Puppy Dog Tales

Breaking the transmission of the tritryps

Why washing dogs and treating body odour might help halt the spread of the tritryps.

Trypanosomes and Leishmania have evolved many cunning mechanisms to ensure they survive and proliferate. They are passed to humans through an insect vector, which has acquired the parasite by feeding on an infected mammalian host – either another human or a wild or domestic animal, such as an armadillo, raccoon, rodent, dog or cat. They also have a complicated life cycle, undergoing several different stages both in the insect vector, and in humans and animals.

A number of Wellcome-funded researchers are studying the ways in which these parasites are transmitted and the ways in which they defeat the mammalian immune system. The ultimate aim is to find new ways to control the disease and break its transmission in the real world.

Leishmania

Humans become infected with Leishmania when bitten by infected sand flies – tiny sand-coloured blood-feeding flies that breed in forest areas, caves, or the burrows of small rodents. If infected, the sand fly transfers around 1000 parasites into the human during its blood meal.

At the Liverpool School of Tropical Medicine, Dr Paul Bates is investigating precisely how the parasite is transmitted from the sand fly to the human, and what happens afterwards.

He and his colleagues have found that a gel produced by the parasite in the gut of the sand fly prevents the insect from feeding properly. This causes it to make more efforts to feed – thus providing more chances for transmission of the parasite.

Dr Bates and colleagues also discovered that the gel is injected into the human with the parasite – and increases the severity of the infection. The crucial factor is a molecule in the gel, called filamentous proteophosphoglycan (fPPG), which interferes with the human immune system.

An important aspect of the immune system is the balance between two arms of the 'T-helper' response. Broadly speaking, the T-helper1 (Th1) response is tailored to intracellular pathogens, such as viruses and some bacteria and parasites. Because these organisms live inside cells, they are not accessible to antibodies, so the Th1 response stimulates other defence mechanisms such as macrophages. The T-helper2 (Th2) system, by contrast, promotes a vigorous antibody response. The two arms are antagonistic, so a Th1 response means a weak Th2 response and vice versa.

"You need a strong T-helper1 response to kill the parasite," explains Dr Bates. "The T-helper2 arm of the immune system can't kill the parasite, leading to uncontrolled disease. The gel pushes the immune response to the non-protective T-helper2 arm rather than the T-helper1 arm. So this parasite is a clever little beastie: it manipulates the sand fly with the blockage to make it feed more, and it manipulates the host`s immune system so that it can spread unchecked."

Dr Bates is now aiming to understand the mechanism in greater detail, with the ultimate aim of developing a vaccine to block the action of the fPPG.

New leads

Across the Atlantic, in the Amazon, attempts to control leishmaniasis have been based on culls of infected domestic dogs, which are a major reservoir host of the parasite. Ironically, points out Dr Orin Courtenay from the University of Warwick, the culling policy may actually be driving the infection.

"For a start, dog owners whose dogs have been culled simply go out and get a new puppy, which is more susceptible to the disease than an adult dog. Then there's the problem of the fact you've only got a few local health networks in Brazil – and it's a big place – so accessibility to houses is a problem. Health workers have to go to the house, take a blood sample from the dog, and send it away for testing. By the time they revisit the house it could be three or six months down the line – by which time the dog will be highly infectious."

Dr Courtenay is looking instead at the possibility of dipping dogs in an insecticide, the pyrethroid deltamethrin (DM), to prevent sand flies from biting them. "Collars impregnated with DM on a slow-release mechanism have been found to be effective in Iran," he says. "But they only last six months, so you have to buy at least two a year. And they're made by commercial companies, so they're too expensive for the local communities. Dipping dogs in the insecticide would be a twentieth of the price."

Dr Courtenay aims to conduct a community-based randomised control trial of around 900 dogs to test the effects of dipping. "I'll look at the mortality rate of sand flies feeding from the dogs after dipping, and whether the insecticide prevents them from feeding. I'll also check for adverse effects on the dogs and conduct public health studies to find out whether dipping local dogs reduces infection in children in the villages." He also plans to find out how long the dipping remains effective, and whether sticking agents help the DM to withstand the Amazon's heavy rainfall.

If the trial indicates that dipping is successful, a follow-on study would look at ways to ensure it becomes a regular activity. "You need to encourage, teach and monitor communities to make sure they are carrying out the intervention properly. That's par for the course when there's no other control for a disease."

Beating the bugs

Chagas' disease (American trypanosomiasis), caused by Trypanosoma cruzi, is the most important cause of heart disease in Latin America. Around 16–18 million people are infected with Chagas' disease, of whom at least 20 000 will die each year.

The early stage of infection is usually mild. Eight to ten weeks after infection, a latent, asymptomatic phase begins, which may last for years. In about one-third of people who get the infection, chronic symptoms develop after 10–20 years. In this phase, the parasite destroys the smooth muscle in the heart and intestines, leading to a huge thickening of these organs because the muscle has to be huge to work; the cells are shattered by the parasite.

The parasite is transmitted by triatomine or 'kissing' bugs which live in wall cracks, thatch roofs and palm-thatched mud outhouses or animal shelters. In the southern cone of Latin America, massive spraying programmes have successfully controlled the triatomine bug. Dr Clive Davies of the London School of Hygiene and Tropical Medicine is looking at the possibility of replicating control of the vector in Colombia.

