Just by living in the world, human beings aresusceptible to disease. Many diseases — forexample, influenza and tuberculosis — are spreadwhen bacteria or viruses pass from one person toanother. Other diseases are acquired geneticallyfrom ones parents, such as cystic fibrosis.
Somediseases, such as heart disease and osteoporosis,develop as humans age. Others humans get from theenvironment; examples are lead poisoning and skincancer due to exposure to the sun. Finally, thereare diseases that can be transmitted from animalsto humans.Lyme disease is one such disease. Casesof Lyme disease, which is transmitted to humansand pets by the bite of a tick, are steadilyincreasing in areas such as the Northeast regionof the United States.
At present, there is novaccine for humans; the best way to prevent Lymedisease is to take precautions against beingbitten by a tick and allowing the tick to remainattached to the skin long enough for the infectionto be transmitted. The story of Lyme disease inthe United States began in 1975, when two mothers,Polly Murray and Judith Mensch, alarmed by thegreat number of cases of joint inflammation in thetheir communities of Lyme and Ease Haddam,Connecticut, contacted public health authorities(7:5). The health department contacted AllenSteere and his colleagues at Yale University, inNew Haven. Steere believed the outbreak mayprovide a clue about the infectious agent orenvironmental toxin that was responsible forarthritis (2:26).One early observation made bySteere was an association between the arthritisand a prior skin rash. A connection was then madebetween this rash and a similar one callederythema migrans, which comes from the bite of thesheep tick, Ixodes ricinus and is frequently foundin northern Europe (7:5). After field studies andpatient surveys were carried out the researchersreleased three essential findings which later ledto the discovery of the infectious agent. First,the disease was seasonal, occurring most commonlyin the summer and much less so in the middle ofthe winter.
In geographic areas such asConnecticut, these findings suggest that the viruswas either a summer virus or an infection carriedby and insect or a tick (7:35). Second, thedisease did not spread from one person in a familyto another.Summer viruses were commonly spreadfrom person to person, especially those living inthe same household.
When a summer virus waseliminated, the focus shifted to the involvementof arthropods (7:37). Third, the disease was muchmore prevalent on one side of the ConnecticutRiver than the other. Since the Connecticut Riverbisects the state, this became an important factorin finding the cause of the disease.
Theresearchers found a good correlation with thefrequency of a certain tick now named Ixodesscapularis. When furthered questioned, theaffected people remembered being previous bittenby a tick (2:27).At first, the infectious agentproducing Lyme disease was thought to have beeneither a virus, protozoan, fungi, or bacteria.Among the possible agents, bacteria was thehighest on the list. European physicians had beentreating patients with medicines that wereeffective against bacteria. However, thesefindings were not originally accepted in theUnited States.
It was only after some of thepatients in Steeres study were successfullytreated with antibiotics, which are effectiveagainst bacteria, but not against viruses,protozoan, or fungi, that bacteria was determinedto be the infectious agent (14:1015). The specificbacteria was found by Willy Burgdorfer, an experton a variety of tick-borne diseases. While workingat Montanas Rocky Mountain Laboratories,Burgdorfer received a shipment of ticks from NewYork.After looking at the contents of the ticksthrough a microscope, Burgdorfer found a wavy formof a spirochete that had never been seen in theIxodes group of ticks before. After determiningthat the same spirochetes were present in tickstaken from Long Island and New Jersey, the newlynamed Borrelia burgdorferi was confirmed as thebacteria that caused Lyme disease (1:47). The tickthat transmits Lyme disease is called Ixodesscapularis. In the northeastern and north-centralUnited States it is often called the deer tickbecause it is found so frequently on deer.
Thesouthern form of I. scapularis is usually calledthe black legged tick.The southern form of thespecies poses less of a threat of infection. Fewerof the southern ticks are infected and they tendto feed on other animal hosts rather than humans(2:43). The ticks that transmit Lyme diseasegenerally live about two years.
I. scapularislarvae, which are not much larger than the periodat the end of this sentence, hatch in the summerfrom eggs laid by the adult female that spring.They usually feed on a field mouse or other rodenthost that summer and into the early fall. Thelarvae then change into nymphs the first year;these nymphs pass through the winter withoutfeeding.the following spring and summer thenymphs feed on a rodent or other small animal. Atleast three out of four Lyme disease cases in theUnited States are from the bite of a nymphal tickthat occurs sometime between May and August.
Mostof the bites go unnoticed because the nymphs areso small–about the size of a poppy seed (2:45).In the second year, the nymph changes to an adultafter feeding. The larger ticks, especially thefemales, are more likely to be noticed by people.I. scapularis adults bite deer and other largemammals, such as humans. The adults feed later inthe year than the nymphs and larvae and may remainactive even as temperature drop to just above zeroin the late fall.
Lyme disease infection occurringin the fall, especially in the northeast, canusually be attributed to the bite of an adult(2:45-46). The Lyme disease spirochetes can remainactive and even multiply inside a ticks body, butseldom are they passed from an adult female to heroffspring. In order for the spirochetes to spreadin nature, an infected tick must feed on anotheranimal, thereby passing the spirochete from thatanimal to other feeding ticks.
Many types ofmammals and birds are capable of hosting thebacteria and of passing it on to other ticks, thuscompleting the vector-reservoir-vector cycle.Because about 99 percent of the larvae of I.scapularis do not carry the bacteria even if theiradult mother of father did, the larvae mustacquire the bacteria by feeding on an infectedhost.
In the case of deer ticks this host isusually a wild field mouse, called Peromyscusleucopus. In some regions more than half the miceare infected with Lyme disease bacteria, thusproviding a continuous reservoir of thespirochetes for many ticks.In high-risk areas forLyme disease, such as New Jersey, the chances thata larva will become infected is as least one infour (13:36). The first sign of the disease in 60to 80 percent of the cases is a rash–a reddishblotch or bulls eye pattern, often no more than 21/2 inches across. If left untreated it may, intwo weeks to a month, expand to four times thatsize. The rash does not always occur at the siteof the bite (11:9). Often it is found at thearmpit, groin, or the back of the knee.
However,in many cases of Lyme disease no rash occurs;therefore it may be necessary to look at otherfactors before making a diagnosis (16:41).Othercommon symptoms include chills, fever, fatigue,and other ….