Parasite recognition

The parasites that affect horses are usually fairly easy to recognize. Some are pathogens while others are opportunist pathogens (Table 3.3).

Table 3.3 Parasites that can be collected from the skin surface, their predilection sites and relative ease of detection

Identification

S. scabiei are round and flattened (Fig. 3.1A) with a rounded head region and short front legs. The first pairs of legs have tiny claws and end with a stalk (the pretarsus) bearing a sucker. The two pairs of posterior legs, which bear long hairs (setae), do not extend beyond the body margin; the dorsal surface bears cuticular folds giving rise to a ‘thumb print’ pattern and is covered with scales and strong spines recognizable in histological sections (Fig. 3.1B).

Figure 3.1 (A) The typical appearance of S. scabiei, showing characteristic setae on the back legs, the pedicle with a small sucker on the front leg, and scales on the dorsal cuticle. (B) The histological appearance can be recognized with the parasites located in dermo-epidermal tunnels.

Life cycle

The entire life cycle takes place on the host. Following mating the female mite tunnels into the sub-corneal layer of the skin. Each tunnel (1.0–2.0 cm in length and just below the skin) contains a single female mite, her eggs and excreted material. The female mite lays around three eggs per day and eggs hatch to six-legged larvae within the tunnels after 3–4 days. The larvae exit the tunnels and successive moulting to eight-legged protonymph, tritonymph and female (600 μm) and male adult (300 μm) stages occurs. Newly moulted adult females mate immediately and create new tunnels by eating skin and tissue fluids. The total egg-to-adult life cycle can be as little as 11 days but up to 3 weeks is usual.

Significance

Sarcoptic mange is a rare but severe condition in horses, largely confined to immunocompromised animals. The disease was historically a very significant cause of epidemic debilitating equine disease and was reportable. Nowadays it is singularly rare and more often associated with immunocompromised horses in contact with other affected animal species.

Burrowing and feeding activity causes intense itching and scratching. Pruritic papular lesions result. These rupture and cause hair loss and crusting.

Dermal pathology can extend over the entire body but the most severe forms of the condition are more often seen on the head and neck (see p. 195).

Identification

The body of P. equi is oval-shaped and the head region is pointed (Fig. 3.2). All stages of this mite are identified by the presence, on the first two pairs of legs, of a three-segmented stalk which ends with a trumpet-shaped sucker. The males are identified by the presence of copulatory suckers in addition to the above features, which are specific to the genus Psoroptes. These characters enable differential identification from Chorioptes equi, a more common mite of equines, and from incidental forage mites, some of which have a broadly similar morphology.

Figure 3.2 Psoroptes spp. mite showing the three-segmented stalk and sucker on the first two pairs of legs; the male has copulatory suckers. Females tend to be smaller than the males and eggs may be seen in the abdomen.

Life cycle

Eggs are relatively large (250 μm) and are produced at about two to three per day. Successive growth and moults occur: eggs, larvae, protonymph, tritonymphs and adults. Adults are often seen in mating pairs in skin scrapings and old moulted cuticles are often abundant. Adults measure approximately 750 μm (females) and 450 μm (males) and are therefore visible to the naked eye when viewed against a dark background. The population may double every 7–8 days.

Significance

Psoroptes equi infestations in the horse occur in regions of thick hair such as the tail and mane but also on the face, ears and other parts. A fluid exudate, which dries leaving crusts, is produced. Thickening and hair loss can occur and is particularly noticeable along the margins of the pinnae. In severe infestations large numbers of active mites can be recovered from the edge of lesions. Psoroptic mange may result in emaciation, head shaking and pruritus (see p. 193).

Identification

Females (Fig. 3.3) (300–400 μm) and males (300 μm) are significantly smaller than P. equi and have a rounded head region. Chorioptes do not have the jointed stalk of Psoroptes and the ending is cup-shaped as opposed to trumpet-shaped. There are long whip-like setae on the reduced third pair of legs in the female. In the male, the two broad posterior lobes are square-shaped and bear numerous long hairs. In heavy infestations all stages of the life cycle including eggs are readily encountered and adult C. equi are often seen as mating pairs.

