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PARASITOS Y ENFERMEDADES PARASITARIAS
DE LOS ANIMALES DOMESTICOS


Dictyocaulus viviparus

Epidemiology

Calves and yearlings turned out to pasture for the first time are highly susceptible to infections with Dictyocaulus viviparus since their lack of previous exposure means they have not developed an immune response. In areas of the world where cattle are primarily raised on pasture, adult animals will have been already exposed to the parasite during their growing phase and will therefore have a strong acquired immunity.

D. viviparus is endemic in temperate areas of the northern hemisphere with mild summers and high rainfall. It is common in cattle-raising areas with permanent pastures used for seasonal grazing. These areas are mostly in northern Europe including the British Isles.

In the United States, D. viviparus is less common than in Europe and often goes undiagnosed because veterinarians are generally unfamiliar with the nematode and the methods used to diagnose infections. In a number of cases reported in the literature, parasitic bronchitis went undiagnosed and untreated for several weeks after the initial reporting of clinical signs (usually sporadic coughing in animals on pasture) because sick animals were usually treated initially for pneumonia of either viral or bacterial origin. Parasitic bronchitis due to D. viviparus has been reported sporadically in North America during the last 50 years. Cases have been consistently reported from the northeast United States, the mid west, the Pacific northwest and Eastern Canada. Many of these more recent  cases were reported in adult lactating dairy cows and all occurred in animals managed on intensive grazing systems. These animals clearly lacked immunity to D. viviparus and we can safely conclude that they received little or no exposure to the parasite during their growing phase and so were still susceptible to infections as adults.

Epidemiological investigations in Britain have concluded that survival of Dictyocaulus and its transmission are dependent on two conditions

  1. Third stage larvae(L3s) may survive over winter on pasture in enough numbers to cause  infections and perhaps disease in susceptible animals turned out to pasture in early spring.

  2. Older animals (yearlings and adults) may serve as carriers over winter in that some adult worms - either as fully mature or hypobiotic immature adults - will survive in the bronchi. Hypobiosis at the immature adult stage (L5) as been documented in mainland Europe, Britain and eastern Canada, although its precise role in transmission has not been determined as yet.

The recent increases in cases of parasitic bronchitis in adult cattle, in Britain may be attributed to several factors

  1. Use of the  lungworm vaccine (Dictol) in Britain was high in the early years after its introduction in 1965.  However, vaccinated animals are still susceptible to infection at a low level even though they are protected against disease. These animals often had small numbers of adult Dictyocaulus in their lungs and passed small numbers of L1s - sufficient to maintain low level contamination of pastures. Over time the annual use-levels of  the vaccine in susceptible calves tapered off. Some of these unvaccinated animals would not be exposed to sufficient levels of infections to initiate a protective immune response and would therefore remain susceptible to infections even as adults. Contamination levels of L3s  eventually build up, on pasture, to the point where disease outbreaks may be seen in susceptible unvaccinated cattle (of all ages). These observations reinforce the point that calfhood vaccination programs must be continued every year with every new group of susceptible calves. The small numbers of L3s on pasture serve to boost the immune response of vaccinated calves throughout their lives but may produce clinical parasitic bronchitis in susceptible animals.

  2. Weather patterns in Britain seem to have changed during the last 10-15 years with dry, warmer summers being more frequent. These conditions have resulted in fewer larvae on pasture so that grazing calves have not only experienced fewer outbreaks of parasitic bronchitis but also have not developed strong immune responses to the parasite. As a result they remain susceptible to the parasite through to their lives as adults.

In areas of the United States where Dictyocaulus is prevalent, summers are not consistently cool and wet leading to variable levels of pasture contamination with L3s depending on temperature and moisture. In warm or hot and dry summers, Dictyocaulus infections and disease will rarely be seen. However, when summers are cooler and wetter than normal, outbreaks of parasitic bronchitis will be seen more frequently. These kinds of changeable weather patterns make it more likely that calves will grow to adults without significant exposure to D. viviparus and will therefore remain susceptible to infections.

There is some evidence that Dictyocaulus larvae may burrow into the soil where it is cooler than on the surface pasture. This behavior may allow larvae to survive periods of inclement weather that would be hostile to their survival and return to the surface pasture when conditions are more favorable.

The vaccine used in western Europe consists of third stage larvae partly attenuated by irradiation and administered orally to calves at least two months old and before they are exposed to infection by turning them out to pasture. Two doses of the vaccine are given 4 weeks apart and vaccinated animals should not be placed on pasture until two weeks after the second dose.

There is also evidence that deer may serve as reservoirs of infection, maintaining   L3s on pasture at sufficient levels to produce clinical disease in grazing cattle.

    

 

Parásitos y enfermedades parasitarias de los animales domésticos
Dr. Colin Johnstone (autor principal)
Derechos de copia © Universidad de Pennsylvania