Lovely Weather for Lungworm

The recent mild and wet weather with heavy rain showers has been perfect for the Dictyocaulus viviparus lifecycle.

In addition to the local, seasonal epidemiology there has been a recent paper in the Veterinary Record by McCarthy and van Dijk (2020) which looks at the long term trends in cattle lungworm diagnoses between 1975 and 2014.  This was based on VIDA recorded diagnoses made through the UK Veterinary Investigation Centres like ours.  Whilst it is acknowledged that there is under reporting in such passive surveillance data, it is recorded in a systematic way that permits the data analysis in this paper.  Interestingly for us the data shows significant increases in the proportion of case numbers in Scotland and Northern England since 2009 with Scotland recording the highest proportion of cases in the UK since then.  Although the peak month for diagnoses is always September, across the UK cases are also increasingly being diagnosed in the winter (December – February). 

If you want to read the full paper it is at the link below:  https://veterinaryrecord.bmj.com/content/vetrec/186/19/642.full.pdf

Some points to note on lungworm diagnostics are as follows:

• Baermann examination of faeces for lungworm L1 larvae is diagnostic and is of particular use in animals meeting the infection for the first time. 
• For animals experiencing repeat infections it is likely that the faecal Baermann test will be negative because the cow’s immune response will arrest the lifecycle within the lungs.  The pathology and disease will be there, but the larvae are not produced. 
• Eosinophilia can be a useful diagnostic aid reflecting an immune response to parasitic infection and also the resulting hypersensitivity response.  Average Eosinophil counts in cattle with clinical disease have been reported to be 1.1 x 109 / litre with a percentage differential of 14% or more. 
• Serology is a useful test to determine if exposure has occurred.  Antibody titres rise 4-6 weeks after infection and remain elevated for 3-4 months.  Antibody titres usually fall to negative over the winter housing period so positive results reflect exposure during the current grazing season.
• If carrying out on farm PM examinations be sure to cut down into bronchi deep into the diaphragmatic lobes to be sure to rule infection out.  Emphysema is one of the common pathologies seen in severe cases. 

Some points to note about husk in general:

• Disease is seen quite commonly in adult cattle with low levels of immunity as a result of either long acting anthelmintics used in young stock or limited  grazing in milking herds.  Beware of the dry cows or low yielders that are turned out to grass! 
• Ensure that young bulls that may have been housed during their rearing period have sufficient anthelmintic cover.
• Recently purchased animals can be at significant risk of husk, especially if their previous grazing history is not known.
• Units maximising the use of grazed grass for cows and youngstock can be at particular risk if immunity is low.  The more grass you eat the more parasites you eat!  If the grass utilisation is such that cattle have to graze closer to faecal pats then the risk might increase further
• After treatment, recovery times can be quite prolonged as a result of significant lung damage, an inability to clear dead worms from the bronchi and an associated hypersensitivity response causing coughing and dyspnoea.
• Some severe cases will not recover at all with chronic suppurative pneumonia the end result. 
• Be aware of the risk of low grade lungworm infections pre-disposing to the common viral and bacterial pneumonias in growing cattle as they are housed this autumn.

Each year we also have discussions with vets about possible anthelmintic resistance developing in Dictyocaulus viviparus worms.  In most situations this apparent failure of efficacy is not as a result of anthelmintic resistance (for example underdosing, or failure to clear worms from the bronchi), however if you have concerns about the possibility of a failure of efficacy or anthelmintic resistance please get in touch with us. 

Posted by Veterinary Services on 17/09/2020