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
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
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.
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
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
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
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
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.