In the United States, disease-carrying ticks* appear to be extending their distributions northward and in some cases, climate warming is suspected to be involved. In mild winters such as that of 2012-13, ticks can build up populations and expand their range to areas where previously, low temperatures prevented overwintering survival and helped determine the northern limits of tick distributions. Higher temperatures tend to accelerate the life cycle of most ticks increasing egg production and population density, and decreasing generation times. Climate also may be influencing disease prevalence indirectly through effects on host animal survival and reproduction. Ticks, of course, are responsible for transmitting numerous human and animal diseases. Three of them are briefly reviewed here with emphasis on ecological aspects. However note that the US Centers for Disease Control and Prevention (CDC) discusses 11 tick-borne diseases occurring in the US while others occur elsewhere especially in the tropics. Medical aspects are only mentioned in passing here. There are many excellent websites describing the disease and associated symptoms, and recommended precautions to take if venturing into tick country.
It would be no more than a small exaggeration to say that over the past few decades, Lyme disease has appeared to explode across North America such that it is now arguably the commonest zoonotic disease on the continent. There are about 30,000 cases diagnosed annually in the US. However ongoing CDC studies suggest that this number may only be a tenth of the actual number! Lyme disease was first characterized in the early 1970s in Connecticut and the disease organism, the spirochete bacterium Borrelia burgdorferi, was identified in 1981.
There are many species of Borrelia found throughout the world. In Europe three species are implicated in Lyme disease including B. burgdorferi. Other Borrelia species cause relapsing-fever in which the bacteria are transmitted to humans either by body lice (principally in the tropics) or by ticks of the genus Ornithodoros (in the western US mountains, southern Europe and parts of Africa). In addition, a newly discovered tick-borne disorder which presents symptoms similar to Lyme disease but without the rash, has been found in three US residents. The bacterium involved is B. miyamoti, which was first detected in ticks from Japan in 1995 and in humans in 2011 in Russia. B. miyamoti has recently been confirmed to infect both the black-legged tick and the western black-legged tick in North America thus perhaps setting the stage for the spread of this illness in areas falling within the range of the tick vectors.
Lyme disease is long-established in Europe. In fact, recent analysis of the genome of Otzi, the 5300 year-old ice mummy discovered in the Italian Alps, revealed the presence of Borrelia genes (in fact, 60% of a whole Borrelia genome), in the remains. However, the disease may not have reached North America from the Old World. Lyme disease may have a longer history in the North America than originally thought given that it went long unrecognized as a distinct disorder, and it produces variable symptoms in humans, some of which are long-delayed. Although ticks were described by explorers and naturalists as common or even abundant in the northern US in the 17th and succeeding centuries, to what degree the ticks served as specific disease vectors is unknown.
Today Lyme disease is widespread and common in the northeastern (Virginia to Maine) and north-central (esp. Wisconsin and Minnesota) states, although still uncommon to rare elsewhere in the US. In the southeastern states, there have been only a relatively few reports of Lyme disease likely because the black-legged (deer) tick (Ixodes scapularis), the disease vector, has historically been uncommon south of central Virginia.
Of the three developmental stages of the tick (besides that of egg-laying), larva, nymph and adult, it is primarily the nymph that transmits the bacterium to humans. Its small size enables it to avoid detection and to feed on the human host for an extended period which increases the likelihood of bacterial transmission. An adequate meal stimulates the development of the nymph to an adult. A blood meal also is required by the larva to reach the nymph stage and by the female adult to lay eggs. The female tick overwinters as an engorged adult in the leaf litter, laying eggs the following spring. The male adult dies in the first year. The larval stage prefers small mammals and birds as hosts and the nymph and adults, larger as well as smaller mammals. If any of these hosts harbor the bacteria or serve as reservoirs for bacterial reproduction, there’s a good chance the tick will become infected and be able to transmit the bacteria to the next host. In areas of high Lyme disease occurrence, 50% of adult deer ticks and 25% of nymphs were found to be infected. Adult ticks prefer deer as hosts but will attack humans. However at this stage the ticks are large enough that most will be spotted by human hosts and killed. The adult is not likely to pass on spirochetes to the eggs so the newly hatched larvae will only participate in the Lyme disease cycle if they bite an already-infected host. This requirement helps somewhat to limit spread of the disease.
Among its many adaptive traits, B. burgdorferi (and likely other species in the genus) does not require iron as a nutrient as do nearly all other organisms. Instead, B. burgdorferi substitutes manganese for iron in its metabolism, enabling it to elude immune system responses of its hosts that fight off pathogens by starving them of iron.
In the past few years, Lyme disease-carrying black-legged ticks have been increasingly encountered in the southern states including Tennessee, North Carolina and Texas and even Florida. The southward spread of the disease may have more to do with vector dispersal via human agency and by habitat change than by climate. However, climate may help explain the spread of the black-legged tick in southern Canada where local areas in British Columbia, Manitoba, Ontario, Quebec, New Brunswick and Nova Scotia support endemic black-legged tick populations known to be infected with B. burgdorferi.
Within Lyme endemic zones in the northeastern US, deer ticks feed on at least 15 species of mammals and ground-nesting birds and also on lizards. Host species vary in their reactions to ticks. Opossums and squirrels are associated with a high mortality of deer ticks and thus qualify as ecological traps. In one study they killed 83-96% of ticks attempting to attach and feed on them. This can lead to a mortality of thousands of ticks per hectare. The native white-footed mice (Peromyscus leucopus), in contrast, are optimal hosts for vectors and pathogens due at least partly to their tolerance- they are far less aggressive towards attaching ticks, and to their ubiquity- they are found even in small forest fragments or woodlots where larger mammals are absent. Here the mice frequently are the dominant warm-blooded animals and tick density is high.
