I love this picture of a female lone start tick and her enormous egg mass not just because of the striking imagery, but because it confronts the viewer with a major difficulty commonly encountered in tick and tick-borne disease control – reducing the dangers of a relatively small but fast reproducing species capable of stealthy and lethal infection. Many public health and entomology initiatives focus on mosquitoes but I’ve always found ticks to be the more fascinating arthropod. They’ve got that essential “creepy” factor but they’re also prodigious reproducers and can transmit a slew of truly nasty diseases. Renewed interest for public health practitioners in these creatures stems from the growing problem of habitat change that is resulting in increasing numbers of habitats and climates well-suited to ticks.
There are 20 species within six genera (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes and Rhipicephalus) of hard, or ixodid, ticks that are responsible for the majority of medical complaints and tick-borne diseases (TBD). Ixodid ticks are graced with the characteristic tough dorsal shield and a four stage life cycle, consisting of the egg, larva, nymph and adult. They’re responsible for an astonishing number of diseases – lyme disease, tularemia, Rocky Mountain Spotted Fever (RMSF), tick-borne encephalitis, Q-fever, Omsk hemorrhagic fever, Crimean-Congo hemorrhagic fever, Colorado tick fever, Texas Cattle fever, African relapsing fever, ehrlichiosis and anaplasmosis. Aside from the sheer number, they’re capable of transmitting the gamut of infectious disease, from protozoan to viral to bacterial.
Indeed, ixodid ticks are considered to have high “vector potential” due to several fortuitous characteristics of their biology and life cycle and are considered to be one of the few arthropods of medical importance, as well as mosquitos, mites and biting flies. Ticks require blood-feeding once during each stage of their cycle and these persistent, slow feeders can feed over several days, giving ample opportunity for pathogen transfer between the tick and its host and vice versa. They’re also fairly laissez faire, easy-going vectors. They have a wide host range, being happy to feed on nearly any vertebrate, from birds to rodents to humans, that they may latch onto while “questing” from nearby vegetation. Indeed, a single tick can feed on three separate hosts over the course of its lifespan, swapping zoonotic diseases willy-nilly amongst disparate species (a).
To boot, they’ve got superb reproductive potential that I mentioned earlier – ticks can produce 3000-4000 eggs following a single host feeding. With the capacity of some species to transovarially transmit disease to their offspring, a single tick can church out thousands of disease vectors. Detecting larva and nymphs that can be as small as a pinhead further compounds the difficulties of the prevention and care of tick bites and TBDs.
One particular disease that is on the rise globally is a single-stranded RNA virus in the Bunyaviridiae family, Crimean-Congo hemorrhagic fever (CCHF), and it has a neat “Hello, World!” story. Reports of a fatal hemorrhagic fever possibly spread by a tick or louse had been circulating in the Central Asian region for centuries, ominously named by the indigenous Uzbeks as khungribta (blood taking), khunymuny (nose bleeding), or karakhalak (black death)(b). A description of the disease by a Tadzhikistan physician written in the 12th century outlines such unfortunate symptoms as blood in the urine, vomit, rectum, gums as well as in the abdominal cavity (b).
It was only until a large epidemic in 1945 during the occupation of the Crimea during World War II that the disease was officially identified by Soviet authorities. Scores of agricultural and hare-hunting fields had been abandoned during the war effort and an ideal tick habitat of weeds and European hares flourished. The population of ticks in the Hyalomma genus exploded, with the larvae feeding on the hares and birds while the adults sought larger mammalian targets, in the form of livestock and wild boar (c). When Soviet troops subsequently occupied the region and tilled the land in late 1944, an epidemic of CCHF exploded with over 200 cases and a 10% case fatality rate (d).
An antigenically-identical isolate from a child patient in the Congo in 1953 is responsible for the disease’s portmanteau name. Indeed, CCHF is quite the international traveler and its geographic range is considered to be the most extensive of any of the tick-borne viruses. Human and animal clinical cases, serological antibodies and viral isolates have been found in nearly 30 countries throughout the former Soviet Union, Africa, China, Southeastern Europe and the Middle East (c). The majority of cases are amongst agricultural and husbandry workers but the clinical picture is complicated by the fact that the disease has multiple modes of transmission, capable of infecting those via contact with bodily fluids from infected patients or livestock (c). As such, CCHF now has the ignominious distinction as the “Asian Ebola” owing to its clinical symptoms and it’s rather high 30% case fatality rate (b)(c).
Climate change and urbanization predictably play a large role in the epidemiology of tick-borne diseases (TBD) and CCHF is no exception. CCHF arrived in Turkey in 2002 and the number of confirmed cases reached to over 700 within five years (d). Researchers are unsure as to the cause of its emergence in this previously unaffected country but point to a spike in warm temperatures in 1998 along with steep decreases in agricultural activity in the Anatolian region due to terrorist activity (d). Scores of fallow fields and a resurgence of the hare and wild pig population have been attributed to the Hyalomma population explosion in the region (d).
Research in the United States focusing on Lyme disease, tularemia and Rocky Mountain Spotted Fever (RMSF) further indicates that increased temperatures and urban development over the past two decades have led to not insignificant increases in clinical incidence and geographical distribution of these diseases (f). The ignominious history and recent epidemic of CCHF in Turkey is just one more reminder that climate change has serious implications for the health of communities. It’s not just getting warmer out here, but enhancing climatic and environmental conditions for dangerous insects and diseases.
For more creepy-crawly info on ticks and the charming diseases that they transmit, please visit these excellent resources.
Emed has a great page on ticks, TBD and prevention tips: http://emedicine.medscape.com/article/786652-overview
Microbiology and Immunology online also has a nice section on tick-borne diseases: http://pathmicro.med.sc.edu/parasitology/ticks.htm
The masters of public health information, the WHO, have a nice page on CCHF: http://www.who.int/mediacentre/factsheets/fs208/en/
(a) Randolph SE. (1998) Ticks are not Insects: Consequences of Contrasting Vector Biology for Transmission Potential. Parasitol Today. 14(5):186-92
(b) Whitehouse CA. (2004) Crimean–Congo hemorrhagic fever. Antiviral Res. 64(3):145-60
(c) Onder E & Whitehouse CA. Crimean-Congo Hemorrhagic Fever: A Global Perspective (Google eBook). Springer, 2007
(d) Dilber E, Cakir M, Acar EA, Orhan F, Yaris N, Bahat E, Okten A, Erduran E. (2009) Crimean-Congo hemorrhagic fever among children in north-eastern Turkey. Ann Trop Paediatr. 29(1):23-8.
(e) Gubler DJ. (1998) Resurgent vector-borne diseases as a global health problem. Emerg Infect Dis. 4(3):442-50.
Randolph, S., & Ergönül, ?. (2008). Crimean-Congo hemorrhagic fever: exceptional epidemic of viral hemorrhagic fever in Turkey Future Virology, 3 (4), 303-306 DOI: 10.2217/174607220.127.116.113