Parasitic Bronchitis - Cattle
Introduction
Parasitic Bronchitis in cattle is caused by the lungworm Dictyocaulus viviparus.
Our knowledge of the epidemiology of disease is far from complete, i.e. there are still outbreaks of parasitic bronchitis that we are unable to explain.
Disease is carried on from one year to the next by, low numbers of L3 overwintering on pasture and from carrier animals (30% yearlings and 5% cows in an endemic area). The sequence of events that leads up to an outbreak of clinical disease are; a few calves in a group pick up overwintered L3 from pasture after turnout, leading to patent infections; the L1 develop to L3 in a dungpat. Then translation of L3 onto the pasture, which is largely by fungus (Pilobilus species) occurs. The remainder of calves are then infected. The infection may cycle 1, 2 or more times before sufficient L3 accumulate on pasture to cause disease (July – September). A large proportion of ingested larvae become inhibited in lungs of calves overwinter, leading to pasture contamination following spring turnout, i.e. “carrier animals”.
Immunity is rapidly acquired following heavy exposure to infection (within a few weeks). There is minimal age resistance with older stock being susceptible if not previously exposed.
The primary infection has a penetration period of one week. Here, the larvae migrate to the lungs and there are no clinical signs. The prepatent period is then one to three weeks and involves the development and migration of larvae. This lead to to bronchiolitis, which produces an eosinophilic exudate. This blocks the passage of air leading to alveolar collapse distal to blockage. The Patent Phase (weeks 4-8), is when the worms mature and become egg-producing. The main lesions are bronchitis (due to adult worms) and parasitic pneumonia (due to aspiration of eggs and larvae → cellular infiltration of polymorphs, macrophages and “foreign body” giant cells). The postpatent Phase (weeks 8-12) is the period at the end of disease when the majority of worms are expelled. In 25% of cases, clinical signs flare up as a result of alveolar epithelialisation, which may be accompanied by interstitial emphysema and pulmonary oedema, or secondary bacterial infection.
Reinfection Syndrome occurs in immune cattle. They will only show clinical signs if exposed to a massive challenge; large numbers of larvae reach bronchioles and are killed by immune response
Clinical Signs
Clinical signs include tachypnoea, coughing (depending on the number of worms) and an increased respiratory rate.
Diagnosis of Parasitic Bronchitis (Calves)
Diagnosis is based on the seasonal incidence, previous grazing history and clinical signs. Definitive diagnosis can be gained by performing a Baerman technique on a faecal sample to identify larvae. Samples need to be taken from both healthy and sick cattle as carrier animals may be important in the epidemiology of disease, e.g. in an endemic area 30% yearlings and 5% cows harbour patent infections, as do vaccinated animals. NOTE: All lungworm-positive faecal samples are potentially significant.
Post mortem examination can also be diagnositc; recovery of worms from lungs by the “Inderbitzen” or lung perfusion technique. Worms are flushed out of lungs by pumping water through pulmonary arteries. Water and worms passed out of trachea collected over sieve. NOTE: Only 200-300 worms are required to cause clinical disease c.f. >40,000 Ostertagia
Diagnosis of Parasitic Bronchitis (Adult Cattle)
Diagnosis is again based on seasonal incidence, previous grazing history and clinical signs. Definitive diagnosis can be achieved by faecal examination using the Baerman technique to identiy larvae. Both healthy and sick cattle should be examined. Blood and Milk examination (ELISA) to look for antibodies can be used, but this has variable results (depending upon Ag used). Herd results are better than individual results in this case.
Grass examination for larvae around dung pats is useful. Response to anthelmintic treatment will provide a retrospective diagnosis.
Treatment and Control
Vaccination – “Huskvac” (Intervet, original vaccine = “Dictol”)
Should be given to first-season calves, >2months old, reared indoors. It is an attenuated oral vaccine (each dose, 1,000 X-irradiated Dictyocaulus viviparus L3). Vaccination is required at 6 weeks of age and again at 2 weeks pre-turnout. NOTE: Never mix vaccinated and non-vaccinated animals. The vaccine in effective at preventing disease, although not 100% effective at preventing infection, i.e. even vaccinated calves may pass a few larvae → boost immunity in vaccinated calves, but could cause disease in non-vaccinated animals. A breakdown in protection can occur due to overwhelming challenge, improper storage or administration of vaccine, concurrent disease and mixing vaccinated and non-vaccinated calves. Therefore other control measures such as trying to keep a clean pasture and following instructions are very important.
Strategic anthelmintic programmes for preventing parasitic bronchitis can also be used. This will entail Ivermectin being administered at 3, 8 and 13 weeks post-turnout. NOTE: Residual activity of 28 days against lungworm. There will be no anthelmintic cover if challenge encountered either early (0-3 weeks) or late (after 17 weeks) in the grazing season.
References
Andrews, A.H, Blowey, R.W, Boyd, H and Eddy, R.G. (2004) Bovine Medicine (Second edition), Blackwell Publishing
Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science
Divers, T.J. and Peek, S.F. (2008) Rebhun's diseases of dairy cattle Elsevier Health Scieneces
Fox, M and Jacobs, D. (2007) Parasitology Study Guide Part 2: Helminths Royal Veterinary College
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences