Dealing with cattle ectoparasites

An ectoparasite lives on the skin outside its host. In cattle, they include lice, mites, ticks and flies.

This article will focus on common UK ectoparasites – in particular, flies and ticks seen during the summer months – and preventive strategies.

Common ectoparasites

Lice and mites are mostly seen in the winter months when cattle are housed. They inhabit the hair of their host permanently and do not survive very long in the environment. They feed on the outer layers of tissue and secretions. Different types include chewing or blood-sucking lice and burrowing or surface mites, and different types can induce varying levels of pruritus.

Heavy louse infestation may itself be a sign of another underlying condition, such as malnutrition or chronic disease. It has been suggested lice should be considered primarily as indicators of ill-thrift rather than being of pathogenic significance themselves¹. Macrocyclic lactones and synthetic pyrethroids are active against both lice and mites², and treatment should be followed by removal of animals from infested buildings.

During the grazing season in the summer months, commonly encountered ectoparasites include ticks and flies. Ticks are usually only present on the host for a few days during blood feeding, but can transmit infections.

These include babesiosis (colloquial term redwater) in the UK, which is caused by the protozoa Babesia divergens; and anaplasmosis (colloquial term tick-borne fever), caused by Anaplasma phagocytophilum, a Gram-negative bacterium³. Both are commonly transmitted by the tick Ixodes ricinus in the UK. Clinical signs include haemolytic anaemia, lethargy, icterus and haemoglobinuria in babesiosis⁴, and usually milder signs in anaplasmosis with fever and general immunosuppression, resulting in secondary infection and abortion¹.

For babesiosis, natural immunity can develop in calves that are born into endemically infected herds; calves exposed to babesiosis between six to nine months will develop immunity. Naive adult cows brought into tick-infected pastures are the most at risk⁵.

The picture is similar for anaplasmosis; however, cattle infected at a young age remain carriers of the bacteria, which are resistant to further clinical disease, but can act as reservoirs for future transmission⁶.

Flies can be classified as blood-feeding or nuisance flies. They will either bite and puncture the skin, or feed on the surface of the skin or wounds. Flies can also act as vectors for bacterial and viral infections, including summer mastitis and bovine keratoconjunctivitis^7,8.

Fly activity may directly cause escape behaviour, making animals move into the shade and become agitated; these changes in behaviour result in reduced time spent feeding and decreased performance in regard to growth and milk yield^9,10.

Ectoparasite control – treatment

Many products are licensed for ectoparasite control and in different formulations. Topical treatment is the most widely used; however, some variations exist within these preparations. Pour-on preparations are applied to the animal’s skin along its back and needs to be repeated throughout the season, dependent on the product’s duration of activity. Fly tags are applied to the ear the same way as an identification ear tag and slowly release the substance over time.

Products mainly used for fly control include synthetic pyrethroids. The synthetic pyrethroids include deltamethrin, cypermethrin, permethrin and alpha-cypermethrin². These products work on the principle of reducing the fly population, meaning the flies have to land on the animal and come into contact with the product to be killed¹¹.

It is essential the products are applied at the start of the summer season at turnout. Once fly populations have built up, it is hard to reduce with late administration. Dosing tends to be a standard volume regardless of size for pour-on and ear tags can either be one or two. The level of environmental challenge is also important – if fly levels are high then the products can be overwhelmed¹².

High-risk pastures for flies include pasture heavily surrounded by woodland or hedgerows¹³ and close proximity to farmyards with muck heaps or slurry lagoons. Pour-on treatments need reapplying regularly – each product is licensed differently and it depends on the level of fly challenge. The impregnated ear tags are licensed to last five months. It is also important not to apply pour-on preparations if rain is forecast immediately post-application.

For tick control, no pyrethroids are licensed for cattle use in the UK. Deltamethrin is in a range of pour-on products that are licensed for tick treatment in sheep, so could be used under the cascade²; however, off-licence withdrawal times need to be observed. Tick control with these products is also difficult due to the limited time ticks are present on the animal.

Alternatives to ectoparasiticides – prevention

Concern has grown about blanket use of pyrethroids worldwide and the impact on other non-target invertebrates – particularly benthic invertebrates^14,15 in water courses. These have been predominantly looking at the use of pyrethroids in crop production; however, pyrethroids applied to cattle can still contribute to environmental contamination if applied preceding rainfall. Ectoparasiticides should only be part of the strategy to protect cattle from parasites with the aim to reduce their use.

Avoidance of high-risk pastures with susceptible animals is key. Evidence has suggested for bovine keratoconjunctivitis, younger animals are more susceptible due to lower immunity¹⁶ and white-faced breeds with non-pigmented conjunctiva are more susceptible¹⁷.

Prioritising lower-risk pasture, exposed open areas and avoiding lowland surrounded with hedgerows and trees, for those animals, could reduce infection risk. Selectively treating those higher-risk animals also could reduce use of ectoparasiticides – that is, younger, white-faced cattle.

Similarly, avoiding high-risk fly environments and using teat sealant for dry cows or in-calf heifers will reduce summer mastitis risk⁸. Avoidance of scrubland or downland for ticks in naive adults or bought-in animals is important and exposure of youngstock to ticks under a year will provide natural immunity if being kept as replacements.

Vaccinations are available for babesiosis^5,18 and bovine keratoconjunctivitis¹⁹, but are not licensed in the UK; some practices have been able to import the babesiosis vaccine to use off-licence. Both have conflicting evidence as to efficacy and depend on the environmental challenge – they are not the only solution and need to be used in conjunction with other control methods. Finally, more recent research includes fly traps on the parlour exit for dairy cows in the US, which have shown promising results^20,21, garlic feeding for potential tick control²² and natural fly repellent sprays²³.

Overall, take-home points are to identify high-risk animals and try to graze them on lower-risk pastures – that is, younger animals on more exposed, windy locations. Apply products early in the season, ideally at turnout to avoid buildup of fly populations and selectively treat higher-risk animals.

For ticks, maintaining a closed herd where possible and grazing young animals on high-risk pastures will enable natural immunity to develop.