Pseudoparasite Example

[Sherman Desrosiers]

PhysicalExamination. Max's physical examination was unremarkable. His attitude and behavior were normal. The owner brought along a sample of foul-smelling liquid feces. A centrifugal fecal flotation examination was performed, and the technician reported the presence of coccidia, roundworms, and hookworms. The veterinarian reviewed the slide (Figure 1) and decided to send the remaining fecal sample to a reference laboratory for a second opinion.

Fecal material often contains artifacts designated as pseudoparasites and spurious parasites. Pseudoparasites are nonparasitic material that looks parasitic. Common pseudoparasites include plant material (Figures 3 and 4), pollen grains, and free-living arthropods (Figure 5). Spurious parasites are true parasites of one species found in the feces of another species because of coprophagy or predation. In this case, both a pseudoparasite and spurious parasites were present, most likely from the ingestion of sheep manure. Ruminant coccidia oocysts and nematode parasite eggs in the manure are passed undamaged through the gastrointestinal tract. Technical staff trained to recognize only small animal parasites will understandably identify these eggs and oocysts as canine parasites. The structure that looks like a Toxocara egg is a pseudoparasite.

Size is also an important criterion for evaluating possible fecal artifacts. In this case, the object that looks like a roundworm egg is too big to be Toxocara. The easiest method of measurement is through use of an inexpensive (less than $100) eyepiece reticle. A reticle is a small glass disc with a scale (usually 50 divisions) that fits into a microscope eyepiece. Approximate sizes can be established by using common parasite eggs as a reference. For example, from a standard text you know that eggs of the canine whipworm, Trichuris vulpis, are 70 to 90 m long. If whipworm eggs measure 7 reticle divisions long on your 10 objective, then each reticle division equals 10 to 13 m. This calculation can be performed for each objective lens.

In cases for which identification of structures is uncertain, the owner should be instructed to prevent the pet from ingesting nonfood materials and to have a second sample examined after a few days.

Yes, and indeed these structures are not canine parasites. The structure that appears to be a Toxocara egg is a pseudoparasite, which is indicated by its large size (approximately 120 m). Also, the outer layer is uneven, with several discontinuities. The egg that resembles the hookworm egg comes from a related family but is an ovine nematode egg. It is larger than the common Ancylostoma species egg, and the presence of other ruminant parasites makes it unlikely that it is an egg of the less common canine hookworm, Uncinaria. On higher-power magnification (Figure 2), the coccidia oocysts can be identified as Eimeria species, not canine Isospora species, by the more oval shape and the presence of the micropyle cap (arrow). Many, although not all, Eimeria species have this cap.

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Common practice in a diagnostic parasitology laboratory involves distinguishing parasitic organisms from various artifacts. Artifacts mean pseudoparasites, supposed parasites and parasitic delusions. Pseudoparasites include undigested leftovers or coincidentally or purposely ingested nonparasitic organisms or their parts. Supposed parasites are wild organisms which were incorrectly identified as the components of faeces. In parasitic delusions, it is impossible to find any kind of parasites while patients describe imaginary parasites in detail. All the above categories of nonparasitic findings including case reports are described and discussed in the article.

Brood parasitism is a subclass of parasitism and phenomenon and behavioural pattern of certain animals, brood parasites, that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the same or of another species, to raise its young as if it were its own, usually using egg mimicry, with eggs that resemble the host's.

The evolutionary strategy relieves the parasitic parents from the investment of rearing young. This benefit comes at the cost of provoking an evolutionary arms race between parasite and host as they coevolve: many hosts have developed strong defenses against brood parasitism, such as recognizing and ejecting parasitic eggs, or abandoning parasitized nests and starting over. It is less obvious why most hosts do care for parasite nestlings, given that for example cuckoo chicks differ markedly from host chicks in size and appearance. One explanation, the mafia hypothesis, proposes that parasitic adults retaliate by destroying host nests where rejection has occurred; there is experimental evidence to support this. Intraspecific brood parasitism also occurs, as in many duck species. Here there is no visible difference between host and parasite eggs, which may be why the parasite eggs are so readily accepted. In eider ducks, the first and second eggs in a nest are especially subject to predation, perhaps explaining why they are often laid in another eider nest.

