Parasite is any organism that lives at the expense of another (host) organism. Some
parasites carry or cause disease.
Parasites are a world-wide major health problem ranking amongst the greater threats to
the well being and survival of mankind. Parasites occur in all natural communities,
sometimes as benign infections, but they may also have massive economic impact by
seriously damaging populations of domestic animals, fish and crop plants. Continuous
effort by the world community of parasitologists is necessary to understand and further
the control of parasites and their vectors.
Parasitic infections are acquired by eating or drinking contaminated food or water,
through direct contact with soil or water containing parasites or their larva, or by
contact with biting insects.
The blood feeding mode evolved independently more than 150 million years ago in at
least 6 different groups of arthropods. These arthropods (insects and ticks), had to solve
independently the problem of how to deal with their hosts' defense against blood loss, the
hemostatic mechanism. Fifty million years ago, dinosaurs went extinct and mammals became
the main food source, bringing with it a new problem posed by blood platelets, which are
much more efficient then bird or reptile (or dinosaur) thrombocytes in preventing blood
loss. Blood feeding arthropods even in the same family had to discover new pharmacological
products to deal with this new feeding problem. Perhaps because of these evolutionary
reasons, a very large diversity of pharmacological products are being discovered in the
saliva of such arthropods, which comprise around 15,000 species in near 600 genera.
Despite decades of concerted international efforts, malaria remains the most
devastating insect-borne parasitic disease of man and until now no acceptable measures has
been devised that can eradicate this killer disease. Because malaria parasite's
development in the mosquito is obligatory, our goal is to disrupt the development of the
parasite in the vector to block the transmission to human.
During the complex development of malaria parasites in mosquitoes new sets of genes are
sequentially expressed in the parasite. Similarly, after blood feeding the biochemical
environments in the mosquito changes dramatically. For successful development and
tranmission to human, malaria parasites must interact with numerous biochemical and
physical factors in the mosquito .
Scientists today are working not only to develop new vaccines against diseases that
still make people sick, but they also are searching for unique ways to get vaccines into
people.
A new approach to malaria is to develop a vaccine that prevents transmission of the
malaria parasite from one infected person to another person. This type of vaccine would be
given to people already infected with the parasite--not to protect them from illness but
to prevent the parasite from infecting someone else.
After a mosquito bites a person whose blood contains the malaria parasite, the mosquito
normally transmits the parasite to someone else when it next feeds. But this vaccine would
block the sexual development of the parasite in the mosquito, so that the parasite could
not cause malaria in the next person bitten by the mosquito.
Other parasites include Giardia lamblia, which is a major cause of intestinal disease
worldwide and the most frequent non-bacterial cause of diarrhea in North America.
Nonetheless, the basic biology of this parasite--including how it ravages the digestive
tract--is poorly understood.
Giardiasis affects three times more children than adults, particularly diapered
children and toddlers being toilet-trained. Families with young children who attend
day-care centers are at greater risk of developing giardiasis than is the general
population.
The usual interval between infection and the onset of acute symptoms ranges from one to
two weeks. In most instances, the individual will experience sudden explosive, watery,
foul-smelling diarrhea; excessive gas; abdominal pain; bloating; nausea; tiredness; and
loss of appetite. Upper gastrointestinal symptoms such as vomiting may predominate. Fever
is unusual. Many infected people, however, have very mild symptoms or no symptoms at all.
Blood or mucus in the stool rarely occurs. Occasionally, the illness may last for months,
or even years, causing recurrent mild or moderate symptoms such as impaired digestion,
especially lactose intolerance; intermittent diarrhea; tiredness and weakness; and
significant weight loss.
Presently, there is no drug available to prevent giardiasis. The following precautions
should be taken if visiting an area where Giardia may exist: drink only boiled or filtered
water, or bottled carbonated drinks; avoid ice and beverages made from tap water; and do
not eat uncooked or unpeeled fruits and vegetables grown locally.
Parasites (in the larval stage) consumed in uncooked, or undercooked, unfrozen seafood
can present a human health hazard. Among parasites, the nematodes or roundworms (Anisakis
spp., Pseudoterranova spp., Eustrongylides spp. and Gnathostoma spp.), cestodes or
tapeworms (Diphyllobothrium spp.) and trematodes or flukes (Chlonorchis sinensis,
Opisthorchis spp., Heterophyes spp., Metagonimus spp., Nanophyetes salminicola and
Paragonimus spp.) are of most concern in seafood. Some products that have been implicated in human infection are:
- ceviche (fish and spices marinated in lime juice)
- lomi lomi (salmon marinated in lemon juice, onions and tomato)
- poisson cru (fish marinated in citrus juice, onions, tomatoes
and coconut milk) - salmon roe
- sashimi (chunks of raw fish)
- sushi (pieces of raw fish with rice and other ingredients)
- green herring (lightly brined herring)
- drunken crabs (crabs marinated in wine and peppers)
- cold-smoked fish and,
- undercooked grilled fish
The process of heating raw fish sufficiently to kill bacterial pathogens is sufficient
to kill parasites.
Freezing (-4F)(-20C) or below (internal or external) for 7 days or -31F
(-35C) or below (internal) for 15 hrs) of fish intended for raw consumption also
kills parasites. FDA's Food Code recommends these freezing conditions to retailers who
provide fish intended for raw consumption.
Brining and pickling may reduce the parasite hazard in a fish, but they do not
eliminate it, nor do they minimize it to an acceptable level. Nematode larvae have been
shown to survive 28 days in a 80 degree F salimeter brine (21% salt by weight).
Trimming away the belly flaps of fish or candling and physically removing parasites are
effective methods for reducing the numbers of parasites. However, they do not completely
eliminate the hazard, nor do they minimize it to an acceptable level.
