Parasitology Notes

Parasitology

1. Introduction

Ectoparasite treatment may aim to control or to eradicate a particular parasite. Eradication is complete removal of a parasite. An attempt was made to eradicate malaria by eradicating the mosquito Anopheles, however this attempt failed due to insecticide resistance, socio-economic factors and poor surveillance.

Control is a reduction of parasite load or vector population, thus reducing transmission. The object is to reduce the disease/pest level to an acceptable level.

Most ectoparasiticides act as neurotoxins. These act on CNS synapses, axons or neuromuscular junctions, causing a spastic or flaccid paralysis. Some examples of neurotoxin ectoparasiticides are listed below:

2. Control of ectoparasites

   1. Fleas

Fleas are of the order Siphonaptera. The cat flea, Ctenocephalides felis, is the most common flea of both cats and dogs.

The lifecycle of the flea means that only the adult stage is present on the animal. The life cycle is also effected by the environment – temperatures of 3C will kill pupae and eggs. Dog and cat fleas are therefore more common in the south of Britain.

Physical control measures include washing pet blankets, rugs and carriers above 60C to remove eggs and larvae. It is also important to maintain a clean environment – for example, hovering to remove eggs from sofas and carpets.

There are chemical control products available that affect the adult, larval and egg stage cycles. An additional environmental spray containing adulticide and insect growth regulator is required to thoroughly eliminate the parasite. As the pupa stage is unaffected by all treatments, it may take up to 3 months to eliminate all fleas.

2. Lice

Lice can be sucking or chewing lice. A list of lice genus is below:

They spend their whole life cycle on the host, so control is simple and effective. Effective products include topical organophosphates, pyrethoids, topical avermectins, ivermectin inject (for sucking lice only) and amitraz for use in cattle and pigs.

It is also important to attent to environmental hygiene and management, as heavy infestations are usually associated with animals in poor health or sanitary conditions. Lice can move off the animal and survive in the environment for a couple of days, and may then be picked up again by the animal.

3. Flies

Blood feeding flies include stable flies, tabanids, horn flies and Culicoidies. Control measures for these flies aim to kill and few flies and to repel. Measures include fly avoidance, repellants, topical insecticides and physical barriers. Some of these flies act as vectors, for example Culicoides as vectors of bluetongue and Schmallenberg virus.

Keds affect sheep and are wingless blood-feeding flies that spend their whole lifecycle on the host. Many are removed by shearing, but they can also be killed by topical insecticides.

Nuisance flies include house and face flies. They can be controlled using topical organophosphates or pyrethroids.

Myiasis is often caused by warble flies (Hypoderma) or blowfly strike (Lucilia sericata) and rarely the house fly (Musca domestica).

Blow flies include the species Lucilia sericata. The females lay eggs into a crevice, carcass or wound, which can take up to 3 days to hatch depending on temperature. Larvae feed directly off the sheep’s skin, causing massive lesions and secondary infections. The larvae then drop off and enter a pupal stage in the soil.

Treatment for blowfly strike include use of topical treatments, including:

• Alpha-cypermethrin (Dysect, zermasect)

• Cypermethrin (Crovect, ectofly)

• Cryomazine (Vetrazin)

• Dicyclanil (Clik)

• Deltamethrin (Coper’s spot on)

• Diazinon (sheep dip)

Warble flies are of the genus Hypoderma, and belong to a group of obligate myiasis (flesh-eating) causing flies in the family Oestridae, which includes many bot flies including the horse bot fly (Gasterophilus) and the sheep nasal bot fly (Oestrus).

Warble fly infection is a notifiable disease in the UK.

1. Ticks

Ticks are divided into hard and soft ticks.

Soft or argasid ticks are nidicolous or nest-living.

Hard or ixodid ticks are non-nidicolous or non-nest -living.

1. Tick borne diseases

Ticks are important as they can be vectors of disease. The following diseases are all tick-borne, with the most important highlighted:

Ixodes ricinus transmits Lyme disease, louping ill virus, Ehrlichia and Babesia in cattle. It is common in the UK.

Dermacentor reticulatus transmits Babesia canis, but is rarely found in the UK.

Rhipicephalus sanguineus transmits Babesia canis and Ehrlichia canis, but is found only in southern Europe.

Lyme disease is caused by corkscrew shaped bacteria called Borrelia burgdorferi. It...

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