Lungworm in Dairy Cattle
What is hoose/husk/lungworm?
Dictyocaulus viviparus is a parasite that invades the respiratory system of cattle and sheep. Previously hoose was thought to be a disease only of young stock, however with increased stocking density, more intensive grazing patterns, changeable weather and the overuse of anthelmintic wormers it is becoming more common in older animals. A high parasitic burden can result in massive financial loss. Milk yield alone can drop by upto 50%, fertility decreases, calving intervals are extended, and not to forget the cost of drugs, labour and often veterinary intervention. Once an animal has recovered from illness, full production may never be fully achieved, many animals are culled or can die as a result of infection with hoose. Outbreaks can be widespread and unpredictable. High risk period is August – October. Replacement heifers and second lactation young cows are at particular risk.
Where do animals pick it up?
Animals come in contact with infective larvae when grazing contaminated pasture. Lungworm thrives in damp, humid conditions. It is unfortunate to say but when the grass is growing, the worms are growing too. The lifecycle of lungworm alternates between a number of life stages and is highly dependent on climatic conditions. An outbreak can commonly occur after an extended dry period, followed by a wet mild period. Cattle ingest infective larvae (L3) into the gut when grazing. Approximately 1 week later L3 will have matured to L4 and can burrow its way from the gut to the lungs via the bloodstream. After a few days L4 mature to L5 which are immature adults. L5 makes its way up to the upper respiratory tract and develops into an adult. L5 is unique in that it has the ability to stay dormant in the upper airways and can over winter there. Adult worms are white, thread like and can be upto 8cm long. Once adult worms are present infection is patent and eggs are produced. The eggs contain immature larvae L1, they are coughed up and swallowed again by the same cow. L1 develops to L2 as it passes through the cow and is excreted in faeces. After one week on pasture, in the right conditions, L2 matures to L3 and the cycle continues. L3 can piggyback on a fungus often found around dung pads, that has an explosive nature, enabling it to be scattered far and wide. Rain also aids dispersal. If L3 are not ingested, some can over winter on pasture if favourable conditions present.
What are the symptoms?
Coughing is one of the main clinical signs noted, along with an increased respiratory rate. Often movement or exertion will exacerbate a cough, e.g cows going for milking, calves coming to the trough. Secondary pneumonia can develop if prompt treatment is not achieved. It can take upto 3 weeks after ingestion of L3 before any symptoms appear. By this time the health of the animal can already be severely compromised. Worms can cause very extensive lung damage so careful monitoring and management must be maintained. One good thing about lungworm in comparison to other parasites is that immunity can develop relatively fast – this is when an animal will be infected but will not develop clinical signs or disease.
How can I test for it ?
Diagnosis is often achieved based on clinical signs, farm history, seasonality, climate, and level of exposure. An upper lung wash (broncho-alveolar lavage) may detect adult worms and mature larvae. During the patent period larvae may be found in faeces using modified Baermann techniques.
How do I treat Hoose?
An animal with a high burden of hoose is already extremely compromised so care must be taken not to tip them over the edge. A mild wormer (e.g.white drench) can be used. This will kill the worms slowly but hopefully is not too strong that it will kill the animal. If secondary pneumonia is also present this will require treatment with anti-inflammatories and possibly antibiotics.
How do I prevent it?
An animal can develop immunity to lungworm, but in order to do this they must be exposed to it. It is difficult to strike the balance between adequate exposure to allow immunity to develop, and over exposure allowing infection to develop. Strategic worming must be employed and overdosing must be avoided, especially young stock. Vaccination should be utilised to help achieve this balance. Never dose animals just before turnout, they pick up lungworm from grazing, if they haven’t been grazing they shouldn’t have lungworm (overwintered larvae are an exception, but you still shouldn’t dose prematurely). Avoid overgrazing and grazing down too short, L3 love to hang out around dung pads and areas that cows would usually choose not to graze, unless forced to. Allow young stock to graze ahead of older stock.
Deworming strategies are difficult to manage. An ideal situation would be to only dose animals once hoose have developed i.e coughing. Then only sick or coughing animals would be treated promptly and the remainder left untreated until they cough. Unfortunately this is not very practical and could select for anthelminthic resistance. Try to extend the dosing interval and use alternating products. The use of a vaccine should be strongly considered for young stock and bought in animals. If vaccinated animals get infected the disease course is much shorter and less serious. Therefore thrive and yield are protected. Vaccines must be used appropriately and planned. They cannot be used with a long acting wormer. Combining vaccination and worming is an effective means to prevent disease in cattle.