The Nematode worms
There are a number of intestinal worms found in the
Ostertagia species have a direct life cycle. The eggs pass in manure; hatch into a first-state (L1) larva, which develop and molt into a second stage (L2) larva, which in turn develop and molt into a third stage (L3) larva. Development from the egg to the L3 larva may occur as rapidly as 1 week under ideal conditions, at which point the L3 larva may then migrate from the manure pat to vegetative forage if moisture is adequate. Following consumption of contaminated grasses by cattle; the L3 larva continues development into L4 and L5 larva in the gastric gland of the true stomach (abomasum). Usually within 18-21 days the young adult worm emerges into the lumen of the stomach. Under certain conditions however the parasite undergoes arrested or inhibited development (hypobiosis) and will not emerge as an adult worm for several months.
In the northern US, contamination of pastures with eggs of Ostertagia begins with early spring grazing, but it not until summer that conditions become right for survival of massive numbers of L3 larva, providing there is adequate moisture to assist the larva in their migration to the forage. In dry summers, the larva cannot escape the dried manure patties until fall rains soften the manure enough to allow for escape and invasion of herbage. With protracted dry spells, the larva may not survive at all. In the southern US, hot dry summers or alternating wet and then dry conditions are not conducive to L3 survival. Heavy larval contamination is more likely from fall until spring, while during summer high mortality of larva provide little transmission of infection. Generally, confinement operations have few significant problems with Ostertagia.
In the northern US, the L3 larva can survive in the moist inner environment of the caked manure, only to be released in falls rains. The L3 larva also can survive under snow pack to infect grasses the following spring.
Ostertagia species, as well as some other nematode parasites, evolved a technique to ensure survival and reproduction of the maximum number of parasites with a condition called hypobiosis. (arrested or inhibited development.) This ability permits the survival of the parasite throughout adverse seasonal effects where it otherwise would succumb to increasing host immunity and adverse moisture or temperature on the pastureland. This hypobiosis occurs principally over the winter in the northern US and the summer in the southern US. The L3 larva remains encysted in the stomach for a protracted period of time; with appropriate changes in the seasons that are more likely to support survival of eggs and larvae on pastureland, the arrested larvae mature and emerge from the gastric glands at a time of year more suited for their continuing survival.
Calves are usually the most susceptible to infection during their first season on pasture but generally develop strong immunity by their second season on pasture. Hence, worm loads are usually a significant problem only to the beef calves grazing contaminated pasture after weaning. Adult cattle are affected very little due to their acquired immunity. Important to remember is that immunity only develops with previous exposure. These older animals may be more seriously affected by Ostertagia if they received minimal exposure as calves; are suffering from some immunocompromizing condition such as severe stress, BVD virus infection, malnutrition or starvation. Cattle raised indoors or in drylots will have little natural exposure to nematode parasites and may not develop good immunity as adults.
The most significant lesion caused by Ostertagia occurs as the adult worms emerge form the gastric glands of the abomasum. Inflammation and destruction of stomach cells leads to loss of digestion ability with subsequent diarrhea and poor condition.
There are two main clinical forms of Ostertagiasis.
Type I occurs as a result of the rapid development from consumption of large number of L3 larva and the emergence of massive numbers of L4 larva from the gastric glands just 3-4 weeks after grazing heavily contaminated pasture. Type I occurs primarily in young cattle up to 18 months of age, and is most common July through October in the northern climates and from January through April in the southern US. Signs coincide with the large numbers of larvae emerging from the gastric glands. Primary signs of disease are loss of appetite, loss of weight, diarrhea and death. Commonly a large percentage of an affected group will show signs of illness.
Type II Ostertagiasis results from the maturation and emergence of large numbers of arrested larvae. This occurs in the early spring in northern climates and in early fall in southern areas. Diarrhea may be intermittent as subsequent waves of larvae mature and emerge from the gastric glands. Usually seen in slightly older animals at 2-4 years of age, the signs include loss of appetite and weight, low blood protein leading to submandibular edema (bottlejaw) and moderate anemia. Signs are usually seen in a relatively small percentage of the group affected (less than 10%).
Signs of subclinical disease include reduced feed intake with decreased weight gain in younger animals. Older animals do not seem to suffer the effects of subclinical disease undoubtedly due to strong immunity to the parasite.
Diagnosis of disease is generally made by history; clinical signs, age groups affected, and time of year. Fecal eggs counts can be helpful in establishing evidence of previous exposure, relative immunity and pasture contamination, but do not necessarily aid in diagnosis in the individual animal. Inhibited larvae and just-emerging adults do not produce eggs, although lesions may be very significant. Other nematode parasite eggs are difficult to distinguish from Ostertagia and may produce an abundance of eggs, which can be misleading. Moderate worm loads can be devastating to younger animals, while usually only very heavy loads will affect mature cattle.
Control strategies require knowledge of the immune status of various groups of cattle
a. Well-fed, mature cows have a high level of acquired immunity to worm burdens, resulting in few total worms (less than 3000) and low egg counts (less than 10 epg or eggs per gram of manure).
b. If adult cattle have fecal egg counts in the hundreds, generally they have either lost immunity or never had it.
c. There is no significant periparturient loss of immunity at the time of calving; unlike sheep which can be severely affected by worm loads at and around lambing, mature cows show only a very mild rise in eggs numbers up to about 20 epg.
d. Young cattle less than 2 years of age have little immunity and suffer large worm burdens (greater than 100,000) with high egg counts most of the time (greater than 100 epg). Young bulls are more susceptible to worms than young steers, which are more susceptible than young heifers. Production and death losses tend to be greatest in young bulls.
Pasture contamination will occur at different levels with different grazing groups.
a. The greatest risk to cattle is young animals grazing pasture.
b. Confinement operations rarely expose animals to significant numbers of Ostertagia.
c. Degree of pasture infectivity is strongly influenced by previous grazing patterns. Young, untreated, nonimmune cattle (particularly bulls) will cause the heaviest contamination of pasture with eggs.
d. Pasture contamination is strongly affected by season. Warm, wet conditions encourage hatch, development and migration of larvae, while drought conditions enhance larval mortality.
e. Once the pasture is contaminated, the larvae survive a long time and pose a threat to all grazing cattle. In the northern US larvae from one year can often survive until late spring the following year. In the southern US larvae survive through fall, winter and spring, but high summer temperatures tend to limit their survival during the summer months.
Management practices affect the worm contamination of pastures.
a. If well fed and managed, most adult cattle will have strong immunity and in the case of beef cows, are usually not a source of pasture contamination for their suckling calves. Fecal egg counts can be helpful in this situation. Low egg counts indicate strong immunity and light pasture contamination. High egg counts indicate something is wrong. There is no economic benefit in deworming mature, immune cattle.
b. Occasionally there will be one or two adults animals that have failed to develop strong immunity. These adults can be treated or culled.
c. Occasionally one or more groups of animals will fail to develop good immunity through lack of exposure such as in confinement; in arid regions; or in drought conditions. These animals will be susceptible to worms if exposed, but will respond to treatment and will hopefully go on to develop immunity.
d. Mature animals can lose immunity though illness, malnutrition, stress or continued lack of exposure. Winter starved adult cattle can lose their immunity and became a serious source of infection for the calves. Both cows and calves should be treated in this case.
e. In the well-managed herd, the suckling calves are not immune, but are exposed to rather low numbers of parasites because they are running with their immune mothers, who pass few eggs to the environment. These suckling calves probably do not require treatment. With heavy pasture contamination from another group of animals, a preweaning treatment of calves may be necessary.
f. Generally, beef calves should be dewormed at weaning time.
g. Certainly there is substantial economic benefit to deworming young beef cattle in the immediate post-weaning period. However, in well-managed, mature, immune cows there is little economic justification to deworm in most cases.
