Trends in Parasitology
Research updateDictyocaulus viviparus: re-emerging or never been away?
Section snippets
Vaccine improvement
McKeand [4] gave several disadvantages of the current irradiated larval vaccine. It does not produce a sterile immunity nor confers life-long protection in the absence of a natural booster. Furthermore, it is relatively unstable and this limits its use. Consequently, a more stable recombinant vaccine against D. viviparus, producing longer lasting immunity would be an improvement. This type of work is particularly pursued in the UK, with a number of interesting articles published during the
Serodiagnosis
Since the 1980s, developments in this area have been achieved mainly in The Netherlands and Germany 8., 9., 10., 11., 12.. Currently, there are two highly specific and sensitive assays available, an ELISA in The Netherlands [8] and a dipstick test in Germany [11]. The dipstick assay can be carried out on whole blood samples, giving a result within 90 mins. The major disadvantage of both assays is that adult infections are required before they test positive, hence infections are not detected
Insufficient development of immunity
In the 1960s, it was noticed that, for example by Michel et al. [15], in the absence of infection, cattle remain susceptible regardless of age. Hence, if lungworm infections are prevented or suppressed during calfhood, it is possible that adult cows attract lungworm disease if exposed. Therefore, with the advent of suppressive parasite control schemes in the 1980s, one of the major lines of research has been to study whether calves grazing on pastures known to be infested with infective larvae
Limitations of controlled field studies
One of the main drawbacks of most studies on the effect of parasite control on immunity development is that most, if not all, used severe challenge of calves to infective larvae while they were being subject to some control strategy. Some of the control strategies allow for some infection getting through in between treatments or for a short period after the treatment regime, resulting in strong immunity development. Furthermore, it has recently been shown that protection can develop even when
Quantifying and defining exposure and protection
Many infections in calves will start with just a few larvae on pasture, resulting in a small proportion of a herd (e.g. only one or two calves) attracting a low level of infection insufficient to cause disease in these animals. Lungworm is a very fecund parasite, and the resulting larval excretion will produce a rapid and dramatic increase in pasture infectivity. This will result in both auto-infection of the calves initially infected and in a primary infection of those not yet infected. The
Host responses and intraspecific parasite variability
Two important aspects should not be neglected. First, there is a huge variability between hosts in their response to infection. Part of this variability will undoubtedly be host genetic in origin. It was recently shown that some calves show high levels of total immunoglobulin (Ig) E following primary infection, whereas others do not. This appears to be related to protection against a challenge infection [7]. The protected animals with high levels of total IgE also showed detectable
Concluding remarks
It does not appear that D. viviparus is re-emerging as a parasite of veterinary importance. This parasite has never been away, but the incidence of outbreaks appears to shift from calves to adult cows. If, for farmers, this is the only result of all efforts on improving parasite control and understanding lungworm epidemiology, then it has been a pitiful achievement. However, much has to be learned. The rapidly advancing molecular techniques and proteomics have a lot to offer to improve our
Acknowledgements
Maarten Eysker is gratefully acknowledged for his as ever critical comments on the manuscript and the many discussions on Dictyocaulus viviparus. Three referees gave highly appreciated comments on an earlier draft of the paper. Literature references have necessarily been selective. Many may justifiably feel their work should have been included in the list, and to those I apologize.
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