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PROTOZOA
Contents: Toxoplasma, Sarcosporidia, Chlamydiae, Rickettsiae
The protozoa are the most primitive organisms of the animal kingdom
and are mostly of microscopical size. They are single-celled and
reproduce by divisison or budding. The protozoan parasites that are of
importance in meat inspection are coccidia, sarcosporidia and
haemosporidia.
TOXOPLASMOSIS
Toxoplasma gondii is a coccidia that is the most widespread and
prevalent parasite in the world. It can infect warm-blooded animals,
including man.
It is responsible for 20.7% of food-borne deaths due to known infectious
agents.
Waterborne outbreaks have also been associated with Toxoplasma in
Canada and Brasil.
Toxoplasma is a minute intracellular parasite belonging to the Sporozoa,
a subphylum of the Protozoa. They possess no locomotor organs or
flagella in the adult stage and have complex life cycles, sexual stages
alternating with asexual ones.
Toxoplasma gondii, the main type, is a crescent-shaped organism 6 m
x 3 m in size that has a wide distribution in domestic and wild animals
and was first described in an African rodent, the gondi, in 1908. Its exact
role in disease, however, was not recognized until 1939, one reason
being that there are great differences in pathogenicity between different
strains of the organism.
Toxoplasma gondii has a sexual life cycle in the intestines of animals,
development taking place in the intestinal epithelium with the formation
of oocysts. The asexual phase, in which trophozoites and tissue cysts
are produced, occurs in most animals, birds and humans, making this
condition a true zoonosis. Both trophozoites and cysts are destroyed by
the usual disinfectants and by desiccation but oocysts are very resistant
and can remain infective for up to 17 months.
The main source of infection is the cat and dog that acquire the
organisms through the consumption of infected rodents or meat
containing oocysts. Incidences as high as 60% in dogs, 35% in cats,
48% in cattle and goats and 30% in pigs have been recorded in various
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countries, the organism apparently causing no disease in many
instances.
Carcases affected with toxoplasmosis generally lose their infectivity very
shortly after death, and freezing and heating of meat quickly inactivates
the parasites. Cooking is, teherefore, an effective means of destroying
trophozoites and cysts, but humans can become infected by eating
undercooked or raw meat.
Infected carcases and feedingstuffs are also important sources of
infection through ingestion, but respiratory infection may also occur.
While most tissues can be invaded, the reticuloendothelial system and
CNS are predilection sites in which the organisms multiply rapidly.
Trophozoites may be present in faeces, blood, urine, saliva and body
fluids.
Infections in animals
These are usually asymptomatic but clinical cases can occur. Congenital
toxoplasmosis causes abortion and neonatal deaths in sheep and
sometimes in goats and pigs. In other animals, especially in dogs and
cats, there is fever, diarrhoea, hepatitis with icterus, pneumonia and CNS
involvement especially under stressful conditions. Some cases show
lymphadenitis and myocarditis.
Lesions
The most common finding is the presence of multiple necrotic
granulomata, mostly in the liver, intestinal ulceration, splenomegaly,
pneumonia and lympadenitis. In sheep, the species most often affected
with abortion, metritis, placentitis and necrotic lesions in the various
organs of the foetus are evident.
A positive diagnosis cannot be made on post-mortem findings alone,
detailed laboratory examination being necessary to identify the causal
organism.
Judgement
Affected organs, and carcases if their condition is poor, should be
condemned.
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Sarcosporidia (sarcosporidiosis, sarcocystosis)
Protozoal parasites of the genus Arcocystis bear a similarity to
Toxoplasma and Besnoitia, are recorded in more than 50 species of
mammals and birds throughout the world and occur frequently in skeletal
and heart muscle, more rarely in the brain and other tissues.
Three species (Sarcocystis hominis, Sarcocystis lindemanni, Sarcocystis
porcihominis) have been recorded in humans (as an intermediate host)
causing illness in the form of diarrhoea, nausea and loss of appetite.
Although sarcocysts have not been regarded as pathogenic for animals,
an experimental work in cattle has produced weakness, anaemia,
cachexia and anorexia with haemorrhages on visceral surfaces and in
muscle and brain.
It is possible that certain underdiagnosed conditions may have been due
to sarcocysts. The chronic disease (dalmeny disease) in which there is
emaciation, submandibular oedema and exophtalmia is believed to be
bovine sarcocystosis.
