THERE ARE many more kinds of parasites in the world than free-living organisms and even when we exclude viruses - which are all parasitic - bacteria and fungi, parasites are still in the majority! Apart from being the raison d'être for parasitologists - like Professor Ralph Robinson in the Department of Life Sciences who studies them - a useful working definition of a parasite is "an organism that benefits directly from another organism and causes it harm." For example, adult fleas on a dog are considered external parasites. They feed on (steal) the animal's blood causing significant irritation and are sometimes responsible for transmitting other parasites e.g. dog tapeworms.
"So how much parasitism is there in the world?" one may ask. The aforementioned fleas and plague-causing bacteria conspired to destroy a third of the European population in the seventeenth century, and malaria, schistosomiasis and African sleeping sickness have sent millions to their graves. Malarial parasites infect more than 400 million people today, mostly in tropical developing countries, with as many as one million deaths annually - mainly among young children and pregnant women.
In addition to causing severe illness and deaths, parasites complicate and even contribute to other illnesses. For example, many persons who succumb to the dreaded HIV/AIDS virus do not die directly from the virus, but from infections with parasites like pneumocystis or toxoplasma that exploit the body's weakened immune system.
Many worms are parasites. They are not the sort we find in the soil nor the insect larvae in mangoes that many people call 'worms', but the kind that occur in people's intestines and elsewhere in the body. Parasitic worm infections are currently estimated to infect 4.5 billion people - more than two thirds the global population!
"So, what sorts of parasites occur in Jamaica?" Thankfully, malaria no longer occurs naturally in Jamaica, nor have we ever had the human blood parasite schistosoma that persists in Hispaniola, Puerto Rico, and some Eastern Caribbean islands, but we do have other parasites - and animals that spread them.
The main research interest of Professor Ralph Robinson, his colleague and former graduate student Dr. John Lindo from the Department of Microbiology, together with their graduate students, is the relationships between humans, animals and the environment as they relate to health and parasitic disease in Jamaica and the Caribbean. Some recent examples of their work include:
This infection is mainly of dogs, but people have been known to be infected outside of Jamaica. The worm parasite, dirofilaria, lodges in the right side of the heart and pulmonary artery of dogs and is of particular concern to security firms that maintain dogs, and dog breeders. Ms. Keisha Singh, a graduate student in the Department of Life Sciences now working in the United States, studied the association between heartworm infections in dogs and mosquitoes that transmit the infection. She found that most heartworm infections occurred in dogs in north-western Jamaica where mosquitoes were also plentiful, while small amounts of infections occurred in the Mandeville area, where there were fewer mosquitoes. The Kingston Metropolitan Area, in comparison, had moderate levels of heartworm infections in dogs and mosquitoes. The infamous, dengue-transmitting mosquito Aedes aegypti was also implicated in transmission of heartworm to dogs.
Reflecting global trends, intestinal worms are, by far, the most common parasites found in humans in Jamaica. In a study of 228 people in Kingston in the 1990s, Ms. Alicia Lyn Sue of the Department of Life Sciences noted that about 20 per cent of persons, mostly children, had low-level infections with roundworms - whipworms, common roundworms, etc. - in their intestines which were strongly related to the practice of soil-eating or having unwashed fresh vegetables. The situation seems to have changed little from that of 42 years ago when Dr. L. S. Grant published his findings of 21-43 per cent infection levels with the same parasites in the West Indian Medical Journal.
However, the good news is that only one per cent of persons now have hookworms, down from a whopping 25 per cent of persons 40 years ago! This is due probably to an increase in shoe/boot wearing over the years, since hookworm larvae infect persons by penetrating bare skin in the soil or grass.
Professor Robinson and Dr. Lindo have also worked on the human threadworm, strongyloides which infects about three per cent of mostly adult Jamaicans. Unlike most other intestinal worm infections, this parasite has been associated with fatalities in Jamaica and elsewhere. They concluded that - although unusual for an intestinal worm infection - intimate contact with an infected person was the most important risk factor for transmission of this parasite. In addition, infection with the HTLV-I virus was associated with increased severity of threadworm infections. This, they showed, resulted from suppression of immune responses that normally control the threadworm by the HTLV-I virus.
FLATWORM INFECTIONS OF TILAPIA
The main obstacle to the success of seawater culture of tilapia - called fresh water snapper - is a parasite called neobenedenia which attaches to the skin of the fish to feed on skin cells and mucus. They cause loss of fish scales, puncture the fish tissues, killing it within a few weeks. Ms. Nicola O'Connor, a graduate student in the Department of Life Sciences, studied infected tilapia to determine whether they develop an immune response that would protect them from neobenedenia. She found that tilapia could, indeed, develop resistance to the parasite, but the problem was that this lasted only a few weeks - not long enough for survival of the fish to harvest time. She noted further that the fish responded to the infection primarily by increasing mucus production, and experiments have shown that the mucus of infected fish contains a protective chemical which provides resistance to the parasite. The nature of this chemical is currently under investigation in collaboration with the Mona Institute of Applied Sciences (MIAS). The parasite is controlled, otherwise, by exposing fish to minute doses of formalin which washes off and kills the parasites and leaves healthy fish.
RAT LUNG WORMS
Angiostrongylus occurs not in the lungs of rats as the common name "rat lung worm" suggests, but rather in the blood in the pulmonary artery close to the heart. Several types of land snails and slugs, and freshwater shrimps, also carry the parasite, and rats are infected by eating them.
The importance of Angiostrongylus lies in the fact that it is the most common cause of eosinophilic meningoencephalitis (meningitis) in humans worldwide. Ms. Cecelia Waugh, a Ph.D. student in the Department of Life Sciences, has studied this parasite for the past four years, following an outbreak of infection among American medical students vacationing in Jamaica in 2000.
Problems arise when humans eat raw or undercooked land snails or slugs that could be infected with the tiny larvae of the parasite. People can also become infected by eating unwashed raw vegetables over which snails have crawled and deposited infective parasite larvae with their slime. Freshwater shrimps also carry the parasites and, if eaten raw, can infect people. A few days after ingestion, the parasite larvae lodge close to the brain and often cause severe illness. Several fatalities have been recorded in Jamaica.
Our investigations revealed that 30 per cent of wild rats in Jamaica carry angiostrongylus in their blood and several species of land snails and slugs are also infected. While most people in Jamaica do not eat raw land snails or slugs some, particularly men, admit to having raw freshwater and saltwater shrimp, as well as mussels directly from rivers and streams. Freshwater and saltwater bait shrimp are also eaten by fishermen.
Notwithstanding the seriousness of the infection, prevention is simple: people should refrain from eating raw snails, slugs or shrimp and ensure that vegetables are well-washed before consumption.