Over time we have discussed many issues that affect nutrition in cattle. One significant factor affecting the nutritional status are disease conditions with certain diseases having a more profound effect than others. Over the next couple of issues we'll examine some of these situations and how these circumstances can affect the animal and subsequently the producer.
It's a fairly well known fact that a significant interaction occurs between disease, nutrition and stress with these three closely linked. We see this as we come to understand the following statements:
1) An appropriate level of nutrition is required to maintain the immune system.
2) Stress in the animal reduces intake as well as increases the excretion of certain nutrients thus reducing the overall nutritional status of the animal subsequently suppressing overall immune response.
3) Disease occurs when animals are subjected to antigens to which they have not been previously exposed and therefore have no established immunity or if the immune response is in a depressed state.
4) Disease conditions, in many cases, reduces feed intake as well as the absorption of specific nutrients thus further depressing nutritional status in the animal.
5) Certain diseases have a direct effect on the absorption process due to its attack of the gastrointestinal tract.
6) Disease conditions create a certain level of stress in the animal.
As you can see from these basic statements, nutrition, disease and stress are irreversibly linked in what can develop into a viscous circle.
Research has shown us that alterations in any of these three will have an effect on the other two factors and a subsequent effect of the overall condition of the animal. This is why, in many cases we cannot only address one of these factors. More often than not, we have to address all three.
Effect of Specific Disease Conditions
Certain disease conditions are well know to have a profound effect on nutritional status of the animal which reduces the intake and absorption of critical nutrients. Diseases of this nature may also have as a subsequent symptom diarrhea, which increases dehydration and also increases the excretion of critical nutrients. A disease that is beginning to make itself known in the beef industry (it's been well-known to the dairyman for quite some time) is Johne's Disease (pronounced “Yonney's,” with a long “o” sound). With the widespread sale and movement of cattle across the country we are seeing an ever increasing incidence of this disease. Let's take a moment and examine this condition somewhat.
Johne's Disease -- Causes and Effect
Johne's disease is a condition of the lower intestinal tract caused by in infection of the bacteria Mycobacterium paratuberculosis. It results in diarrhea, loss of weight and ultimately death. Most commonly the animals infected are ruminants: animals that are herbivores, have three or four chambered stomachs, and chew their cud such as cattle, sheep, goats, deer, elk, antelope, camels, llamas, and alpacas. Additionally, there have been a few reports of M. paratuberculosis infecting pigs, horses, and non-human primates. Some recent reports claim to have detected genetic components of M. paratuberculosis in humans and there is some theory that Johne's may be similar in nature to Crohn's disease in humans.
Calves become infected for life by ingestion of M. paratuberculosis contaminated material. After several years the infection causes inflammation and thickening of the intestinal wall, preventing it from functioning normally. This results in the animal's inability to absorb the nutrients that are presented to the small intestine. Remember that this is where the majority of nutrient absorption takes place. The animal's immune system is not capable of controlling the infection because of the aggressive nature of the organism and because of immune response suppression due to reduced absorption of critical nutrients. Eventually the organism spreads from the intestinal tract to nearby lymph nodes and other body tissues including the uterus, the mammary gland and the fetus in pregnant females. When the infection spreads to the mammary gland, milk and colostrum produced will be contaminated with the organism. Therefore, calves nursing the contaminated milk or colostrum are at a high risk of infection with Johne's. Eventually as the animal ages the infection leads to chronic unthriftiness (increasingly poor feed efficiency) and severe diarrhea which eventually leads to death. The tissues of cattle slaughtered during this period can also be contaminated with M. paratuberculosis.
Spread of the Disease
At this point in time, the most common method of spreading this disease is from one animal being sold into another herd. M. paratuberculosis bacteria are known as 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 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. As stated before, 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 the organism is the most common way animals become infected.
Another source of infection as mentioned before is milk from an infected female. 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 is a direct function of time spent with the mother and/or how often they are fed milk from infected females. In dairies where the calves are removed from the cow at only a day or two of age, the opportunity for infection is actually lower than in beef herds where calves generally remain on the cow up until six to seven months of age. 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.
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).
Additionally, extent and duration of exposure to feces and milk from infected adult animals directly affects the likelihood of M. paratuberculosis transmission. Clean, dry, calving environments limits the possibility of infection transmission. Conversely, dirty maternity pens or fecal contamination of feed and water supplies will promote spread of the infection.
One point that needs to be made clear that is a complicating factor of this condition is that infection is not obvious in younger animals. In other words, a producer way not be aware that a given animal has this condition until it is substantially older and has infected a major portion of his herd.
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 percent of dairy cattle and 1.8 percent of beef cattle harbored a M. paratuberculosis infection. Regional surveys report 3 to 18 percent 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.
The economic impact of the disease is obvious. For commercial producers it leads to:
a) Premature culling of clinical or infected animals - the overall cull rate in the infected herd is increased and results in retention of less productive animals within the herd that would normally be culled in a non-infected herd.
b) Infection heavily taxes the immune system thereby reducing the animal's resistance to other diseases. This results in increased veterinary costs.
c) Decreases weight and salvage value at slaughter
d) Results in restriction on movement (sales) in infected herds
Purebred or Seedstock producers suffer these same risks plus a few others:
a) Selling of infected animal's puts the buyer at risk and can result in legal consequences for the seller.
b) Infected animals, in addition to transmitting the disease, will also be very poor performers. Bulls ultimately “fall apart.” This results in dissatisfaction by customers even if their herds do not become infected.
Johne's Disease is a serious condition and should not be taken lightly. Just as a producer is careful to purchase cattle that are free from infection of diseases such as Bang's, we must also guard against Johne's as well as other conditions. This brings up concern over purchasing animals from sources where the disease status is unknown. When purchasing cattle for reproductive purposes from sales it is highly advisable that they be evaluated by a veterinarian prior to introduction into the herd. The long range effects that introduction of an infected animal to the herd can have are significant and even devastating.
In the next issue we'll look at more of these disease conditions and how they can affect productivity.
Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, Texas. He can be reached at P. O. Box 653 Sulphur Springs 75483, by phone at (903) 885-7992 or by e-mail at firstname.lastname@example.org.