Brood parasitism is an evolutionary strategy that relieves the parasitic parents from the investment of rearing young or building nests for the young by getting the host to raise their young for them. This enables the parasitic parents to spend more time on other activities such as foraging and producing further offspring.[1]

Among specialist avian brood parasites, mimetic eggs are a nearly universal adaptation. The generalist brown-headed cowbird may have evolved an egg coloration mimicking a number of their hosts.[2] Size may also be important for the incubation and survival of parasitic species; it may be beneficial for parasitic eggs to be similar in size to the eggs of the host species.[3]

The eggshells of brood parasites are often thicker than those of the hosts. For example, two studies of cuckoos parasiting great reed warblers reported thickness ratios of 1.02 : 0.87[4] and 1.04 : 0.81.[5] The function of this thick eggshell is debated. One hypothesis, the puncture resistance hypothesis, states that the thicker eggshells serve to prevent hosts from breaking the eggshell, thus killing the embryo inside. This is supported by a study in which marsh warblers damaged their own eggs more often when attempting to break cuckoo eggs, but incurred less damage when trying to puncture great reed warbler eggs put in the nest by researchers. Another hypothesis is the laying damage hypothesis, which postulates that the eggshells are adapted to damage the eggs of the host when the former is being laid, and prevent the parasite's eggs from being damaged when the host lays its eggs.[6] In support of this hypothesis, eggs of the shiny cowbird parasitizing the house wren and the chalk-browed mockingbird and the brown-headed cowbird parasitizing the house wren and the red-winged blackbird damaged the host's eggs when dropped, and sustained little damage when host eggs were dropped on them.[7]

Most avian brood parasites have very short egg incubation periods and rapid nestling growth. In many brood parasites, such as cuckoos and honeyguides, this short egg incubation period is due to internal incubation periods up to 24 hours longer in cuckoos than hosts. Some non-parasitic cuckoos also have longer internal incubation periods, suggesting that this longer internal incubation period was not an adaptation following brood parasitism, but predisposed birds to become brood parasites.[8] This is likely facilitated by a heavier yolk in the egg providing more nutrients. Being larger than the hosts on hatching is a further adaptation to being a brood parasite.[5]

Bird parasites mitigate the risk of egg loss by distributing eggs amongst a number of different hosts.[9] As such behaviours damage the host, they often result in an evolutionary arms race between parasite and host as they coevolve.[10][11]Some host species have strong rejection defenses, forcing the parasitic species to evolve excellent mimicry. In other species, hosts do not defend against parasites, and the parasitic mimicry is poor.[12]

Intraspecific brood parasitism among coots significantly increases the reproductive fitness of the parasite, but only about half of the eggs laid parasitically in other coot nests survive. This implies that coots have somewhat effective anti-parasitism strategies.[13] Similarly, the parasitic offspring of bearded reedlings, compared to offspring in non-parasitic nests, tend to develop much more slowly and often do not reach full maturity.[14]

Given that the cost to the host of egg removal by the parasite is unrecoverable, the best strategy for hosts is to avoid parasitism in the first place. This can take several forms, including selecting nest sites which are difficult to parasitize, starting incubation early so they are already sitting on the nests when parasites visit them early in the morning, and aggressively defending their territory.[15]

Once a parasitic egg has arrived in a host's nest, the next most optimal defense is to eject the parasitic egg. This requires the host to distinguish which eggs are not theirs, by identifying pattern differences or changes in the number of eggs.[16] Eggs may be ejected by grasping, if the host has a large enough beak, or by puncturing. When the parasitic eggs are mimetic, hosts may mistake one of their own eggs for a parasite's. A host might also damage their own eggs while trying to eject a parasite's egg.[17]

Among hosts that do not eject parasitic eggs, some abandon parasitized nests and start over again. However, at high enough parasitism frequencies, this becomes maladaptive as the new nest will most likely also be parasitized. Some host species modify their nests to exclude the parasitic egg, either by weaving over the egg or by rebuilding a new nest over the existing one. For instance, American coots may kick the parasites' eggs out, or build a new nest beside the brood nests where the parasites' chicks starve to death.[13] In the western Bonelli's warbler, a small host, small dummy parasitic eggs were always ejected, whilst with large dummy parasitic eggs, nest desertion was more frequent.[18]

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