sexual reproduction of the parasite takes place being a carnivore such as
cats, dogs or humans, and an intermediate host such as cattle, sheep or
pigs in the tissues of which the asexual cysts are formed.
Two species can infect humans:S. hominis, which infects cattle, and
S. suihominisfrom pigs. Although symptoms are usually mild, they can
include nausea and diarrhoea. Beef and pork which have been ade-
quately cooked lose their infectivity.
In the case ofToxoplasma gondii, the definitive host is the domestic or
wild cat but many vertebrate animals including humans are susceptible
to infection by the oocysts shed in their faeces. Thus herbivores can
become infected by eating grass and other feedstuffs contaminated by cat
faeces and, once infected, their tissues may remain infectious for life.
Although foodborne infection in humans may be rare, it could occur
through consumption of raw or undercooked meat, especially pork or
mutton.
Toxoplasmosis is usually symptomless or associated with a mild
influenza-like illness in healthy humans, but infection can be serious in
immunocompromised people.
8.3 Toxigenic Algae
Although strictly speaking the term algae should now be used as a
collective term for a number of photosynthetic eukaryotic phyla, for the
purposes of this section the prokaryotic cyanobacteria, or blue-green
algae, will also be included.
A number of planktonic and benthic algae can produce very toxic
compounds which may be transported to filter-feeding shellfish such as
mussels and clams, or small herbivorous fish which are food for larger
carnivorous fish. As these toxins pass along a food chain they can be
concentrated and it may be the large carnivorous fish which are caught
for human consumption which are most toxic. In the case of shellfish, the
toxins may accumulate without apparently harming the animal but with
potent consequences for people or birds consuming them.
A number of distinct illnesses are now recognized including PSP
(paralytic shellfish poisoning), NSP (neurotoxic shellfish poisoning),
DSP (diarrhoeal shellfish poisoning), ASP (amnesic shellfish poisoning)
as well as ciguatera fish poisoning. The toxins implicated in the various
forms of shellfish poisoning are not only undetectable organoleptically
but are also generally unaffected by cooking.
8.3.1 Dinoflagellate Toxins
Planktonic dinoflagellates may occasionally form blooms containing
high numbers of organisms when environmental conditions such as
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