Figure 3.3 (A) Chorioptes equi are usually easy to recognize by their physical appearance (though very similar to Psoroptes). (B) There are invariably many very active parasites to be seen. It is important to ensure that other parasitic mites are not confused with non-parasitic forage mites, which may also be present in the skin brushings.

Figure courtesy of Dr Marianne Sloet.

Life cycle

The life cycle of C. equi is similar to that of P. ovis. Egg, larval, two nymphal and adult stages occur. The period from egg to adult takes approximately 3 weeks. Mites may survive off the host and transmission may occur via the bedding as well as by direct contact.

Significance

The mites prefer the lower parts of the limb and sometimes the head and base of the tail. Horses with feathered legs are frequently affected. Erythema, crusting, ulceration and alopecia with itching is common. Infected horses frequently stamp their legs (see p. 191).

Identification

Demodex spp. mites are minute (100 μm) with a tapered and pointed abdomen, the cuticle of which is striated. The stumpy legs, which end in claws, and the cigar-shaped bodies are recognizable in stained skin sections (Fig. 3.4).

Figure 3.4 Typical Demodex mites obtained from skin scrapings (see p. 63); they can sometimes be found in a histological section of normal skin.

Life cycle

Demodex spp. usually exist as harmless commensals in the hair follicles and the sebaceous glands of the skin. The life cycle, which takes 18–24 days to complete, consists of eggs, two larval stages (a protonymph and tritonymph), and the adult stages. Males are rarely seen but all the life cycle stages may be present in a single hair follicle. Demodex spp. are unable to survive away from the host’s body and so transmission is very slow; it is suggested that infection is acquired by foals at or around birth but little is known about the disease in horses.

Significance

Although Demodex spp. are usually regarded as non-pathogenic, significant clinical disease can result if the health and immune status of the animal is compromised. Most cases are reported to be in horses receiving long-term glucocorticoid therapy. Infestation may present as scaling and alopecia; papules and pustules may form on the face, shoulders, neck and limbs (see p. 196).

Identification

Trombiculids (Fig. 3.5) are very small (200 μm) and are sometimes noticed in groups as pale orange-brown specks on the skin. Close examination will reveal six-legged larvae having a scutum and body covered with very small feathered hairs.

Figure 3.5 Trombicula spp. larvae are small, orange in colour and can be seen on the skin. Only the larvae attack mammalian hosts, where they can cause a hypersensitivity response (see p. 198).

Life cycle

The adults and nymphs are non-parasitic and predate upon the eggs and larvae of various types of soft-bodied insects. Eggs are laid in well-drained soil and newly emerged larvae quest, in a similar manner to ticks, on the tops of grass stems awaiting a passing host. The mouthparts pierce the skin, producing an exudate on which they feed for several days. In northern Europe, there is one generation of larvae each year which are very active in late- to mid-autumn. Horses are parasitized through contact with grasses when feeding and during exercise.

Significance

Infestation, usually on the limbs and face, causes pruritus with erythema, and sometimes a severe exudative dermatitis out of proportion to the severity of the infestation. Hypersensitivity may result with pustules, scabbing and hair loss (see p. 198).

Identification

These soft-bodied mites are oval-shaped and typically possess numerous and very long whip-like hairs on the posterior body margin (Fig. 3.6). Teeth are usually prominent on the palps. These features are important in distinguishing forage mites from the mites that cause mange.

Figure 3.6 A variety of mites occur in bedding and forage and on pasture. (A) These parasites are Acarus siro and are common in hay and straw. (B) This mite is a typical bedding-related mite. These are sometimes significant causes of limb and face dermatitis and irritation.

Significance

When in contact with horse skin, mites may cause a hypersensitivity-related dermatitis or even abrade the skin to cause a temporary erythema. There may also be pruritus, papules and crusts. The most obvious lesions occur close to the hay or straw that harbour the mites, i.e. the face and distal limbs (see p. 198).

