Johne's Disease - The Silent Killer: Far Below the Tip of the Iceberg
Life on a farm is anything but simple. The full and hectic schedule can be enough to deter most from this way of life; those that do manage can find it a rewardingly frustrating, hard, and turbulent life with non-stop rounds of controlled chaos that few can really appreciate. But on up to 68% of dairy farms in the United States, there can be a silent, infectious, and deadly shroud that has devastated many a farm and herd. This is the cockroach of microbes: able to survive in conditions that would normally make even the hardiest of pathogens cringe and wither. And that is the most alarming thing of all- how do you kill something so determined to survive?
The cost that a single pathogen can incur on that already struggling livelihood, the Dairy Industry, is hard to fathom. Every year the economical impact of Johne’s Disease (JD) is more than $200 million dollars. These costs range from testing, money used to buy/breed the animals, veterinarian costs, and the hit to the pocketbook from the reduction of milk production. A JD infected cow may produce 1,000 to 6,000 lbs. less milk during a given lactation, and overall will produce significantly less than their healthy counterparts. Cows that are in the earlier stages of the disease may experience less impacted milk production, perhaps 300-400 less lbs in a lactation period. That loss increases as the disease matures. This loss in production is lost income for the farmer, which makes identification and isolation of JD positive animals so critical.
Johne’s Disease was first discovered in 1826, but it wasn’t until 1894 that it was identified as a disease in ruminants, or animals with multi-chambered stomachs that chew their cud. It is a terminal, lingering killer. Animals that may appear to be the epitome of health often show no signs (at least initially) for years of the inner war raging on in their bodies. Most likely infected as babies from their dams, or contaminated birthing areas, these animals won’t show signs until later, in adulthood. Peak incidence of symptoms occurs between four and seven years of age. New infections are unlikely after the calf is six months old.
There has been no documented evidence to date of what actually triggers the change from latency to active infection within the adult, but once this little beast wakes up – death is assured. It is not the infection itself that kills the animal, but rather the immune response to the organism and its subsequent effect- inflammation of the intestinal lining. This constant inflammation causes the lining to thicken to the point where it can no longer absorb the nutrients the body requires to stay alive. The animal slowly starves to death even though its eating habits change little. It is at this point of infection that the animal will exhibit symptoms of diarrhea and weight loss. From there on out it is a downward spiral for the animal. Unfortunately, the fecal matter is one of the main transmission routes the Gram-positive pathogen, known as Mycobacterium avium subspecies paratuberculosis, or MAP for short. Other transmission connections include the calf nursing from an infected dam in utero and tainted birthing areas.
MAP is an opportunistic organism that invades the specialized ileal tissues in the digestive tract called Peyer’s Patches to cross into immune cells called macrophages. It is the ability to hide within these gobblers that partly makes MAP so resilient. They not only survive the defenses of macrophages but can grow and multiply within them. In general, there are three reasons that help explain MAP’s constant elusion from immune detection. 1) The cell wall that demonstrates its chemical uniqueness by making it resistant to destruction or penetration, 2) Counter-secretions that neutralize or avoid the antimicrobial properties inside the macrophages, and 3) Chemicals that are released by MAP to modify or suppress the immune response of its host. It is that immune response (to the indication of MAP) that inevitably contributes to the dysfunction and pathology that are the ultimate result of the infection.
It is a known fact that Johne’s is a silent devastation. When an animal or animals are bought from a seller portraying their herd to be test negative- or that seller fails to even know the negative status of the livestock, the consequences can be dire. Fortunately, more and more states are recognizing liability by holding the seller responsible for negligence. But proactiveness is a two-way street- should you take the risk of buying from a herd whose JD status is unknown, isolate them until tests can be preformed but remember that positive results can take months to appear? During that time you could be putting the healthy animals at risk. It is better to purchase from known negative herds than it is to save a few dollars now and then, because it will cost you more in the end.
Infection in the herd is also known as “The Iceberg Phenomenon”. This illustrates vital concepts in recognizing the potential impact that Johne’s can have on the herd. Allowing this to remain unchecked means disease progression will exponentially increase over time. For every obvious case of JD, at least 15-25 other animals are likely to be infected but only 30-40% of all the infected cattle can be accurately diagnosed, even with the most sensitive of tests. Take for instance a herd size of 100: if 25-30 animals test positive on a fecal culture, then it is likely that at least 50% of the herd is infected at some stage of JD. The downside to these tests is that the culture itself takes so long to grow (three months or more) and is virtually undetectable in its early stages.
Ideally, those suspected to be infected would be separated from the herd until their status is confirmed or denied; but in an industry where every pound of milk counts towards the bottom line, some is considered to be better than none. But it is that “some” that puts the entire herd in danger. There is no substitute for sanitation. Once Johne’s has found its home on a farm, it can take over five years to remove JD’s presence. Immediate and aggressive culling of all positive animals after a herdwide fecal test can be very effective in initially reducing the exposure to the rest of the herd. Prevention starts with the calf. Enforcing calving in areas that are free of manure, clean, and well bedded is very key. Young calves and heifers should be housed separately from adult cattle if possible, as well as removing the calf immediately from the dam and maternity area are some of the most effective ways to reduce the occurrence of infection.
It is a delicate balance. While a farmer may not be able to afford extensive culling of their herd, they can’t afford not to. MAP can survive outside the environment of a host for a year or more. Practical management goals include: addressing the issue at the first sight if there’s suspicion, prevention of infection, hard-hitting hygiene supervision (including manure removal), and better grazing lands. It’s important to realize Johne’s is a clinically invisible disease, and through fecal culture tests and blood tests (ELISA), immediate culling for all animals under the age of two and of all positive animals is recommended for a typical commercial dairy herd. It is up to the farmer to actively fight Johne’s once it has been detected. To ignore the problem is to open Pandora’s Box.
Most states now offer financial assistance for testing, so check with your local agency to see if it is offered. Prevention may be crucial, but early intervention can mean the difference between a salvageable herd and a decimated business.