Epidemiology of Johne's Disease

Host range

The cause of Johne's disease, Mycobacterium paratuberculosis, has a broad host range. The type of animals most commonly infected are ruminants: animals that are herbivores, have 3 or 4 chambered stomachs, and chew their cud. Examples of ruminants include: cattle, sheep, goats, deer, elk, antelope, camels, llamas, and alpacas. Johne's disease has been reported in several species of wild ruminants, both captive and free-ranging. In addition, there have been a few reports of M. paratuberculosis infecting pigs, horses, and nonhuman primates. Some recent reports claim to have detected genetic components of M. paratuberculosis in humans. The significance of these findings has yet to be determined, however.

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Prevalence

Johne's disease has been reported on every continent. Virtually no country or region of the world can claim freedom from the disease. The reported prevalence of infected animals is at least partially a reflection of the diligence with which veterinarians and animal owners look for the disease.

In the USA, one national survey found that 2.6% of dairy cattle and 1.8% of beef cattle harbored a M. paratuberculosis infection. Regional surveys report 3 to 18% of dairy cattle as infected. Surveys of this type are affected by the type of diagnostic test used for testing animals and the nature of the study design. The only published survey of animal herds (as opposed to individual animals) was done on dairy herds in Wisconsin using a blood test called the ELISA. In that survey, one-third of herds showed evidence of having one or more infected animals.

In the state of Victoria, Australia, 14-17% of dairy herds are estimated to be infected with M. paratuberculosis. Johne's disease is common in dairy herds in most European countries as well.

The prevalence of Johne's disease in small ruminants like sheep, goats and farmed deer is less well studied. However, based on the experience and opinion of several experts, the disease is fairly common.

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Sources of infection

M. paratuberculosis bacteria are obligate "parasites". This means that the only place they can grow and multiply in nature is inside an animal. When M. paratuberculosis leaves an animal, for example in the feces, it can survive for a long time in the environment, but it can not multiply once outside the animal. Consequently, the primary source of infection is infected animals.

Herds or flocks of animals acquire the infection by addition of an infected animal. The longer that animal remains part of the herd or flock, the greater the opportunity for transmission of M. paratuberculosis bacteria to other animals. As the infection progresses, the frequency and number of M. paratuberculosis bacteria being excreted increases.

M. paratuberculosis infects the intestine, thus feces (manure) is the most common vehicle for exit of the bacterium from the animal. In feces, M. paratuberculosis can remain alive for over a year, depending on environmental conditions. Ingestion of feces containing M. paratuberculosis is the most common way animals become infected.

Milk from infected female animals is a second source of M. paratuberculosis infection. The likelihood of M. paratuberculosis being excreted in milk of animals increases with time as the infection progresses. The probability of young animals becoming infected by drinking milk from infected cows, does, or ewes is a direct function of time spent with the mother and/or how often they are fed milk from infected females. In animals where husbandry practices allow young animals to remain with their mothers and nurse naturally, the chances for transmission of the infection are greatest. M. paratuberculosis may be excreted directly into the mother's milk or, it might be on the outside of the teats from contamination with infected feces.

Pond water contaminated with feces of animals infected with M. paratuberculosis is another potential source of infection. A less likely, but possible, infection source is pastures contaminated with infected feces.

Johne's disease has been reported in free ranging wildlife, but their role in the ecology of Johne's disease is not known. Similarly, it is not known if wild birds can become infected or transfer M. paratuberculosis bacteria between farms.

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Transmission of infection

Risk factors affecting the probability of transmission of M. paratuberculosis between animals have not been measured. Consequently, only general statements can be made about circumstances that promote transmission of infection.

Animal age is perhaps the most well recognized factor affecting M. paratuberculosis transmission. In cattle, there is an age-dependent increase in resistance to M. paratuberculosis infection. This means it takes a larger dose of the bacterium to infect an adult (over 2 years-old) than it does to infect a young animal (0 to 6 months-old). This may also be true for small ruminants, but it is not as well studied and clinical reports suggest there is a greater susceptibility of sheep, goats and deer to this infection, even as adults.

Extent and duration of exposure to feces and milk from infected adult animals directly affects the likelihood of M. paratuberculosis transmission. Clean, dry, birthing environments and housing of young animals away from the adult herd or flock limits the possibility of infection transmission. Conversely, dirty maternity pens or fecal contamination of feed and water supplies will promote spread of the infection.

A computer model was developed that predicts the rate of spread of M. paratuberculosis infections in herds of dairy cattle. A detailed description of the model was published (Preventive Veterinary Medicine, 11:131-146, 1991) and an example is included here:

Epidemiological graph The curve shown is for a herd of 60 milking dairy cows with average quality herd management, where one replacement heifer is bought per year from a source with a 5% prevalence of M. paratuberculosis infections. The model illustrates that it takes a long time for the infection to get started in a herd, but once that happens M. paratuberculosis can spread through the herd quickly if no actions are taken to control it.

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Selected references

  1. Beeman, K.B., B.J. Huber, and H .W. Leipold. 1989. Johne's disease (paratuberculosis) in sheep. Comp.Contin.Educ.Pract.Vet. 11:1415-1422.

  2. Belknap, E.B., D.M. Getzy, L W. Johnson, R.P. Ellis, G.L. Thompson, and W.P. Schulaw. 1994. Mycobacterium paratuberculosis infection in two llamas. J.Am.Vet.Med.Assoc. 204:1805-1808.

