被致命的人类寄生虫感染的狗闻起来更容易成为昆虫的载体 

New research suggests female sand flies that pass the protozoa that causes visceral leishmaniasis to humans are attracted to affected canines

新的研究表明，通过原生动物导致人类内脏产生利什曼原虫病的雌性沙蝇被（被）感染的犬类吸引

Brazil is home to some 52 million dogs, most of whom are well-loved pets, and  many of whom are hosts to Leishmania infantum, a parasite that is incredibly harmful to humans. Hundreds of Brazilians die every year when they are bitten by sand flies that obtained the parasite from dogs. Visceral leishmaniasis, the disease the parasite causes, leads to cutaneous sores on the skin, infections of organs including the spleen and liver and sometimes death.

巴西有近5200万只狗，其中大部分是宠物狗，这些狗里面很多都是利什曼原虫幼虫的宿主，这种寄生虫对人类极其有害。每年有很多的巴西人被沙蝇叮咬致死，这些沙蝇正是从狗身上获得的寄生虫。利什曼原虫病是寄生虫引起的疾病，该病导致皮肤生疮，使得包括脾脏，肝脏在内的器官感染，而且有时会致死。

Now, research published today in PLOS Pathogens reports that dogs infected with the parasite may smell more attractive to female sand flies, suggesting L. infantum could be manipulating its hosts as it travels from one to the next. Experts say the results could have implications for controlling the often-deadly tropical disease.

今天发表在PLOS Pathogens的研究报告上提到，被寄生虫感染的狗对于雌性沙蝇来说更具吸引力，这表明利什曼原虫幼虫可以在从一个宿主到另一个宿主的过程中操纵它的宿主。专家表示，这些结果可能对控制这种经常致命的热带病有作用。

Visceral leishmaniasis is thought to kill between 20,000 to 40,000 people a year, says Christine Petersen, the director of the Center for Emerging Infectious Diseases at the University of Iowa, who was not involved in the new study. At least 80 percent of South American cases occur in Brazil, where leishmaniasis has been spreading since the 1980s, most likely carried by people and their pets as they migrated from rural to urban areas. Brazil’s favelas—crowded, low-income urban settlements often neglected by authorities—as well as poor areas of India and East Africa are hotbeds for the disease. “The people who are at the most risk of this disease are really in the most impoverished parts of the world,” Petersen says.

美国爱荷华大学新兴传染病中心主任Christine Petersen认为内脏利什曼病每年导致20，000至40，000人死亡，但他并未参与这项新研究。南美至少有80％的病例发生在巴西，那里的利什曼原虫病从20世纪80年代以来一直在蔓延，很有可能是人们以及他们的宠物在从农村迁移至城市的过程中携带而来的。巴西的贫民窟——这个经常被当局忽视的拥挤而又低收入的城市住区——和印度和东非地区一样，是利什曼原虫病的温床。Petersen说：“患这种疾病风险最高的人们实际上在世界上最贫穷的地区。”

Gordon Hamilton, a medical entomologist at Lancaster University who co-authored the study, published a study in Scientific Reports in 2017 showing that in laboratory settings, female sand flies were more attracted to hamsters infected with L. infantum. About half of the rodents developed an odor that was more attractive to female sand flies. But hamsters, which are a model organism widely used in laboratory experiments, are not a natural host of the parasite. “It’s more relevant to [study] dogs, which the parasite has evolved along with,” Hamilton says.

兰卡斯特大学的医学昆虫学家Gordon Hamilton是该研究的共同作者，在2017年的《科学报告》中发表了一项研究表明在实验室环境中，雌沙蝇会更容易被感染婴儿利什曼原虫的仓鼠吸引。大约一半的啮齿动物产生了一种对雌性沙蝇更具吸引力的气味。但是这些仓鼠是实验室中被广泛用作实验的模式生物，并不是寄生虫的天然宿主。Hamilton说：“它与[研究]狗更相关，因为寄生虫是与之一同进化的。”

In the new study, the researchers collected hair and blood samples from 133 infected and healthy dogs in the Brazilian city of Governador Valadares, located in an industrialized area where visceral leishmaniasis is common. The scientists isolated volatile organic chemicals—the molecules that convey scent—from 30 of the hair samples, half of which were from infected dogs, which they then exposed to male and female sand flies in a laboratory. The flies were given a choice between approaching the scent molecules from infected or healthy dogs. While the male flies showed little preference between the odors, females chose the infected dogs’ odor almost twice as often as healthy dogs’. “Every single infected dog we tested was more attractive to females than the uninfected dogs,” Hamilton says.

在这项新研究中，研究员从巴西Governador Valadares市的133只被感染的以及健康的狗中收集了毛发和血液样本，该市位于利什曼原虫病常见的工业地区。科学家从30个毛发样本中分离出挥发性有机化学物质（传递气味的分子），其中一半来自被感染的狗，然后将这些化学物质释放在实验室的雌雄沙蝇中。雄性沙蝇对于这种气味只表现出一点偏爱，而雌沙蝇选择被感染狗的气味的的几率几乎是健康的狗的两倍。

The evidence that female sand flies are more attracted to infected dogs than male flies is important, says Hamilton, because females are the ones who feed on the blood of dogs and humans and transmit the parasite. Male flies do not.

