That cough is so Husky

Author: Kurt Arden BVM BVS BVMedSci(Hons) CertHealthSci MRCVS

Dictyocaulus viviparus is a bovine lungworm which causes the disease known as Husk, Hoose, Dictyocaulosis or Parasitic Bronchitis.

This disease is increasingly common and is predominantly seen in first season calves, although yearlings and adult cattle can also be affected. Mild cases suffer from reduced weight gain and decreased milk production, whilst severe cases result in death.

The parasite has a relatively simple life cycle; adult worms live in the windpipe and lungs of infected animals. Females lay eggs which are coughed up and swallowed, eggs hatch in the intestinal tract and larvae are passed in the hosts faeces, they then go through further developmental stages on pasture. New hosts acquire infection through ingesting larvae from infected pasture. The larvae migrate from the intestinal tract to the lungs where they grow into adults and the cycle begins anew. It takes around 3 ½ weeks for the parasite to be detected following infection; also known as the prepatent period.

Farms will often see disease being carried on from one year to another, this is because of the larvae's ability to overwinter on the infected pasture, as well as due to carrier cattle. Commonly, cattle at turn out ingest overwintered larvae on the pasture, around 4 weeks later these individuals start shedding larvae which infects the remainder of the herd, this process continues with the majority of clinical disease being seen in the summer months. Carrier animals, cattle who had adults in their lungs over winter, also compound the problem by shedding larvae in spring as well.

Clinical signs vary depending on the stage of the infection process, during the penetration phase, which takes up to 1 week and is where larvae infect an animal and move to the lungs, no signs are seen. The prepatent phase, which lasts between 4 and 8 weeks is where larvae develop in the lungs and block the airways, causing the characteristic clinical signs of a husky cough and difficulty breathing. Finally, the post patent phase, which can last anywhere between 8 to 12 weeks, is where the majority of worms are destroyed by the body's immune system, some milder clinical signs such as a persistent cough can be seen.

Your vets will most likely diagnose lungworm through getting a thorough history of past grazing, seasonality and clinical signs. Commonly a faecal sample will be taken and examined to identify larvae; ideally both clinically ill and apparently healthy animals will be examined, due to carriers. A bulk milk sample can also be used to look for antibodies to lung worm, this will however only tell you if the cattle have previously been exposed to lung worm, it doesn't guarantee active infection. Post-mortem examination is also a very usual tool to identify lung worm in server cases resulting in deaths.

Treatment of clinically infected cattle requires a wormer such as any product containing ivermectin. Residual lung damage will persist for some time. Also, secondary bacterial infections are a common side effect to lung worm burden, and these will require separate treatment strategies.

As with most of veterinary medicine, prevention is better than cure, especially for productivities sake, thankfully a commercial vaccine is available ‘Huskvac' (Bovilis). This product requires two doses given 4 weeks apart (ideally the last dose given 2 weeks before turnout) as immunity develops two weeks after the final dose. It can also be used for yearly re-vaccination if required (again administered two weeks before turnout). This isn't always necessary as immunity should be maintained year to year by exposure to larvae on pasture.

Strategic anthelmintic treatment is another preventative option however the issue here is that each treatment lasts 28 days only, meaning reinfection is likely to occur within a grazing season, unless multiple dosing is undertaken, increasing the risk of developing resistance.

Although disease in adult cattle is often thought to be mild, this really depends on the larval burden of infected pastures, with heavily infected pastures resulting in widespread fatalities. This can be because there is minimal age resistance to the disease, with older stock being susceptible if not previously exposed, or because of re-infection syndrome. Reinfection syndrome occurs in immune cattle, immunity is effectively acquired following exposure, these animals will only get clinical disease if exposed to a very large larvae burden, in this case the immune response to the larvae is huge and it is the immune response which causes the disease. Signs are similar to the disease itself with weight loss, reduced milk yields, coughing and increased breathing being the main signs.

In addition to reinfection syndrome, there are several other reasons why a breakdown in prevention is possible, these range from improper storage or administration of vaccines, to concurrent disease affecting the cattle, or the mixing of vaccinated and un-vaccinated cattle. It is crucially important to never mix vaccinated and non-vaccinated animals, as vaccinated animals may retain a low worm burden, which through the life cycle, will result in clinical disease in the non-infected individuals.

By following vaccine protocol and having a base understanding of the parasites lifecycle it is possible to avoid this costly and fatal disease.

Multi-buy pricing available for Huskvac orders. Call the office on 01306 628215 for more details.