Psoroptes ovis

Surface mites of the genus Psoroptes infest cattle and sheep, free-ranging ungulates and rabbits around the world.  Each host species seems to have its own species or strain of mite, although cross-infections are possible experimentally, and might occur very rarely, in the field.  The life cycle of the mites occurs on the host, although transmission through stages that survive in the environment cannot be excluded.  In general, psoroptic mange in sheep (sheep scab) is a much more serious problem than it is in cattle. 

In Canada, sheep scab has not been detected for many decades, but remains on the CFIA list of annually notifiable diseases.  Psoroptic mange used be seen occasionally in cattle in Canada, but with the advent of the avermectins (ivermectin and others) in the 1980s, the requirement for reporting the infestation in this host to the authorities lapsed, and there are no good recent data for its occurrence in cattle in this country.  It is assumed to be very rare or absent.

In the United Kingdom sheep scab was thought to have been eradicated in the early 1950s but it re-appeared in 1973, probably as a result of the inadvertent importation of infested animals.  It is now endemic and has proved very difficult to manage effectively.

Psoroptes mites are not known to be zoonotic.

Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Subclass: Acari
Order: Astigmata (Sarcoptiformes)

As well as the Order Astigmata, the Subclass Acari consists of the Order Ixodidae containing the hard (Family Ixodidae) and soft (Family Argasidae) ticks, and several other Orders (Mesostigmata, Prostigmata and Orbatidae) containing the parasitic and free-living mites. Other members of the Order Astigmata of importance in veterinary medicine include Chorioptes, Otodectes and Sarcoptes.

In recent times in Canada, Psoroptes ovis has been found ONLY on cattle, although in other parts of the world the mite also infects horses, sheep and goats, as well as camelids.   Infestation of sheep with Psoroptes is known as sheep scab, and is a major scourge in some parts of the world.  It seems that each host species has their own species or strain of mite which appear to be host specific.

A related species, P. cuniculi, is common in the ears of rabbits in many parts of the world, including Canada.

Adult Psoroptes ovis measure up to approximately 750 μm in length and can just be seen with the naked eye on a microscope slide, but not on the animal. Psoroptes ovis are surface mites and in the adult stage they have four pairs of long legs that extend past the body margin, and at the tip of some of the legs is a long, jointed pedicel(pretarsus) supporting a terminal "sucker".

Psoroptes ovis and psoroptic mange occur in cattle in many parts of the world, especially in animals that are closely confined. This mite is much less common in cattle in North America than it was 30 to 50 years ago, probably because of the widespread use of effective endectocides (for example, the avermectins). Apparently P. ovis has not been seen in Canada in any host for some time, although the situation in cattle is unclear as the infestation is no longer reportable to the authorities.  Sheep scab remains on the CFIA list of annually notifiable diseases.

Experimentally, Psoroptes mites can easily be transmitted among its various host species, for example cattle to sheep and vice versa, cattle to rabbits and cattle and goats to sheep, but in nature these cross-species infections either do not occur or are very rare..

The entire life cycle of P. ovis occurs on the host. The mites live on the skin surface, where adult females lay eggs, which hatch to release a six-legged larva, which develops to the adult stage. The life cycle can be completed in a few weeks.

Life Cycle: Psoroptes ovis

Transmission of P. ovis is direct animal-to-animal or sometimes by fomites such as fences, scratching posts and grooming equipment. Experimental evidence indicates that adult P. ovis can survive for at least a couple of weeks away from the host. As with lice, P. ovis populations on cattle usually decrease over the spring and summer, although if the animals are prevented from grooming, this doesn’t happen. Some animals will retain small mite populations in the inguinal and similar regions and serve as a source of infestation the following fall.

Cattle with very small numbers of mites may be normal, and clinical signs typical of mange are more common in the winter, particularly in confined animals. Unlike Chorioptes bovis, P. ovis does pierce the skin during feeding, and ingests blood and tissue fluids. The activities of these mites on the skin surface can cause severe pruritus, which is often the primary clinical feature. Skin lesions include papules and pustules, alopecia, oozing, crusting and sometimes lichenification, and result from both the activities of the mites and the host’s attempts to relieve the pruritus.

Sheep scab is characterised by small, vesicular skin lesions, which develop scabs, and intense pruritus (itching), with attempts at relief sometimes leading to extensive loss of fleece, self-trauma and secondary bacterial infection of the wounds.  Lesions are usually first observed along the back, but often spread and may involve the whole body.  Severe sheep scab can so interfere with the normal functioning  of the host, particularly feeding, that they rapidly lose condition and may die.

Sheep scab is a major problem for sheep in many parts of the world, NOT including Canada or the US.  As a result of an earlier mandatory eradication program, sheep scab was absent from the UK from the early 1950s until 1973, when it returned - probably with imported animals, and it is now endemic.

SHEEP SCAB IN CANADA IS ANNUALLY REPORTABLE TO FEDERAL AUTHORITIES

 

 

History and clinical signs can be very helpful, but recovery of adult mites, or even fragments, from a skin scraping, and microscopic examination, confirms the diagnosis. The pedicels on the legs (long and jointed for Psoroptes) are the basis for differentiation from Chorioptes.

In Canada, injectable doramectin (DECTOMAX), ivermectin (VARIOUS), and moxidectin (CYDECTIN), and topical (pour-on) moxidectin (CYDECTIN) are approved for psoroptic mange in cattle. Some of these products have milk withdrawal and slaughter delay requirements.  

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

There are no products approved in Canada for Psoroptes in sheep and no demand for teatment as ovine psoroptic mange (sheep scab) does not occur here.

Effective control of psoroptic mange in cattle depends on the rapid isolation and treatment of infested animals, together with treatment of all contacts and, where possible, adjusting management practices to minimize transmission. This may include leaving areas housing infested animals empty for several weeks.

Human infestations with P. ovis have not been reported.