I’m back with everybody’s favorite topic – worms, of course! I would put away that food you are munching on until you have finished reading this blog!
I want to share with you results from a simple comparative study that I carried out on common goat dewormers. It’s very unscientific but I thought it might be useful to some goat owners. As I mentioned in my previous goat blog, worms (parasites) are the number one problem affecting meat and dairy production worldwide, particularly in the Southern Hemisphere and Third World countries. It’s a major problem because goat worms are becoming resistant to all major groups of chemical dewormers. The one parasite that’s the most serious threat is a guy called Haemonchus contortus, also known as the barber pole worm, or the blood worm. It makes a living sucking blood from the lining of your goat’s stomach and can cause severe anemia and, ultimately death, if not controlled.
Here’s an image of the culprit and some of its eggs captured with a fluorescence microscope. Ultraviolet (UV) light is causing the worm and eggs to “fluoresce”, or emit light in the visible portion of the spectrum. To give you an idea of scale, the long dimension of the eggs is about 100 micrometers, or 0.1 mm. This is a very ingenious application of the property of fluorescence to an important veterinary problem. I am sure that many of you have seen fluorescent mineral displays in geology departments and museums. It’s the same principle. Unlike some minerals, however, Haemonchus eggs are not naturally fluorescent. Therefore, the key is to find a chemical substance that (1) binds only to Haemonchus eggs in the fecal sample, and (2) fluoresces in UV light. In this case, the eggs are coated or “stained” with a plant protein (lectin) called peanut agglutinin. You can read a press release about the development and application of this technique at http://www.vet.uga.edu/pr/sheep-parasite.php.
Haemonchus is an example of a strongyle worm (small roundworm) found primarily in hotter, more humid climates. Unfortunately, its eggs cannot be distinguished from those of other species in this group based on size, or morphology (shape). The fluorescent staining method is a breakthrough in goat and sheep parasitology because it will now be possible to rapidly and inexpensively detect the presence of Haemonchus eggs and take the necessary steps to control this pathogenic organism (one worm can lay up to 10,000 eggs per day!).
I have set up my own small parasitology lab at our farm. It’s easy to do, relatively inexpensive and avoids the high cost of fecal tests, currently about $50.00 per test (quantitative analysis) at veterinary clinics in our area. I also determine my own egg counts for my mustang horses. It’s easy to learn to identify the most common parasites. For example, web sites such as the Maryland Small Ruminant Page, provide a number of general parasitology links with images and tutorials.
This is a photo of my lab for determining fecal egg counts using the McMaster technique. Other than a microscope, you will need a standardized fecal solution (in this case, sodium nitrate, or Fecasol, S.G. 1.20) for floating the eggs, McMaster slides, an assortment of glassware, a strainer and funnel, and some exam gloves. I use the Modified McMaster Technique described by Ray Kaplan DVM, PhD and James Miller DVM, PhD in the article Modified McMaster Egg Counting for Quantitation of Nematode Eggs. I modified their procedure slightly by including a step to filter out the solid fraction in the fecal solution. This makes it easier to extract the liquid fraction and load the McMaster slide. When interpreting the results of the McMaster technique, it is important to remember that egg counts might not reflect the actual number of worms present. Refer to The RVC/FAO Guide to Veterinary Diagnostic Parasitology for a list of the factors that can affect egg counts. This web site is also a good source for other information about fecal analyses of small ruminants.
The objective of my experiments was to determine the dosage of chemical dewormers necessary to reduce the population of intestinal parasites by at least 80% after 1-2 weeks. This is referred to as the fecal egg count reduction test (FECRT) in the scientific literature. There are number of different types of parasites in goats but only those eggs belonging to the strongyle group were counted. These are the most common eggs observed in fecal analyses of goats.
The current recommendation by parasitologists is to use one dewormer until FECRT < 80%, indicating a significant resistance problem. Rotation of dewormers is discouraged.