Hamilton说，雌沙蝇比雄沙蝇对被感染的狗更有吸引力的证据是很重要的，因为雌性是以人与狗的血液为食并传播寄生虫的，而雄沙蝇则不是这样。

“This is the first study to confirm that odor of infected dogs is significantly more attractive to female sand flies than to male sand flies,” says Filipe Dantas-Torres, a veterinary parasitologist at the Aggeu Magalhães Institute in Recife, Brazil, who was not involved in the study.

巴西累西腓市Aggeu Magalhães研究所的兽医寄生虫学家Filipe Dantas-Torress说，“这是第一个证实被感染的狗的气味对雌沙蝇比对雄沙蝇更具吸引力的研究。"，他没有参与这个研究。

Dantas-Torres says that while dogs are the primary hosts for L. infantum, the parasite can infect a wide range of hosts, including livestock and cats, which could also serve as reservoirs for the disease. Parasites often have a life-cycle that involves multiple host organisms—such parasites cause most zoonotic and emerging infectious diseases—and some of these can manipulate their hosts to help ensure they are passed on to the next unfortunate victim. Such manipulation can affect host behavior in bizarre ways. For example, the lancet fluke hijacks the nervous system of ants to ensure the insects make themselves vulnerable to predators so they are eaten. And the Plasmodium parasites that cause malaria make their human hosts smell more attractive to biting insects, who pass them to the next host.

Dantas-Torres表示，狗是利什曼原虫幼虫的主要宿主，该寄生虫可以感染许多种宿主，包括家畜和猫，他们也可以作为该病的宿主。该寄生虫通常具有涉及多种宿主生物的生命周期——这种寄生虫会导致大多数人畜共患传染病以及新兴传染病——并且其中的一些可以操纵它们的宿主以助于确保它们传播到下一个不幸的受害者身上。这种操作会以奇怪的方式影响宿主的行为。比如，柳叶吸虫劫持了蚂蚁的神经系统以确保这些昆虫更容易被食肉动物吃掉。引发疟疾的疟原虫使它们人类宿主的气味对于叮咬类的昆虫更具吸引力，昆虫会把它们带给下一个宿主。

One limitation of the recent study is that the dogs included were mixed-breed. Dantas-Torres says that some dog breeds may attract different biting insects more than others, and the study’s reliance on mutts could have missed this. Petersen says it is also possible the parasite was not directly manipulating the dogs’ odor; infected dogs might smell more attractive to sand flies because as the disease progresses, the dogs can develop renal complications, which can be malodorous. But Hamilton says that regardless of parasite load or observed symptoms, infected dogs were significantly more attractive to the female sand flies than uninfected dogs. Even dogs that had low levels of parasite infection still had a change in their smell that attracted female sand flies.

最近研究的一个局限性是狗包括混种狗。Dantas-Torres提到，某些种类的狗相对于其他的犬种可能更吸引不同的叮咬昆虫，而该研究对杂种狗的依赖可能错过这一点。Petersen表示，寄生虫也可能没有直接操纵狗的气味；被感染的狗可能闻起来对沙蝇更有吸引力是因为随着病情发展，狗会出现肾脏并发症，这也是恶臭的。但是Hamilton说，不论是从寄生虫负载量还是观察到的症状来看，被感染的狗明显比未感染的狗更对雌沙蝇有吸引力。甚至寄生虫感染水平较低的狗也仍会改变它们的气味从而吸引雌沙蝇。

Both dogs and L. infantum were brought to South America from the Mediterranean basin by European colonizers. In Europe, the parasite is carried by a species of sand fly that is not as effective of a vector as the one in Brazil, but scientists don’t know why, Hamilton says.

狗和婴儿利什曼原虫都是由来自地中海盆地的欧洲殖民者带来南美洲的。Hamilton说到，在欧洲，这种寄生虫被一种沙蝇所携带，但传播不像在巴西一样有效，专家不知道其中原因。

Dantas-Torres says that understanding such host-parasite-vector interactions is fundamental to designing more comprehensive approaches to control parasites such as Leishmania protozoa. Canine infections are largely controlled with topical repellents such as insecticide-laced collars, with vaccinations being a second choice. Petersen says that vaccinations could help prevent dogs from contracting the disease the parasites cause, but they wouldn’t prevent them from carrying L. infantum and potentially passing it on to humans via sand flies. In the highest-impacted areas in Brazil, infected dogs are sometimes culled to control the spread of disease.

Dantas-Torres说，理解这种宿主-寄生虫-载体的相互作用是设计更全面的控制像利什曼原虫这样的寄生虫方法的根本。犬类感染可以通过局部驱虫剂得到很大程度的控制，比如带杀虫剂的项圈。接种疫苗是第二选择。Petersen说，接种疫苗可以帮助防止狗感染寄生虫引起的疾病，但是疫苗不能防止狗携带利什曼原虫幼虫并且可能通过沙蝇将其传给人类。在巴西受灾最严重的区域内，被感染的狗有时被清除以控制疾病的传播。

Hamilton says the new study’s findings could present new avenues of infection control for L. infantum. He is currently involved in a project that uses pheromones to attract sand flies to areas where they can be killed. “If we can identify what the chemicals are that make infected dogs more attractive, we can potentially combine that with the synthetic pheromone to make it even better,” he says.

Hamilton说，这项新的研究发现可能为控制利什曼原虫幼虫感染提供了新的途径。他正在参加一个项目，该项目使用信息素吸引沙蝇去它们的死地。“如果我们可以验证什么化学物质可以使被感染的狗更具吸引力，我们就可能将其与合成信息素结合使它甚至更好，”他说。

 —— 本文译自史密森尼杂志，仅用于学习和交流目的