SHEEP MITES: biology, prevention and control. Sheep scab, sheep mange. Psoroptes, Sarcoptes, Chorioptes, Psorergates, Demodex
Last Updated on Wednesday, April 17 2013 15:42
Written by P. Junquera
Species, distribution and prevalence of mites on sheep
The most important parasitic mite species of sheep are:
- Psoroptes ovis that causes psoroptic mange, also called sheep scab: worldwide
- Sarcoptes scabiei var. ovis that causes sarcoptic mange, also called scabies: worldwide
- Chorioptes ovis that causes chorioptic mange, also called leg mite, foot scab: worldwide
- Psorergates ovis, responsible for psorergatic mange, also called itch mite. Especially in Australia, New Zealand, South Africa, North and South America.
Demodex ovis, responsible for sheep demodectic mange can have local importance. In most cases it causes no clinical symptoms and has little or no economic impact on sheep flocks.
Some sheep mite species have been eradicated in certain regions, e.g. Psoroptes ovis in Australia, Canada New Zealand and the USA, Psorergates ovis and Chorioptes ovis in the USA.
Sheep mites are not vectors of other pathogens, i.e. they do not transmit microbial diseases as many other livestock parasites do.
Infestations with mites are technically called acariosis or acariasis, both on animals and humans.
Click here to learn more about the general biology of mites.
Biology and life cycle of sheep mites
Psoroptes ovis (sheep scab)
Adult Psoroptes mites are ~0.75 mm long, i.e. they are usually only recognizable under the microscope. As for all mites, development goes through various larval and nymphal stages. A female mite lays 1 to 3 eggs a day, a total of about 50 to 100 eggs in her lifetime. Adult life lasts for about 50 days. The shortest life-cycle duration from eggs to eggs of the next generation is about 10 to 14 days.
Psoroptes mites do not dig tunnels in the skin. In the past it was thought that they pierce the skin of their hosts. Today it is believed that they do not pierce the skin, but that the mite feces cause an allergic reaction of the host's skin, which reacts producing exudations and skin thickening and hardening (lichenification) with formation of papules, scales and crusts (excoriations), mostly with wool loss. The mites feed on the exudates and secretions produced by the affected skin. Large scabs may develop that spread to cover the entire body in 2 to 3 months if left untreated. Mites concentrate at the edge of the growing scabs.
As all mite species, Psoroptes mites spend their whole life on the same host. Transmission within a herd is mostly by physical contact. Mites do not actively jump or crawl from one host to another one, but are passively transmitted when animals come in close contact. Nevertheless, psoroptic mites and eggs can survive 2 to 3 weeks off the host (e.g. in tags of fallen wool, on fence posts, etc.) by suitable conditions (maximum of 12 weeks by cold weather). This means that sheep can pick mites or eggs from their environment, especially from those objects that affected sheep use for rubbing, e.g. fence posts. But there are no external vectors that transmit the mites, e.g. insects, worms, rats, mites, birds, etc., as it happens with many other parasites.
Sheep scab is a serious and very harmful sheep disease. Lesions often affect the back, the flanks and the shoulders. Infestations remain often unnoticed until wool loss becomes evident, which mostly means that the whole flock is probably already infested. Affected animals suffer from intense itching (pruritus) and react vigorously scratching, biting and rubbing against objects, which causes injuries that can be infected with secondary bacteria. All this leads to weight loss and wool loss, reduced milk production, and general weakness that makes the affected animals more susceptible to other diseases. Left untreated it is often fatal, especially for lambs. Hides of affected animals are downgraded or rejected at slaughter.
In regions with a cold winter (e.g. most of Europe, Northern China, etc.) sheep scab is a typical winter pest. Outbreaks often peak in late winter or spring. Flocks confined indoors during the winter months offer an ideal environment for mite development (constant temperature and high humidity thanks to the long wool) and transmission inside the flock (crowding and close physical contact). The short life cycle allows quite sudden outbreaks that can be devastating if left untreated.
The disease remains latent during the summer months. Once the sheep are outdoors, no more crowded, exposed to wind and sunlight, and shorn, the environment becomes adverse to mite development and transmission. However, a few mites often survive in protected areas of the sheep body (e.g. in the ears or the perineum).
Diagnosis is based on the presence of the previously mentioned symptoms, but has to be confirmed examining skin scrappings of affected parts under the microscope for visualization of the mites.
Psoroptic mites are not infectious for humans, dogs and cats.
Sarcoptes scabiei var. ovis
Sarcoptic mites of sheep are a species-specific strain of Sarcoptes scabiei, a mite species that infests also cattle, pigs, other livestock and also humans. This means that it can be transmitted to humans. They are less abundant on sheep than psoroptic mites.
Sarcoptic mites are very small (0.3 to 0.5 mm) and can be seen only under the microscope. As all mite species, sarcoptic mange mites spend their whole life on the same host. Mites do not actively jump or crawl from one host to another one, but are passively transmitted when animals come in close physical contact. However, sheep can pick mites from the immediate environment or fomites. There are no external vectors that transmit the mites, e.g. insects, worms, birds, etc., as it happens with many other parasites.
