For this study, the researchers looked at Mycobacterium avium subsp. paratuberculosis (MAP), the pathogen responsible for Johne’s disease. They built network models for between-farm direct contacts using data on cattle exchange and on indirect contacts using data for on-farm visits by veterinarians.

The researchers combined data from 6,524 on-farm visits performed by 50 veterinary officers and 14,053 visits performed by 203 veterinary practitioners. They found that while indirect contacts are less efficient in transmitting diseases compared to direct contacts, the two networks operate independently, creating a need to account for both routes in developing plans for the control of potential epidemics where indirect contacts represent an effective route of disease transmission. The authors point out that most farms receive or move cattle only a few times each year, while veterinarian visits can be much more frequent.

The two networks of direct and indirect contacts also have a synergistic relationship, the researchers note, as the infection potential derived by combining the networks of direct and indirect contacts is considerably larger than the one computed by using only cattle movements or the veterinary network. Direct contacts can act as a “bridge” joining different clusters of potentially infectious contacts due to veterinarians. “Our results highlighted the importance of considering indirect contacts to adequately model between-farm spread of infections,” the authors note.

The researchers add that use of data on personnel visits could improve the surveillance and control of epidemics such as FMD and avian influenza through modelling approaches.

Read the full article online from PLOS Computational Biology.