Identification

Adults are relatively large – up to 1 mm in length. The body cuticle is tougher than that of the soft-bodied permanent species such as Psoroptes spp. and the legs are spider-like legs that protrude well beyond the body margin (Fig. 3.7). A diagnostic feature that separates this mite from the similar types (below) is the anal shield, which is large and D-shaped (Fig. CD3 • 1); this feature is only seen if the specimen is cleared (see section examination for mange mites of horses). Poultry mites are fairly easy to see with the naked eye or with a magnifying glass, appearing bright red when fully blood-engorged and almost black when the blood meal is more than 24 hours old. Pigeons’ nests within or near stables may be a transient source of D. gallinae in autumn when young birds are fledged.

Figure 3.7 (A) Dermanyssus gallinae found on the distal limb of a horse showing facial and limb pruritus. (B) The horse was stabled in contact with domestic poultry.

Life cycle

Mites are fully fed within a few hours of arriving on their host. They then drop off quickly. Eggs are laid near nests and roosts. Newly hatched larvae do not feed but undergo successive moults to the nymph and adult stages in as little as 7–10 days. Populations can build up rapidly in bird accommodation.

D. gallinae populations can survive for up to 35 weeks in the absence of their usual avian hosts; accommodation can therefore remain infested for long periods. Horses (and humans) are likely to be bitten by dispersing hungry mites.

Significance

There may be pruritus, papules and crusts. Diagnosis is difficult because mites are seldom recovered from animals (see p. 199).

Identification

The legs are long and the body shape very similar to that of D. gallinae. The anal plate is pear-shaped and not D-shaped as in D. gallinae. The bodies of these species are densely covered with short hairs (Fig. CD3 • 2).

Life cycle

These species are usually associated with rodents and their nests but they will feed on the blood of other animals. They may become problematic if rodent populations near stables are high.

Significance

There may be papules and crusts with pruritus. Diagnosis is difficult because mites are seldom recovered from animals. The implications of finding them of course is that rodent populations are high and control should be instituted.

Identification

There are no distinct body regions. The head and thorax are fused to form the basis capituli. Examination of the structure and shape of the mouthparts can confirm the specimen as a tick and is also useful in determining the genus.

Some tick genera have a metallic, shiny scutum whereas others do not. Species of ticks in subtropical/tropical genera Ambylomma and Hyalomma are often very large – up to 1.0 cm in size even when flat and unengorged – and can be easily identified by their overall size and very long mouthparts. The scutum of Amblyomma spp. has a metallic appearance; that of Hyalomma spp. is dull and non-metallic. Both these genera are usually found in the perianal region and the underside of animals and may appear like bunches of grey grapes, 1–1.5 cm in size when full of blood. Ixodes spp. are non-metallic, dark red, and have long mouthparts; Ixodes spp. occur in temperate climates.

Boophilus spp. are the smallest of all the tick genera. They are non-metallic bluish grey in colour and possess much reduced mouthparts. Dermacentor spp. are a metallic-coloured species, the mouthparts of which are club-shaped. Haemaphysalis spp. are non-metallic and have laterally expanded palps. Rhipicephalus spp. are non-metallic, reddish in colour with a characteristic hexagonal shape to the basis capituli. Rhipicephalus spp. may be found inside the ears.

Soft ticks such as Otobius and Ornithodorus spp. appear crab-like and have a leathery, textured cuticle. The mouthparts are recessed under the tick and cannot be seen from above.

Life cycles

Hard and soft ticks comprise the parasitic stages of larva, nymph and adult. Soft ticks have a multi-host developmental cycle. Each stage of the soft tick life cycle feeds several times and for only a few minutes on each occasion. Hard ticks are far more important and their life cycle is now described in detail. Adult hard ticks spend the first 24 hours on the host without imbibing blood and at this point females are more or less flat and 5–10 mm in size. Females attach to the host, attract males and become fully engorged with blood after 6–12 days. Engorged adult ticks may reach 1 cm in size (Fig. 3.8); they then drop off the host and produce thousands of eggs in vegetation.

Figure 3.8 An engorged ixodid tick.

The resulting hatched larvae are very small (less than 1.0–1.5 mm in size), and become only slightly larger when blood-feeding is completed within 2–5 days. Engorged larvae drop off the host and moult to nymphs. Nymphs are 1–3 mm in their unfed state but swell to 3–6 mm following feeding. Nymphs are fully blood-engorged after 3–8 days and then drop off to moult to the adult stage.