The cyst lie within or between individual muscle fibres and have a
characteristic cigar shape, known as Miescher’s tubes. The larger cysts
may lie loose in the perimuscular connective tissue and are globular,
oval, or the shape of a bean. Though the size of the cyst is determined
by its age and the host, its basic structure remains the same, namely a
wall that may be striated and consists partly of sarcolemma, a cavity that
may or may not be subdivided by septa, and contents that are viscous or
gelatinous and opaque to milky white or yellowish in colour.
The cyst contains sporoblasts along the periphery and varying numbers
of sickle-shaped sporozoites that are nucleated and known as Rainey’s
corpuscles.
The distribution of Sarcocysts is world-wide with the incidence
significantly higher in older animals, in stall-fed cattle and in pigs fed on
swill. Numerous species names have been allocated to sarcocysts but
the justification for this is still in doubt inasmuch as cross-transmission to
cats, dogs, pigs and fowls can occur via the excreta of pigs previously
fed on sarcocyst-infested meat.
The larger cysts render infested meat aesthetically objectionable but
smaller cysts are only visible when degeneration and calcification occur.
Lesions of focal eosinophilic myositis, characterized by spindle-shaped
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greenish areas that may coalesce to form lesions several centimetres in
diameter, may be caused by degenerating or decomposing sarcocysts,
but similar lesions frequently occur without any apparent connection with
sarcocysts. It has been shown that other parasitic organisms may elicit
eosiniphilic myositis and the conditions sarcosporidioisis and eosinophilic
myositis are not synonymous.
Sarcocystsi miescheriana occurs in pig. The Miescher’s tubes measure
up 4.5 m long by 0.35 mm in width. The muscles of the abdominal wall,
diaphragm and the masticatory and skeletal muscles are most often
affected. The larger cysts are visible to the naked eye as light grey dots
on cross-section and spindle-shaped on longitudinal section.
The cysts may be confused with trichinella cysts but calcified sarcocysts
are the commonest form of calcification occurring in the muscles of the
pig.
Sarcosporidiosis was at one common in swill-fed pigs in the USA, with
an incidence of 75% in some parts compared with 5% in grain-fed pigs.
The incidence has fallen markedly since it became compulsory to boil
swill before use.
This parasite was at one time found in 20-40% of pigs in Denmark but
now rarely recorded.
Sar. tenella, a parasite of sheep occur mainly in the tongue, pharynx,
larynx, diaphragm and skeletal musculature. The parasite is frequently
encountered in the oesophagus of older sheep.
Sarcocystis blanchardi and mirsuta and cruzi are found in the
musculature of cattle, mainly in active muscles such as the tongue,
heart, diaphragm and masseter muscles at post-mortem tend to
disappear during overnight chilling. Cases of eosinophilic myositis have
been attributed to infection with sarcocysts, the two conditions can often
coexist.
Sarcocysts (betrami) would appear to be worldwide in occurrence in
horses that act as intermediate hosts, with the dog as the definitive
host, as in hydatid disease. The infection is maintained through the
feeding of uncooked horse meat and offal to dogs.
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Judgement
Sarcocystis lindemanni, hominis and suihominis occur in humans where
allergic muscular swellings and bronchial asthma may be due to
degenerating sarcocysts. There is no proof that consumption of
sarcocyst-infested meat leads to human sarcosporidiosis and it is
probable that humans are an unsuitable host. Contamination of food by
excreta of carnivorous animals is more likely to cause human infestation.
No practicable inspection procedures can detect microscopical forms of
Sarcocystis and in most countries where infestation is slight the carcase
may be passed for food after the removal and condemnation of all
affected tissue (partial condemnation).
Incision into various muscles (as in cysticercosis) is necessary in order to
determine the extent and severity of the infestation which if generalized
requires total condemnation on aesthetic grounds. In case of
eosinophilic myositis a similar inspection procedure and judgement
should be applied.
The distribution of Sarcocystis species has led the US Food Safety and
Inspection Service to use the oesophagus of sheep (not required for
human consumption) as an indicator for visible infection in carcases. In
the USA prevalence of infected oesophagi in one study of adult sheep
was 0.1%. It is now the practice for all carcases with infected oesophagi
to be detained for further examination and disposition.
The USDA is now evaluating streamlined inspection for broilers and
feedlot cattle.
