Posted by Administrator on June 25 2007 11:02
CURRENT STATUS OF ANTHELMINTIC RESISTANCE IN GASTROINTESTINAL NEMATODES OF CATTLE AND POTENTIAL IMPACTS ON CATTLE PRODUCTION
Author: Louis C. Gasbarre, PhD. USDA-ARS
I work with the Agricultural Research Service of the USDA in Beltsville, Maryland. We have been looking at some alternative ways to control internal parasites in cattle. I want to talk to you about what parasites do and what are we currently doing to control them; some studies that are indicating that some of the things we have been doing aren’t exactly right; and talk to you about the future.
You all know what gastrointestinal nematodes are. If anyone thinks they can get rid of them, they would be the first. Parasites have an extraordinary ability to remain on pasture. Parasites aren’t like a lot of things. They don’t have to run from danger or hunt down food. They have to do one thing and only one thing, and that is reproduce at a high rate. And they do it very well. They are rudimentary in all of their systems except for one, their reproductive system.
Parasites have had millions of years to co-evolve with their hosts. They have taken advantage of that time to ensure that they can reproduce at high levels.
Parasitic diseases are quantitative diseases. They are not like the foot-and-mouth disease virus where once you are infected there is a problem. In this case it’s all about numbers. The most important thing in the life cycle from a producer standpoint is how many larvae are available to the host.
Parasites have adapted to conditions that best enhance forage growth. So if you want to know if parasites are being transmitted, ask if forage is growing.
Historically, there have been two ways to control parasites: one is to modify how many eggs are passed in the feces and the other is to alter how many survive the time between when they hit the pasture and when they are ingested. Prior to about 30 years ago, most of the effort was in modification of pastures and parasite survival.
Fecal egg counts
I want to explain to you what a fecal egg count is and what it isn’t so you can better evaluate data in terms of the assay that is being used. Several years ago we did a four-year study on a cow-calf operation. We tested the calves from birth to weaning for fecal egg counts. Three times in the grazing season, May, July and just before weaning in September, we went to those calves, put them through the chute, took a grab sample from the rectum, and we did this for three consecutive days at each of the 3 sampling months.
Given this sampling program there are four things that will influence the value you get from a sample: the year you took it, the individual animal you took it from, the day you sampled and the error that is inherent in the assay.
The following chart partitions each of those and shows you about what percent of variance is accounted for each of those factors. In this particular study, there were 200 samples each day, times three days, multiplied by three times per year which is 1800 times per year or roughly 7,200 fecal samples from approximately 800 different animals.
What you see is the year didn’t make a lot of difference. About 9% of the total variance was due to year-to-year variation. The day you sampled, the three consecutive days, had very little effect on the mean.
The biggest source of variation was variation between different animals. But there also was a huge amount of sampling variation. This doesn’t mean that fecal egg counts are of no use but it does indicate that the whole procedure from the time you collect to the
time you are done has a number of steps that have error associated with them. In other words if you are going to use fecal egg count data, you better do all you can to account for this error.
Effects of parasites
The single most important effect on a production basis is the suppression of appetite by the parasites. We know that parasite infections will depress appetite, partly due to interaction with the immune system. Generation of immune responses with things like TNF-alpha and a number of other cytokines will tend to suppress appetite.
Other effects involve making the intestine a poor place for absorption. The mammalian body’s first response to anything foreign is to get rid of it. You get rid of things in the gastrointestinal tract most effectively by forcing them out. Parasites are not a tumor or circulating virus or anything where you can attack it immunologically with very specific mechanisms. Instead the goal is to force them out. The best way to do that is to have an explosive diarrhea because the nematodes aren’t that good swimming upstream.
As soon as the parasite is in the gut stimulating the immune response, you get a bunch of physiologic changes. There is an increase of muscle mass in the intestine, a movement of fluids out instead of in because you are trying to flush things out, and changes in the epithelium to more mucus secretion.. With the fluid you’re also putting out a lot of salts and proteins, and all of the sudden you have an animal that may be doing everything right immunologically, but it sure isn’t going to absorb nutrients very effectively. From the animal’s standpoint, getting rid of the parasite is good, but from a producer standpoint, if it does it by way of diarrhea, that’s not very good.