In this way, most hard tick species parasitize three different animal hosts in order to complete their life cycle (three-host ticks), which may be as short as a few months in the subtropics or as long as 3 years in temperate climates.

In contrast, Boophilus spp. are ‘one-host’ ticks, feeding and moulting on the same host. The larva remain on the host, mature and drop off the host as an engorged adult.

Some species of Hyalomma and Rhipicephalus spp. are two-host ticks.

Significance

Hard ticks tend to feed at specific sites, especially the ears, face, neck, axillae, groin, distal limbs and tail, but although the different species have preferential feeding sites, ticks may be found anywhere on the body. Due to the time spent on the host, one-host and two-host species are likely to induce a more severe host reaction than three-host species. The cutaneous signs associated with hard tick feeding in horses include focal necrosis and a variable (mild–severe) inflammatory response. Papules, pustules and ulcerations occur. The larvae of Boophilus microplus are reported to cause a local hypersensitivity reaction with papule formation at the site of feeding. Tick feeding can cause abscesses by the introduction of surface bacteria. There may be subsequent infection with Staphylococcus. The lesions created by ticks may be attractive to screw-worm (fly) larvae in the tropics and subtropics (see p. 212).

The larvae and nymphs of the soft tick Otobius megnini can produce a severe otitis externa.

Identification

Stomoxys spp. are 5–7 mm, brown/grey and house-fly-like in appearance (Fig. 3.11). The abdomen is chequered and the thorax striped. Males and females are similar and possess a typical strong proboscis which is black, shiny and forward pointing. The palps are short and thin and very small compared to the length of the proboscis.

Figure 3.11 Stomoxys calcitrans, the stable fly.

Life cycle

Both males and females are active throughout the day; individual female flies take several blood meals in order to mature their eggs and this interrupted feeding pattern is an important aspect of disease dissemination.

Stomoxys spp. breed in sheltered animal manure mixed with straw and urine but other varieties of rotting matter provide suitable breeding substrates, provided it is protected from rain. Larvae feed and moult from the first instar to the second and then to the third instar. Larval development may take as little as 7 days. The third-stage larva pupate and adult development within this is temperature-dependent. Stomoxys spp. overwinter as puparia in temperate regions but breed all year round in warm climates. In northern Europe, population levels peak in August/September and flies can be seen pestering housed and grazing horses. They can be recognized at rest ‘sunning’ themselves on exposed walls, farm machinery and beams.

Significance

Stable flies inflict a painful bite and are one of the most annoying pests of horses, mostly attacking the lower body and legs. Horses react by stamping the legs, biting and shivering the skin. The presence of cattle and pigs, on which this species also readily feeds, can influence the numbers of S. calcitrans pestering horses.

Identification

Lyperosia spp. are the smallest of the biting muscids. The palps are the same length as the proboscis. The head is rather small compared to the overall body of the fly and the wings are elongated (Fig. 3.12). Flies may appear embedded in the hair along the backline.

Figure 3.12 Lyperosia irritans. Note that flies are very small, being around 4–5 mm long. The wings are set very flat.

Life cycle

The life cycle of Lyperosia spp. is highly adapted to ectoparasitism; the flies congregate to feed and rest on the backs of animals. Flies leave the host to deposit eggs in fresh dung as soon as it is dropped on pasture. The life cycle in dung is similar to that of the other species, so that new populations may emerge every 2–4 weeks, depending on temperatures and conditions. Adult flies do not infest animal accommodation; Lyperosia irritans does not bite horses inside stables.

Significance

The flies can cause irritation. Severe infestations may cause anaemia. Presence of cattle pasture may influence the numbers pestering horses.

Identification

Flies in the family Tabanidae are all large and stout-bodied, between 10 and 15 mm in size. Tabanus spp. are the largest of the Tabanidae, and measure 1–2 cm (Fig. 3.13). Like all tabanids the eyes are semi-moon-shaped and iridescent, the antennae are three-jointed and the wings are clear. Haematopota spp. are smaller (Fig. CD3 • 3A) and the wings are mottled. In Chrysops spp. (Fig. CD3 • 3B), the wings are usually banded.

