Initially prepared and edited by Don Hansen and Christine Rossiter of the AABP Food Safety Committee

In cattle, Johne's disease is an infectious bacterial disease primarily affecting the intestinal tract and associated lymph nodes of ruminants. Johne’s disease should be viewed as a herd problem as well as an individual animal problem because much of the infection is subclinical in nature. Infection is common in domestic ruminant species and has been reported in many species of captive, free-ranging, and exotic ruminants world-wide i.e., cattle, sheep, goats, deer, mtn. goat, elk, antelope. Pathology and signs do vary across species. Isolated cases have been reported in nonruminants including, horses, rabbits, and nonhuman primates, but these species are not believed to be significant reservoirs of Johne’s disease.

Isolation of the bacteria or components of its genetic DNA from people with Crohn’s disease (CD) are reported but the significance as related to CD remains to be determined.

Johne’s disease is caused by a mycobacterium called Mycobacterium avium subspecies paratuberculosis (Map). It is a distant relative of the bacteria Mycobacterium bovis, tuberculosis and leprae that cause TB in humans and animals and leprosy in humans. Map does not cause TB or leprosy. Map is very similar to another Mycobacterium called Mycobacterium avium. One genetic feature that separates Map from M. avium is copies of a unique DNA element named IS900. DNA probe detection of the IS900 unit is also used to identify Map infection.

After ingestion, Map bacteria are taken up by intestinal mucosal cells, especially in Peyer’s patches in the ileum, and immediately engulfed by resident macrophages. Map multiplies slowly within the macrophage and if infection is successful it manages to destroy the macrophage and continue to multiply. Map has clever ability to evade the immune response of the macrophage even after it is turned on. However, not all infections are successful and the immune response in some exposed individuals prevents progression of the initial infection. These mechanisms are largely unknown but do involve dose and number of exposures, strain, immune biology of the individual etc. As organisms multiply they are released and excreted in the feces, which leads to contamination and their accumulation in the animal’s environment, feed and water.

The primary source of contamination and exposure to Johne’s disease that leads to infection is directly from infected animals who are multiplying and shedding the bacteria. The organism does not multiply once it is outside the animal but it is well protected by its’ cell wall and can survive a year or more in most moderate environments. It is more susceptible to degrading by ultraviolet, heat, and drying.

It must be emphasized that, because of the slowly progressive nature of the infection, signs of Johne’s disease may not be seen until years after initial infection. Cattle may be infected for years before they show any signs of disease.

When they finally do occur, the signs of Johne’s disease are intermittent bouts of diarrhea, which eventually becomes chronic, accompanied by weight loss and typically a good appetite. Some infected animals initially just appear unthrifty. Affected cattle do not generally have a fever. The signs of this disease can easily be confused with several other diseases. The development of noticeable signs may occur with stressful events such as calving, feed changes, relocation, etc. There is some data to suggest that subclinical stages may result in a decline in performance, especially in milk production in the last lactation before culling.

Mycobacteria are taken up by specialized cells (M cells) in the ileum. M cells present the bacteria to macrophages and lymphocytes in Peyer’s patches. The cellular immune system reacts to the invasion by recruiting more macrophages and lymphocytes to the site. Lymphocytes release a variety of cytokines, which enhance the bacterial killing ability of the macrophages. Macrophages fuse into large cells and with lymphocytes infiltrate infected tissues in large numbers. This leads to the granulomatous thickening of the intestine. In later phases of the immune response, bacteria escape macrophages into the intestinal lumen and animals begin to shed Map in feces. Eventually the bacteria also escape into the vascular system which stimulates antibody. This humoral immunity however does not play a significant role in controlling Map because of its intracellular localization. 

Ultimately the body’s ineffective immune response to Map yields a combination of factors which contribute to the development of clinical disease in an individual. The factors are poorly understood but involve the inflammation and impaired nutrient absorption from the intestine, interaction of systemic and gut immune components, and the eventual collapse of the cellular immune response allowing the unhindered proliferation of the infection.

The obvious answer is a herd exhibiting cows with chronic diarrhea and/or weight loss in the face of good appetites. However, some animals may be infected, appear normal and be culled before they show any clinical signs. So, some owners may never realize their herd is infected. A complaint in these herds could be that herd production not as high as it should be.

In attempting to find the cause of low herd production, tests for Johne’s disease on several poor-doing animals would be advised. In other herds, owners may see one or more cows with diarrhea or weight loss and suspect Johne’s disease as a possible cause. In chronically infected herds, for each animal with clinical disease there may be five to 15 other animals with subclinical infection and no signs of Johne’s disease at all. See figure 2.