Included in this study were Rumatel (Morantel Tartrate), Panacur, Moxidectin paste (Quest Gel) and liquid (injectable form), Ivermectin and Dectomax. For the time being, I did not consider Levamisole (not recommended for milking goats), or some of the other benzimidazoles (unsafe for pregnant animals, e.g., Valbazen). I normally don’t dewormed until the level of anemia is equal to, or greater than, “3″ based on the FAMACHA system (“5″ being severely anemic). The FAMACHA test compares the color of the lower eyelid to a color chart to determine the level of anemia from Haemonchus infection. Using the FAMACHA system, I deworm once or, at most, twice per year. If you are not familiar with this test, or parasite control in general, you can read about it at the web site for the Southern Consortium for Small Ruminant Parasite Control (SCSRPC) (http://www.scsrpc.com).
Summary of results from comparative study:
Rumatel (Morantel Tartrate): Only dewormer approved by FDA as an additive to goat feed. Advantage is that there is no withdrawal period for milk. Recommended dosage on bag is 2 lbs. In fact, it takes 2 lbs daily for three successive days to achieve a FECRT > 90% after 2 weeks. Good alternative if you are already feeding processed food or milking your goats. I recently switched to Goat Care-2X medicated pellets. Same stuff but more concentrated so one treatment (4 oz per 50 lbs weight) will produce the same results for FECRT.
Panacur: One dose at 2-3X body weight is ineffective (well known from goat literature). Three successive daily doses at 2-3X body weight will reduce FECs by 90% after 2 weeks (for any horse owners out there, this is the Panacur Powerpack for goats!).
Moxidectin paste versus liquid (injectable form): FECRT for both dewormers resulted in 85-90% reduction in egg counts after two weeks. Dosage for paste is 2X body weight, that for liquid only 1X, as recommended by SCSRPC. Downside to using the liquid is cost for a small herd— about $110.00 for 200 ml (ouch!). Fort Dodge holds the patents on this stuff so a generic equivalent will not be available on the market in the foreseeable future. Philosophy behind using the liquid is that less is required (it has a longer persistence in the body) so parasite drug resistance will develop more slowly. As it turns out, the injectable form of Moxidectin is no longer recommended for use by the SCSRPC because of its greater withdrawal times (120-130 days) for consumption of meat. See their web site for more details. Drug withdrawal times for meat and dairy goats are also listed in this table compiled by Dr. Seyedmehdi Mobini, Fort Valley State University.
Ivermectin and Dectomax (same chemical group): Not effective in significantly reducing FECs for my small herd, even when included as part of a rotational program.
A few concluding remarks about the drug class of macrocyclic latones (MLs), which includes two closely related chemical groups – the avermectins and milbemycins. Ivermectin and Doramectin (“Dectomax”) are avermectins whereas Moxidectin is a member of the milbemycin group. Sometimes the three are group together as “avermectins” but because of its structural formula Moxidectin should strictly be referred to as a milbemycin.
Why all this boring stuff about chemistry? There is a very good reason if you want to understand more about the effectiveness of these drugs. The chemical difference between the avermectins and milbemycins gives rise to a unique property – Moxidectin is about 100Xs more lipophilic (fat loving) than Ivermectin! Doramectin is also less lipophilic than Moxidectin but more lipophilic than Ivermectin.
Because Moxidectin is concentrated in the fatty tissue it has a longer persistence in the body which accounts, in part, for its effectiveness as a dewormer. Given the lipophilic nature of Moxidectin, it would be interesting to know the body fat:body weight ratio in goats relative to that of other livestock. It seems that a high ratio would favor Moxidectin over Ivermectin.
Keep in mind that Ivermectin has been used extensively in goats since the 1980s so one might expect a greater parasite resistance to this drug. Moxidectin was the last of the MLs to be introduced. Therefore, parasite drug resistance should develop more slowly. It is interesting that none of the MLs are effective in controlling Haemonchus in the Southern Hemisphere. Perhaps, we are next if we don’t make judicious use of dewormers, and employ effective pasture management practices that disrupt the life cycle of parasites.
So far we have only talked about chemical dewormers. There are other alternatives such as herbal remedies and pasture plants but how effective are they in controlling internal parasites? More on this topic at the SCSRPC web site.
As a disclaimer, I need to mention that I am not a veterinarian and cannot provide medical advice for your goats. All the information presented here is based on work with my diary goats and what I have read in the scientific literature.