Figure 3.13 Tabanus spp. are some of the largest flies and can reach 1.5 cm long. The wing pattern is a good identifying feature.

Life cycle

The larvae are carnivorous and develop from eggs laid underneath hanging vegetation in or near wet areas, muddy patches or marshes. There is usually only one generation of adult flies annually in temperate climates; emergence starts on warm days in early summer and feeding activity, on warm, humid days, is greatest in July and early August. Tabanids overwinter as late-stage larvae in Europe. Adult emergences overlap in tropical and subtropical zones, but are influenced by wet and dry times of the year. Females of some tabanid species do not require an initial blood meal to produce eggs, a phenomenon known as autogeny.

Significance

Despite their large size, tabanids fly silently and appear on any part of the horse’s body but very often it is the lower legs and the heels which are bitten. Haematopota spp. are also attracted to the sides of the face and neck and other parts where the host may have difficulty in dislodging the attacker. Tabanids usually make several attempts to bite and readily move between animals, making them effective vectors of disease pathogens. Feeding results in bleeding which attracts nuisance flies. Horses tolerate tabanids to different extents but may flee from the persistent attention (gadding).

Identification

Glossina spp. are similar in colour to house flies but have a slim body and are larger (Fig. CD3 • 4). They possess a forward-pointing proboscis and hold their wings over the dorsal abdomen in a scissor-blade posture. The wings also have a characteristic hatchet cell not seen in other blood-sucking or nuisance fly species.

Life cycle

Both males and females feed several times during a highly specialized reproductive strategy – a single, ground-dwelling larva is produced during each 21-day cycle. Adults emerge from puparia after 30 days. Several larvae are produced by a single fly.

Significance

Skin lesions and intense annoyance result from feeding behaviour, the extent of which depends on the level of fly challenge, but their principal importance is as vectors of cyclically-transmitted animal trypanosomes.

Identification

Adult flies are approximately 10 mm in length and are pale reddish brown (Fig. 3.14). They have small heads with bulging eyes and the proboscis is forward-pointing. Hippoboscids feed on the host for long periods (see p. 210).

Figure 3.14 Hippobosca equi. These parasites are extremely tough – it is almost impossible to kill them by hand! It is ill advised to try to dislodge or kill them by slapping the horse’s skin. The horse may resent the hit more than the flies.

Life cycle

Both sexes feed on blood. Females leave the host and deposit a single larva in soil or leaf litter. A female may produce up to six larvae in her life time. Flies are most abundant in the summer months in temperate zones.

Significance

The preferred feeding sites are the perineum and between the hind legs. Flies may be well tolerated by donkeys and horses that are exposed to high, persistent challenge throughout the fly season. This species can be a source of intense irritation in other circumstances, e.g. when horses are moved into infested areas (see p. 210).

Identification

Adult are 3–10 mm long, slender with long legs, threaded antennae and scales on the wings. Mosquitoes have a very long proboscis flanked by palps.

Life cycle

Only females feed on blood. Eggs are laid in batches on still surface water. The larvae and puparia of anophelines require unpolluted fresh water sites whereas culicines breed in stagnant, organic-rich water. The eggs of Aedes spp. have adapted to survive desiccation and are laid on moist substrates and not directly on water itself.

Mosquito larvae and pupae may be easily recognized as they rest and breath just under the water surface and ‘loop’ away downwards if disturbed. There is a distinct summer activity pattern in temperate zones and woodland with ponds or salt marshes providing breeding sites. In tropical/subtropical zones there may be large increases in fly activity weeks after the seasonal rains.

Significance

Mosquitoes can significantly contribute to fly ‘nuisance’. Animals may vary in their ability to tolerate mosquito bites; some individuals may simply show local redness and swelling whereas others may have a severe hypersensitivity to saliva (see p. 284).

Identification

Culicoides spp. are very small flies (1–4 mm) and may not be noticed flying around or feeding on horses. Many species have mottled wings (Fig. CD3 • 5). Sandflies are small flies with hairy wings which are held erect above the thorax while feeding.

