Arboviruses: (AR-thropod Bo-rne Viruses) Definitive Arthropod host Both Hosts become infected Some viruses very specific in host-vector relationship Some very generalist A. Cyclopropagative Transmission: The parasite undergoes cyclical changes and multiplies within the vector, i.e., there are both developmental changes and multiplication of the parasite. B. Cyclodevelopmental Transmission: The parasite undergoes cyclical changes within the vector but does not multiply, i.e., there are only developmental changes of the parasite without multiplication. C. Propagative Transmission: The parasite multiplies within the vector without any cyclical changes, i.e., the parasite increases in number within the vector but does not undergo any developmental changes. Transmission by Insect vectors Insect ingested (double pored tapeworm) 1) As insect bloodfeeds, parasites in the salivary glands injected into the host along with the saliva (Plasmodium, Arboviruses). 2) As insect bloodfeeds, parasites in the mouthparts recognize the host as suitable and forcibly exit the mouthparts and enter the host (filarial nematodes). 3) As insect feeds in engorges and defecates, parasites in feces enter host (Chagas disease) 4) As insect feeds it regurgitates compounds from its crop and the parasites enter the host (Plague bacterium) 5) Parasites in 3 and 4 remain in the GI tract and do not come in contact with the tissues that are important in the invertebrate immune response to these parasites. Arboviruses: Not a taxonomic grouping- many families of viruses included Epidemiologically related, not phylogenetically related Why study arboviruses 1) Human health 2) Wildlife health 3) Spread wildlife-humans/domestic animals Arboviruses • Infect two physiologically different hosts: – invertebrate vector – persistent infections; few cytopathic effects – vertebrate host – high titred infections and massive cytopathic effects. • Arboviruses cause no measurable detrimental effects on insect vectors Yellow Fever Exists in Tropical areas – Central America- in primates Vectors traditionally are insects of the genus Hemogogus Mosquitoes feed on primates in treetops- cycle- no problem. If virus enters spider monkeys- can be lethal=indication of problem Humans enter picture- but we do not spend significant periods in the treetops So how does it enter humans? Yellow fever is a mosquito-borne infection characterized by hepatic, renal, and myocardial injury, bleeding, and a high case-fatality rate. The disease occurs in tropical South America and sub-Saharan Africa. Maintained by zoonotic transmission between sylvatic mosquitoes and nonhuman primates, the virus cannot be eradicated, and prevention of human infection requires high vaccination coverage. Recently the incidence of yellow fever has increased dramatically. The true incidence is believed to far exceed the reported incidence. Yellow fever remains a public health problem because of failure to implement effective immunization, particularly in Africa. The recent upsurge in yellow fever activity has been associated with an increased risk for infection among non-immunized travelers. In 1996, two tourists infected in the Amazon region of Brazil died after returning home. Fatal and other cases in tourists were also reported in previous years. Other cases were likely missed, misdiagnosed as viral hepatitis, treated abroad, or not reported. Symptoms: incubation period of three to six days. There are then two disease phases. While some infections have no symptoms whatsoever, the first, "acute", phase is normally characterized by fever, muscle pain (with prominent backache), headache, shivers, loss of appetite, nausea and/or vomiting. After three to four days most patients improve and their symptoms disappear. However, 15% enter a "toxic phase" within 24 hours. Fever reappears and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes and/or stomach. Once this happens, blood appears in the vomit and faeces. Kidney function deteriorates; this can range from abnormal protein levels in the urine to complete kidney failure with no urine production. Half of the patients in the "toxic phase" die within 10-14 days. The remainder recover without significant organ damage. Yellow fever is difficult to recognize, especially during the early stages. It can easily be confused with malaria, typhoid, rickettsial diseases, haemorrhagic viral fevers (e.g. Lassa), arboviral infections (e.g. dengue), leptospirosis, viral hepatitis and poisoning (e.g. carbon tetrachloride). Sylvatic (or jungle) yellow fever: In tropical rainforests, yellow fever occurs in monkeys that are infected by wild mosquitoes. The infected monkeys can then pass the virus onto other mosquitoes that feed