A number of studies show that as native animal species diversity decreases, disease risk for both tick-borne and other parasitic diseases including mosquito-borne West Nile Virus, and encephalitis increases. The mechanism governing this observation is not well understood. However it seems likely that in the case of Lyme disease, increasing diversity means moving from a community dominated by white-footed mice to one that includes at least some species that reduce tick survival through grooming. Therefore the tick load is dispersed among species varying in tolerance for the parasites, lowering tick and Borrelia density. In Florida, many black-legged ticks feed on the abundant populations of reptiles which are not reservoirs of the disease, thus helping to limit disease spread to warm-blooded organisms.
One factor explaining the rise in prevalence of Lyme disease is believed to be the large increase in white-tailed deer populations in the northern and central states in recent decades. Their numbers have increased from about 500,000 nationwide in the early 1900s to more than 15 million today.
White-tailed deer are viewed as “dead-end hosts” for the black-legged tick which gets its required blood meal from the deer and then drops off without seeking another host because the female tick is now physiologically ready to lay eggs. The eggs will hatch the following spring. Deer are readily attacked by adult ticks of both sexes and mating often takes place on the host deer. However, the deer show no symptoms of Lyme disease and cannot directly infect another host. Apparently the spirochete cannot build up populations in deer. Thus ticks attacking deer cannot transmit the bacterium to other hosts, but because the far more mobile deer can assist its tick parasite in getting around and can support many ticks, reducing the deer herd in a Lyme disease-plagued area is an important step in limiting the disease.
Other diseases are borne by ticks in the southeastern states. Ehrlichiosis is a serious disease with fatal effects in about 2.8% of infected humans and also affects other large mammals including dogs and livestock. It was first described in 1987. Symptoms in humans resemble that of spotted fever (see below). Ehlichiosis is difficult to diagnose because of its variable symptoms. The disease-causing organisms are several species of bacteria in the genus Ehrlichia carried principally to humans by the lone star tick (Amblyomma americanum), an aggressive biter and the commonest tick species in the southeastern states. Larval, adult and nymphal stages can bite humans and, if infected, transmit the disease. Separate diseases are actually involved, both recognized only in the last 25 years or so. The lone star tick feeds on large mammals, especially man, hogs, deer and dogs. Like other tick vectors, the larvae are tiny mite-like creatures that are easily missed when attached to the host. The lone star tick also is implicated in the recently recognized “Southern Tick Associated Rash Infection” which manifests a rash similar in appearance to Lyme disease, but does not produce the arthritic and neurological symptoms.
One of the bacteria species, E. canis, has caused ehrlichiosis in dogs, wolves, jackals, cats and lemurs bitten by the brown dog tick (Rhipicephalus sanguineus) while E. chaffeensis has infected humans, deer, dogs and coyotes. As in the case of Borrelia bacteria, Ehrlichia species and their tick vectors are disseminated by white-tailed deer, which often bring the parasites into contact with humans and pets. Formerly confined to the southern states, the lone star tick is moving north, along with its bacterial parasite. In the mid-Atlantic states, both the lone star and the black-legged tick are now common, leading, in some cases, to misdiagnosis of disease symptoms.
The brown dog tick readily enters homes on dogs, and has been known to bring the bacterial parasite into homes as well. Canine ehrlichiosis cases are being increasingly reported further north than the Gulf Coast region where the disease was once thought to be confined. The disease is now apparently also in the UK based on the finding of two infected dogs in 2012 that were never abroad or in contact with infected animals.
Rocky Mountain Spotted Fever
Rocky Mountain Spotted Fever, a potentially lethal disease that infects humans, cats and dogs, is caused in the US by Rickettsia rickettsii, a bacterium-like, rod-shaped microbe, which like viruses, lives only inside the host’s cells. Despite its name, this disease is widely distributed in the New World, ranging from Canada southward into South America. Other Rickettsia species caused spotted fever diseases in Australia, the Mediterranean, Africa and Japan. In the US, spotted fever is transmitted by the American dog tick (Dermacentor variabilis), and the Rocky Mountain wood tick (D. andersoni). Both species readily bite humans.
D. variabilis is found in the Midwest and eastern states and in California and D. andersoni is mainly a tick of the central and northern Rocky Mountains west to the Pacific Northwest. D. andersoni can also transmit the Colorado tick fever virus. Like other disease-carrying ticks, Dermacentor ticks bite (take a blood meal) from an infected host and the adults overwinter in the soil or leaf litter. Wildlife seldom contracts the disease but can carry the bacterium. Dermacentor larvae feed primarily on rodents including white-footed mice and take two to eight days to complete a meal. Nymphs prefer rodents, especially voles, raccoons, possums or pets. Both stages can survive months without feeding and larvae may enter diapause (similar to hibernation) in advance of unfavorable conditions. Pets may carry the tick home, spreading the disease to humans. Only a relatively “few” dog ticks are infected but the disease, if untreated, can be life-threatening to humans. In the southern states, spotted fever is more common than Lyme disease. The adults of D. andersoni are the principal transmitters of Rickettsia microbes to humans. The same tick species serves as one of several routes of transmission of a bacterium found in mammals, especially rodents, rabbits and hares, that causes tularemia, a potentially serious disease in humans.
*Ticks are not insects but are related to them. Ticks are more closely related to spiders and very closely related to mites. All these animals mentioned are eight-legged except insects which are six-legged.
Among the references consulted are: