(Husk, Hoose)

This disease, which is caused by the lungworm, Dictyocaulus viviparus, is characterised by bronchitis and pneumonia. Typically, dairy and dairy cross calves are affected during their first grazing season particularly if they have had access to old pastures. Parasitic bronchitis is particularly prevalent in temperate areas where there is a high rainfall.

The pathogenesis of D. viviparus infection may be divided into 4 phases:

1. Penetration Phase: Days 1-7 

2. Prepatent Phase: Days 8-25 

3. Patent Phase: Days 26-60 

4. Post-patent Phase: Days 61-90

CLINICAL SIGNS

Days 22 onwards- Frequent bouts of coughing; increased rapidity and depth of breathing. The respirations are harsh and correlate with the increased respiration rate; in severely affected cases, squeaks and crackles are heard over the posterior lung lobes. Occasionally, subcutaneous emphysema is detected. If death occurs, it is usually between 20 and 25 days post-infection.

The severity and duration of the clinical signs in any outbreak are dependent mainly on the number of larvae ingested and the rate of their ingestion. Other factors include age of the animal affected, the plane of nutrition and weather. In an affected group, differing degrees of clinical severity are apparent because of differing levels of infection, e.g. a few animals mildly affected, most moderately affected, a few severely affected.

Mild group infection - occasional/frequent coughing when calves are made to stand up or when chased.

Moderate group infection - frequent bouts of coughing at rest, increased rate and depth of breathing.

Severe group infection - rapid respiration rate, difficulty breathing brought on or exacerbated by exercise, frequent bouts of harsh coughing. Although many cases may have a slightly elevated temperature this is usually due to the damage produced by the lungworms and not to a secondary bacterial infection.

It is usually the smallest calves within a group which are most severely affected. A massive infection in this type of calf can present as a sudden onset breathing difficulties and death, due to acute heart failure, can occur during the subsequent 24-48 hours. Such cases are often in the pre-patent phase but in general, most calves are suffering from the patent disease when clinical signs are first noticed.

After drug therapy, the frequency of coughing decreases, the breathing rate eventually returns to normal and the depth of breathing becomes much less noticeable. However, full return to normality can take weeks and even months in severe cases.

About six weeks after clinical signs are initially noticed, a proportion (20-25%) of convalescent individuals, which had been severely affected, may develop sudden onset respiratory distress with mouth-breathing. Squeaks and crackles can be heard over the posterior lobes and the temperature is normal. These calves usually die from acute heart failure within 24-96 hours of their first being seen to be ill.

The name given to this form of the disease is Post-Patent Husk^.

EPIDEMIOLOGY

Generally only calves during their first grazing season are clinically affected, since on any farm where disease is endemic, older animals have a strong acquired immunity.

In the vast majority of husk incidents, it is dairy calves or dairy-cross calves being reared for beef, which are affected. However, under certain circumstances, single suckled calves and adult cattle can develop clinical disease.

There are two main ways by which D.viviparus infection can persist on endemic farms.

Overwintered larvae: in the British Isles, larvae can survive on pasture from autumn until late spring in sufficient numbers to initiate infection. A similar effect may result when infected manure is spread onto grass in the spring.

Carrier animals: small numbers of adult worms can survive in the bronchi of infected animals for several months. 

Another factor which is involved in the dissemination of larvae is the fungus Pilobolus. This occurs in bovine faecal pats and when the spores are ripe, the sporangia which can carry up to 50 larvae, are discharged into the air and onto the surrounding herbage.

This is based on the clinical signs, the time of year and a history of grazing on permanent or semi-permanent pasture.

Dictyocaulus larvae can be detected in the faeces of cases with patent infections.

When calves are found dead and parasitic bronchitis is suspected, post mortem examination of the lungs should show characterisitic signs of lungworm infestation.

TREATMENT

The anthelmintics available for the treatment of parasitic bronchitis and pneumonia are levamisole, ivermectin, or any of the modem benzimidazoles used orally i.e. oxfendazole, fenbendazole or albendazole. These drugs have all been shown to eliminate both mature and developing lungworms with a consequent amelioration of clinical signs and a reduction in faecal larval counts.

For maximum efficiency these drugs should be used as early as possible in the treatment of the disease since clinical signs associated with pulmonary pathology are not rapidly resolved by the removal of adult lungworms.