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Chlamydiae (see also Avian psittacosis and Ornithosis)
The organism belonging to the genus Chlamydia include those
previously called the psittacosis-lymphogranuloma venereum-trachoma
group (PLT organisms). Like the Rickettsiae they occupy a position
between the bacteria and the viruses.
They are tiny, non-mobile, spherical and Gram-negative organisms
whose infective forms (elementary bodies) are found within the cells in
diseased animals and humans.
The Chlamydia are widely distributed in animals and birds and besides
causing psittacosis in birds (parrot family) and in humans are
responsible for ornithosis in poultry as well as enzootic abortion in ewes
and cattle, neonatal diarrhoea in calves, infectious keratoconjuctivitis in
sheep, sporadic bovine encephalomyelitis and polyarthritis (transmissible
serositis) in sheep, calves and pigs. These diseases are caused by
Chlamydia psittaci.
The Chlamydiae cause serious disabilities in humans (trachoma and
inclusion conjunctivitis) that is probably the most common form of
human blindness, and lymphogranuloma venereum, both being due to
Chlamydia trachomatis.
Enzootic (endemic) abortion of ewes
An infectious disease of sheep manifested by abortion and still-births
occurring in Europe and USA. It is judged to be the most commonly
diagnosed cause of ovine abortion in the UK where it is of great
economic significance.
Aetiology
A strain of Chlamidya psittaci, the cause of psittacosis (ornithosis), a
form of bovine abortion in the USA and polyarthritis in calves, lambs and
foals. Chlamydiae are increasingly being implicated in diseases in
animals and humans. They are especially dangerous to pregnant women
in whom infection causes abortion and critical illness. Great care must be
exercised by all handling infected animals and material.
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Lesions
In addition to the presence of dead foetuses there is inflammation of the
placenta with irregular thickening and necrosis of cotyledons. The
foetuses may show subepidermal haemorrhages.
Judgement
Active disease: total condemnation. As for Salmonella and Rickettsia,
the infectivity of these organisms for humans must be borne in mind.
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Rickettsiae
The Rickettsiae and Coxiella burnetti are tiny pleomorphic parasitic
organisms closely allied to the bacteria and viruses. Like bacteria they
contain RNA and DNA, glucosamine and muramic acid in their cell walls,
enzymes for metabolic activities, the ability to reproduce by binary fission
and are sensitive to antibiotics and antiseptics.
While Rickettsiae are Gram-negative, Coxiella burnetti is Gram-positive
and is more resistant to heat, antiseptics and drying than the other
Rickettsiae. Coxiella burnetti. at one time termed Rickettsia burnetti,
occupies a genus itself.
Rickettsiae are found in many types of animals (ticks, lice, fleas, mites,
birds and mammals) and are responsible for serious diseases in animals
and humans.
Humans: Epidemic typhus (Rickettsia prowasekii), French fever
(Rickettsia quintana), and the Spotted fevers (Rickettsia rickettsi and
conorii) that can also affect animals (horses, dogs and rodents), Scrub
Typhus (Rickettsia tsutsugamushi) and Q fever (Coxiella burnetti).
Animals: Tick-borne fever (Rickettsia bovin and ovina), Heartwater
(Rickettsia ruminantium), Ovine and caprine contagious ophtalmia
(Rickettsia conjuctivae).
Q Fever (Query fever)
The cause of the zoonosis, Q Fever, in humans, Coxiella burnetti, has,
like other rickettsiae, a wide host range in nature, being reported in both
domestic (cattle, sheep and goats) and wild mammals and their
ectoparasites such as ticks.
Spread of infection is effected from wild animals, ticks, etc. and also by
direct and indirect contact between domestic animals and by ingestion of
infected milk and food contaminated by discharges.
Since abortion is the main event in Coxiellosis, humans become infected
through aerosol transmission of infected particles, by handling aborted
animals and materials and by ingestion of infected milk.
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Symptoms
Although abortion occurs in sheep and goats, it has not yet been
reported in cattle, in spite of the fact that coxiella organisms can be shed
at parturition and in milk, having established themselves in the
reproductive tract.
Apart from abortion, anorexia is the only clinical sign observed in sheep
and goats.
Lesions
No specific post-mortem changes are noted except perhaps for a mild
placentitis.
Judgement
Total condemnation.