Johne’s is an infectious disease which typically passes through four stages. These stages help explain why infected animals appear healthy for so long and diagnostic tests can’t detect early infection. (Figure 1) Stage I is the initial infection: the animal is infected, not showing signs of disease, and not likely to be shedding bacteria into the environment. The stage is not detectable by diagnostic tests. In Stage II, the infection is progressing but the animal still does not show any clinical signs. The organism however may begin to be excreted in high enough numbers to be detected by fecal culture but only the exception will have antibody detectable by serology. These animals begin to be infectious to animals in contact. Animals in Stage III show early signs of disease and many diagnostic tests detect infected animals as positive. Stage IV is the obvious clinical end-stage of infection. It is readily recognized, terminal, and most animals are positive on most diagnostic tests.

1. The most common source of infection is feces or manure. Except under unusual circumstances infection begins in the first few months of life. Animals become more resistant with age, though resistance is never complete. If given a sufficiently large dose of the bacteria adults can become infected. Nevertheless, under normal animal husbandry conditions, exposure to such large doses probably rarely occurs. This means, for example, that an infected adult dairy cow in a stanchion or tie-stall barn is not going to infect the cow standing next to her. Infected cows are primarily the main source of infection for calves. In a beef herd, large doses of pathogens may be deposited on ground-fed forages thus increasing the risk for other animals and especially youngstock.

The vast majority of infections in young animals are acquired by ingestion of the bacteria. This happens accidentally when they consume manure containing the bacteria. Suckling manure contaminated teats, licking contaminated bars in the stall where they are born, or being housed in a location where they have access to manure from the adult herd are ways young animals have opportunities to ingest this microbe.

Johne’s disease typically enters a herd when infected, but healthy-looking, animals come in. As the disease progresses in that animal, the frequency and number of bacteria being excreted increase. Every day, billions of Johne’s microbes may be excreted from an animal in Stage III or IV of the disease. The infection spreads to calves and herd mates without the owner's knowledge. Eventually signs of the disease may be recognized in one or more animals.

2.  Another source of infection is milk and colostrum from infected dams. The likelihood of Johne’s bacteria being excreted in milk of infected females increases as the disease progresses. Studies suggest that 36% of Stage III and IV cows could have Johne’s microbes in their colostrum and milk. In beef herds, where calves remain with their mothers and nurse daily, the chance for transmission of the infection through colostrum and milk is high. These bacteria may be excreted directly through the mother's milk or, it might be present in manure of the teats, udder or anywhere calves may attempt to suckle.

3. In-utero exposure may be a source of infection for calves. It is possible for a fetus to become infected in-utero if the dam is in the later stages of disease. Studies have shown that, in disease Stages III and IV in the dam, 8% to 40% of fetuses were infected from their mothers in-utero. Infected fetuses can abort, or they can be born and not display their infection until they are adults. How in-utero infections affect diagnostic tests on those animals that survive to adulthood is not known. Risk for infection of the fetus is low from mothers in disease Stages I and II.
   
4. Water and feed. Water or forages that are contaminated with manure from infected animals are additional potential sources of infection. These sources are not well quantified and their risk must be assessed by estimating the degree of contamination, prevalence of infected animals, and age susceptibility of exposed animals. Potential sources to consider include shallow, stagnant water sources, forages contaminated during delivery or feeding, poorly drained bogs, over-grazed or heavily manure-contaminated pasture, hay crops sprayed with lagoon waste from infected herds, etc.

A debate has developed over the last 15 years as to whether or not Map could play are role in causing human Crohn’s Disease (CD). CD in people is a chronic inflammatory disease of the intestinal tract and in that way bears some resemblance to Johne’s disease in ruminants. CD is most prevalent in the northern, more industrialized regions of Europe and North America. Its incidence has trended upward over the last four decades. CD typically affects younger people, from adolescence to 35 years old and an quarter to half million people in the US are estimated to suffer from CD. The cause of CD is still to be discovered, but is believed to involve multiple factors including genetic predisposition plus exposure to some environmental or infectious factor(s).

It is generally agreed that CD is the consequence of the intestinal immune system over-responding to some stimulus. Map, other infectious agents, and bacterial or allergic components of the normal intestinal stream are all hypothesized as potential factors in initiating or prolonging the immune inflammatory response in CD. Anti-inflammatory drugs, surgical removal of affected intestine, immunomodulating agents, and multi-antibiotic regimens are all used to reduce the inflammation in CD. There is no cure however and individuals with CD alternate periods of remission with flare-up for life.

