Lungworm in Cattle
Introduction
Lungworm infections are less common than those of gut worms. Outbreaks of disease can be severe and difficult to predict, but are often associated with wetter summers and following periods of rainfall. Disease risk normally peaks between July and October. In some herds, clinical lungworm may not be seen for many years. This can lead to complacency about prevention measures and, in a year when the conditions favour the parasite, outbreaks can occur. Usually, calves in their first grazing season are at greatest risk of disease, although older animals can still become infected. Often, but not always, it is characterised by the classic “husk” cough.
Cause
The bovine lungworm (Dictyocaulus viviparus) causes disease when infective larvae consumed on contaminated pastures migrate from the gut to the lungs causing tissue damage and inflammation. Mature adults ultimately reside in the large airways and produce larvated eggs. Some of these eggs are aspirated (inhaled) into the smaller airways, causing further inflammation and lung damage. Otherwise, eggs and larvae are coughed up, swallowed and passed out in faeces as larvae. These larvae develop to their infective stage in as little as 5 days under favourable conditions. Infective stage larvae may then continue to reside within faecal pats in drier conditions to be dispersed rapidly and in large numbers once milder, wetter conditions are present, meaning pasture contamination can go from being low to high very rapidly. Under such circumstances, disease risk can be very high, with susceptible animals consuming large numbers of larvae in a short space of time. This can be made worse where gutworm infections are also present. Where exposure is more gradual, immunity will develop over a period of around two months. This immunity requires re-exposure over subsequent grazing seasons or it can be lost. Consequently, if turned out onto heavily contaminated pastures even adult cattle can be at risk of “reinfection syndrome” caused by large numbers of migrating larvae in the lungs.
Lungworm larvae do not generally survive long on pastures and are killed off in dry summers and cold winters. Pastures are usually re-contaminated the following season by infected carrier animals. These are usually second season grazers, but can also be adults harbouring low level infection with no clinical signs. It is also thought lungworm larvae can be spread by other means such as animal’s feet, vehicle wheels, boots and possibly even on the wind.
Figure 1. Lungworm lifecycle. Image: www.cattleparasites.org.uk
Welfare and Economics
In outbreaks of clinical disease large numbers of animals may be affected, severely compromising animal welfare in the group as a whole. Clinically infected animals will show significant distress and difficulty breathing and are at risk of further complications from such as secondary bacterial infections. In cases with very heavy infection animals may die within 1-2 days of developing signs. Economic losses can be very high when outbreaks occur through mortality, impaired growth rates, permanent lung damage and the costs of treatment. It has been estimated that outbreaks of disease can cost £50-£100 per animal, or £5.2 million to the UK as a whole annually.
Figure 2. A lungworm outbreak in first year grazers can be very costly through deaths, lung damage, severe growth checks and the cost and labour for treatment. Image: Phil Scott
Clinical Signs and diagnosis
Lungworm is most common in first year grazers exposed for the first time. However, it should be considered for any animals at pasture with a persistent cough, since immunity can wane with a lack of exposure. Early signs of infection include widespread coughing in the group, initially after exercise then at rest, increased respiratory rate and difficulty breathing, rapid loss of weight and body condition and milk drop in lactating cattle. Sudden exposure of animals without immunity to large numbers of lungworm larvae on heavily contaminated pastures can cause sudden death, or a sudden onset of severe breathing difficulty. This generally occurs in calves but can affect all ages of animal. The head is usually extended with the tongue out, frothy saliva and rapid shallow breaths. Animals have difficulty with any exertion and may have the characteristic “husk” cough. These animals are also at risk from secondary bacterial pneumonia.
Figure 3. Lungworm should be considered for any cattle at pasture with a persistent cough
Diagnosis of lungworm can be based on farm history and clinical signs in an at-risk group, with treatment given on the basis of this suspicion to prevent further deterioration and clinical disease. Lungworm infection can be confirmed by identification of larvae in faecal samples (using a different technique to the standard faecal egg count for gutworms). However, this will only detect patent infection from around 25 days post infection. Severe or even fatal disease can occur well before this. A blood antibody test is also available for lungworm.
Figure 4. Live lungworm larvae can be identified from a faecal sample.
In the cases of sudden death, or death following respiratory signs, lungworm larvae may be identified in the lower airways and adults are easily identifiable in the upper airways at post-mortem.
Figure 5. Lungworm are easily identified during post mortem examination. Image: Phil Scott
Prevention and control
On farms with no history of lungworm infection, it is advisable to minimise the risk of its introduction through bought-in stock. To achieve this, make sure adequate quarantine measures are in place. The COWS groups have published guidelines on quarantine treatments in cattle.
On farms with a history of lungworm infection, vaccination offers a valuable tool for protection against disease in calves. Since the lungworm vaccine is live, it must be purchased fresh ahead of each grazing season. Planning and ordering the number of doses is therefore required well in advance of the grazing season.
All calves over 8 weeks of age entering their first grazing season should be given two doses of lungworm vaccine four weeks apart, with the second dose being given at least two weeks before turnout. It should be noted these animals require some natural infection to develop a fully protective immunity. Reasons for a lack of natural challenge in previous years may include turn-out to uninfected pastures, or clearance of lungworm infections due to anthelmintic treatments to control gut roundworms. Consequently, in some instances a further one-off vaccination may be indicated for youngstock entering their second grazing season. It is also important to consider the potential risk posed to animals brought onto farms from lungworm-free premises.
Prevention of lungworm through pasture management is not as easy as it is for gut roundworms, but following an outbreak of clinical disease the pasture in question should be considered contaminated and grazing by at-risk animals avoided for the remainder of the season and, ideally, the start of the subsequent season. Anthelmintic dosing strategies to control gut roundworms in first season calves are usually effective in controlling lungworm infections also. However, as previously discussed, youngstock require some level of exposure to lungworm at pasture to gain protective immunity for subsequent grazing seasons, meaning choice of wormers and dosing regimen should be considered carefully, particularly in the case of with products with residual activity, or pulse-release boluses.
Treatment
Most worming products are effective against lungworm and to date no resistance has been reported to any class of wormer in the UK. Only the clear wormers (group 3- MLs) have residual activity, so animals treated with a one-off group 1-BZ or 3-LV must be moved to clean pasture to prevent immediate reinfection. Where outbreaks of clinical disease occur, all animals within the affected group should be treated with an anthelmintic. Clinically affected animals should be removed from the pasture and placed on either clean grazing or housed in a well-ventilated building. Animals with severe respiratory signs need to be examined and treated by a vet. Treatment with wormers in heavy infections can result in a mass die-off of parasites in the lungs which can worsen the condition and increase breathing difficulty, making additional treatments to control lung inflammation, control secondary bacterial pneumonia and aid breathing essential.
Acknowledgements
Author: Emily Simcock BSc (hons) BVSc (hons) MRCVS
Images: Phil Scott BVM&S DVM&S DIPECBHM CERTCHP DSHP FRCVS