Control option for beef weanling cattle include:
a. Single rumen retention device at weaning or pasture turnout. (Ivomec SR bolus)
b. Treat at weaning or pasture turnout and repeat at 3 week intervals with a nonavermectin dewormer such as fenbendazole (Panacur or Safegard) albendazole (Valbazen), levamisol (Levasole or Tramisol), oxfendazole (Synanthic) or morantel (Nematel or Rumatel)
c. Treat at weaning or turnout on pasture and repeat at 5-6 week intervals
with an avermectin product such as ivermectin (Ivomec), eprinomectin (Eprinex), moxidectin (Cydectin), or doramectin (Dectomax).
d. Treat once and move immediately to uncontaminated safe pasture. Safe pasture is land that has not been grazed by cattle within 12 months; although other species may have grazed on it.
e. Alternate grazing with other species.
Fecal egg counts can be performed on 5-6 animals per management group as a technique to determine immunity and pasture contamination. They are of limited value in determining actual worm burden in the individual. Some worms produce large number of eggs per day (10,000 for Haemonchus) while Ostertagia and Trichostrongylus may lay only 100 –200 eggs per day. However, over 1 million hypobiotic larvae of Ostertagia have been found in the abomasum of cattle. These larvae can cause devastating damage upon their emergence from the stomach glands, yet their numbers are impossible to estimate because they produce no eggs at this stage.
Various dewormers will be effective for these parasites. The avermectin products such as Ivomec and Dectomax have good control over hypobiotic larvae, while other families of drugs are highly effective against the adult worms. Generally the drugs that are individually dosed are more effective because the animal can be dosed accurately for its body weight. Medicated blocks, feed or drinking water cannot ensure that each animal receives the correct dose. Proper dosing kills more parasites and avoids selection of resistant populations.
The avermectin products are highly effective and easy to use. However there may be considerable effect on the environment from these drugs. These products are almost totally excreted in the manure and kill both worms and insects at extremely low concentrations. Very slow manure degradation has been observed in pastures grazed by avermectin-treated cattle, with subsequent reduction in grazing areas due to the presence of the dung patty for a protracted period of time. Ivermectin contamination of water may also pose a threat to marine live and is toxic to fish at extremely low concentrations.
The avermectin products, however, are without a doubt one of easiest dewormers to use, particularly the pour-on variety; and have the broadest spectrum of effect. The pour-on products in this family will control intestinal nematodes (the roundworms) as well as the external parasites such as lice and mange mites, and will destroy grub larva before they produce damage to the hide. The pour-on avermectins will also control certain flies for a limited amount of time after application, and it is here that the presence of the product in the manure may be an advantage. The larva of some parasitic flies may be inhibited from development in the manure for 2-4 weeks or even more. Several studies have shown that moxidectin has the least effect on developing fly larva; eprinomectin and ivomectin have an intermediate effect, with doramectin having the longest lasting effects on fly larva in the manure.
If concern for environmental effects is a high priority, say in animals on rotational grazing systems where rapid breakdown of manure is important, or if recently treated animals will have access to waterways, then avermectin products probably are not ideal. Levamisole also comes in a very easy to use pour-on, and any number of other extremely effective oral dewormers are available to control the intestinal worms. If treatment for lice or flies is desirable, there are also a multitude of excellent products that are relatively easy to deliver, ranging from low-dose topical nonsystemic products (Cylence or Durasect) to dust bags or oil rubs. If cattle grubs need to be treated, the systemic topical organophosphates are extremely effective, although their margin of safety does not approach the avermectin products. (This does not mean these products are not safe, it means you cannot overdose them indiscriminately.)
Intestinal worms can be a serious burden to cattle, particularly young stock.
However, indiscriminate deworming of all cattle on the property may be overkill and economically unnecessary. Young stock are at highest risk for illness due to parasites and need to be treated accordingly, but consultation with a veterinarian may pinpoint the problem animals on any given farm, allowing strategic deworming of select groups of animals, at select times of the year. Expense can be minimized and maximum worm control can be obtained.