The mites dig tunnels beneath the skin. Their saliva has potent digestive enzymes that dissolve the skin tissues. They feed on the resulting liquids. They do not suck blood. Adult females deposit their eggs in tunnels, which hatch in 3 to 5 days. The whole development through several larval and nymphal stages can be completed in less than 2 weeks. Adults live for 2 to 3 weeks. Off the host the mites survive only a few days.
Harm to sheep can be considerable, although usually not as severe as from Psoroptes mites. Mite digging causes skin irritation, which is enhanced by allergic reactions to the saliva. The affected skin develops pimples and papules that become crusty, and shows hardening, thickening, and building of folds. Infestations often affect non-wooly skin and frequently start on the head to later spread along the neck and the fore legs.
Sarcoptic mange is also a winter pest in regions with a cold season, for the same reasons and with similar dynamics as previously mentioned for sheep scab.
Chorioptic sheep mites (also called "leg mites", or "foot scab") are quite abundant worldwide, but less harmful than psoroptic or sarcoptic mites. They are not transmitted to humans.
They are also very small (0.4 to 0.6 mm) and can only be seen under the microscope. They have chewing mouthparts and neither suck blood, nor dig tunnels as sarcoptic mites, but bite the outer skin layers and feed on skin debris, fat, lymph or exudates. The whole development through several larval and nymphal stages can be completed in about 32 weeks. Off the host the mites survive only a few days.
Preferential sites or chorioptic mites are the hoofs and lower part of the legs; occasionally they also affect the scrotum, the face and the lips. Affected parts show formation of scales and crusts. Rams are often more affected and spread the disease in the flock, especially if they are permanently confined. Severely infested rams may suffer from partial paralysis and low reproductive performance.
Itching is not as severe as with psoroptic and sarcoptic mange, and the scratching and biting reactions of affected animals are also less vigorous. Secondary bacterial infections are seldom. As a consequence, economic impact on animal productivity (weight gain, milk production, etc.) is lower than for sarcoptic or psoroptic mange, but not irrelevant in case of heavy infestations.
Chorioptic mange is also a typical winter pest in regions wilt a cold winter. Outbreaks usually peak in late winter and early spring. Once the animals leave the winter confinement, chorioptic mange mostly recedes spontaneously.
Psorergates ovis, the Australian itch mite, is even smaller than other sheep mites (<0,2 mm long). It affects particularly Merino sheep in Australia, but also elsewhere (e.g. New Zealand, South Africa, North and South America, etc.).
The life cycle is completed in 4 to 6 weeks. Survival off the host is limited to a few days. Whereas immature mites remain mostly below the epidermis, adult mites are on the skin surface, move freely and spread the infestation throughout the host's body. Transmission mainly of adult mites is by contact, mostly from freshly shorn sheep to shorn or wooly sheep.
Harm to sheep flocks can be considerable. Infested sheep suffer intense itching and bite the wool of affected body parts. This causes wool loss or damage than can reduce wool production and/or quality by up to 40%.
Prevention and control of sheep mites
As already mentioned, motes are often a winter problem in regions with a cold season (e.g. Canada, most of the US and Europe, etc.). Crowding during indoor confinement means closer physical contact between the animals, which makes mite transmission easier. It also means a more humid hair coat, which is also favorable for mite development. Feeding is often deficient during the cols season, and confinement stresses the animals. These factors make the animals less resistant to mite infestation, especially to prosoptic mange.
For all these reasons, keeping the animals well fed and in good health and hygienic conditions is crucial to reduce the risk of winter outbreaks, or at least to limit the harm that such outbreaks can cause.
As soon as the animals go back to pasture in spring exposure to sun reduces the humidity in the hair coat, which slows down mite development, and without crowding mire transmission is significantly reduced.
For reasons yet unknown, a few mites often survive such unfavorable summer conditions in a few animals within a herd, without showing any clinical signs. Once the herd goes back to the winter quarters in fall, these animals will transmit the mites to the rest of the herd if preventive measures are not taken.
Therefore it is highly recommended to preventively treat the herds in late autumn. All animals in a herd must be treated, because it is impossible to know which are the carrier animals.
If a herd is free of mites, contamination can only come from cattle brought in. Consequently, to avoid contamination all incoming animals must be treated against mites, also those that went to the a fair or to the market and came back unsold: they may have picked mites from other sheep. Two injections with a macrocyclic lactone (e.g. doramectin, ivermectin, moxidectin) with 7 to 10 days interval should do the job, but keep the animals isolated until 10 days after the second injection. Remember that sheep may be infected with mites without showing clinical signs! Topical sprays and pour-ons are not reliable for controlling psoroptic mites.
Vacated facilities where sheep infected with psoroptic mites were hold must be kept vacant for at least 2 weeks. This is necessary to allow surviving mites or eggs to die before arrival of new stock.
