Calves at grass from midsummer to autumn are most vulnerable to the disease, but heavy infections in animals of any age, not previously infected and having developed an immunity, will produce clinical signs.
It is most prevalent in dairy type calves, but also common in weaned beef calves. Between10- 40% of herds have been reported to be infected with lungworm in studies in Sweden, Switzerland and Denmark, the prevalence in the UK is less clear.
- Causes of Bronchitis, Parasitic >
- Effects of Bronchitis, Parasitic >
- Diagnosis of Bronchitis, Parasitic >
- Treatment & Control of Bronchitis, Parasitic >
- Medication/Vaccination for Bronchitis, Parasitic >
Causes of Bronchitis, Parasitic
Parasitic bronchitis (lungworm, husk) is caused by Dictyocaulus viviparus. Female worms in the host’s lungs produce eggs containing fully developed larvae. These are coughed up and swallowed and are passed in the faeces.
These become infective within a much shorter time, approximately 1 week, compared to the eggs of gastrointestinal nematodes, and hence in optimal conditions can produce a rapid increase in their numbers. Larvae can be dispersed to adjacent areas from faecal pats by ascending the common faecal fungus Pilobolus and being propelled into the air on discharge of the sporangium.
Infective larvae can remain viable in soil as well as on pasture over the winter. 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.
Differences in temperature and rainfall can dramatically alter the amount of infective larvae present on pasture. A small number of adult worms and hypobiotic larvae can survive and overwinter in infected animals, despite the host having immunity to reinfection, only to mature and propagate larvae during the following spring.
In contrast to 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.
Effects of Bronchitis, Parasitic
In adult dairy cattle, milk drop may be a typical clinical sign. By the third week, severely affected cattle do little else except stand in a characteristic head-extended position with rapid shallow breathing and frequent coughing.
Harsh lung sounds are heard over the ventral lung field. There may, or may not be a patent infection (larvae in faeces) when clinical signs are seen.
Clinical disease in adult dairy cows can result in severe losses, mainly in terms of reduced milk production and potentially even the death of the cow.
Diagnosis of Bronchitis, Parasitic
Clinical signs may vary from occasional coughing to severe respiratory distress, and are a reflection of the number of infected larvae ingested during a relatively short period. Typically, the peak incidence of clinical cases occurs in late summer and early autumn though can persist until winter.
Treatment & Control of Bronchitis, Parasitic
Treatment:
- In the case of disease, kill all stages of Dictyocaulus viviparus with a suitable anthelmintic treatment e.g. Levamisoles, benzimidazoles and probenzimidazoles.
- Treat severely affected animals with antibiotics on the advice of the veterinary surgeon to control secondary bacterial pneumonia.
- If anorexia is present, use rehydration with electrolytes.
The worst affected animals should be housed, especially in poor weather conditions.
Control:
- Lower stocking rates.
- Closed herd policy or treatment of introduced stock, as they may bring in the disease to herds that do not have it.
- Do not run young calves with older calves.
- Avoid lush wet pastures or paddocks with swampy areas if possible.
- Use rotational grazing if possible, moving at weekly intervals, followed by adult immune cattle.
- Blood samples can be analysed for antibody to D. viviparous and faecal larvae counts can be used to assess exposure from early summer through the grazing season.
- Vaccination may be required on farms where the disease is endemic. Vaccinated animals need to be exposed to parasitic challenge following immunisation to ensure the development of adequate immunity.
Medication/Vaccination for Bronchitis, Parasitic
There are two major methods available to control the lung worms, namely vaccination or anthelmintic suppression of infection. Immunity can be stimulated by the use of live vaccines, which take the form of 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. Although the vaccine induces good protection against clinical disease, it does not completely prevent all worms from natural infection from completing their life cycle, indeed, development of immunity is dependent on exposure to larvae at pasture following vaccination.
As a result the parasite is maintained at a very low level 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 all farms.
The use of the vaccine has declined, and regular anthelmintic treatments to suppress infection have increased in recent years.
There are potential problems with this method as some animals build up insufficient immunity in the first year of their lives to prevent disease in the second or later years. Suggested combination approaches of vaccination and dosing include: turn calves out onto pasture following vaccination in spring/early summer, then dose with an anthelmintic and move to aftermaths or clean grazing in mid-summer; turn calves out as before but move calves each month from the start of July to aftermaths or grazing grazed by adult cattle or sheep.
However, this may possibly select for anthelmintic resistance, if calves always return to clean grazing after treatment. In order to kill all stages of Dictyocaulus viviparus, anthelmintic treatment is essential. Levamisoles and the more recently developed benzimidazoles and probenzimidazoles, such as oxfendazole, fenbendazole, albendazole and netobimin, are suitable. Moxydectin is also effective and it is considered to be less environmentally harmful than the other drugs of this group.
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 may 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.
This info was adapted from Defra’s Compendium of Animal Health & Welfare in Organic Farming, which is a collaborative effort led by Duchy College, Cornwall with VEERU at The University of Reading and The Faculty of Veterinary Medicine, University of Glasgow.
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