on them. These infected wild mosquitoes bite humans entering the forest resulting in sporadic cases of yellow fever. The majority of cases are young men working in the forest (logging, etc). On occasion, the virus spreads beyond the affected individual. Intermediate yellow fever: In humid or semi-humid savannahs of Africa, small-scale epidemics occur. These behave differently from urban epidemics; many separate villages in an area suffer cases simultaneously, but fewer people die from infection. Semi-domestic mosquitoes bite both monkey and human hosts. This area is often called the "zone of emergence", where increased contact between man and infected mosquito leads to disease. This is the most common type of outbreak seen in recent decades in Africa. It can shift to a more severe urban-type epidemic if the infection is carried into a suitable environment (with the presence of domestic mosquitoes and unvaccinated humans). Urban yellow fever: Large epidemics can occur when migrants introduce the virus into areas with high human population density. Domestic mosquitoes (of one species, Aedes aegypti) carry the virus from person to person; no monkeys are involved in transmission. These outbreaks tend to spread outwards from one source to cover a wide area. Humans cut down forest- increase contact between humans/Hemogogus Vectors brought into contact with humans Infected primates brought into contact with other insects including Aedes aegypti Aedes aegypti loves humans In the 60’s eradication programs were started to eradicate Ae. aegypti Very successful- now returned to many areas Jungle cycle: transmission in primates/few humans by Hemogogus/ Ae. aegypti Urban cycle: humans and Ae. aegypti Once cases show up- health authorities enter with vaccines Yellow fever outbreak reported in Paraguay Health officials reported an outbreak of yellow fever in Paraguay, with seven confirmed cases in San Pedro and four as yet unconfirmed cases in San Lorenzo. In addition, the number of cases reported by neighboring Brazilian health authorities has more than quadrupled during the past two months, with 13 deaths reported. Yellow fever is a viral disease transmitted by mosquitoes. Although a vaccine exists, there is no cure for the disease that infects about 200,000 people annually. "As with malaria, the yellow fever outbreaks highlight the urgent need for carefully controlled insecticide spraying programs," said Richard Tren, director of Africa Fighting Malaria. "These programs should have been strengthened to sustain progress. "Decades of anti-insecticides pressure culminated in 1997 when the World Health Assembly passed a resolution to reduce the use of insecticides in disease control," added Tren. "The resurgence of yellow fever is an unfortunate consequence of that resolution.“ The World Health Organization said it has sent 4 million doses of vaccine to Paraguay, along with an epidemiologist, virologist and other emergency management experts. Yellow Fever Trivia In 1793, yellow fever ravaged Philadelphia, killing 10% of its residents. The British lost 20,000 of 27,000 in 1741 in Mexico, French lost 29,0000 of 33,000 trying to acquire Haiti and Mississippi valley Yellow fever killed 20,000 people in New Orleans and Mississippi in 1853, and another 20,000 there in 1878, when the epidemic ran as far north as Memphis. May have led France to negotiate to give up the Louisiana Purchase Yellow fever and malaria forced France to abandon completion of the Panama canal, William Gorgas developed mosquito control program in Cuba- copied in Panama In the 20th century, US and Canada vanquished malaria and yellow fever with a combination of quarantine, medicine, hygiene, and chemical pesticides. DDT made the biggest difference. But the mosquito always returns, often carrying new diseases. Arboviruses: Dengue virus • Geographically wide spread arbovirus • 2.5 billion people at risk • 50-100 million new infections annually • ~500,000 cases of DHF • No vaccine, no drugs Dengue and dengue hemorrhagic fever (DHF) are caused by one of four closely related, but antigenically distinct, virus serotypes (DEN-1, DEN-2, DEN-3, and DEN-4). Infection with one of these serotypes does not provide cross- protective immunity, so persons living in a dengue-endemic area can have four dengue infections during their lifetimes. Dengue is primarily a disease of the tropics, and the viruses that cause it are maintained in a cycle that involves humans and Aedes aegypti, a domestic, day- biting mosquito that prefers to feed on humans. Infection with dengue viruses produces a spectrum of clinical illness ranging from a nonspecific viral syndrome to severe and fatal hemorrhagic disease. Important risk factors for DHF