Current evidence is inadequate to determine health risk

Current evidence cannot support or reject a causal relationship between Map and CD. Associations do not offer much guidance about the causal relationship between two factors. CD is a complex disease and extensive research will ultimately be needed to resolve the question of its cause. The proposed associations with Map are provocative however and have helped highlight the need for more research into the cause of CD and have caused the cattle industry to be more concerned about Johne’s as a potential food safety issue. Some of the issues surrounding a possible association between Map and CD are summarized below.

Map DNA in CD patients’ tissues

DNA detection methods have permitted researchers to look for the presence Map in tissue samples by identifying a unique genetic (DNA) component of the bacteria. Several studies in the last decade have reported finding Map DNA in intestinal tissues taken from CD patients a higher percent of the time than from tissues from control patients with intestinal diseases other than CD. Overall results are conflicting, however, since a near equivalent number of studies have reported different findings including no Map DNA in CD or non-CD patients, Map similarly present in both groups, and other Mycobacteria also present in each group.

Map bacteria cultured from a few Crohn’s patients

Successful attempts to grow Map from CD patient specimens are rare and the 6 to 8 reported isolations required months or years of incubation. If Map is present in human tissue, proponents of the association suggest that it exists in low numbers and in an adapted form. This form does not grow in the laboratory and lacks a cell wall, which explains why it also cannot be visualized by standard microbiologic methods.

Does milk pasteurization kill Map?

Epidemiologic studies have not offered much evidence for specific factors that could be associated with CD, including milk consumption. However, Map has been cultured from cows with clinical Johne’s disease, thus the role of milk as a possible exposure factor has been considered. In 1996, UK researchers reported that Map DNA was detected in 7% of samples collected in a survey of retail milk samples. Identification of Map DNA does not indicate how much is present or whether the bacteria is living or not. Subsequently, US and UK researchers conducted experiments on milk samples spiked with Map to determine if current high-temperature, short-time (HTST, 72C for 15 seconds) milk pasteurization procedures were adequate to kill Map. Results were conflicting using different methods.

US studies used a turbulent flow lab-scale commercial pasteurizer and reported 100% kill of Map. UK researchers used a holder tube method and showed survival of low numbers of organisms. In 1998, FDA reviewed the data and stated that commercial HTST pasteurization in the U.S. eliminates the hazard from raw milk products. The batch holder method used by UK researchers is argued to provide less efficient heating than commercial methods.

The UK Food Standards Agency commissioned an investigation of the Map in 1000 retail milk samples, by DNA and culture. In efforts to resolve the pasteurization controversy, investigators have focused on improving detection of Map by combining techniques to concentrate and grow Map with DNA detection. Full results of the UK study are anticipated in 2001 and preliminary reports indicate that viable Map have been recovered from a small percent of raw and retail milk samples. The Food Standards Agency, an independent advisory board in the UK, did not recommend changes in advice on milk consumption or pasteurization procedures but will next actively explore all possible controls to reduce the risk of exposure to Map while research on the association with CD continues.

A case of Map before Crohn’s

A 1998 published medical case reported the story of a young boy who developed CD five years after being evaluated for enlarged lymph nodes and possible TB. Reevaluation of the original lymph nodes after CD was diagnosed revealed that the DNA IS900 sequence, which is unique to Map, was present with other Mycobacteria in the lymph nodes. This is the only reported case of Map being demonstrated prior to the development of CD.

Multiple antibiotics may help in Crohn’s

Clinical trials are underway to analyze the effect of treatment with multi-antibiotic regimens on remission of signs and disappearance of CD inflammation. Some patients on long term anti-mycobacterial and broad spectrum antibiotics have had extended periods of remission and reduced symptoms for up to four years after treatment. Typical remissions in CD are months to a couple of years. If antibiotic treatments were curative, it would support that a bacterial organism(s) may be at least partially responsible for CD symptoms.

Epidemiologic data not helpful about cause

There have been many epidemiological studies conducted on CD, but none have examined specific Map exposure factors such as direct contact with animals that could have Johne’s disease. The following are examples of some of the more interesting associations that have been reported but have not shed much light on possible causes for CD:

- a US study in one state reported a geographic overlap between diagnosed CD patients and dense dairy cattle areas

-a "hot spot" of CD in the UK exists in a region where water supplies can be contaminated by ag-runoff from cattle operations.

- the incidence of CD has a north-south gradient, being highest in northern regions of Europe and North America (highly industrialized countries). Incidence in these regions may now be leveling off but increasing in southern regions.

- Sweden has one of the highest incidences of CD and one of the lowest of Johne’s disease.

- family members have a higher risk of CD, which supports a similarity in both genetics and or exposure to some factor.