These parasites are extraordinary stimulators of the immune system and because the immune system is a finite body, it can only handle so many things at a time and is highly regulated; you will see loss of certain aspects of the immune response in animals that are heavily or moderately parasitized.
Controlling parasitism
Since the advent of the modern dewormers and the development of the concept “we’re not going to treat disease anymore, instead we’re going to prevent disease,” there has been a move towards the idea of strategic deworming.
The older system of parasite control was based upon the idea that when animals saw pasture in the spring, parasite transmission would be initiated. Over time the parasites numbers would start to build up and at some point you would hit a gray zone where productivity was affected. This would be noticed by the producer and they would then go in and treat the animals. You would stop the production loss, but the problem was by that time there were billions of larvae on the pasture and they would immediately be reinfected. You would then in a cycle of periods of economic disadvantage. The idea behind strategic deworming which was to use treatments prophylactically. The important thing was to use the drug early in the grazing season to keep the buildup from happening until later when seasonal effects would reduce parasite numbers. The idea was that we’re not treating the animals anymore, we’re treating the pasture.
This works best at critical times in the parasite life cycle, when can we put the most pressure on the parasite. This has led to efforts to use drugs that last longer to keep the pressure on. Many felt that not only can we control disease, we can also clean up the pastures and not have to worry about it in the future.
There are some problems with this approach. One is that the optimal temperatures for transmission for different parasites aren’t necessarily the same. For instance, in our studies in Maryland, optimal transmission takes place very early in the season for Ostertagia, the dominant parasite pathologically. So the optimal strategic treatment is going to be front-loaded with treatments early in the spring for great control of Ostertagia and Cooperia. But this does very little for Nematodirus whose transmission doesn’t start until about June. By that time, the early-season treatment is gone and you miss the parasite and must apply later treatments. So the obvious effect is that you treat more often and spread it out.
A second big change is how we do business is the realization of the effects of subclinical parasitism. The following table shows some general figures regarding the economic return gained by of controlling subclinical parasitism. Using the cost of deworming as of about two years ago, in an intensive rotational grazing system the economic gain, for every 1,000 head in this system, was about $50,000 worth of increased profit.
Expression of immunity
I want to bring in the concept of how parasites and the host interact and the basis for this interaction. Cattle generally will regulate parasite transmission. They can do this either by reducing the number of parasites that colonize them or they can damage the worms enough that the worms do not release as many eggs, so they reduce the fecundity of the worms.
If you look at the same cohort of animals year after year you’ll see that the mean egg count drops. In the first year, of one study, calves had a mean of over 200 eggs per gram. As yearling heifers, that mean drops significantly. As you follow that group which is getting smaller, you’ll see that the egg count decreases. As the exposure to the parasites increase, there is an immune response and it reduces the egg output which is good from a herd standpoint.
We also know that even within that age group or cohort that there is an over-distribution of the parasite. The distribution is not the bell-shaped curve that we saw in college. Instead it’s a negative binomial, which is defined by the fact that the standard deviation exceeds the mean. In other words, lots of low values and a few very high values.
So most of the animals have been successful in developing a herd immunity and others are the focus of most parasite transmission. In a calf population, we estimate that about 25-30% of the calves account for the majority of the eggs on the pasture.
As the age of the animal increases and immunity matures, it’s becomes even more skewed. So when you are dealing with 5 or 6-year old cows, a very small percent, less than 10%, will account for all of the eggs that are being passed by that age group.
The question we had is: is this the result of chance or is it controlled by the genetic makeup of the animals? We found a strong genetic component to those variations. Using bulls over a number of years and then categorizing their offspring as to whether they are those highly susceptible or not, you can develop an odds ration of a particular animal producing those calves.