For the time being there are no vaccines that will protect sheep by making them immune to the mites. There are no repellents, natural or synthetic that will keep mites away from sheep. There are no biological control means for controlling sheep mites (or any other mites of livestock and pets). And there are no traps for catching sheep mites, for the simple reason that they spend their whole life on the animals and therefore there are no stages in the environment searching or waiting for a host.
Psoroptes ovis (sheep scab)
Psoroptic mange remains one of the most harmful pests of sheep in many countries with moderate climate. This in spite of numerous past eradication campaigns in the last century. Eradication was and remains successful in a few countries (e.g. Australia, Canada, New Zealand, USA) but not everywhere. It was also declared eradicated in the UK and Hungary, but it re-appeared again. Eradication was achieved through a package of control measures that included compulsory dipping of flocks with approved acaricides, very strict quarantine measures, complete sacrifice and burning of infested flocks, etc.
Under the conditions prevailing in most flocks overwintering indoors the mites can complete their life cycle in 10 to 12 days. Due to crowding the whole flock can become infested in a few weeks, which can cause sudden outbreaks. Before the introduction of injectable acaricides, such a sudden winter outbreak could be devastating, because dipping with acaricides was usually impossible during the cold winter months, and sprays or pour-ons are not effective enough to control such outbreaks. Nowadays injectable macrocyclic lactones (e.g. doramectin, ivermectin, moxidectin) are effective enough (and expensive) to control such outbreaks.
Due to the high cost of such injectables traditional autumn dipping of the flocks remains the preferred option for sheep scab prevention in many regions. To ensure effective prevention the acaricide must reach the mites everywhere in the body of all sheep, including inside the ears, where mites are often found after the summer months on pasture and from where they can easily re-infect the flocks during winter confinement.
Nevertheless, a few mites may survive dipping below the scabs, where the thickened skin can hinder penetration of the acaricide. And mite eggs are anyway not killed by acaricides. All this means that to be effective, the acaricide must have enough residual effect to kill those surviving mites, ideally about 4 weeks. This is essential because re-dipping the flock 4 weeks later is often not practicable or too expensive.
In many countries approval of parasiticides for dipping against sheep scan must accomplish very strict efficacy requirements, and many products approved for mite control on pig or cattle may not be approved for sheep scab control, because they do not ensure such strict requirements. Approved dips contain mostly a few organophosphates (e.g. diazinon, phoxim) or synthetic pyrethroids (e.g. flumethrin).
To ensure adequate control the use instructions in the product label must be strictly followed, especially regarding dip vat replenishment. It is also a must that:
- The sheep remain at least 1 minute in the dip
- Sheep heads are completely submerged at least once
- The dip is freshly prepared
- The sheep have a minimum wool length (not less than 2 weeks off-shears)
Preventive fall treatments against mites control lice as well, which are also a winter pest in many regions. It is also important to treat with acaricides the premises, objects, and equipment that have been in contact with the sheep (e.g. boxes, pens, fencing, trailers, etc.), since mites or mite eggs may survive off the host for several weeks (depending on the species).
However, for reasons regarding workers and environmental safety the use of sheep dips has been strongly restricted in several regions (e.g. UK, Australia), especially those dips with organophosphates. The new regulations strongly discourage the use of dips in favor of injectables. Complying with such regulations can make the use of dips more expensive than the use of injectables.
Injectable macrocyclic lactones are highly effective against psoroptic mites. A single doramectin injection (300 mcg/kg bw), or 2 ivermectin or moxidectin injections (at 200 mcg/kg bw 7 days interval) are enough to ensure complete control. The major disadvantage is that they are substantially more expensive that topical products.
Drenches with macrocyclic lactones do not control psoroptic mites at the usual dose (500 mcg/kg bw). The reason is that the blood levels achieved after drenching are often lower and the residual effect is shorter than after injection.
In certain countries the product requirements for sheep scab control are less strict, and sprays or even pour-ons may be approved. Such products may provide sufficient control in regions without winter confinement of the flocks, but will not ensure complete protection.
Other sheep mites
Most products previously mentioned for controlling sheep scab are also effective against other sheep mites (Sarcoptes scabiei, Chorioptes ovis, Psorergates ovis). Amitraz dips and sprays are also effective against these other mite species. However control of chorioptic mange with injectable macrocyclic lactones is mostly insufficient: pour-ons work often better than injectables.
As for sheep scab, it is also very important to follow the use instructions in the product label, especially regarding dip vat replenishment. However, these other mites are usually less harmful than sheep scab and the consequences of incomplete control are less dramatic.
|If available, follow more specific national or regional recommendations or regulations for mite control.
Click here to learn more about the general features of parasiticides.
Resistance of sheep mites to parasiticides
There are reports on field resistance of Psoroptes ovis (sheep scab) against some organochlorines and organophosphates in Argentina, and against a few organophosphates and synthetic pyrethroids in the UK. However it does not seem to be a widespread problem and such products are still effective in many properties in these countries.