by George Looby, DVM
Among the routine management procedures that must be done in a sheep flock or a herd of goats is to develop a sound program for parasite control. This may sound like a very simple process but there are many roadblocks that stand in the way of achieving a good, easily managed program.
Many years ago, the approach to parasite control was pretty simplistic — if your critters had worms you gave them some wormer and if that didn’t seem to do the trick you gave them some more. When that approach no longer seemed to be effective researchers of husbandry practices began to take a deeper look at the entire area of parasite control. What has been found is that there are numerous interrelated practices which contributes to an effective program of parasite management and control.
Worms can survive a terrific amount of punishment and there will always be some that manage to survive. Shepherds and other livestock managers have been concocting a variety of medications over many years to aid in eliminating the variety of worms that sheep and goats are prone to. Consider dosing your animals with copper sulfate, tetrachlorethylene, nicotine sulfate or phenothiazine either singly or in combination.
Of the many worms found in the GI tract of sheep and goats, the Barber Pole Worm (Haemonchus contortus) is considered to be the most serious to the health of small ruminants. It is a blood sucker and when present in adequate numbers it can cause severe anemia. One symptom of which is a descriptive lesion called “bottle jaw”. This is an accumulation of fluid under the jaw. This fluid is altered blood in which most of the red blood cells have been destroyed by the worm infestation. One factor that may delude even the most experienced observers is that there is no diarrhea with a Barber Pole Worm infestation.
Female Barber Pole Worms are prodigious egg layers. Researchers who carry out such studies observe that 200 mature females can lay 1 million eggs a day. Once the eggs mature, the larvae hatch out. These represent the infectious stage of the parasite. This process takes from five to seven days depending on the weather, cooler weather slowing down the process. It is worth while noting that worms as a group do not do well in a loose housing environment as almost all require grass to complete their life cycles. A further note is that all grazing animals have worms but worms tend to be species specific.
The life span of an adult is a few months and once their food supply is used up they die. They react in a positive way to warmth — the hotter it is the more they wiggle thus using up their food reserves. Larvae go into the stomach with the onset of cold weather and enter a dormant stage called hypobiosis. This selective process allows the larvae to overwinter and transition into adults with the onset of warm weather in the spring.
Control of the Barber Pole Worm depends in large measure on the proper management of permanent pasture. Dependence on available wormers alone cannot be expected to achieve the results most owners are looking for but it is important to be aware of what each of the classes is and what their respective advantages and disadvantages are. Three classes to note are the benzimidazoles in which may be found thiabendazole (Tresaderm), the imidazothizoles (Levamasole) and macrocylic lactones. One of the more common products found in the latter group is Ivermectin. There is an array of product names to sort through making the selection process somewhat difficult. Working closely with your veterinarian can help in clearing some of the confusion.
With very few exceptions, intestinal parasites have the ability over time to develop resistance to any given wormer that is given for their control but these products still remain an important part of any parasite control program. There always seem to be a few worms that have an innate ability to adapt to any control program to which they might be exposed. It seems that with most conventional worming programs that a certain percentage of the total worm population will be resistant to that treatment and survive into the next season. This means that a greater percentage of the total population the following year will be resistant.
A relatively new concept has emerged whereby a certain percentage of the total worm population is allowed to survive to provide a base of worms which are not resistant of the wormers being used in a given operation. This management program has been dubbed “refugia”. In one program of this type 10 percent of the flock is allowed to go untreated. The worms from untreated animals will mate with those worms showing a high degree of resistance. Many of the resulting offspring will be susceptible. The goal here is not to eliminate all of the worms in a herd or flock but to keep the worm population at a level where its impact on overall animal health will be minimized.
For those operators fortunate enough to have a large amount of land available, keeping the stocking level on any parcel low would be an excellent strategy. For those with limited land, a dry lot operation could be developed remembering that grass is a key element in the overall control program. For those with a number of species on the farm pasture rotation between species is a good idea. Horses are not going to pick up most sheep and goat parasites and vice versa. Poultry are excellent at picking up larvae so if they are allowed to free range they can contribute considerably to a control program. Surprisingly, haying can aid in parasite control. Parasite larvae are physically limited in how high they can climb on a blade of grass with four to six inches being their limit. If grass is mowed and harvested as hay at this stage there will be a considerable larval kill. The hotter the weather the higher the rate of kill. If pastures are mowed at the optimal time they should recover to allow for late summer, fall pasture with a somewhat reduced parasite load. The timing here is largely location dependant.
Goats are natural browsers and should be allowed to do so as it is possible that some of the leaves that they will eat have some anti-parasitic properties. Animals fall into various categories which determine how they should be managed. Dry open ewes and does tend to be the most immune and they should be used to clean up pastures in the fall. At weaning move lambs to what might be considered the safest pastures as they are the group at greatest risk for exposure. Another group considered to be at high risk are the first lactation group.
A test developed in Turkey is an aid in the detection of anemia in parasitized sheep. Basically it is a color matching test where the colors on a chart are matched against those of the mucous membranes of the eye. The colors on the hand-held chart range in five stages from a healthy pink to a ghastly white. Using a simple technic outlined in the instructions the chart is held against the exposed mucous membranes and assigned a grade ranging from 1 to 5 with grade 1 being the best. Each animal in the flock or herd is assigned a grade from which appropriate management decisions can be made. As an example, those animals with a score of 4 or 5 might be considered as candidates for the cull list.
It is suggested that animals in category 1 and 2 not be wormed unless they are showing outward physical signs of other possible problems. Animals that are in category 3 should be wormed if they are lambs or kids, pregnant or lactating ewes or does, if greater than 10 percent of the total flock or herd scores a 4 or 5 or if an animal is in poor body condition.
Another look at worming of sheep and goats
by George Looby, DVM