When the disease is severe and well-established in a number of calves the farmer should be aware that anthelmintic treatment, while being the only course available, may exacerbate the clinical signs in one or more animals with a possible fatal termination. The reasons underlying this are still under study but are apparently associated with the death, dissolution and aspiration of adult worm material into the respiratory bronchioles and alveoli.

In more serious cases the prognosis must be guarded and the farmer informed that a proportion of these animals may not recover whilst some of those that do so may remain stunted. As well as being treated with an anthelmintic, severely affected animals may require antibiotics if they are fevered and may be in need of hydration if they are not drinking. The use of finadyne (flunixin) in severely affected animals will aid recovery.

The only proven and reliable method of preventing the development of parasitic bronchitis is to immunise all young calves with the commercially available lungworm vaccines (Huskvac) prior to their going out to grass for the first time. Both are irradiated, live, larval vaccines which are given orally to calves aged two months or more. Two doses of vaccine (1,000 larvae in each) are given with an interval of four weeks between doses. In order to allow a sufficiently high level of immunity to develop, vaccinated calves should be protected from challenge until two weeks after their second dose. The vaccine should not be given to calves obviously affected with infectious calf pneumonia.

Although vaccination is effective in preventing clinical disease, it is not 100 per cent effective against challenge and small numbers of lungworms may be found in the bronchi of vaccinated calves. Consequently the pastures remain contaminated and it is important that all calves should be vaccinated before going to grass and that a vaccination programme, once undertaken, must be continued annually. Although these pasture larvae will effectively boost the immunity of vaccinated calves, they can produce clinical disease in susceptible animals.

Because of the unpredictable epidemiology of husk control grazing management alone is not practical.

Using some of these techniques, calves are extremely unlikely to develop parasitic bronchitis although occasionally disease may occur at the end of a long grazing season. Also provided they have been exposed to sufficient lungworm infection they will acquire a degree of immunity which will protect them during subsequent years.

HUSK IN SINGLE-SUCKLED CALVES

The importance of husk in this type of animal appears to be on the increase as an increasing number of beef cows calve down in the autumn.

Calves born in August and September are usually weaned during May and June and from weaning until they are housed or sold in October/November, they are likely to be exposed to infection with D.viviparus larvae. Clinical disease may become evident around September/October around the time the calves are sold at the autumn calf sales and parasitic bronchitis can be an important complicating factor in cases of Transit Fever. Spring-born single suckled calves which are grazed with their dams until they are housed or sold do not commonly develop severe clinical parasitic bronchitis. However, coughing as a result of a mild D.viviparus infection is common.

This is relatively uncommon although there appears to be an increasing incidence in recent years, especially in dairy animals.

The patent form of the disease is by far the most commonly encountered although the prepatent and postpatent forms have also been recognised. The most common presenting sign in lactating females is a reduction in milk yield together with the onset of breathing problems.

For adult cattle to develop husk, they must have been shielded from a significant natural challenge for several grazing seasons. Clinical disease results when such animals are exposed to a significant larval challenge e.g. when cows or heifers are put onto a field where calves suffering from clinical husk have grazed previously.

Individual cows can also develop husk by encountering a massive challenge e.g. while being kept in the calf paddock or run with calves, because they are due to calve or they require daily attention or treatment.

The drugs available for treatment are similar to those discussed for calves but in selecting a drug for adult dairy cows one must consider the withdrawal period for milk intended for human consumption.

This form of the disease is relatively common as a mild syndrome, but much less frequent as a severe syndrome.

When a partially immune animal is suddenly exposed to a massive larval challenge, significant numbers of larvae may reach the lungs and migrate to the bronchioles where they are simultaneously killed by the animals own immune response.

The mild syndrome is characterised by frequent coughing and slightly increased respiratory rate.

In the severe syndrome, lactating dairy cattle have a marked reduction in milk yield and increased rate and depth of respiration. 

This form of the disease can also occur in younger cattle such as vaccinated calves or second year stirks and adults which have acquired immunity either through vaccination or/and natural infection, when they are exposed to a massive larval challenge. The source of this larval challenge is often a heavily contaminated field on which calves suffering from clinical parasitic bronchitis have grazed during the previous year. The following year the vaccinated calves, second year stirks or adults when exposed to these large numbers of larvae may develop clinical signs of the reinfection syndrome.

Treatment is as described for husk in adult cattle.