Life cycle

Culicoides spp. breed in any decaying vegetable matter, notoriously in compost heaps. Sandfly breeding sites are leaf litter, humus and associated decaying matter. The life cycle is typically dipteran and can be completed in 3 weeks or less.

Significance

The very common skin disorder insect bite hypersensitivity (‘sweet itch’ Queensland itch) is frequently caused by hypersensitivity responses to the bites of Culicoides (see p. 284). This is a predictable highly pruritic condition in susceptible horses with self-inflicted skin damage through rubbing on posts, etc. The withers, mane, tail, face, ventral midline and other regions are often affected, leading to hair loss, excoriation and crusting which leads to hyperkeratosis and thickening of the skin.

Sandflies may also cause significant dermatitis in horses where this fly is endemic but this is usually transient (although some hair loss may persist) and usually far less severe than insect bite hypersensitivity (see p. 209).

Identification

Simulium spp. are small flies (2–4 mm) with a characteristic humped thorax. The wings are clear and the antennae short. Adults are dark in colour but some species are brown rather than black.

Life cycle

Eggs are deposited in gelatinous aggregations on vegetation in slow to fast running waters, in which the larvae and puparia develop. The adults emerge in late summer and bite the underside of horses or inside the ears, depending on the species. Black-flies are active throughout the day.

Significance

Simulium spp. can cause general dermatitis at feeding sites and crusting, particularly inside the ears and in the ventral midline (see p. 210). They may be vectors of the virus that causes pinnal acanthosis (see p. 136). Swarm attacks can be fatal to horses.

Identification

Females of Musca spp. have the same gross appearance as Stomoxys spp. but the proboscis is of the non-biting type, ending in a pad for mopping up secretions. In Musca spp., the two longest wing veins join at the wing margin forming a V-shape. Males also feed and have a yellow abdomen.

Life cycle

The life cycle of Musca spp. is very similar to that of Stomoxys spp. However, Musca spp. breed in animal dung on pasture or in other decaying organic matter. The life cycle may take as little as 14 days in the summer, with up to four generations in one season. Flies overwinter as adults.

Significance

Feeding occurs on the face and muzzle and around the corners of the eye. Eye irritation can lead to kerotoconjunctivitis (fly eye or ‘pink eye’) either directly or through transferred bacterial infections.

Musca spp. are also vectors of the stomach worm Habronema spp. the larvae of which cause granulomatous cutaneous lesions around the face (see p. 217). Musca spp. also transmit the eye worm Thelazia spp.

Identification

Female H. irritans are 4–5 mm in size and have a pale green abdomen with a yellow wing base (Fig. CD3 • 7). Males are slightly larger and black in colour. H. albipunta resemble H. irritans but are 3–4 mm in size and the abdomen has a bluish hue.

Life cycle

The larvae and puparia of H. irritans develop in soil and there is a single emergence each year. Males emerge in mid-July and swarm around horses outside stables. Females emerge several weeks later, feeding on secretions around the eyes and muzzle but the entire body may be attractive. The mouthparts are adapted to abrade the skin and this species may initiate small wounds or enlarge existing ones, particularly on warts and sarcoids. They often feed on blood released around the head of a biting fly. H. irritans may be present on horses until late October. H. albipunta has a similar life cycle to Musca autumnalis, breeding in animal dung on pasture, and there may be more than one generation each year.

Significance

H. irritans causes major annoyance. The intensity of fly challenge due to H. irritans may be increased by the presence of adjacent cattle and sheep pastures, particularly near wooded river meadows, on which populations of these flies are particularly abundant. H. irritans causes intense irritation and associated cutaneous damage. Feeding around the corners of the eye can result in a conjunctivitis (fly eye) and they may be a vector for Moraxella spp. bacteria.

Identification

Lucilia sericata are metallic green flies (Fig. 3.15 and Fig. CD3 • 9A) and measure approximately 10.0 mm. Their eggs are creamy white, approximately 1 mm in size and may be seen in groups or scattered on the skin. Second- and third-stage larvae a are 5–10 mm in size and are segmented and barrel-shaped. The puparia are tan-brown.

Figure 3.15 Lucilia sericata (arrow) feeding on a recently dead carcass. Note other species of fly are also present.