  3. Braum, P.K., C.D. Buergelt, R.C. Littell, S.B. Linda, and J.R. Simpson. 1990. Use of an enzyme-linked immunosorbent assay to estimate prevalence of paratuberculosis in cattle of Florida. J.Am.Vet.Med.Assoc. 196:1251-1254.

  4. Chiodini, R.J. and H.J. Van Kruiningen. 1983. Eastern white-tailed deer as a reservoir of ruminant paratuberculosis. J.Am.Vet.Med.Assoc. 182:168-169.

  5. Chiodini, R.J. and H.J. Van Kruiningen. 1986. The prevalence of paratuberculosis in culled New England cattle. Cornell Vet. 76:91-104.

  6. Collins, M.T. and I.R. Morgan. 1991. Epidemiologic model of paratuberculosis in dairy cattle. Prev.Vet.Med. 11:131-146.

  7. Collins, M.T., D.C. Sockett, W.J. Goodger, T.A. Conrad, C.B. Thomas, and D.J. Carr. 1994. Herd prevalence, geographic distribution of, and risk factors for, bovine paratuberculosis in Wisconsin. J.Am.Vet.Med.Assoc. 204:636-641.

  8. de Lisle, G.W., G.F. Yates, and D.M. Collins. 1993. Paratuberculosis in farmed deer: case reports and DNA characterization of isolates of Mycobacterium paratuberculosis. J.Vet.Diagn.Invest. 5:567-571.

  9. Dierckins, M.S., D.M. Sherman, and A. Gendron-Fitzpatrick. 1990. Probable paratuberculosis in a sicilian ass. J.Am.Vet.Med.Assoc. 196:459-461.

  10. Fawcett, A.R., P.J. Goddard, W.A.C. McKelvey, D.Buxton, H.W. Reid, A. Greig, and A.J. Macdonald. 1995. Johne's disease in a herd of farmed red deer. Vet.Rec. 136:165-169.

  11. Frisby, H.R. and D.T. Berman. 1985. Design and interpretation of a slaughter survey of Wisconsin cull cows to estimate the prevalence of Mycobacterium paratuberculosis infection. Proc.Ann.Mtg.U.S.Anim.Hlth.Assoc. 89:458-474.

  12. Gay, J.M. and D.M. Sherman. 1992. Factors in the epidemiology and control of ruminant paratuberculosis. Vet.Med. 87:1133-1139.

  13. McClure, H.M., R.J. Chiodini, D.C. Anderson, R.B. Swenson, W.R. Thayer, and J.A. Cook. 1987. Mycobacterium paratuberculosis infection in a colony of stump tail macaques (Macaca artoides). J.Infect.Dis. 155:1011-1019.

  14. Merkal, R.S., D.L. Whipple, J.M. Sacks, and G.R. Snyder. 1987. Prevalence of Mycobacterium paratuberculosis in ileocecal lymph nodes of cattle culled in the United States. J.Am.Vet.Med.Assoc. 190:676-680.

  15. Riemann, H., M.R. Zaman, R. Ruppanner, O. Aalund, J.B. Jorgensen, H. Worsaae, and D. Behymer. 1979. Paratuberculosis in cattle and free-living exotic deer. J.Am.Vet.Med.Assoc. 174:841-843.

  16. Saxegaard, F. 1990. Experimental infection of calves with an apparently specific goat-pathogenic strain of Mycobacterium paratuberculosis. J.Comp.Path. 102:149-156.

  17. Seitz, S.E., L.E. Heider, W.D. Hueston, S. Bech-Nielsen, M. Rings, and L. Spangler. 1989. Bovine fetal infection with Mycobacterium paratuberculosis. J.Am.Vet.Med.Assoc. 194:1423-1426.

  18. Streeter, R.N., G.F. Hoffsis, S. Bech-Nielsen, W.P. Shulaw, and M. Rings. 1995. Isolation of Mycobacterium paratuberculosis from colostrum and milk of subclinically infected cows. Am.J.Vet.Res. 56:1322-1324.

  19. Sweeney, R.W., R.H. Whitlock, and A.E. Rosenberger. 1992. Mycobacterium paratuberculosis cultured from milk and supramammary lymph nodes of infected asymptomatic cows. J.Clin.Microbiol. 30:166-171.

  20. Sweeney, R.W., R.H. Whitlock, and A.E. Rosenberger. 1992. Mycobacterium paratuberculosis isolated from fetuses of infected cows not manifesting signs of the disease. Am.J.Vet.Res. 53:477-480.

  21. Van Kruiningen, H.J., R.J. Chiodini, W.R. Thayer, J.A. Coutu, R.S. Merkal, and P.L. Runnels. 1986. Experimental disease in infant goats induced by a Mycobacterium isolated from a patient with Crohn's disease. A preliminary report. Digest.Dis.and Sci. 31:1351-1360.

  22. Williams, E.S., J.C. DeMartini, and S.P. Snyder. 1985. Lymphocyte blastogenesis, complement fixation, and fecal culture as diagnostic tests for paratuberculosis in North American wild ruminants and domestic sheep. Am.J.Vet.Res. 46:2317-2321.

  23. Williams, E.S., S.P. Snyder, and et al. 1983. Experimental infection of some North American wild ruminants and domestic sheep with Mycobacterium paratuberculosis: clinical and bacteriological findings. J.Wild.Dis. 19:185-191.
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