Since the northern countries of Latin America are poorer and lack the resources for national spraying campaigns, the Colombian government recently conducted a national survey of Chagas' disease risk, looking at the distribution of bugs and infection rates in school children, and identified municipalities that were a priority for control.

However, since the survey involved sampling a few villages, and using those to estimate the average risk of an entire municipality, the data do not allow precise targeting of specific villages at risk. Dr Davies is therefore aiming to develop a computer-based predictive map at village level, to help local practitioners to decide where, with their limited resources, they should carry out sampling and spraying.

"I'll use environmental data, such as satellite images of land cover and meteorological maps, and census data and socioeconomic measures like house design (mud or brick) to predict risk," he explains. "I'll also be measuring the effects of different policies of sampling and spraying."

The northern countries of Latin America face another obstacle. While the triatomine bugs in the south are purely domestic, the vector in Venezuela and Colombia is Rhodnius prolixus – a bug that lives not only in people`s homes, but also in nearby palm trees. As a result, there is a chance that bugs from palms might reinfest houses – and this is something else Dr Davies wants to monitor. "I want to measure reinfestation rates and find out whether some bugs survived the spraying, whether they've come from another house – or whether they've come from the palm trees outside."

Such work could be of long-term benefit. "The challenge here is to sustain the control effort and find a cheap and effective way of staying vigilant for reinvasion once you've eradicated the bugs. Unfortunately, once there are no bugs left, or very few of them, the government won't want to 'waste' further money on sampling programmes – but we need to check for the possible return of the bugs."

Supporting Dr Davies's epidemiological studies, Dr Michael Gaunt, also from the London School, is investigating the genetics of Rhodnius prolixus vectors in palm trees and the domestic environment. His studies suggest that the insect in the houses is a sub-species of the one in the palms, which has implications for the effectiveness of spraying.

Aided by the new genome sequences, Dr Gaunt is also looking at genetic exchange and recombination between trypanosomes. Interestingly, he notes that unlike the offspring of other organisms, which receive half their DNA from each parent, the progeny of T. cruzi receive a full genome from both parents. The duplication is then followed by 'genome erosion' – another area Dr Gaunt aims to explore. "We don't know the rate of genetic erosion. Is it sudden loss from each? Or is random DNA lost over time?"

Genome duplication is likely to be the mechanism leading to the huge biodiversity among trypanosomes. "Trypanosomes are the most biodiverse group of organisms on earth," explains Dr Gaunt.

I could feel it creeping up my vein…

Most tourists don’t get Leishmania, says Dr Richard Reithinger, an infectious disease consultant in Virginia, because only a minority of sand flies are infected – usually below one per cent. It’s mainly people who live in the countries and are exposed to it all the time. But you get a few backpackers trickling back with leishmaniasis each year, because they were in the wrong place at the wrong time and got bitten.

He should know – he was one of them. I was doing field work on Leishmania in Brazil so I was catching sand flies in light traps at night.

I obviously got bitten by an infected one because I got the classical clinical picture of cutaneous leishmaniasis: two little red nodules, like a mosquito bite, which then ulcerated and oozed.

On his return to the UK (at the time he was at the London School of Hygiene and Tropical Medicine), Dr Reithinger went to the Hospital of Tropical Disease in Gower Street for treatment.

In countries where Leishmania is endemic, you generally are an out-patient. The treatment is injected intramuscularly or straight into the lesion. Depending on the treatment protocol, you get up to 21–28 shots on consecutive days. But in the UK, you are registered as an inpatient and you get 28 days of IV treatment.

The medication, sodium sibogluconate, is a heavy metal derivative and can be toxic. I could feel it creeping up my vein, Dr Reithinger remembers. I got aching in all my bones, throughout my body, like flu, which faded over the day as the sodium sibogluconate got excreted. You also get headaches and your liver enzymes shoot up. IV patients have to be monitored for heart rhythm and liver enzymes.

He attracted a lot of attention from hospital staff, who rarely see cases of cutaneous leishmaniasis. I got loads of visits from all the hospital staff – registrars and medical students. It was like being an animal in a zoo.

Luckily, after 28 days, the lesions had closed. Only small crevices remain, rather than the disfiguring scars that many people in endemic countries end up with. And there’s one advantage. If you’ve had Leishmania once, you may be immune for life. says Dr Reithinger. So he can go back to catching sand flies without trepidation.

Scents for sand flies

There seems no end to the artfulness of the Leishmania parasite in ensuring that it can thrive and spread. At the University of Keele,

Dr Eduardo Rebollar-Tellez and Professor Richard Ward have found that hamsters infected with Leishmania emit odours that makes them more attractive to sand flies – and thus more likely to get bitten and infected.

Now at the University of Yucatan in Mexico, Dr Rebollar-Tellez is collecting odours from infected and uninfected mammals in Mexico, where Leishmania is endemic, and sending them back to Professor Ward and his colleague Dr Gordon Hamilton in Keele. Since the odours are highly volatile, Professor Ward and Dr Hamilton are using gas chromatography to analyse and compare their compositions.

We want to identify exactly which odours are attracting the sand flies, discover their composition, and find the chemicals responsible, explains Dr Rebollar-Tellez. Then we might be able to develop a new trap to use in the field to control sand flies.

Image: dog with visceral leishmaniasis, courtesy of O Courtenay/IHIC.

Puppy Dog Tales

Breaking the transmission of the tritryps

Leishmania

New leads

Beating the bugs

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