Also known as: Dictyocaulus viviparus, Parasitic Bronchitis, Husk Show Parasitic bronchitis (lungworm, husk, hoose) is caused by a roundworm called Dictyocaulus viviparus and is associated with high mortality. Calves on grass from midsummer to autumn are most vulnerable to the disease, but heavy infections in animals of any age who have not previously been exposed, will produce clinical signs. It is most often seen in dairy calves, but also common in weaned beef calves. Around 40% of herds have been reported to be infected with lungworm in studies in Sweden (Höglund et al., 2004), Germany (Schnieder et al., 1993) and the Netherlands (Ploeger et al., 2000). Infections are more prevalent in wetter areas, especially those in the west of the British Isles (Dijk, 2004; Taylor and Andrews, 1992). Epidemiology of Lungworm in CattleFemale worms in the host’s lungs produce eggs containing fully developed larvae (See lifecycle below). They hatch in the bronchi, are transported up the wind pipe, swallowed and are passed in the feces. They can develop to infectivity in approximately 1 week (Dijk, 2004), which is much faster than most of the eggs of gut worms (gastrointestinal nematodes) . Larvae are dispersed to the area around fecal pat by the common fecal fungus Pilobolus. They are propelled into the air on discharge of the sporangium (Eysker, 1991; Eysker and De Coo, 1988). Infective larvae can remain viable in soil as well as on pasture over the winter (Duncan et al., 1979; Oakley, 1979). Larvae are very sensitive to dehydration, therefore, most of them die after 2-3 weeks in the summer if conditions are dry, although survival in autumn can be much longer (Dijk, 2004). Differences in temperature and rainfall can dramatically alter the amount of infective larvae present on pasture, although optimal conditions can produce a rapid increase in pasture larval numbers. When ingested the infective larvae migrate to the lungs, provoking a strong immune response that is short-lived, unless the animal has previously been expose to the larvae within the past six months. D. viviparus can grow for a few millimeters in length to centimeters in about 3-4 weeks. A small number of adult worms and hypobiotic larvae can survive and overwinter in infected animals. In contrast to gut worms (gastro-intestinal nematodes), silent carriers of lungworm are the most important source of larval contamination of pasture rather than larvae surviving at pasture over winter (Dijk, 2004). David (1997) identified several risk factors from a survey of 32 mainly dairy herds (10-210 cattle) in England with a history of lungworm outbreaks. These included:
However, the epidemiology of lungworm remains less predictable compared to gastrointestinal nematodes. In recent years, increasing numbers of clinical cases of lungworm have been reported in adult dairy cows (Dijk, 2004). This is thought to be due to failure of development of immunity, either through reduced vaccine use or due to increased reliance on long acting anthelmintics, which prevent development of larvae to a stage when lasting immunity is stimulated. Cases of clinical lungworm have even been reported in housed calves (Crawshaw and Smith, 2003). Immunity consists of two phases:
Reinfection syndrome: However, even in the face of immunity, a massive infestation of larvae can penetrate the lung faster than the immune system can respond and lead to development of “Re-infection husk”. In theory, although resistance to invasion of larvae can develop when animals are under anthelmintic treatment, it is unlikely that it is sufficient to stimulate long-term immunity (Dijk, 2004; Höglund et al., 2003; Taylor, 2000). Clinical signs of Lungworm Infection
Control and Prevention of Lungworm in CattleMixed results have been obtained from rotational grazing to control lungworm. D viviparus larvae can last as little as 4 weeks in short swards on hot sunny conditions, but in cooler years, with longer herbage they can survive >12 months. Rotation of grazing once a week over 6 paddocks during the grazing season prevented the occurrence of severe lungworm infections and permitted the development of immunity to Dictyocaulus viviparus, but rotation of grazing once every two weeks failed to control lungworms (Eysker et al., 1992, 1993a). Simply put, rotational grazing is unreliable, as its success is largely dependent on the climate and sward. Also rotational grazing does not lead to the development of immunity to gastro-intestinal parasites and therefore cannot be recommended as a general approach to parasite control (Dijk, 2004). There has been research into biological control in recent years (Hertzberg et al., 2002). Henriksen (1997) found an 86% reduction in larval release from fecal cultures by nematode-trapping fungi (Henriksen et al., 1997). Laboratory investigations of Aphodius beetles (Scarabaeidae) as biological control agents of lungworm have been conducted by researcher in Ireland (Gormally, 1993). Unfortunately, none of these methods turned out to be practical in commercial herds. Generally there are two major methods available:
VaccinationImmunity can be stimulated by the use of live vaccines (Bovillis Huskvac, MSD Animal Health), which take the form of two doses of 1000 infective L3 larvae irradiated by gamma-irradiation. Vaccination is aimed at first season grazing animals, but can also be safely used in pregnant older animals if required (Holzhauer et al., 2005). Although the vaccine induces good protection against clinical disease, it does not completely prevent all worms from natural infection from completing their lifecycle, indeed, development of immunity is dependent on exposure to larvae at pasture following vaccination. As a result the larval pasture levels should be remain low on pasture grazed by vaccinated cattle. On farms where calves have suffered lung damage due to viral pneumonia, vaccination may be inadvisable because of the possibility of exacerbation of the existing lesions. With the unpredictable epidemiology of lungworm and limited efficacy of strategic grazing protocols, vaccination is often the most reliable and cost effective method of control on farms. Other limitations are the short shelf-life of the vaccine and the short window of availability. This makes it difficult to introduce a vaccination program in spring born beef calves. Anthelmintic treatmentsRegular anthelmintic treatments are potentially problematic with this as some animals will build up insufficient immunity in the first grazing season and may become ill second or later years (Andrews and de Wolf, 1994; David, 1993; Eysker et al., 1993b; Jacobs, 1993; Taylor et al., 1997; Wilkinson, 1992). Similar findings have been made when using the Michel’s `dose and move’ system (Eysker et al., 1995, 1997). Suggested combination approaches of vaccination and dosing include:
There is some debate as to establishing the threshold for treatment of cases of parasitism (Thamsborg and Roepstorff, 2003; Vercruysse and Claerebout, 2001). A therapeutic threshold identifies animals with clinical disease, a production-based threshold measures effects on productivity and a preventive threshold is used to predict and control future infection. In sustainable farming systems, it may not be appropriate to merely consider production at the expense of immune development, although it should be remembered that widespread clinical disease is unacceptable (Thamsborg and Roepstorff, 2003). Treating Lungworm in CattleIn order to kill all stages of Dictyocaulus viviparus, anthelmintic treatment is essential. All anthelmintic groups will target lungworm. However, you should follow the COWS (Control of Worms Sustainably) guidelines to ensure effective treatment. Severely affected animals may require antibiotic treatment to control secondary bacterial pneumonia and, if anorexia is present, rehydration with electrolytes. Non-steroid anti-inflammatory agent should also be used as a support therapy. The worst affected animals should be housed, especially in poor weather conditions. Treatment may initially exacerbate the signs due to mortality of worms in the airways. Additionally, despite treatment, some cases may relapse a few weeks after treatment due to the severity of the lung pathology. Good Practice Based on Current KnowledgeMethods that can be used to prevent lungworm outbreaks are:
Dictylocaulosis viviparus References
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