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Gastrointestinal nematode infection is completely different depending on the species and age of ruminant and the environment. The disease, control measures, and drugs used must be focused on different ages and environments.
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Infection and disease are not synonymous. We try to prevent disease and ensure the animal’s immune system is stimulated, not overwhelmed.
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Different drugs should be used against the parasites most likely affected by the specific drug and administered how and when it will
Veterinary Clinics of North America: Food Animal Practice
Gastrointestinal Nematodes, Diagnosis and Control
Section snippets
Key points
Transmission
In the pasture, larval development is temperature and moisture dependent. Larvae are more active at higher temperatures. Optimal development occurs near 100% relative humidity and 22 to 26°C (72–79°F) in 7 days; longer in cold weather. The development is fast in summer but life expectancy is short. Development is slow during the winter, but survival is lengthy. Larvae survive only a month in humid tropics. Larvae may survive in low temperatures on pasture up to 1 year but usually 2 to 6 months.
Disease
Disease is seldom due to just one species but rather to the cumulative effects of a mixed Infection. Outbreaks, however, due to a predominance of a single species of parasite are common.
The economic loss associated with this disease is usually associated with a failure to gain weight or produce milk at an optimum rate. Occasionally disease is manifested by death or severe weight loss. However, the primary loss is decreased production. Nutritional interactions are important with this complex, as
Immunity
Acquired immunity may be manifest as a retardation or inhibition of development of larval nematodes (hypobiosis), but is usually seen as a reduction in establishment of infection with smaller worms and suppressed egg production. Immunity is not a 100% reduction in worm numbers but protection from disease. Most animals in a herd will manifest some level of immunity. Some species of nematodes provoke an early strong resistance to infection, others require repeated exposure. Genetic selection of
Cattle nematodes
Most infections in cattle are a combination of several parasitic species but a few stand out as being more economically important than others. The most important nematode parasite of cattle in temperate regions of the world is Ostertagia ostertagi. The parasite is pathogenic and may cause severe clinical disease and, even when encountered in comparatively low numbers, it causes anorexia and an inability to efficiently convert forage into milk or meat. The economic importance of this parasite
Small ruminant nematodes
In North America, fewer nematodes are associated with disease, but the economic impact is far greater than in cattle. Two species stand out where sheep, goats, camelids, deer, or exotics are raised: Haemonchus contortus and Trichostrongylus colubriformis. Haemonchus kills by exsanguination and Trichostrongylus causes ill thrift and diarrhea. In cooler moist regions Teladorsagia circumcincta may be an important pathogen causing weight loss and diarrhea. To be certain, in some geographic areas,
Ostertagia
Ostertagia ostertagi is the most important species of nematode throughout most of the temperate world and may even cause clinical disease in adult cattle. The strategies directed toward the control of parasitic gastroenteritis are based on Ostertagia, but the timing must be different geographically. For instance, Ostertagia is transmitted in the northern United States and Canada during the summer and autumn, and during the winter and spring in the southern United States and is absent from south
Diagnosis of gastrointestinal nematodes
How do we know that cattle have problems with gastrointestinal nematodes? If cattle lose weight, become anemic, or have diarrhea or other clinical signs, we can certainly put parasitism on the list and look for eggs produced by adult worms. Eggs are found, so that is the problem. Maybe, how many eggs? Which host? What time of year? Answering those questions will increase the chances of ruling in or out parasites. Fecal egg counts in small ruminants can be highly suggestive, with counts greater
Control programs for gastrointestinal nematodes of cattle
Approaches designed to lessen the chances of acquiring a large number of worms should be the goal of any control program. The program must address the species, environment, nutrition, management, and time of year. When dairy or background calves leave a barn/stable or dry lot and enter a pasture, parasitic gastroenteritis infection is a major concern. In climates in which calves are stabled or kept in dry lots, introduction to pasture and parasites can be overwhelming. The first 4 months on
Grazing management
With the availability of inexpensive, safe, and effective dewormers, most of the control of parasitic disease has relied on the use of drugs rather than management. With high-intensity grazing systems and the selection of cattle for maximum production of meat or milk, this approach has generally served well. However, if lessons can be learned from small ruminants, the sustained use of these drugs will require some level of management in the selection of which animals to treat and when to treat
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Disclosure Statement: Dr T.M. Craig provides service for, and has done research and consultation with, several pharmaceutical companies. He owns no stock nor has any monetary affiliation with any.