In this analysis you designate someone as the reference. In this cast there was an Angus bull called Scotch Cap that was used often a few years ago. We designated him as the reference. The odds of him producing a calf that was a high egg shedder as compared to himself was one. If we used other bulls, another produced 17 calves over that period and none of them fell into what we consider a high category. So the odds based on calf numbers were about one in four. If you use Scotch Cap you get four times as many susceptible animals as you would if you used the bull Independence. This is just a small selection of the number of bulls used over that period and there was about a 20-fold difference from top to bottom.
The heritability trait is about 30% which is about the same as milk production in cattle, moderately heritable. This means there is a very strong likelihood changing the genetics will change the parasite transmission.
To further test this we developed a resource population to identify genomic regions that control resistance to the parasites. From these studies we know there are six chromosomes of the 29 bovine chromosomes that contain structural variations that influence whether the animal will pass a lot of eggs or have a lot of parasites. This is not just the egg count, this is a profile, including things like serum pepsinogen for Ostertagia. On these 6 chromosomes there are a total of eight different loci or locations. The recent sequencing of the bovine genome has given us a powerful tool to identify exactly what these differences are. The initial sequencing was done using a cow that belongs to ARS in Montana. By sequencing enough nucleotides to cover her genome six times, she is sequenced with enough coverage to give you the whole genome six times. You then line up all the pieces that match like a huge jigsaw puzzle.
With the completion of the genome sequence the next step is to identify how individuals differ. One of the most powerful ways to do this is by using a variation called a single nucleotide polymorphism (SNP).
You take the initial sequence and go to another animal, such as a jersey cow, and you sequence her and find that same piece of genetic material. Looking closely you will find places where there was a switch during DNA replication and a different base was used. This may not do anything physiologically in terms of negative or positive effect, but this is a permanent marker for that spot. The good news is that happens all the time. Probably every several hundred bases you have one of these. So scattered throughout the genome, these changes allow you to identify a unique piece of chromatin or genetic material.
You also know that when reproduction takes place things tend to get segregated in groups. You don’t just randomly do bases. You get chunks. In other words, you can develop a half-map. You can identify specific chunks of certain size containing specific genetic structures. By this time next year there will be available a commercial way to look for 60,000 of these SNP’s across the entire genome. If you know that in a given area there is some genetic variation you can then begin to identify whether it’s a change in the gene or number of copies of the gene, a deletion of something important, or something else.
How would you apply that in the case of parasites? The best way to control a parasite infection cost-effectively is to manage intensively the susceptible animals. The animals that are responsible for the bulk of the transmission. The problem right now is you can’t easily identify them. Large numbers of samples cost a lot of money. Tests like this will identify animals almost immediately. As soon as it’s typed for anything else you’ll know if it’s an animal that will potentially give you parasite problems. It gets you away from the concept of treating everybody.
In areas of enough parasite pressure you could select for resistant animals, but you will also probably get susceptibility to something else. The genetic test will give you a more precise tool by which you can make treatment decisions and for a growing segment of the industry such as the organic sector, it gives some tools that they don’t have.
Dr. Gasbarre received his PhD in zoology in 1978 from the University of Maryland, College Park. He was a Rockefeller Foundation Postdoctoral Fellow at the WHO Immunology Research and training Center, Lausanne, Switzerland. From 1981 to 2003 he has been a microbiologist with the USDA Agricultural Research Service, at the Animal Parasitology Institute, Livestock and Poultry Science Institute and Animal and Natural Resources Institute, Beltsville, Md.
Since 2003, Dr. Gasbarre has been research leader, Bovine Functional Genomics Laboratory, USDA-ARS, Animal and Natural Resources Institute. He is a member of numerous professional organizations, including American Association of Veterinary Immunologists (president, 1996), American Association of Veterinary Parasitologists (president, 1999), Conference of Research Workers in Animal Disease, International Society for Animal Genetics and the Society for Mucosal Immunology.